There are two kidneys in man. They are bean-shaped. There is a distinct advantage of two kidneys in human body. If one kidney fails, the other can still carry out the function of excretion. The left kidney is placed a little higher than the right kidney. The kidneys are solid, reddish-brown, about 10 cm long organs which lie in the abdominal cavity, one on either side of the vertebral column. Each kidney weighs about 150 g. The outer surface of the kidney is convex while the inner surface is concave. There is a concave depression called hilus through which the arteries and veins enter and leave the kidney.

Internal Structure and function

Each kidney is enclosed in a thin, fibrous covering called capsule. A renal artery brings oxygenated blood to the kidney along with the nitrogenous waste material. After filtration, the purified but deoxygenated blood leaves the kidney through a renal vein. The excretory products are collected in the kidneys and are eliminated in the form of urine. It passes down the two tubular ureters (about 30 cm long), which open separately into the urinary bladder. The urinary bladder is a bag-like structure in which urine is stored. It is situated in the cavity of pelvis. Its size and position vary with the amount of urine it contains. The bladder has three openings, two from the ureters and one to the urethra. The urine is released from the urinary bladder periodically via the urethra to the outside. Two distinct regions can be seen in the section of kidney-an outer dark, granular cortex and an inner, lighter medulla. The space from where the ureter leaves the kidney is called pelvis.

Each kidney is made up of numerous (about one million) coiled tubules, known as uriniferous tubules or renal tubules or nephrons. In other words, bnephron is the structural and functional unit of kidney. Each nephron is made up of a malpighian body and a long tubular part. Malpighian body has a cup-like structure called Bowman’s epithelial cells. A network of capillaries forms the glomerulus which is found Bowman’s capsule. Filtration of blood takes place in the Bowman’s capsule when the blood passes through the glomerulus. The filtrate includes small molecules; large molecules are retained in the blood. Thus, the Bowman’s capsule acts as a dialyzing bag. Tubular part of the nephron consists of proximal convoluted tubule, a U-shaped loop of Henle and a distal convoluted tubule. As the filtrates pass down the proximal tubule, much of the salts and all of glucose are reabsorbed and brought back into the blood. When the filtrate passes down along the loop of Henle, certain amount of water is also reabsorbed. Some compounds are secreted in the distal tubule. The filtrate which passes beyond the distal tubule is called urine.


Kidneys play an important role in regulating the composition of blood. They maintain mineral balance, exact proportion of water and acid-alkali balance in the blood. They regulate the osmotic pressure of the body fluids and concentration of salts in the blood. It is very essential because if the concentration of salts within the body fluids exceeds that within the cells, water passes out of the cell to the fluids. As a result of this, cells shrivel and die. On the other hand, if the concentration of salts in the fluids is less than the cells, water passes into the cells making them swell and burst.

Interesting points about excretion through kidney

(i) About 130 ml of filtrate are formed per minute in the glomeruli of the two kidneys of man.

(ii) About 99% of the water of the filtrate is reabsorbed as it passes down the nephron.

(iii) Body salts excreted in human urine may amount to 2.2% and urea 6% of the volume of urine.


(iv) The yellow colour of urine is due to a pigment called urochrome.

(v) About 1600 ml urine is excreted by an adult in 24 hours. A nephron is a 5-cm long tubule.

(vi) Urination is called micturition. This is a reflex action (quick action) controlled by the spinal cord.


In dialysis or haemodialysis, one of the arteries of the patient is connected to a cellophane tube lying in a bath of isotonic solution with blood. The other end of the cellophane tube is connected to a vein of the patient. As the blood passes along the cellophane tube, urea and other nitrogenous compounds diffuse out into the dialysis solution.

Artificial Kidney

The principle of dialysis is applied for purifying he blood of those patients whose kidneys are either damaged or do not function. Through surgery, the damaged kidney is removed and in its place another kidney from the healthy body of a donor is transplanted. In some cases the service of an artificial kidney is utilized. In such a kidney the blood of a patient is passed through cellophane tube suspended in salt solution. This salt solution is similar in composition to blood plasma. As the blood flows through cellophane tube, the impurities of blood such as ammonia and urea are filtered out, while proteins and other useful substances are retained within the tube. Such filtering of blood is haemodialysis. The purified blood is returned to the body after a series of such dialysis. In this process some anticoagulants have to be added to the blood during dialysis.


Take a cellophane tube or bag. Fill it with a solution of sodium chloride and starch, tie the bag with a thread and suspend it in a beaker containing distilled water. Let it remain like this for a few hours. Now test the water in the beaker and that in cellophane bag for the presence of starch and common salt. For testing common salt add silver nitrate to the water. It will produce white precipitate of silver chloride. Iodine solution is added to water for the test of starch. Water will change into dark blue color, if iodine is added to the water having starch. After these tests you will find that starch is present in the cellophane bag, while common salt has come out and mixed in the distilled water of beaker. It shows that cellophane is semi permeable and allows only the molecules of common salt to move across it. The pores in the cellophane are large enough to let the sodium and chloride ions pass through freely, but are smaller in size than the large molecules of starch.