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What is the structural and functional unit of the kidney?
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Answer: Option D — Nephron
Detailed Solution
The kidneys are two bean-shaped organs, each about the size of a fist, located on either side of the backbone in the abdomen, just below the ribcage. They are the main organs of the excretory system. They filter the blood to remove waste products, such as urea, uric acid and creatinine, and excess water and salts, and form urine.
Each human kidney contains about 1 million (10 lakh) tiny filtering units called nephrons. A nephron is the structural and functional unit of the kidney, because it carries out the whole process of urine formation. Each nephron has two main parts:
Renal corpuscle (Malpighian body): made up of the glomerulus, a network (tuft) of tiny blood capillaries, and Bowman's capsule, a cup-shaped structure that surrounds the glomerulus.
Renal tubule: a long, coiled tube that includes the proximal convoluted tubule (PCT), the loop of Henle and the distal convoluted tubule (DCT). It opens into a collecting duct.
Urine is formed in three steps:
Ultrafiltration (glomerular filtration): blood is filtered under pressure in the glomerulus, and the filtrate (water, glucose, salts, urea and other small molecules) passes into Bowman's capsule. The kidneys filter about 180 litres of fluid a day.
Selective reabsorption: useful substances, such as glucose, amino acids, most of the water and salts, are reabsorbed into the blood from the tubule, mainly in the PCT. About 99% of the filtrate is reabsorbed.
Tubular secretion: some wastes and ions are actively secreted into the tubule.
Finally, only about 1-1.5 litres of urine is produced each day. It passes through the ureters to the urinary bladder and is expelled through the urethra. The hormone ADH (vasopressin) from the pituitary controls water reabsorption. When kidneys fail, blood is cleaned artificially by dialysis (haemodialysis).
Why not the other options?
- ✗A neuron is the structural and functional unit of the nervous system.
- ✗An alveolus is the tiny air sac of the lungs where gas exchange occurs.
- ✗A villus is a finger-like projection in the small intestine that absorbs digested food.
Quick tip
Remember: Nephron = unit of the kidney (glomerulus + Bowman's capsule + tubule), about 1 million per kidney; neuron = nervous system; alveoli = lungs; villi = small intestine.
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Human Body Systems — Preparation
This question tests your understanding of Human Body Systems, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the systems of the human body: the circulatory, respiratory, excretory, nervous, endocrine, skeletal and muscular systems, the sense organs, and important facts about blood, blood groups, bones and hormones. Common questions ask about the number of bones in an adult (206), the largest and smallest bones (the femur and the stapes), the universal donor blood group (O negative), which gland is called the 'master gland' (the pituitary), and which hormone controls blood sugar (insulin). The functional unit of the kidney is the nephron. Practise the human body systems questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.