The urinary system, also known as the excretory system, is concerned with the removal of water-soluble waste products from the body in the form of urine. The various components or organs of the urinary system are associated with the production, storage, and then expulsion of urine from the body. At the same time, the system also takes part in several vital functions of the body.
The Urinary System and its Functions
The main components of the urinary system are two kidneys, two ureters, the urinary bladder, two sphincter muscles and the urethra. Excretion of nitrogenous waste products is the main function of the urinary system, which, if not eliminated from the body, can become toxic and cause the death of an individual. Along with this crucial function, maintaining the volume of blood, blood pressure and blood pH, and stimulating the synthesis of red blood cells and vitamin D, are its other functions. Each organ performs some specific functions, and here we shall learn about them.
Removal of Metabolic Wastes and Excess Water
The primary function of the kidneys is the removal of metabolic wastes formed within the cells of the body. We know metabolism occurs within every cell of our body, and nitrogenous wastes, such as urea, uric acid, and ammonia, are generated as a result of this process. These toxic compounds are dissolved in the bloodstream, and are filtered out by the kidneys.
- As shown in the diagram given above, the renal artery supplies oxygenated blood to each kidney. If you look at the internal structure of a kidney, you'll find that it is made up of over a million nephrons, which are its structural and functional units.
- The filtration of urine takes place in the nephrons, where excess water, dissolved salts, and electrolytes, are also removed from the blood, along with the metabolic wastes.
- The filtered blood then flows out of the kidneys through the renal vein, and reaches the heart through the inferior vena cava.
- The excess water that has toxic wastes dissolved in it, flows out of the kidneys as urine.
The process of regulating the levels of water and dissolved salts in the blood, is termed as osmoregulation. The kidneys play a key role in homeostasis by regulating the amount of water in the body, and the concentration of dissolved ions in the blood. Homeostasis involves regulating various internal conditions, in order to maintain a constant internal environment.
- Water forms around 70-90% of our body cells, and the cell and tissue fluids are composed of water. Since blood is also a tissue, the percentage of water in the blood needs to be regulated.
- To achieve this, the kidneys retain the amount of water required by the body, and expel the rest of it as urine.
- The kidneys regulate the composition of blood and tissue fluids by the selective reabsorption of essential substances such as glucose, certain amino acids, and ions of sodium, calcium, and bicarbonate. The reabsorption depends on the current levels of the substances in the blood.
- Higher levels of water in the blood can cause swelling in the tissues due to water retention, and this condition is termed as edema. On the other hand, secretion of excess water with urine, can cause the blood volume to decrease, depriving the vital organs of constant supply of oxygen.
- The kidneys carry out osmoregulation by two processes, namely filtration and selective reabsorption.
- The process of filtration occurs in the glomerulus of the nephron, which is a coiled mass of capillaries, surrounded by a funnel-shaped structure known as the Bowman's capsule. Small molecules like water, glucose, urea, sodium chloride, amino acids, etc., pass through the walls of the glomerulus, into the renal tubule. This is known as the glomerular filtrate.
- The process of selective reabsorption takes place in the proximal convoluted tubule of the nephron. It is a method to recover water and other essential molecules from the glomerular filtrate. The quantity of water reabsorbed depends on whether the quantity of water in the blood is more or less, and this is detected by the hypothalamus.
- When there is too little water in the blood, the hypothalamus detects it and signals the pituitary gland to release more antidiuretic hormone (ADH). This causes the kidneys to reabsorb more amounts of water, and the concentration of blood decreases. In such a scenario, the urine formed is less in volume, and is highly concentrated.
- When there is too much water in the blood, the hypothalamus signals the pituitary gland to secrete less ADH. As a result, the kidneys reabsorb less water, and the blood concentration increases. In such a scenario, more urine is formed and it is dilute.
Secretion of Hormones
The kidneys also act as an endocrine gland and secrete the hormones erythropoietin, renin, and calcitriol.
Regulation of Blood Pressure
The kidneys regulate blood pressure by regulating the volume and concentration of blood. The hormone renin plays an important role in controlling blood pressure, as it regulates the amount of salts retained in the blood, along with the expansion and contraction of blood vessels.
Regulation of Blood pH
The kidneys regulate the acidity of blood by releasing H+ ions with urine, and conserving bicarbonate ions (HCO3-).
Production of Red Blood Cells
The kidneys secrete a hormone erythropoietin that triggers the production of red blood cells (RBCs) in the bone marrow.
Synthesis of Vitamin D
The kidneys produce a hormone calcitriol which is bioactive vitamin D, and is essential for bone development.
- The ureters are narrow tubes that arise from the kidneys and descend to the urinary bladder. Each ureter is attached to the renal pelvis of the respective kidney.
- The function of the ureters is to transport urine from the renal pelvis of the kidneys to the urinary bladder.
- The urine flows downwards through the ureter due to the rhythmic contraction and relaxation of the smooth muscles of its walls. The ends of the ureters extend into the bladder.
- Improper functioning of the ureters can lead to cystitis or infection of the kidneys.
- The urinary bladder is a triangular, hollow organ that is located in the lower abdominal region.
- The function of the urinary bladder is to store urine until it is expelled from the body.
- The urinary bladder expands when it is full, and reduces in size when empty.
- Circular muscles, known as sphincter muscles, prevent the leakage of urine from the bladder. These muscles are located at the opening of the bladder into the urethra.
- The nerves present in the bladder control the process of urination or micturition. When the bladder is full and it is time for urination, the nerves of the bladder transmit this information to the brain. The brain then signals the bladder muscles to contract and the sphincter muscles to relax, so as to facilitate urination.
- The urethra is the tube through which urine is expelled from the body.
- It is 8 inches long in males and 2 inches long in females.
- The flow of urine through the urethra is controlled by the internal and external urethral sphincter muscles.
- In males, the urethra also serves as the tube through which semen is ejaculated.
The urinary system functions in proper coordination with other organs, like the skin, lungs and intestines, to excrete all types of waste products generated in the body, while carrying out the vital life processes. Any kind of injury or damage to any part of the urinary system can impair the whole process of excretion, leading to accumulation of toxic substances, and giving rise to a number of health problems.