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What is meant by excretion ? Explainthe process of formation of urine. |
| Answer» <p>The elimination by an organism of waste products, such as carbon dioxide and urea, resulting from metabolic processes. Higher animals have specific organs of excretion, such as the lungs and kidneys. In plants and many lower organisms, waste is eliminated by diffusion to the outside environment.</p> <p>Urine formation occurs in the kidney in three stages:filtration,reabsorption, andsecretion.</p><p>Filtration</p><p>Stage 1: filtration. Thekidneyis the body's blood filtering system. Blood vessels visit the kidney and enter a special ball of capillaries called theglomerulus. The glomerulus is nestled within a region of the kidney called theBowman's Capsule. This is where filtration occurs. As blood is pushed through the tiny capillaries, the high-pressure forces some things to pass through the capillary walls. The walls act as a sieve or a filter. Hence, it is called filtration.</p><p>Water, sugar, salts, amino acids, nitrogenous wastes, and other tiny things enter the kidney as a substance called thefiltrate. Cells and large blood proteins that cannot fit through remain in the blood vessels. The filtrate entering the kidney is like pre-pre-urine.</p><p>Reabsorption</p><p>Stage 2: reabsorption. The filtrate enters the kidney in the proximal tubule. This region of the kidney is special because many things can be removed from the filtrate. These valuable things are recollected, or reabsorbed, by the body.</p><p>Glucose, certain salts, vitamins, hormones, and amino acids are restored to the body and will not be included in urine. Sometimes, if the body has too much of something then the extra sugar or salt will stay in the filtrate. For example, diabetics with high levels of blood glucose may have glucose in their urine since it cannot all be reabsorbed. The filtrate after reabsorption is like pre-urine</p><p>Secretion</p><p>Stage 3: secretion. The filtrate then passes through a really neat structure called theLoop of Henlewhere it gains and loses water and salt. As it leaves the Loop of Henle, it enters thedistal tubule, where secretion occurs.</p> | |