The urinary system is that part of the body which includes the two kidneys and the two ureters and the bladder in addition to the urethra which leads the urine outside the body. The two kidneys are the most important organs of the urinary system on the functional level. The kidneys are the site of formation of the urine. The filter the blood from waste products of the metabolism inside our body.
The site of filtration of blood is called glomeruli. The filtration process occurs due to the presence of hydrostatic pressure that the blood exerts on the wall of the glomerulus in the nephron. Much of what is filtered is reabsorbed back into the circulation in the tubules of the nephron. This reabsorption occurs via either diffusion as occurs with water or via active transport as occurs with electrolytes such as sodium and potassium ions.
The urine composition is an invaluable marker for kidney health and pathology. The main components of urine are water and the molecule urea in addition to creatinine which is a marker for kidney failure. Elevated levels of creatinine in the blood is a marker for kidney failure.
Water and urea are also reabsorbed by the kidney tubules into the blood circulation. Urea is the main constituent of urine in addition to water. Urea is formed in the liver from ammonia. Urea is a much less toxic compound than ammonia. Ammonia is formed also in the liver by a deamination process of amino acids.
In the case of liver damage ammonia accumulates in the blood. Thus leading to hepatic encephalopathy in which ammonia acumulates in the brain of affected individuals.
Patients that have kidney failure often require a procedure which is called dialysis in which urine is removed manually through filtration the blood by an artificial kidney from waste products in the blood.
Diseases of the kidney can affect the composition of urine. For example, in the nephrotic syndrome proteins are observed in the urine of patients affected by this type of disease. In addition, in the nephritic syndorme red blood cells are observed in the urine of affected individuals.
Cancer of the kidney is characterized endocrinically by the presence of excessive amount of the hormone erythropoitien. This hormone is know to stimulate the production of red blood cells. Thus in certain types of kidney cancer a polycythemia or increased viscosity of the blood is observed or high hematocrit ratio is observed.
One of the common medical conditions that affects the urinary system is the presence of stones anywhere in the urinary system from the kidneys to the bladder. Small stones that are formed in the kidneys can pass to the ureters. Thus obstructing the flow of urine alon the urinary pathway. This condition can cause hydronephrosis or accumulation of fluid in the kidneys.
The presence of stones in the ureters can be very painful. It is treated with medications that solubilize the stones. Stones can also form in the bladder itself.
The urinary bladder functions as a reservoir of urine. It does not have an apparent metabolic role in absorption. All the urinary system is lined with transitional epithelium which does not have any role except as a reservoir for urine.
A pheochromocytomalike condition can affect the bladder giving clinical symptoms similar to those observed with pheochromocytoma of the adrenal gland.
Cancer of the bladder can occur due to smoking or due to metaplasia of the transitional epithelium. Smoking renders the absorption of polycyclic aromatic compounds which are carcinogenic. In addition metaplasia can transform the normal transitional epithelium of the bladder into a carcinogenic or abnormal epithelium.
