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Excretory System in Man:

Our excretory system consists of kidneys, blood vessels that join them, ureters, urinary bladder and urethra. They help produce and excrete urine.

There are two bean-shaped kidneys that lie in the abdominal cavity, one on either side of the vertebral column. The kidneys are reddish brown. Each of them is about 10 cm long and weighs about 150 g. Although they weigh less, they receive a lot of blood for filtration.

A volume of blood nearly equivalent to that in the whole body passes through the kidneys every four or five minutes. The kidneys produce urine to filter out the waste products, like urea and uric acid, from the blood.

Urine leaves each kidney through a tube called ureters. The ureters from both the kidneys are corrected to the urinary bladder that collects and stores urine. Ureters carry urine from the kidneys into the urinary bladder. The urethra is a canal that carries urine from the bladder and expels it outside the body.

Internal Structure of a Kidney:

Each kidney is enclosed in a thin, fibrous covering called the capsule. A renal artery brings blood into the kidney, along with nitrogenous waste materials. After filtration in the kidney, the purified blood leaves the kidney through a renal vein.

Two distinct regions can be seen in the section of a kidney:

(1) An outer, dark, granular cortex and (2) an inner, lighter medulla. The hollow space from where the ureter leaves the kidney is called the pelvis. Each kidney is made up of numerous (about one million) coiled excretory tubules, known as nephrons, and collecting ducts associated with tiny blood vessels. A nephron is the structural and functional unit of a kidney, having three functions— filtration, reabsorption and secretion.

A cluster of thin-walled blood capillaries remains associated with the cup-shaped end of each nephron tubule. These capillaries bring blood from the body to the nephron for filtration. The network of capillaries spreads over the nephron tubules also. These capillaries finally carry purified blood to the body.

Structure and Function of a Nephron: 

A nephron consists of a long coiled tubule and the Malpighian corpuscle. The tubule of the nephron is differentiated into the proximal convoluted tubule, Henle’s loop and the distal convoluted tubule. The distal tubule opens into the collecting duct.

At the proximal end of the nephron is the Malpighian corpuscle, which consists of Bowman’s capsule and the glomerulus. Bowman’s capsule is a double-walled cuplike structure which surrounds the dense network of blood capillaries called the glomerulus.

The process of excretion in nephron:

The process of excretion may be divided into three stages- tubular secretion.

Filtration:

Filtration of blood occurs under high pressure in the nephrons of the kidney. Blood enters the glomerulus through the afferent arteriole (with a wider lumen) and leaves through the efferent arteriole (with a narrow lumen). Therefore, blood passes through the glomerulus under pressure. This results in filtration of blood.

Water and small molecules are forced out of the walls of the capillaries of the glomerulus and Bowman’s capsule and enter the tubule of the nephron. Large molecules remain in the blood of the glomerulus. The filtrate contains water, glucose, salts, urea, vitamins, etc. It is called the glomerular filtrate.

Selective reabsorption:

Some molecules of the glomerular filtrate are selectively reabsorbed into the blood. The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped Henle’s loop and the distal convoluted tubule. It contains many useful substances such as glucose, amino acids and salts.

These are reabsorbed by a process, which requires energy. Without reabsorption, these nutrients could have been lost with the urine. The filtrate now contains urea, some salts and water. Reabsorption of solutes into the blood increases the water concentration of the filtrate.

Then water is reabsorbed into the blood by the process of osmosis, and the osmotic balance is restored. The amount of water reabsorbed depends on the amount of excess water in the body and that of the dissolved waste to be excreted.

This reabsorption of water from the filtrate to maintain the water balance of the body fluid is known as osmoregulation. In this way the kidneys serve as water-conserving organs. After reabsorption from 180 L of filtrate in the kidney, only 1-2 L of urine is produced.

Tubular secretion:

Some nitrogenous waste products like creatinin and some other substances like potassium ions are removed from the blood by the distal convoluted tubule, and are then added to the urine. This is called tubular secretion.

Control of excretion:

The urine that is formed continually collects in the urinary bladder. As the bladder expands, its pressure creates an urge to pass urine through the urethra. As the bladder is muscular, the urge to urinate is under voluntary nervous control.

Kidney Failure

The kidneys may be damaged due to infection, injury, diabetes, and extremes of blood pressure. A damaged kidney cannot function efficiently to remove urea, ions, water, etc., from the blood. This malfunctioning results in the accumulation of toxic wastes like urea (uremia), which can lead to death.

One of the ways to treat kidney failure is to use a ‘dialysis machine’ that acts as an artificial kidney. It has a long tube like structure made of Cellophane suspended in a tank (dialyser) of a fresh dialysis fluid (dialysis). The Cellophane tube is partially permeable and therefore allows solutes to diffuse through. The dialysis fluid has the same concentration as normal tissue fluid, but nitrogenous wastes and excess salts are absent.

During dialysis, the blood of the patient is withdrawn from an artery and cooled at 0°C. It is maintained in a liquid state by adding an anticoagulant and by other special treatments. It is pumped through the dialysis machine. Here, the nitrogenous waste products from the blood diffuse into the dialysis fluid. The purified blood is then warmed to the body temperature and pumped back into the patient’s body through a vein.

The dialyser is specific for each patient to avoid infections. Dialysis through an artificial kidney has to be carried out at frequent intervals. This process of purification of blood is called haemodialysis.

A dialysis machine works like a kidney except that no selective reabsorption takes place in the former