In cardiac muscle the action potential is caused by opening of the voltage activated fast sodium channels and the L-type calcium channels, The heart is composed of the atrial syncytium and the ventricular syncytium., Cardiac muscle is refractory to restimulation during the action potential., Each atrium is a weak primer pump for the ventricle, Potentials are conducted from the atrial syncytium into the ventricular syncytium only by the A-V bundle

Phase 3 (rapid repolarization), calcium channels open and slow potassium channels close., Phase 4 (resting membrane potential) averages about −90 millivolts., Phase 2 (plateau), calcium channels open and fast potassium channels close, Phase 0 (depolarization), fast sodium channels open, Phase 1 (initial repolarization), fast sodium channels close.

Transport of calcium back into the sarcoplasmic reticulum is achieved with the help of a calcium–adenosine triphosphatase (ATPase) pump. , Calcium entering the cardiomyocytes triggers the release of calcium into the sarcoplasm from the sarcoplasmic reticulum. , The strength of contraction of cardiac muscle depends to a great extent on the concentration of calcium ions in the extracellular fluids , The duration of contraction of cardiac muscle is mainly a function of the duration of the action potential., The calcium release channels in the sarcoplasmic reticulum membrane are also called ryanodine receptor channels.

Each cycle is initiated by spontaneous generation of an action potential in the sinus node , There is a delay of more than 0.1 second during passage of the cardiac impulse from the atria into the ventricles, The atrial contraction usually causes an additional 20 percent filling of the ventricles, The delay of action potentials conduction on A-V node allows the atria to contract ahead of ventricular contraction., The heart beating at a very fast rate does not remain relaxed long enough to allow complete filling of the cardiac chambers before the next contraction

The ventricles fill with blood during diastole, During the middle third of diastole, only a small amount of blood normally flows into the ventricles , Approximately 60 percent of the blood in the ventricle at the end of diastole is ejected during systole, During the period of isovolumic relaxation the cardiac muscle tension is increasing but little or no shortening of the muscle fibers is occurring. , During the period of isovolumic contraction no ventricular emptying occurs

The semilunar valves (i.e., the aortic and pulmonary artery valves) prevent backflow from the aorta and pulmonary arteries into the ventricles during diastole. , The papillary muscles prevent the bulging of A-V valves too far backward toward the atria during ventricular contraction. , Because of the rapid closure and rapid ejection, the edges of the aortic and pulmonary valves are subjected to much greater mechanical abrasion than are the AV valves., The A-V valves (i.e., the tricuspid and mitral valves) prevent backflow of blood from the ventricles to the atria during systole., For anatomical reasons, the semilunar valves require rather rapid backflow for a few milliseconds to cause closure.

About 0.16 second after the onset of the P wave, the QRS waves appear as a result of electrical depolarization of the ventricles, The P wave is caused by spread of depolarization through the atria., The electrocardiogram waves are electrical voltages generated by the heart and recorded by the electrocardiograph from the surface of the body. , The T wave occurs slightly before the end of ventricular contraction, The QRS complex begins slightly before the onset of ventricular systole.

During diastole, normal filling of the ventricles increases the volume of each ventricle to a volume called the end diastolic volume., The remaining volume in each ventricle at the end of systole is called the end-systolic volume. , As the ventricles empty during systole, the volume decreases about 70 milliliters (the stroke volume output)., The ejection fraction is usually equal to about 0.6 (or 60 percent). , The fraction of the end-diastolic volume that is ejected is called the ejection fraction

Increasing during fast ejection, Decreasing during isovolumic ventricular contraction, Increasing during third heart sound, Increasing after opening aortic valve, Decreasing during rapid inflow

Increasing slightly during atrial contraction, Increasing after open aortic valve, Increasing after closing the mitral valve, Decreasing during isovolumic relaxation, Decreasing during ventricular filling

Closes at the beginning of isovolumic contraction, Closes after atrial contraction, Closure generates the first heart sound, Opens at the beginning of rapid inflow, Opens at the end of isovolumic relaxation

When the left ventricle contracts, the ventricular pressure increases rapidly until the aortic valve opens., After the aortic valve has closed, the pressure in the aorta becomes equal to left ventricular pressure. , The pressure in the aorta decreases slowly throughout ventricular filling., An incisura occurs in the aortic pressure curve when the aortic valve closes., Before the ventricle contracts again, the aortic pressure usually has fallen to about 80 mm Hg (diastolic pressure).

The first hears a sound has a low vibration pitch and it is long-lasting., When the valves close, the vanes of the valves and the surrounding fluids vibrate under the influence of sudden pressure changes, giving off sound that travels in all directions through the chest. , When listening to the heart with a stethoscope one does not hear the opening of the valves., When the aortic and pulmonary valves close at the end of systole, one hears the second heart sound. , When the ventricles contract, one first hears a sound caused by closure of the A-V valves

During period of ejection (Phase III) the volume of the ventricle decreases from endsytolic volume to end-diastolic volume., Period of filling (Phase I) extends from end-systolic volume (50 milliliters) to the enddiastolic volume (120 milliliters)., During period of isovolumic relaxation (Phase IV) the intraventricular pressure decreases without any change in volume., At the end of the period of ejection, the aortic valve closes, and the ventricular pressure falls back to the diastolic pressure level. , During period of isovolumic contraction (Phase II) the pressure inside the ventricle increases to equal the diastolic pressure in the aorta (80 mm Hg).

For cardiac contraction, the preload is usually considered to be the end-diastolic pressure when the ventricle has become filled., The degree of tension on the muscle when it begins to contract, which is called the preload., The load against which the muscle exerts its contractile force, which is called the afterload., Approximately 70 to 90 percent of heart muscle energy is normally derived from oxidative metabolism of fatty acids., Oxygen consumption has also been shown to be nearly proportional to the tension that occurs in the heart muscle during contraction multiplied by the duration of time that the contraction persists.

Stretch of the right atrial wall directly increases the heart rate by 10 to 20 percent. , Strong sympathetic stimulation can increase the heart rate in young adult humans from the normal rate of 70 beats/min up to 180 to 200., Cardiac output often can be increased more than 100 percent by sympathetic stimulation, Within physiological limits, the heart pumps all the blood that returns to it by way of the veins, Sympathetic stimulation can the force of heart contraction thereby increasing the volume of blood pumped and increasing the ejection pressure.

Excess calcium ions cause the heart to move toward spastic contraction. , Excess potassium in the extracellular fluids causes the heart to become dilated and flaccid and also slows the heart rate., Increased body temperature, such as that which occurs when one has fever, greatly increases the heart rate. , Elevation of potassium concentration to only 8 to 12 mEq/L can cause severe weakness of the heart, abnormal rhythm, and death. , Large quantities of potassium also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle.

Without the calcium from the T tubules, the strength of cardiac muscle contraction would be reduced considerably., The sarcoplasmic reticulum of cardiac muscle is less well developed than that of skeletal muscle, Cardiac muscle begins to contract a few milliseconds after the action potential begins, At the end of the plateau of the cardiac action potential, the influx of calcium ions to the interior of the muscle fiber is suddenly cut off. , The T tubule action potentials in turn act on the membranes of the longitudinal sarcoplasmic tubules to cause release of calcium ions.

Increased myocardial strength (β1 adrenergic receptors) , Increases the overall activity of the heart, Thermogenesis (β3 adrenergic receptors), Cardioacceleration (β1 adrenergic receptors), Vasoconstriction (alpha adrenergic receptor)

After opening of the aortic valve, the intraventricular volume rapidly decreases, After closing of the mitral valve, the intraventricular pressure rapidly increases, After closing of the aortic valve, the intraventricular pressure rapidly increases, After opening of the mitral valve, the intraventricular volume rapidly decreases , If both the mitral and aortic valves are closed, the intraventricular pressure does not change.

The Purkinje fibers could discharge at an intrinsic rate somewhere 15 and 40 times per minute. , The sinus node is the pacemaker because it has the fastest rate of rhythmical discharge., The sinus node is almost always the pacemaker of the normal heart., The A-V nodal fibers could discharge at an intrinsic rate of 40 to 60 times per minute, Other parts of the heart than sinus node, that exhibit intrinsic rhythmical excitation.

The normal rhythmical impulses of the heart are generated in the sinus node., In normal conditions, all portions of the ventricles are contracting almost simultaneously., The rhythmical and conductive system of the heart is susceptible to damage by heart disease, especially by ischemia of the heart tissues., The human heart has a special system for rhythmic self-excitation and repetitive contraction., The internodal pathways conduct impulses from the sinus node to the atrioventricular node.

After the action potential is over the inward-leaking Na+ and Ca2+ overbalance the K+ outflow., Between heartbeats, influx Na+ causes a slow rise in the membrane potential of sinus nodal cells. , The rhythmical discharge of a sinus nodal fiber could continue throughout a person’s life. , Self-excitation of sinus node cells generates their own action potential., The inherent leakiness of the sinus nodal fibers to Na+ and Ca2+ causes the selfexcitation.

The atrioventricular node delays impulse conduction from the atria to the ventricles., The ends of the sinus nodal fibers connect directly with surrounding atrial muscle fibers, Usually, muscle bridges penetrate the fibrous barrier between atrial and ventricular syncytium boosting the cardiac impulse to re-enter the atria from the ventricles. , The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junctions, A special characteristic of the A-V bundle is the inability of action potentials to travel backward from the ventricles to the atria.

Parasympathetic (vagal) stimulation slows the cardiac rhythm and conduction , Norepinephrine stimulates beta-1 adrenergic receptors, which mediate the effects on heart rate. , The beta-1 adrenergic stimulation increases the permeability of the fiber membrane to sodium and calcium ions., Sympathetic stimulation increases the cardiac rhythm and conduction, If the vagal stimulation is strong enough, it is possible to stop the rhythmical self-excitation of the entire heart.

Before the blockage, the Purkinje fibers had been “overdriven” by the rapid sinus impulses and, consequently, are in a suppressed state., Delayed resumption of heartbeat is called Stokes-Adams syndrome., A new pacemaker usually develops in the Purkinje system of the ventricles and drives the ventricular muscle at a rate of 15 to 40 bpm. , A pacemaker elsewhere than the sinus node is called an “ectopic” pacemaker. , After sudden A-V bundle block, the Purkinje system does not begin to emit its intrinsic rhythmical impulses until 5 to 20 seconds later.

In the ventricular myocardium: norepinephrine → hyperpolarization. , In the A-V node: norepinephrine → ↑ conduction velocity , Stimulation of beta-1 adrenergic receptors → ↑ permeability for Na+ and Ca2+., In the sinus node: norepinephrine → accelerate self-excitation. , Norepinephrine → stimulates beta-1 adrenergic receptors in the heart.

They are all similar to the standard limb lead recordings, except that the recording from the aVR lead is inverted. , When the positive terminal is on the left leg, it is known as the aVF lead., When the positive terminal is on the left arm, the lead is known as the aVL lead. , When the positive terminal is on the right arm, the lead is known as the aVR lead. , Two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal.

The QRS complex is caused by potentials generated as the repolarization wave spreads through the ventricles. , The QRS complex is caused by potentials generated when the ventricles depolarize before contraction., No potential is recorded in the ECG when the ventricular muscle is either completely polarized or completely depolarized., The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins., The T wave is caused by potentials generated as the ventricles recover from the state of depolarization.

The time between the beginning of the P wave and the beginning of the QRS complex is the interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles. , The interval from the beginning of the QRS complex to the end of the T wave is called the Q-T interval., The time between the beginning of the P wave and the beginning of the QRS complex is called the P-R interval., The Q-T interval ordinarily is about 0.35 second., The normal P-R interval is about 0.16 second.

The standard bipolar limb leads record positive T waves., In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal is connected to the left arm., The standard bipolar limb leads record positive P waves., To record limb lead III, the negative terminal of the electrocardiograph is connected to the left arm and the positive terminal is connected to the left leg., To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal is connected to the left leg.

Extending the ECG to allow assessment of cardiac electrical events while the patient is ambulating during normal daily activities is called ambulatory electrocardiography., Improvements in digital technology permit transmission of digital ECG data and rapid “online” computerized analysis of the data as they are acquired., Ambulatory ECG monitoring is typically used when a patient demonstrates symptoms that are thought to be caused by transient arrhythmias or other transient cardiac abnormalities. , Standard ECGs provide assessment of cardiac electrical events over a brief duration, usually while the patient is resting., Ambulatory ECG monitoring is typically used when a patient demonstrates symptoms as chest pain, syncope (fainting) or near syncope, dizziness, and irregular heartbeats.

When a vector is exactly horizontal and directed toward the person’s left side, the vector is said to extend in the direction of 0 degrees., When the vector extends from above and straight downward, it has a direction of +90 degrees, When the vector extends from the person’s left to right, it has a direction of +180 degrees , When the vector extends straight upward, it has a direction of −90 degrees., In a normal heart, the mean QRS vector, is about +59 degrees.

Lead III has an axis of about +120 degrees, Lead II has an axis of about +60 degrees, Lead aVF has an axis of about +90 degrees, Lead aVL has an axis of about -30 degrees, Lead I has an axis of about 0 degrees

The outer apical surfaces of the ventricles repolarize before the inner surfaces, The greatest portion of ventricular muscle mass to repolarize first is the entire outer surface of the ventricles, especially near the apex of the heart., The normal T wave in all three bipolar limb leads is positive., The high blood pressure inside the ventricles during contraction greatly reduces coronary blood flow to the endocardium slowing repolarization in the endocardial areas., The positive end of the overall ventricular vector during repolarization is toward the apex of the heart.

Shift to the left occurs quite frequently in obese people., Shift to the right occurs normally in tall people. , Shift to the left occurs at the end of deep expiration., Shift to the right occurs at the end of deep inspiration. , Shift to the left occurs when a person lies down.

The left bundle branch is blocked, The left ventricle hypertrophies as a result of congenital heart conditions in which the left ventricle enlarges while the right ventricle remains relatively normal in size , The left ventricle hypertrophies as a result of aortic valvular stenosis , The left ventricle hypertrophies as a result of hypertension, The left ventricle hypertrophies as a result of aortic valvular regurgitation

Fluid in the pericardium, Diminished myocardial mass, Pulmonary emphysema, Pleural effusion , A series of old myocardial infarctions

Flows between the pathologically depolarized and the normally polarized areas, even between heartbeats, Could be caused by mechanical trauma, Could be caused by infectious processes that damage the muscle membranes , Could be caused by ischemia of local areas of heart muscle, Could appears in the ECG during an attack of severe angina pectoris

The T wave becomes abnormal when the normal sequence of repolarization does not occur, When conduction of the depolarization impulse through the ventricles is greatly delayed, the T wave is almost always of opposite polarity to that of the QRS complex., Changes in the T wave during digitalis administration are often the earliest signs of digitalis toxicity. , One means for detecting mild coronary insufficiency is to have the patient exercise and to record the ECG, noting whether changes occur in the T waves. , Mild ischemia causes shortening of depolarization of cardiac muscle that can cause T-wave abnormalities.

Shift of the pacemaker from the sinus node to another place in the heart , Blocks at different points in the spread of the impulse through the heart, Spontaneous generation of spurious impulses in almost any part of the heart , Abnormal pathways of impulse transmission through the heart, Abnormal rhythmicity of the pacemaker

Increased body temperature , Severe blood loss, Weakening of the myocardium, Toxic conditions of the heart, Stimulation of the heart by the sympathetic nerves

In patients with carotid sinus syndrome, the baroreceptors in the carotid sinus region of the carotid artery walls are excessively sensitive. , The well-trained athlete’s heart is often larger and considerably stronger than that of a normal person. , The heart rate increased and decreased no more than 5 percent during quiet respiration., If body temperature increases more than 40.5°C, the heart rate may decrease because of progressive debility of the heart muscle because of the fever. , Sinus arrhythmia can result from any one of many circulatory conditions that alter the strengths of the sympathetic and parasympathetic nerve signals to the heart sinus node.

When there will be an atrial P wave but no QRS-T wave. , When the action potential is sometimes strong enough to pass through the bundle into the ventricles and sometimes not strong enough to do so, When there are “dropped beats” of the ventricles., Could be caused by an abnormality of the bundle of His-Purkinje system. , When conduction through the A-V bundle is slowed enough to increase the P-R interval to 0.25 to 0.45 second.

Ischemia of the A-V bundle, Extreme stimulation of the heart by the vagus nerves, Inflammation of the A-V node or A-V bundle , Compression of the A-V bundle by scar tissue, Ischemia of the A-V node

The P waves become dissociated from the QRS-T complexes., After sudden cessation of A-V conduction the ventricles start their own beating following a delay as a result of overdrive suppression. , Complete block of the impulse from the atria into the ventricles occurs in the A-V node or A-V bundle. , Because the heart does not pump any blood for 5 to 30 seconds, most people faint a few seconds after complete block occurs., The ventricles “escaped” from control by the atria and are beating at their own natural rate.

Possible causes of ectopic foci are toxic irritation of the A-V node, Purkinje system, or myocardium caused by infection, drugs, nicotine, or caffeine., Premature ventricular contractions (PVCs) has QRS complex is prolonged and has a high voltage., A-V nodal premature contractions have the same significance and causes as atrial premature contractions., The interval between the premature contraction and the next succeeding contraction is slightly prolonged (a compensatory pause). , When the heart contracts ahead of schedule, a deficit in the number of radial pulses occurs when compared with the actual number of contractions of the heart (pulse deficit).

The long QT syndrome (LQTS) increases a person’s susceptibility to developing ventricular arrhythmias called torsades de pointes. , In torsades de pointes, premature ventricular beats lead pauses, postpause prolongation of the Q-T interval, and arrhythmias, The acquired forms of LQTS could be associated with hypomagnesemia, hypokalemia, or hypocalcemia., The congenital forms of LQTS could be caused by mutations of sodium or potassium channel genes. , The long Q-T interval may trigger arrhythmias, tachycardia, and in some instances ventricular fibrillation.

ECG could show almost normal QRS-T complexes but totally missing or obscured P waves for an A-V nodal paroxysmal tachycardia. , Is believed to be caused most frequently by re-entrant “circus movement” pathways., ECG could show an inverted P wave before each QRS-T complex for an atrial paroxysmal tachycardia. , Is associated with rapid rhythmical discharge of impulses that spread in all directions throughout the heart., ECG has the appearance of a series of ventricular premature beats occurring one after another without any normal beats interspersed for ventricular paroxysmal tachycardia.

ECG shows no tendency toward a regular rhythm of any type., Results from cardiac impulses that have gone out of control within the ventricular muscle mass, Could be initiate by ischemia of the heart muscle, of its specialized conducting system, or both , Makes the ventricular chambers to remain in an indeterminate stage of partial contraction with pumping either no blood or negligible amounts, The most serious of all cardiac arrhythmias

Decreased the efficiency of ventricular pumping by 20 to 30 percent, Is frequent caused by atrial enlargement , ECG demonstrates irregularity of ventricular rhythm , Has the same mechanism of as ventricular fibrillation, ECG could show either no P waves from the atria or only a fine, high-frequency, very low voltage wavy record

Shows on ECG that QRS complex follows an atrial P wave only once for every two to three beats of the atria, giving a 2 : 1 or 3 : 1 rhythm., Shows on ECG strong P waves because of contraction of semicoordinate masses of muscle. , Because the signals reach the A-V node too rapidly, there are usually 2 to 3 beats of the atria for every single beat of the ventricles., Is caused by a circus movement in the atria, Causes a rapid rate of contraction of the atria (200 to 350/minute).

Can cause death, Is a final serious abnormality of the cardiac rhythmicity conduction system, Can be caused by severe myocardial disease , Results from cessation of all electrical control signals in the heart , from anesthesia responds to prolonged CPR.

Transport hormones from one part of the body to another , Serve the needs of the body tissues, Transport nutrients to transport waste products away. , Transport nutrients to the body tissues , Maintain an appropriate environment in all the tissue fluids of the body for survival and optimal function of the cells.

Blood flow to the kidney is related to its excretory function, which requires that a large volume of blood be filtered each minute. , Blood flow to the kidney is in excess of its metabolic requirements. , The heart and blood vessels, in turn, are controlled to provide the necessary arterial pressure to cause the needed tissue blood flow. , The heart and blood vessels are controlled to provide the necessary cardiac output to cause the needed tissue blood flow., The rate of blood flow through many tissues is controlled mainly in response to their need for nutrients.

Arteries are transporting oxygenated blood, Is also called the peripheral circulation, Veins are transporting deoxygenated blood , Supplies blood flow to all the tissues of the body except the lungs., Is also called the greater circulation.

The veins function as conduits for transport of blood from the venules back to the heart , The venules collect blood from the capillaries, The arterioles are the last small branches of the arterial system , The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid, The function of the arteries is to transport blood under high pressure to the tissues

84 percent of the entire blood volume of the body is in the systemic circulation , 64 percent is in the veins, 7 percent is in the systemic arterioles and capillaries , 13 percent is in the arteries, , 16 percent is in the heart and lungs

Under resting conditions, the velocity in the capillaries is 1000 times lower than in the aorta , Aorta has a cross-sectional area of 2.5 cm2, If of blood flow is the same, the velocity of blood flow is inversely proportional to vascular cross-sectional area, The cross-sectional area of the veins are much larger than those of the arteries , Capillaries have the highest cross-sectional area

The mean pressure in the aorta is high (about 100 mm Hg) because the heart pumping is pulsatile. , In the lungs, the blood in the pulmonary capillaries is mixed with oxygen and other gases in the pulmonary alveoli, The average “functional” pressure in most vascular beds is about 17 mm Hg. , The arterial pressure alternates between a systolic pressure (120 mm Hg) and a diastolic pressure (80 mm Hg) because heart pumping is pulsatile. , The mean pressure is about 0 mm Hg in the superior and inferior venae cavae where they empty into the right atrium of the heart.

Cardiac output is the sum of all the local tissue flows., Blood flow to most tissues is controlled according to the tissue need , Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control., The microvessels act directly on the local blood vessels, dilating or constricting arteries and veins. , Nervous control of the circulation from the central nervous system and hormones provide additional help in controlling tissue blood flow.

The nervous signals cause contraction of the large venous reservoirs to provide more blood to the heart , The kidneys play an additional major role in pressure control by regulating the blood volume, The kidneys play an additional major role in pressure control by secreting pressurecontrolling hormones., The nervous signals increase the force of heart pumping , The nervous signals cause generalized constriction of the arterioles.

When pressure gradient is constant, the blood flow through a blood vessel is higher if the resistance is lower., Blood flow through a blood vessel is determined by pressure gradient along the vessel, and vascular resistance. , The blood flow is directly proportional to the pressure difference but inversely proportional to the resistance., The pressure gradient represents the pressure difference of the blood between the two ends of the vessel, which pushes the blood through the vessel., The vascular resistance is the impediment to blood flow through the vessel.

The flow may then become turbulent when it passes by an obstruction in a vessel, when it makes a sharp turn, or when it passes over a rough surface. , In the laminar flow or streamline flow with each layer of blood remaining the same distance from the vessel wall., Blood flow means the quantity of blood that passes a given point in the circulation in a given period of time. , When laminar flow occurs, the velocity of flow in the center of the vessel is far greater than that toward the outer edges., The ultrasonic Doppler flowmeter is capable of recording rapid, pulsatile changes in flow, as well as steady flow.

The mean left atrial pressure averages 2 mm Hg., Blood pressure means the force exerted by the blood against any unit area of the vessel wall. , In the pulmonary system, the net pressure difference is 14 mm., The normal right atrial pressure is about 0 mm Hg, The mean pulmonary arterial pressure averages 16 mm Hg.

The conductance of the vessel increases in proportion to the fourth power of the diameter. , The rate of blood flow is directly proportional to the fourth power of the radius of the vessel., Resistance is the impediment to blood flow in a vessel., The diameter of a blood vessel plays by far the greatest role of all factors in determining the blood pressure through a vessel. , Conductance is a measure of the blood flow through a vessel for a given pressure difference

Poiseuille’s Law shows that the rate of blood flow is inversely proportional with viscosity of the blood. , Increasing hematocrit markedly increases blood viscosity., Anemia decreases the blood viscosity and increases the blood flow., The greater the viscosity, the lower the flow in a vessel if all other factors are constant., The plasma protein concentration and types of proteins in the plasma, could affect the blood viscosity.

Blood pumped by the heart flows through blood vessels arranged in series and in parallel., The arteries, arterioles, capillaries, venules, and veins are collectively arranged in series., Brain, kidney, muscle, gastrointestinal, skin, and coronary circulations are arranged in parallel, and blood flow through each tissue is determined by the resistance in that tissue(/organ)., When blood vessels are arranged in series, flow through each blood vessel is the same. , Adding more blood vessels to a parallel circuit increases the total vascular resistance.

Inversely proportional to the length of the vessel, Directly proportional to the fourth power of the radius of the vessel, Directly proportional to the pressure difference between the ends of the vessel , Inversely proportional to the viscosity of the blood, Depending mainly by the diameter of the blood vessel

The conductance of the vessel increases in proportion to the fourth power of the diameter. , Removal of a kidney reduces the total vascular conductance and cardiac output while increasing total peripheral vascular resistance., Conductance is the exact reciprocal of resistance., Small changes in vessel diameter markedly change its conductance. , The total conductance for blood flow is the sum of the conductance of each parallel pathway.

The distensible nature of the arteries provides smooth, continuous flow of blood through the very small blood vessels of the tissues. , The distensible nature of the veins provides a reservoir for storing large quantities of extra blood. , The distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations. , All blood vessels are distensible., The most distensible of all the vessels are the veins.

Compliance is equal to distensibility times volume., Because the walls of the arteries are thicker and stronger than those of the veins, the veins are about eight times more distensible than the arteries. , The compliance represent the total quantity of blood that can be stored in a given portion of the circulation for each mm Hg pressure rise. , Vascular distensibility normally is expressed as the fractional increase in volume for each millimeter of mercury rise in pressure. , The compliance of a systemic vein is about 24 times that of its corresponding artery because it is about 8 times as distensible and has a volume about 3 times as great.

One-half liter of blood can be transfused into a healthy person in only a few minutes without greatly altering the function of the circulation. , When the arterial system of the average adult person is filled with about 700 milliliters of blood, the mean arterial pressure is 100 mm Hg, When the arterial system of the average adult person is filled with only 400 milliliters of blood, the pressure falls to zero., In the entire systemic venous system, the volume normally ranges from 2000 to 3500 milliliters. , A change of several hundred milliliters in the volume of the venous system is required to change the venous pressure only 3 to 5 mm Hg.

An increase in vascular smooth muscle tone caused by sympathetic stimulation increases the pressure at each volume of the arteries and veins. , Sympathetic inhibition decreases the pressure at each volume of the arteries and veins. , Enhancement of sympathetic tone reduces the vessel sizes enough that the circulation continues to operate almost normally even when as much as 25 percent of the total blood volume has been lost. , Sympathetic control of vascular capacitance is highly important during hemorrhage., An increase in vascular tone throughout the systemic circulation can cause large volumes of blood to shift into the heart.

Means that a vessel exposed to increased volume at first exhibits an increase in pressure, but delayed stretching allows the pressure to return toward normal., Is one of the ways in which the circulation automatically adjusts itself over a period of minutes or hours to diminished blood volume after serious hemorrhage., Is a characteristic of all smooth muscle tissue and is called stress-relaxation. , Is a valuable mechanism by which the circulation can accommodate extra blood after too large a transfusion. , Is a valuable mechanism by which the circulation can accommodate extra blood when necessary.

The compliance of the arterial tree normally reduces the pressure pulsations to almost no pulsations by the time the blood reaches the capillaries., Were it not for distensibility of the arterial system, the new ejected blood would have to flow only during cardiac systole, and no flow would occur during diastole. , Were it not for distensibility of the arterial system, all ejected blood would have to flow through the peripheral blood vessels almost instantaneously, only during cardiac systole., Tissue blood flow is mainly continuous with very little pulsation., The difference between the systolic pressure and the diastolic pressure (about 40 mm Hg), is called the pulse pressure.

The total distensibility of the arterial tree , The stroke volume output, The character of ejection from the heart during systole, The compliance of the arterial tree , The mean arterial pressure

Lower compliance of the arterial system, Greater stroke volume output , Greater ratio of stroke volume output to compliance of the arterial tree , Greater amount of blood that must be accommodated in the arterial tree with each heartbeat, Arteriosclerosis

The pulse pressure increases to more than 100 mm Hg because aortic regurgitation. , Sharp incisura when the aortic valve closes , Before the ventricle contracts again, the aortic pressure decrease to about 80 mm Hg (diastolic pressure). , Sharp upstroke from 80 mm Hg, following the opening of aortic valve, The aortic pressure can fall all the way to zero between heartbeats because aortic regurgitation

Transmission of the pressure pulse in the arteries represents spreading of the wave front of distention along the arteries during systole., The velocity of pressure pulse transmission in the normal aorta < in the large arterial branches < in the small arteries , The pressure pulse is simply a moving wave of pressure that involves little forward total movement of blood volume , The velocity of pressure pulse transmission is slower if the compliance is the greater, The velocity of transmission of the pressure pulse is 15 or more times the velocity of blood flow

Only when the aortic pulsations are extremely large, or the arterioles are greatly dilated can pulsations be observed in the capillaries., The resistance damps the pulsations because a small amount of blood must flow forward at the pulse wave front to distend the next segment of the vessel. , The compliance damps the pulsations because the more compliant a vessel, the greater the quantity of blood required at the pulse wave front to cause an increase in pressure. , The degree of damping is almost directly proportional to the product of resistance times compliance, The progressive diminution of the pressure pulsations in the periphery is called damping of the pressure pulses.

It is not equal to the average of systolic and diastolic pressure , It is determined about 60 percent by the diastolic pressure and 40 percent by the systolic pressure , It is more closely approximated as the average of systolic and diastolic pressures at very high heart rates., At all ages is nearer to the diastolic pressure than to the systolic pressure, It is the average of the arterial pressures measured millisecond by millisecond over a period of time

As long as the cuff continues to compress the arm with too little pressure to close the brachial artery, no sounds are heard. , When the cuff pressure is great enough to close the artery during part of the arterial pressure cycle, a sound is then heard with each pulsation., Just as soon as the pressure in the cuff falls below systolic pressure blood begins to flow through the artery beneath the cuff during the peak of systolic pressure., The clinician determines systolic and diastolic pressures through indirect means, usually by the auscultatory method., A stethoscope is placed over the antecubital artery and a blood pressure cuff is inflated around the upper arm.

The Korotkoff sounds are believed to be caused mainly by blood jetting through the partly occluded vessel and by vibrations of the vessel wall., As long as this cuff pressure is higher than systolic pressure, the brachial artery remains collapsed so no Korotkoff sounds are heard., When the cuff pressure is great enough to close the artery during a part of the arterial pressure cycle, Korotkoff sounds are heard with each pulsation. , Many clinicians believe that the pressure at which the Korotkoff sounds completely disappear should be used as the diastolic pressure. , When the cuff pressure is gradually reduced, as soon as these sounds begin to be heard, the pressure level indicated by the manometer connected to the cuff is about equal to the systolic pressure.

A slight extra increase in systolic pressure usually occurs beyond the age of 60 years., The kidneys are primarily responsible for the long-term regulation of arterial pressure., The kidneys exhibit definitive changes with age, especially after the age of 50 years. , The progressive increase in pressure with age results from the effects of aging on the blood pressure control mechanisms. , The slight increase in systolic pressure beyond the age of 60 years results from decreasing distensibility of the arteries, which is often a result of atherosclerosis.

Provide passageways for flow of blood to the heart. , Are capable of constricting and enlarging., Make the stored blood available when it is required by the remainder of the circulation, Help to regulate cardiac output by propelling blood forward by means of a venous pump. , Could store either small or large quantities of blood.

If the right heart is pumping strongly, the right atrial pressure decreases. , Blood from all the systemic veins flows into the right atrium of the heart., Any factor that causes rapid inflow of blood into the right atrium from the peripheral veins elevates the right atrial pressure., Weakness of the heart elevates the right atrial pressure., The pressure in the right atrium is called the central venous pressure.

Increased large vessel tone throughout the body with resultant increased peripheral venous pressures, Dilation of the arterioles, which decreases the peripheral resistance and allows rapid flow of blood from the arteries into the veins, After massive transfusion of blood , Serious heart failure , Increased blood volume

Large veins have so little resistance to blood flow when they are distended that the resistance then is almost zero and is of almost no importance. , The pressure in the neck veins often falls so low that the atmospheric pressure on the outside of the neck causes these veins to collapse. , Most of the large veins that enter the thorax are compressed at many points by the surrounding tissues so that blood flow is impeded at these points. , Veins coursing through the abdomen are often compressed by different organs and by the intra-abdominal pressure., The veins from the arms are compressed by their sharp angulations over the first rib

In the arm veins, the pressure at the level of the top rib is usually about +6 mm Hg because of compression of the subclavian vein as it passes over this rib., In the standing position, the venous pressure in the sagittal sinus at the top of the brain is about −10 mm Hg. , When a person is standing, the pressure in the right atrium remains about 0 mm Hg., The veins inside the skull are in a noncollapsible chamber and thus they cannot collapse., In an adult who is standing absolutely still, the pressure in the veins of the feet is about +90 mm Hg.

If a person stands perfectly still, the venous pump does not work , Every time a person moves the legs or even tenses the leg muscles, a certain amount of venous blood is propelled toward the heart. , The valves in the veins are arranged so that the direction of venous blood flow can be only toward the heart, The “venous pump” is efficient enough that under ordinary circumstances the venous pressure in the feet of a walking adult remains less than +20 mm Hg. , If a person stands perfectly still the pressures in the capillaries increase greatly, causing fluid to leak from the circulatory system into the tissue spaces

When valves of the venous system become “incompetent”, the pressure in the veins of the legs increases greatly because of failure of the venous pump. , The valves of the venous system may become “incompetent” or even be destroyed when the veins have been overstretched by excess venous pressure which can occur in pregnancy. , The valves of the venous system may become “incompetent” when the veins have been overstretched by excess venous pressure lasting weeks or months, which can occur when one stands most of the time. , When the lack of complete closure of valves occurs, the pressure in the veins of the legs increases greatly, which further increases the sizes of the veins and finally destroys the function of the valves entirely., When the function of the valves is destroyed entirely, the person develops “varicose veins,” which are characterized by large, bulbous protrusions of the veins beneath the skin of the entire leg, particularly the lower leg.

When the venous and capillary pressures become very high, the leakage of fluid from the capillaries could cause edema in the legs. , For people with varicose veins, continual elevation of the legs to a level at least as high as the heart could can be of considerable assistance in preventing the edema and its sequelae. , Whenever people with varicose veins stand for more than a few minutes, the venous and capillary pressures become very high., The edema prevents adequate diffusion of nutritional materials from the capillaries to the muscle and skin cells., As a result of decreased diffusion of nutritional materials from the capillaries to the tissue cells, the muscles to become painful and weak, and the skin may even become gangrenous and ulcerate.

Venous pressure often can be estimated by simply observing the degree of distention of the peripheral veins., When the right atrial pressure becomes increased to as much as +10 mm Hg, the lower veins of the neck begin to protrude., When the right atrial pressure becomes increased to as much as +15 mm Hg, essentially all the veins in the neck become distended. , In the supine position, the carotid arteries are never distended in the normal quietly resting person. , In the sitting position, the neck veins are never distended in the normal quietly resting person.

If the pressure at the tricuspid valve falls, the right ventricle fails to fill, its pumping decreases, and blood dams up in the venous system until the pressure at the tricuspid level again rises to the normal value., At the level of the tricuspid valve gravitational pressure factors caused by changes in body position of a healthy person usually do not affect the pressure measurement., When a person is lying down, the tricuspid valve is located at almost exactly 60% of the chest thickness in front of the back (the zero pressure reference level for a person lying down). , If the pressure at the tricuspid valve rises slightly, the right ventricle fills to a greater extent, causing the heart to pump blood more rapidly and therefore to decrease the pressure at the tricuspid valve back to normal value. , The heart acts as a feedback regulator of pressure at the tricuspid valve, the reference level for pressure measurement.

The large abdominal veins can contribute as much as 300 milliliters to circulating blood volume. , The venous plexus beneath the skin, can contribute several hundred milliliters to circulating blood volume. , The spleen can decrease in size sufficiently to release as much as 100 milliliters of blood into other areas of the circulation., The venous system serves as a blood reservoir allowing the circulatory system to function almost normally even after as much as 20% of the total blood volume has been lost., The liver sinuses can release several hundred milliliters of blood into circulation.

The red pulp of the spleen is a special reservoir that contains large quantities of concentrated red blood cells, trapped by the trabeculae., The concentrated red blood cells can be expelled into circulation, whenever the sympathetic nervous system becomes excited and causes the spleen and its vessels to contract. , The spleen has two separate areas for storing blood: the venous sinuses and the pulp. , The venous sinuses can swell the same as any other part of the venous system and store whole blood., As much as 50 milliliters of concentrated red blood cells can be released into the circulation, raising the hematocrit 1-2% if sympathetic nervous causes the spleen and its vessels to contract.

The pulp of the spleen contains many large phagocytic reticuloendothelial cells, and the venous sinuses are lined with similar cells, When the blood is invaded by infectious agents, the reticuloendothelial cells of the spleen rapidly remove debris, bacteria, parasites, and so forth., The large phagocytic reticuloendothelial cells from spleen function as part of a cleansing system for the blood. , The phagocytic reticuloendothelial cells from spleen perform their cleansing function acting in concert with a similar system of reticuloendothelial cells in the venous sinuses of the liver., In many chronic infectious processes, the spleen enlarges and then performs its cleansing function even more avidly.

In the splenic pulp, the capillaries are so permeable that red blood cells, oozes through the capillary walls into a trabecular mesh, forming the red pulp. , The fragile red blood cells cannot withstand the thorough squeezing to which they are subjected as they pass through the splenic pulp before entering the sinuses., After erythrocytes rupture, the released hemoglobin is digested by the reticuloendothelial cells of the spleen. , The red cells could become trapped by the trabeculae, while the plasma flows on into the venous sinuses and then into the general circulation., Many of the red blood cells destroyed in the body have their final demise in the splenic pulp

A systemic vein has a volume of blood about 3 times as great as its corresponding artery., In the pulmonary circulation, the pulmonary vein distensibilities are similar to those of the systemic circulation. , The walls of the arteries are thicker and stronger than those of the veins, so the veins are much more distensible than the arteries., The compliance of a systemic vein is about 24 times that of its corresponding artery., A systemic vein is about 8 times as distensible as its corresponding artery.