The kidney is an indispensible organ in the body of healthy individuals. It is important although dialysis in end stage disease such as chronic renal failure can increase the life expectancy in affected individuals which are undergoing renal dialysis. The importance of the kidney stems from the fact that it rids individuals from the toxic materials in their blood which are metabolic products that are obtained in the liver.
The functional unit of the kidney is the nephron. Each of the two kidneys is composed of millions of nephrons that specialize in the filtering of the blood forming the urine. Each nephron in the kidney is composed of a glomerulus which is connected to renal tubules.
In the glomerulus the process of blood filtration from its waste products takes place. The process of filtration in the glomerulus takes place by diffusion under the effect of the blood hydrostatic pressure that the blood exerts on the glomerular wall. In the tubules of the nephron many processes take place in which compounds which are important and which were filtered in the glomerulus are reabsorbed back into the circulation.
The process of reabsorption can take place by one of two methods. These are by diffusion and by active transport. Water is usually reabsorbed into the circulation by diffusion. Other molecules and in particular electrolytes are reabsorbed by active transport. Examples are sodium and potassium ions in addition to hydrogen ions.
The function of the kidney is tightly regulated by hormones that are secreted by the various glands in the body. The rate of filtration and reabsorption of electrolytes and water are also under the hormonal control from the pituitary gland and the adrenal gland.
The hormone that is secreted by the posterior pituitary gland and which functions by regulating the filtration and reabsorption of water by the kidney cells is called antidiuretic hormone or ADH. This hormone is very critical for the amount of water that is filtered and reabsorbed in the tubules of the kidney.
An increased amount of secretion of this hormone can cause a retention of excessive amount of water by the kidney. Thus leading to hypertension and subsequent edema. On the other hand, a deficient secretion of this hormone by the posterior pituitary gland leads to excessive loss of water in the kidney tubules into the urine. Thus causing a state of polyurea and dehydration. This hormone, thus, controls the blood pressure of the body by regulating the amount of water that is secreted by the kidney into the urine.
The other hormone that is also functional in the regulation of the kidney function is called aldosterone. This hormone is secreted by the adrenal cortex in response to hypovolemia. Its secretion induces the kidney to increase the reabsorption of sodium ions in the kidney tubules in exchange for potassium ions which are excreted in the urine.
This reabsorption of sodium ions does not take place by diffusion but instead by an active transport process. Along with the reabsorbed ions of sodium water also diffuse in the same direction passively and is reabsorbed also into the circulation. The net result of this process is elevation of sodium concentration in the blood with concomittent elevation in the blood pressure due to the rise in the amount of water that is reabsorbed in the kidney tubules. Edema can also develop as a result of the elevation in the blood pressure.