The kidney is a bean-shaped organ that filters the blood to eliminate waste products and excessive fluids and produces urine. Functions of the kidney are essential for the body and its well-being. Despite the common thought that their only function is the urine production role of the kidneys extends far beyond that. The complicated functions of this organ are discussed below.
Usually, there are two kidneys in the body located on the right and left in the retroperitoneal space (the space located in the abdomen behind the peritoneal cavity). Each kidney is about 11 cm in length and weight of 115-170 g and is covered with a fibrous capsule and protected by the adipose capsule (layer of fat). The blood is supplied to the kidneys by the renal arteries. The kidney consists of an outer layer called the renal cortex and inner medulla. Urine produced by the nephrons – functional units of the kidney and drained into the calyceal system, flows to the pelvis of the kidney, ureter and finally reaches the bladder where it is stored until a person voids.
Functional structure of the kidney
Each kidney is composed of approximately 1 million nephrons, which produce urine. The nephron is composed of the renal corpuscule which includes the glomerulus of the capillaries and the Bowman’s capsule and numerous tubules: proximal convoluted tubule, the loop of Henle, distal convoluted tubule and collecting duct which finally empties into the calyceal system.
Functions of the kidneys
- Urine production;
- Elimination of waste products, drugs and toxic substances from the blood;
- Maintenance of blood pH;
- Regulation of blood osmolality and minerals concentrations, the volume of fluid in the body;
- Blood pressure regulation;
- Erythropoietin and renin production;
- Vitamin D metabolism and calcitriol production;
Urine production and related functions
By producing urine the kidneys eliminate excess water, minerals (sodium, potassium, calcium, phosphates) and waste products including some drugs. On the other hand, the mechanism of urine production allows retaining the substances necessary for the body, such as protein, glucose, sodium, and bicarbonate. Thus, homeostasis is regulated and maintained – the blood composition remains stable with the necessary concentration of the solutes, regular fluid volume and sustain acid-base balance (with a pH level between 7.38 and 7.42).
Filtration occurs in the renal corpuscle. Here water and the solutes move through the capillary wall and the wall of the Bowman’s capsule into its lumen forming the primary urine.
Primary urine flows through the tubules where the reabsorption of water and some solutes (such as the glucose, amino acids, albumin, sodium, phosphate, etc.) takes place.
In the distal tubule, some substances such as sodium, bile acids, urea, and some drugs are secreted. Bicarbonate and hydrogen ions are also secreted within the tubules – this process is required to regulate the blood pH and maintain acid-base balance.
Sodium reabsorption and secretion along with changes of water reabsorption maintain electrolyte-water balance in the body (blood osmolality). This function of the kidney is regulated by the antidiuretic hormone produced by the pituitary gland.
Finally, urine reaches the collecting duct and is excreted. If needed, some amounts of water may be reabsorbed in the collecting ducts as well.
Erythropoietin is a glycoprotein secreted by the kidneys in response to hypoxia, which stimulates red blood cell production by the bone marrow. Of note, this hormone is also involved in wound healing.
Renin production and blood pressure regulation
Kidneys also regulate blood pressure. Hormone renin produced by this organ is a part of the renin-angiotensin system responsible for the arterial pressure regulation. Therefore, in response to the changes in blood flow, the kidneys secrete renin, which in turn causes the release of angiotensin II – a substance that constricts the blood vessel. Simultaneously, water and sodium absorption increases and, as a result, blood pressure rises.
Vitamin D metabolism
Vitamin D in the body is converted into its active form – calcitriol (vitamin D3) in the kidneys. Calcitriol increases calcium absorption in the intestines and phosphate reabsorption in the kidney.