Overview of Kidney Physiology

The kidney has several apparent physiological roles that are related to maintaining homeostasis of the body.  It is composed of filtering units that are called nephrons.  Nephrons in turn are composed of glomeruli  and tubules.  The filtration of blood occurs in the glomerulus while reabsorption of important molecules occurs in the tubules of the nephron. 

The filtarion of blood in the glomeruli is restricted to small molecules such as amino acids and urea.  Large molecules such as proteins are not filtered under normal function of the kidney.  Under pathological conditions of the kidney such as occurs in the nephrotic syndrome, large molecules are filtered such as proteins in addition to small molecules. 

Reabsorption of molecules in the kidney tubules is in most cases an active transport process.  In cerain cases it is processed by diffusion as occurs with water.  Glucose and electrolytes are transported using molecular pumps that use ATPase enzymes which catalyze and drive the transport process of compounds against their concentration gradient. 

One of the various roles of the kidney is to filter waste products of the body metabolism such as urea and creatinine.  Under pathological conditions of the kidney such as occurs in chronic renal failure there is retention of creatinine and urea in the blood. 

Creatinine is usually a marker for kidney failure.  Its level in the blood is elevated in chronic renal failure.  Other compounds are also filtered in the glomerulus such as glucose, but this compound is then reabsorbed in the kidney tubules using active transport.

The other role to the kidney is the synthesis of glucose from amino acids.  This process is called gluconeogenesis.  It occurs in the kidney as well as the liver.  In kidney and liver failures a state of hypoglycemia can occurs due to the lack of mechanism for the synthesis of glucose in these teo organs. 

Gluconeogenesis is an anabolic process because it participates in the production of glucose.  This is in contrast to glycolysis which is a catabolic process because it degrades glucose.  In addition gluconeogenesis is a process which consumes energy in contrast to glycolysis which releases energy.   

The kidney has an important physiological role in the regulation of blood volume and pressure in addition to maintaining electrolytes composition in the blood.  The main modulator of blood volume is a hormone called the antidiuretic hormone or ADH. 

ADH is usually secreted from the posterior pituitary gland in response to low blood volume or high blood osmolality.  ADH stimulates the retention of water in the kidney tubules.  Under pathological conditions of increased ADH secretion there is overretention of water in the kidney tubules.  Thus predisposing to hypertension and edema.

When the level of ADH is low there is oversecretion of water in the kidney tubules.  Thus predispoing to a state pf polyurea and hypotension. 

Another polypeptide hormone that is important in regulating blood volume but to a lesser extent than ADH is called atrial natriuretic peptide.  This hormone is secreted by the atria of the heart.  It is secreted in response to high blood volume and causes urination which lowers the blood pressure.

The third hormone that is important in blood electrolytes balance as well as water is called aldosterone.  This hormone is usually secreted by the adrenal cortex and functions by stimulating the conservation of sodium ions in the blood in exchange for potassium which is excreted in the urine.

Hypersecretion of aldosterone can cause excessive conservation of sodium and water.  Thus causing a state of hypertension. 

Two other horomones which are secreted by the kidney and which have roles in blood cells homeostasis and bone metabolism are called erythropoitien and calcitriol.  Erythropoitien usually stimulates the production of red blood cells.  Calcitriol is important in the mineralization of bone.  It does so by increasing the rate of absorption of calcium in the small intestine.