Kidney Function

Role of Kidney
The healthy balance of our body’s internal chemistry is in large part due to the work of the kidneys. Each kidney is about the size of your fist and weighs about 150 grams.


Figure: Left – Kidneys position; Right – Detailed view of Kidneys

Kidney function

1. Elimination of Waste Products

Blood flows into the kidneys through the renal (renal refers to the kidney) artery. An artery is a blood vessel which carries blood away from the heart and through the body. Thus the renal artery takes blood from the heart and delivers it to the kidneys. Once the blood reaches the kidneys, waste products are filtered out into the urine and the cleansed blood flows back to the heart through the renal vein. Veins carry blood flowing from various organs back to the heart just as an artery carries blood flowing away from the heart. Within the kidneys, there are very small filtering units called nephrons. Approximately 1 million nephrons are found in each kidney.

The nephron is composed of a small cluster of blood vessels called the glomerulus (where the actual filtering process occurs) and a minuscule tube called a tubule. Almost 180 liters of blood is filtered every day. The fluid filtered from the blood plasma is modified and in fact largely reabsorbed as it travels along the tubules. Cells and large molecular substances like proteins do not get filtered. The remainder (the urine) finds its way into the ureter, a larger tube draining all the tubules, which carry the urine to the bladder from each kidney. Every two minutes the entire blood supply circulates through the kidneys, and is thus continuously cleansed.

2. Production of Red Blood Cells and Bone Formation

The Production of red blood cells and the formation of healthy bones are both dependent on the proper functioning of our kidneys. First, the kidneys release the hormone erythropoietin which promotes the maturation of erythrocytes (red blood cells) in the bone marrow. A shortage of this hormone may cause anaemia. The measurement that gives information on the number of red blood cells in the body is called the hematocrit. Patients generally experience fatigue when their hematocrit is low. Secondly, the kidneys affect bone formation by regulating calcium and phosphorus concentrations in the blood and by producing an active form of Vitamin D, which promotes calcium absorption from the food.

3. Regulation of Blood Pressure

High Blood pressure (hypertension) can be both a cause and a consequence of kidney disease.Prolonged high blood pressure damages the blood vessels of many organs and can lead to heart disease and stroke as well as kidney failure.Control of the body’s blood pressure is another function of the kidneys. These organs monitor concentrations of sodium, potassium, calcium, phosphorus, chloride and bicarbonate as well as body fluid levels and remove excess amounts of salts and fluids in the urine.

When the kidneys fail to perform these vital functions, the blood pressure may rise and swelling may occur. In severe cases heart failure may result. The kidneys also secrete a substance called rennin. Rennin stimulates the production of a hormone which causes an elevation in blood pressure. When the kidneys do not work properly, too much rennin may be produced and this can lead to high blood pressure.

4. Control of the Body’s Chemical and Fluid Balance

The waste by-product of this filtering process, as previously mentioned, is called urine. When the kidneys fail to function properly, waste products which should be eliminated in the urine are retained in the blood. This results in a very serious condition known as uremia. Symptoms of uremia can be expressed in every organ of the body and  includes nausea, weakness, fatigue, disorientation, shortness of breath and swelling in the arms and legs.

Some of the waste products that build up in the blood can be measured and their increased concentration can be used to evaluate the seriousness of the problem. The substance most commonly measured for this purpose is called creatinine. This substance is a waste product that results from the metabolism in the muscle tissues. Kidney disease is often associated with high levels of creatinine.