Kidney is the chief excretory organ of the body. Each human being has two kidneys (exceptions are there; one in every 1000 individuals has only one kidney). Kidneys are situated on either side of the vertebral column in the middle of the back below the ribs.
Each kidney weighs approximately 150 g. Kidney receives its blood supply through renal artery, which is a branch of aorta. Venous drainage from the kidney reaches inferior venae cava through the renal veins. Urine formed in the kidney is initially collected in an inner cavity of kidney called renal pelvis, which is collected by a tube like structure called ureter. Ureters from both kidneys drain into urinary bladder where urine is stored. Urinary is bladder is emptied when an individual passes urine.
Urine formation occurs in functional units called nephrons. There are one million nephrons in each kidney. Each nephron has a filtering unit (membrane like structure) called glomerulus and a long tube like structure connected to it (See picture, nephron –functional unit of kidney). Process of urine formation begins in the glomerulus where the blood is filtered. The filtered fluid will contain small molecular weight substances apart from water. Blood cells and proteins are not removed as the glomerulus does not allow free their passage because of the large molecular size. During the passage of the filtered fluid through the nephron, most of the water and other essential substances are reabsorbed back into the blood. Thus eventually urine will have only the waste products and water. Urea, creatinine, oxalate, guanidoacetic acid are some of the metabolic waste products removed in the urine.
Does the kidney have any other functions?
Apart from excretory functions kidney has several other important vital functions.
1. Regulation of blood pressure and body fluid status
Kidney is the important organ in the regulation of blood pressure. Kidney controls salt and water excretion. Accumulation excess salt and water is characterized by the development of high blood pressure. Kidney produces a hormone called rennin, which is associated with hypertension. A reduced function of the kidney is also associated with the development of high blood pressure. Kidney has an important role in the maintenance of the composition of body fluids. Impairment of kidney function can cause abnormalities in sodium and potassium and acid base status.
2. Production of erythropoetin
Kidney produces a hormone called eryhropoetin, which is required for the red blood cells in the bone marrow. Kidney failure results in reduced production of this hormone resulting in anaemia.
3. Vitamin D metabolism
Kidney produces an enzyme, which converts vitamin D to its biologically active form, which is essential for bone formation.
How are the kidney functions checked?
Kidney functions can be checked my measuring the blood levels of certain substances, which are normally excreted by the kidney. An increase in the concentration of these substances suggests reduced functioning of the kidney. The commonly done tests and their normal levels are given below.
Test– Normal Value
Urea– 25- 35 mg/dl
Creatinine– 0.7-1.3 mg/dl
Creatinine clearance– 125 ml/mt
Creatinine is a product of muscle metabolism. As children have lower muscle mass, they are likely lower values for creatinine. Creatinine clearance can be estimated by measuring the total amount of creatinine excreted in the urine in 24 hours.
The most accurate method for measuring kidney function is by measurement of glomerular filtration rate (GFR). GFR is the sum total of the filtration done by all the nephrons (filtering units). The normal value for GFR is 125 ml/mt. Creatinine clearance is approximately equal to GFR. The most accurate way of estimating GFR is by administering a radioisotope and estimating the radioactivity using a gamma camera.