The excretory system of humans is also called the urinary system. It is formed of one pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys filter blood to produce urine and the ureters carry urine from kidneys to urinary bladder. The bladder temporarily stores urine until it is released from body. Urethra is the tube that carries urine from urinary bladder to the outside of body.
1. Structure of Kidney
Kidneys are dark-red, bean shaped organs. Each kidney is 10 cm long, 5 cm wide and cm thick and weighs about 20 grams. They are placed against the back wall of abdominal cavity just below diaphragm, one on either side of vertebral column. They are protected by the last 2 ribs. The left kidney is a little higher than the right.
The concave side of kidney faces vertebral column. There is a depression, called hilus, near the centre of the concave area of kidney. This is the area of kidney through which ureter leaves kidney and other structures including blood vessels, lymphatic vessels and nerves enter and leave kidney.
The longitudinal section of the kidney shows two regions. Renal cortex is the outer part of kidney and it is dark red in colour. Renal medulla is the inner part of kidney and is pale red in colour. Renal medulla consists of several cone shaped areas called renal pyramids. Renal pyramids project into a funnel-shaped cavity called renal pelvis, which is the base of ureter.
The functional unit of the kidneys is called nephron. There are over one million nephrons in each kidney. There are two parts of a nephron i.e, renal corpuscle and renal tubule.
The renal corpuscle is not tubular and has two parts i.e. glomerulus and Bowman’s capsule. Glomerulus is a network of capillaries while Bowman’s capsule is a cup-shaped structure that encloses glomerulus.
The renal tubule is the part of nephron which starts after Bowman’s capsule. Its first portion is called the proximal convoluted tubule. Next portion is U-shaped and is called the Loop of Henle. The last portion of renal tubule is the distal convoluted tubule.
The capillaries of the glomerulus arise from the afferent arteriole and Glomerulus is a network of capillaries while Join to form the efferent arteriole.
The distal convoluted tubules of many nephrons open in a single collecting duct. Many collecting ducts join together to form several hundred papillary ducts which drain into renal pelvis.
2. Functioning of Kidney
The main function of kidney is urine formation, which takes place in three steps:
The first step is pressure filtration. When blood enters the kidney via the renal artery, it goes to many arterioles, and then to the glomerulus. The pressure of blood is very high and so most of the water, salts, glucose and urea of blood is forced out of glomerular capillaries This material passes into the Bowman’s capsule and is now called glomerular filtrate.
Blood cells and proteins are not filtered through the glomerular capillaries because they are relatively larger in size
The second step is the selective re-absorption. In this step about 99% of the glomerular filtrate is reabsorbed into the blood capillaries surrounding renal tubule. occurs through osmosis, diffusion and active transport. Some water and most of the glucose is reabsorbed from the proximal convoluted tubule. Here, salts are reabsorbed by active transport and then water follows by osmosis. The descending limb of loop Henle allows the reabsorption of water while the ascending limb of Loop of Henle allows the reabsorption of salts. The distal convoluted tubule again allows the reabsorption water into the blood.
The third step is the tubular secretion. Different ions, creatinine, urea etc. are secreted from blood into the filtrate in renal tubule. This is done to maintain blood at a normal pH (7.35 to 7.45)
At the final stage urine is only 1% of the originally filtered volume The typical volume of urine produced by an average adult is around 1.4 litres per day.
After the above mentioned steps, the filtrate present in renal tubules is known as urine. It moves into collecting ducts and then into pelvis.
|Chloride ions||1.87 g/l|
|Sodium ions||1.17 g/l|
|Potassium ions||0.750 g/l|
|Other ions and compounds||Variable amounts|
3. Osmoregulatory Function of Kidney
Osmoregulation is defined as the regulation of the concentration of water and salts in blood and other body fluids. Kidneys play important role in osmoregulation by regulating the water contents of Relate too much sugar blood. It is an important process as excessive loss of water concentrates the body fluids whereas excess intake of water dilutes them.
When there is excess water in body fluids, kidneys form dilute (hypotonic) urine For this purpose, kidneys filter more water from glomerular capillaries into Bowman’s capsule. Similarly less water is reabsorbed and abundant dilute urine is produced, brings down the volume of body fluids to normal.
When there is shortage of water in body fluids, kidneys filter less water from glomerular capillaries and the rate of reabsorption of water is increased. Less filtration and more reabsorption produce small amount of concentrated (hypertonic) urine. I increases the volume of body fluids to normal. This whole process is under hormonal control.