Anatomy And Physiology

Anatomy Physiology



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Edema is an accumulation of fluid in the body cavities and the skin such as the pleural cavity of the lungs in which it is called hydrothorax and pleural effusion.  Also fluid can accumulate in the peritoneal cavity causing a condition which is called ascites.  In addition, fluid can accumulate in the pericardial cavity in the heart causing pericardial effusion or accumulation of fluid in the heart covering layers.

An edema which affects the whole body is called anasarca.  Fluid can also accumulate in the skin which can be characterized by the production of depression in the skin after the application of pressure to the skin surface by a finger. 

The blood in the arteries is maintained there by two opposing forces that each works in a different direction.  The first is the hydrostatic pressure which the weight of the bloods exerts on the walls of the blood vessels tend to force the blood out of the arteries while the plasma colloid osmotic pressure tend to force the blood in the opposite direction inside the arteries. 

Th forces usually cancel each other under normal conditions.  Edema can result due to increased hydrostatic pressure as occurs in increased blood pressure or hypertension due to the accumulation of fluids in the arteries or it can occur due to high blood pressure that can result due to narrowing of the arteries such as occurs in the medical condition of pheochromocytoma. 

In pheochromocytoma chronic secretion of the hormones epinephrine and nrepinephrine causes sustained elevation in blood pressure.  Edema can be localized to a special tissue or it can be generalized  in the whole body.  Example of localized edema occurs in skin inflammation in which the edema occurs due to secretion of vasodilating compounds such as histamine by the inflammatory cells causing local formation of fluid accumulation at the site of inflammation. 

Also allergic reactions such as occurs in the allergic condition of anaphylaxis.  An edema in this condition occurs in the form of skin hives or wheals as a result to the dilatation of blood vessels by the action of vasoactive amines that are released by mast cells at the site of inflammation. 

Also due to the dilatation of the blood vessels a condition of low blood pressure such as shock can occur.  This condition can be corrected by the administration of epinephrine and norepinephrine which both function by narrowing the blood arteries.  Thus contributing to ameliorating the shock condition by raising the blood pressure in the arteries. 

The types of edema that were discussed so far are due to localized form of the edema which affects specific parts of the body.  Generalized edema is another type of edema in which there is universal acumulation of fluid in the body cavities.  This can occur due to sodium and water retention in the body by the action of the hormone aldosterone or it can be caused due to increased amount of water in the body by the action of the hormone vasopressin on the kidney to promote the conservation of water in the body with subsequent development of hypertension and edema.

One type of edema which occurs due to retention of sodium and water in the body is the cardiac edema.  In cardiac failure the ability of the heart to pump blood to the general circulation is impaired.  The net result to this condition is the development of hypovolemia with deficient blood perfusion of the body organs including the two kidneys which are the site where the response to the hypovolemia occurs.

As a result of the low blood prefusion to the renal artery a state of hypovolemia occurs in which the kidney cells begin to secrete the hormone renin in response to the hypovolemia .  Renin in turn stimulates the adrenal cortex to secrete the hormone aldosterone which functions to restore blood pressure to normal by increasing the amount of sodium and water in the blood.  It does so by stimulating the reabsorption by the kidney tubules.

As a result a generalized type of edema occurs which can be corrected by the administration of appropriate type of diuretics.  Also generalized edema can occur due to the lack of proteins in the blood.  This can occur due to liver failure or due to the nephrotic syndrome.  The lack of appropriate amount of the protein albumin in the blood can cause diminished plasma colloid osmotic pressure which allows for the hydrostatic pressure inside the arteries to overcome the plasma colloid osmotic pressure. 

This in turn allows for the fluid in the arteries to leak to the outer side of the arteries causing edema.  This type of generalized edema is especially manifested in liver failure in which proteins synthesis is diminished.  Also it is manifested by the nephrotic syndrome in which hypoproteinemia occurs due to the escape of large amount of proteins in the kidney glomeruli into the urine. 


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