Excretion is the biological process by which metabolic waste products are removed from the body. In humans, the excretory system — primarily the kidneys — filters blood, removes nitrogenous waste (urea) and excess water, and produces urine. This topic is covered in the CBSE Class 10 Life Processes chapter.
The human excretory system consists of: (1) Two kidneys — bean-shaped organs located in the abdominal cavity that filter blood. (2) Two ureters — tubes connecting kidneys to the bladder. (3) Urinary bladder — stores urine temporarily. (4) Urethra — tube through which urine exits the body.
The nephron is the functional unit of the kidney. Each kidney contains approximately one million nephrons. A nephron consists of: (1) Bowman's capsule — a cup-shaped structure containing the glomerulus (a capillary tuft). (2) Proximal convoluted tubule. (3) Loop of Henle (descending and ascending limbs). (4) Distal convoluted tubule. (5) Collecting duct.
Urine formation involves three steps: (1) Glomerular filtration — blood is filtered under high pressure in the glomerulus; water, glucose, amino acids, salts, and urea pass into Bowman's capsule as the glomerular filtrate. (2) Tubular reabsorption — useful substances (glucose, amino acids, most water and salts) are reabsorbed back into the blood in the proximal tubule and Loop of Henle. (3) Tubular secretion — additional waste substances (H⁺ ions, K⁺ ions) are secreted into the tubule from surrounding capillaries.
When kidneys fail, dialysis is used to filter blood artificially. In haemodialysis, blood is passed through a dialyser that uses a semi-permeable membrane to remove waste and excess fluid. Kidney transplant is a permanent solution for kidney failure. Understanding these medical links makes the topic relevant to real-world biology.
Plants do not have a dedicated excretory system. They use different strategies: (1) Excess water is removed by transpiration through stomata. (2) Waste products like resins, gums, and latex are stored in old xylem, bark, or leaves that fall off. (3) Some waste is stored in cellular vacuoles. (4) Oxygen (waste of photosynthesis) and CO₂ (waste of respiration) are released through stomata and lenticels.
A nephron is the basic structural and functional unit of the kidney responsible for filtering blood and forming urine. Each kidney contains approximately one million nephrons.
An average adult produces about 1 to 1.5 litres of urine per day, though the kidneys filter approximately 180 litres of blood daily. Most of the filtrate is reabsorbed.
If both kidneys fail, waste products accumulate in the blood, which can be fatal. Haemodialysis (using an artificial kidney machine) or a kidney transplant is required to sustain life.
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