Sodium ions are very important for the normal functioning of the human body. If the concentration of sodium in the extracellular space falls too low we may die due to the neurological symptoms that accompany the condition such as coma and seizure. Sodium ions are the most abundant positively charged ion in the extracellular space in the body.
It has two main physiological functions in the body. These are: monitoring and controlling the cell membrane resting electric potential and its initiation of the nerve and muscle cells. Thus it functions by conserving the function of the nervous system and the muscular system as far as contraction and relaxation is concerned.
Sodium ion is an antagonist to both potassium and hydrogen as far as concentration is concerned. Elevated level of sodium is accompanied by depletion of both potassium and hydrogen. Sodium is slightly excreted in the urine. Approximately 1% of filtered sodium in the kidney is excreted in the urine. The rest is reabsorbed back into the circulation.
The other function of sodium ions is the maintenance of blood pressure through a regulation of its concentration in the body using a hormone that is called aldosterone. This hormone is crucial for the homeostasis of sodium in the body. Aldosterone is usually secreted by the cortex of the adrenal gland. It is secreted in response to low blood volume or hypovolemia.
The secretion of aldosterone leads to the conservation of sodium in the kidney tubules. It prevents its secretion in the urine by reabsorbing it through an active transport process. It is reabsorbed there in exchange for potassium and hydrogen which are excreted in the urine.
In Conn's syndrome or hyperaldosteronism an excessive amount of sodium is conserved in the kidney tubules due to the elevated level of aldosterone in the blood. Along with sodium that is reabsorbed is also water reabsorbed. Thus causing a state of hypertension and subsequent edema.
On the other hand in adrenal insufficiency syndrome in which aldosterone is not secreted in sufficient amount in the adrenal cortex due to a destruction of the cortex by an immune reaction low level of sodium is reabsorbed in the kidney tubules. Thus elevated amount of sodium is excreted in the urine in exchange for potassium and hydrogen which are reabsorbed. Thus causing a state of hyperkalemia. This condition can lead to hypertension due to the depletion of water as well which accompanies sodium lost by diffusion or passive transport.
The two major disorders of sodium in the body are hyponatremia and hypernatremia. Hyponatremia can develop due to adrenal cortex insufficiency. In addition, it can occur due to excessive secretion of the antidiuretic hormone by the posterior pituitary gland. This syndrome is called: Syndrome of inappropriate ADH antidiuresis. Hyponatremia can also develop as a result of deficient sodium in the diet.
Hypernatremia on the other hand can develop due to salt intoxication or due to Conn's syndrome which was mentioned earlier. In addition it can develop due to excessive loss of water in the urine as occurs in diabetes insipidus. Diabetes insipidus is a disorder of the pituitary gland in which the hormone ADH is secreted in deficient quantities from the posterior pituitary gland. Thus causing a state of polyurea and dehydration due to excessive loss of water in the urine. This process leads to the concentration of sodium ions in the extracellular space with subsequent development of hypernatremia.
The other function of sodium besides its function in controlling blood pressure is its function in regulating the function of the nervous system. It makes so by maintaining the cellular membrane resting electric potential. In addition it modulates the occurrence of the action potential of nerve and muscle cells.
It does this through a complex mechanism that involves in addition to sodium ions potassium ions as well. In addition this process involves molecular pumps that use sodium and potassium. These molecular pumps along with ion channels of sodium and potassium control the function of nerve impulses along the nervous system path.