Many chemical reactions take place inside body cells. Some products of these chemical reactions are not needed by the body. They may even be harmful if they accumulate in the body. Their removal from the body is called excretion.
Apart from kidneys, lungs, skin and liver also remove wastes. Sweat glands in the skin remove excess salts when we perspire. Lungs remove carbon dioxide.
In human beings, excretion is carried out by an organ system known as the urinary system or the excretory system:
Two bean shaped kidneys, located below the diaphragm in the abdomen and towards the back.
Two excretory tubes or ureters, one from each kidney.
One urinary bladder, ureters open into it.
A muscular tube called urethra arises from the bladder. The urinary opening is at the end of urethra.
Each kidney is made of tube like structures called nephrons (renal tubules). A nephron is the structural and functional unit of the kidney. The cup-shaped upper end of nephron is called Bowman’s capsule, has a network of capillaries within it called glomerulus. Glomerulus is a knot of capillaries formed from the artery which brings blood containing wastes and excess of water to the kidney.
Bowman’s capusle leads into a tubular structure. The tubular part of the nephron or renal tubule has three sub-parts, the proximal convoluted tubule (PCT), a thinner tube called loop of Henle and the distal convoluted tubule (DCT). Blood capillaries surround these tubules.
Filtration and reabsorption are two important processes of excretion.
Blood entering the glomerulus gets filtered in the Bowman’s capsule and is called the nephric filtrate. The red blood corpuscles and proteins do not filter out. They remain in the blood stream.
The filtrate entering the renal tubule not only contains waste but also useful substances. The useful substances get reabsorbed from the tubule into the blood capillaries surrounding the tubule. Excess water and salts like sodium and chloride also get reabsorbed into the blood from the renal tubule. Thus, waste alone which is primarily in the form of urea enters into collecting tubules from various renal tubules. It is the urine.
From the kidneys, the urine enters the ureters to reach the urinary bladder where it is temporarily stored. Urine is thrown out periodically through the urinary opening.
Kidneys not only excrete nitrogenous wastes but also regulate the water content of the body (osmoregulation).
Keep the normal mineral balance in the blood. When this balance is upset, a person can fall sick.
A person gets sick if the balance of substances such as mineral ions, water or even hormones inside the body is upset. Maintenance of the correct amount of water and mineral ions in the blood is termed osmoregulation.