Life Processes

EXCRETION

• Nutrients from various sources are digested and metabolized in the body.
• After metabolization, the body sorts out useful and toxic substances.
• Accumulation of toxins may be harmful and so the body removes all metabolic waste by the process called excretion.
• Different organisms have different modes of excretion. Most unicellular organisms eliminate waste from their body through diffusion.
• In the human body which is considered to be an exceptional machine, a lot of processes (respiration, circulation, digestion, etc.) take place simultaneously.
• So, a lot of waste products produced are in different forms like carbon dioxide, water, and nitrogenous products like urea, ammonia, and uric acid.
• Also, the chemicals and other toxic compounds from medications and hormonal products are produced.
• So, a simple diffusion or evaporation is not sufficient to eliminate the waste from the human body.
• The excretory system is a specialized system for excretion in complex organisms such as animals including humans.
• The excretory system of human beings consist of the following main organs:
• Two kidneys
• Two ureters
• Bladder
• Urethra

Kidneys:

• Blood contains both useful and harmful substances. The useful substances are separated by kidneys through reabsorption and toxic substances are released through urine. Each kidney is made up of a large number of structural filtration units called nephrons.
• The nephron has a cup-shaped bag at its upper end which is called Bowman’s capsule. The lower end of Bowman’s capsule is tube-shaped and it is called a tubule. The Bowman’s capsule and the tubule taken together make a nephron. The Bowman’s capsule contains a bundle of blood capillaries which is called glomerulus (plural glomeruli).
• The function of glomerulus is to filter the blood passing through it. Only the small molecules of substances present in blood like glucose, amino acids, salts, urea and water, etc, pass through the glomerulus and collect as filtrate in the Bowman’s capsule. The large molecules like proteins and blood cells cannot pass out through the glomerulus capillaries and hence remain behind in the blood. The function of tubule of nephron is to allow the selective reabsorption of the useful substance like glucose, amino acids, salts and water into the blood capillaries (which surround it). The waste material like urea remains behind in the tubule. It does not get reabsorbed into blood capillaries.
• Only the waste substance urea, some unwanted salts and excess water remain behind in the tubule. The liquid left behind in the tubule of nephron is urine. The nephron carries this urine into the collecting duct of the kidney from where it is carried to ureter. From the ureter, urine passes into urinary bladder. The urinary bladder is controlled by the Central Nervous system. Urine is stored in the bladder till it contracts on receiving brain signals. Urine is finally passed out of the body through the urinary opening called urethra.
• Due to certain reasons like poor blood flow to the kidneys, infections, injuries, etc. the kidneys fail to perform their functions. A patient with kidney failure is treated periodically on a kidney machine

Dialysis

The procedure used in artificial kidney in place of normal is called dialysis.

Besides the urinary system, skin, lungs, and even eyes helps in excreting waste products in different forms. Excretion through the skin or sweating releases water, urea, and other salts. Similarly, gaseous wastes such as carbon dioxide, nitrogen, etc. are exhaled through the lungs.