Excretion is a process that ensures the elimination of metabolic products from the body that cannot be used by the body.
The allocation of metabolic products is carried out by various organs:
- carbon dioxide and water vapor are removed from the human body through the lungs;
- water, urea, ammonia, salts are excreted through the sweat glands;
- through the intestines (with feces) salts of toxic heavy metals are removed from the body.
But the main excretory organs are the kidneys, through which liquid metabolic products (protein breakdown products containing nitrogen, excess water, some salts and other substances) are removed. Thus, the kidneys maintain the water-salt balance in the body.
The urinary system consists of the kidneys, ureters, bladder, and urethra (urethra).
The kidneys are paired bean-shaped organs located at the posterior wall of the abdominal cavity at the level of the 1st and 2nd lumbar vertebrae.
The concave edge of the kidneys faces the spine. At this point, blood vessels enter and exit the kidney. There is also a cavity called the renal pelvis.
In the kidney, there are external, cortical, and internal, cerebral, layers. The kidney is covered with connective tissue and fatty membranes.
The adrenal glands are adjacent to the upper pole of the kidney (these are glands belonging to the endocrine system).
The ureter is a thin long tube 6–8 mm in diameter with elastic muscular walls. Through the ureters (from the right and left kidneys), urine flows into the bladder.
The bladder is a hollow muscular organ that stores urine and excretes it out through the urethra. Its volume in an adult is 300-700 ml.
The bladder is located in the pelvic area. In the lower part, the bladder narrows and passes into the urethra.
The thick, smooth muscle wall of the bladder expands when it fills with urine and contracts when urinating occurs.
The exits from the bladder and urethra are thickened (sphincters). When the bladder fills, its walls stretch, the sphincter relaxes, and the urethra opens, releasing urine.
Young children are unable to control the flow of urine from the bladder. This process takes place in them reflexively: a signal is sent from the receptors to the brain that the bladder is filled with urine. The brain sends back impulses to the muscles in the walls of the bladder that cause them to contract. The volume of the bladder decreases and urine is removed from it.
Later, the urinary reflex begins to control the higher parts of the brain, and the process becomes arbitrary. Typically, the urge to urinate in an adult occurs when about 0.5 liters of urine is collected in the bladder.