An organ consists of two or more types of tissues, The stomach consists of all four main types of tissues: epithelial, connective, muscle and nervous, The muscle system includes striated muscles, smooth muscles and the cardiac muscle, The urinary system consists of the kidneys, the bladder and the associated urinary tracts

The human body contains three serous membranes: the pleura, the pericardium and the peritoneum, Each of the three serous membranes has a parietal and a visceral sheet, The parietal sheet of the peritoneum lines the abdominal and the pelvic cavities, The space between the peritoneal sheets is called peritoneal cavity, the space between pericardial sheets is called pericardial cavity

The nucleus delimited by the membrane is present in eukaryote cells, The nucleus is present in all the cells of the human body, with the exception of red blood cells (erythrocytes), The dense mass of the nucleus containing RNA (ribonucleic acid) is called nucleolus, During the interphase period, when chromosomes cannot be distinguished from one another, the dispersed DNA mass and its associated proteins from the cell nucleus are called chromatin

In the gastrointestinal tract, cells divide often, The cell cycle has two important phases: the interphase and mitosis, The cell cycle is a repeating series of events including cellular growth and reproduction, The contents of the nucleus is directly involved in cell reproduction