Water is the major part of our body. It is found inside cells as well as outside the cells, and in the plasma of blood. Intracellular water amounts approximately two thirds while the rest are found in the extracellular space, which is composed of two parts: the blood plasma and the interstitial water, or water in the space between cells.
Loss of water from the intracellular part such as occurs in osmosis when salt concentration is higher outside the cells than inside them will have as a consequence cellular contaction and the clinical manifestation of lethargy and possibly coma as well in certain cases.
Loss of the extracellular water such as occurs in blood loss in bleeding will lead to hypotension as well as little supply of blood to various organs such as the kidney and liver.
The cellular membrane is not freely permeable, and it is not permeable to every compound. But there is selective transport, and active transport across the cell membrane that use ATP as an energy source to drive the transport process of certain electrolytes against their concentration gradient.
We expect then that electrolytes will not have equal concentations inside and outside the cell. Sodium is the main ion outside the cells, while potassium is predominantly found inside the cells.
Regarding ions, proteins are considered as ions because they have negative charge on them. Biphosphates are the major ions inside the cells, while chloride and bicarbonte ions are chiefly found outside the cells. This is so again due to active transport across the cellular membrane.
Water level in the vascular system is regulated by a hormone called arginine-vasopressin (AVP). An alternative name to this hormone is anti-diuretic hormone (ADH). This hormone is secreted by the posterior pituitary gland (hypophysis) in response to to low blood volume in the arteries or hypovolemia. It is also secreted in response to levels above normal of osmolality. As a result there is conservation of water from being wasted in the urine through the action of ADH on the renal tubules.
In disorders of this hormone such as happens in diabetes insipidus, when the seretion of this hormone is not sufficient there is as a result excessive loss of water in the urine through multiple urination during the day. In this condition there is no sufficient conservation of water due to the lack of ADH.
The opposite condition to diabetes insipidus is called syndrome of inappropriate antidiuresis or SIAD. In this condition there is excessive secretion of ADH due to several causes such as a malignancy. The result is infrequent urination. The clinical picture is hypertension due to the excessive conservation of water.
Sodium contributes to the regulation of body fluid volume and osmolality through the action of two hormones. These are aldosterone and atrial-natriuretic hormone. Aldosterone is normally secreted due to hypovolemia ( low blood volume) It stimulates sodium reabsorption in the renal tubules along with water. Sodium is conserved due to the action of aldosterone. Sodium is conserved in exchange for potassium which is secreted in the urine. The mechanism to this exchange is sodium/potassium and sodium/hydrogen pumps which use ATP as energy source to drive this transport.
Atrial natriuretic hormone is a protein hormone secreted by the cells of the heart. It is secreted in response to hypervolemia or high than normal blood volume. It stimulates the secretion and water and so restoring homeostasis.
Sodium pathophysiology is either hyponatremia ( low than normal level of sodium in the blood) or hypernatremia ( higher than normal level of sodium in the blood). Hyponatremia can develop by either excessive loss of sodium or by excessive water retention. When sodium is lost there is an accompanying lost of water by passive diffusion.
Sodium loss can be due to several reasons. Vomitting and diarrhea are two reasons. Taking medications such as those against aldosterone which normally functions by conserving sodium. Hyponatremia due to water retention is mostly observed in the syndrome of inappropriate antidiuresis or SIAD. This condtion is found in certain malignancies and was discussed previously.
Hypernatremiais defined as above than normal levels of sodium in the blood. It is formed either due to excessive water loss or due to excessive sodium intake. Diabetes insipidus was discussed previously. Another condition that leads to hypernatremia is hyperaldosteronism or so called Conn syndrome. The excessive secretion of aldosterone leads to more than usual conservation of sodium and with it water thus leading to hypernatremia and hypertension.
Potassium is as was previously mentioned mainly found inside cells. It contributes significantly to the resting potential of the cell membrane. Any disturbance in the potassium intracellular concentration can have fatal efects.
Hyperkalemia or higher than usual concentration of potassium in the blood can be caused as a response to several conditions. These are: renal failure and in particular chronic renal failure. The accumulation of potassium in the extracellular space causes hyperkalemia. This condition can cause cardiac or heart failure due to the sensitivity of heart muscle cells to any change in the concentration of potassium inside the cell.
Another contributing factor to hyperkalemia is hyperaldosteronism. The excessive loss of sodium due to the hypoaldosteronism leads to excessive conservation of potassium via sodium/potassium pumps. This in turn causes hyperkalemia.
In the case of acidosis which is an excess of acid in the extracellular space, there is a shift of hydrogen ions to the inside of the cells in exchange for potassium. The acid is then neutralized using bicarbonates inside the cell. The shift of potassium outside the cell causes hyperkalemia.
Another cause of hyperkalemia is tissue damage such as caused in Rhabdomyolysis ( muscle disease). As a result of the cellular destruction all the potassium inside the cells go outside the cells then contributing to hyperkalemia. Chemotherapy has also the same effect by destroying cells and malignant tissues. X-ray therapy has also the same effect.
The treatment of hyperkalemia is by infusion of insulin which is know in its action on potassium by letting it flow inside the cells and so restoring normal levels of potassium.