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There are 4 basic processes that occur in the kidney: filtration, re-absorption, secretion and excretion.
Filtration is where active transport occurs due to change in blood flow and pressure. It mostly occurs due to hydraulic pressure. This is key for the kidneys function to remove toxins and waste from the plasma efficiently. Usually this occurs throughout the nephron system. Within the Glomerulus and Bowman’s capsule where electrolytes, water, glucose, amino acids and ions are filtered.
Re-absorption is a two-step process which begins with the active extraction of useful substances from the tubule fluid, then the transport of the substances from the interstitium (connective tissue surrounding the nephron) back to the bloodstream. Within the proximal convoluted tubule, re-absorption occurs when the pH needs to be maintained and ions are reabsorbed back into the bloodstream. Amino acids, K ions and glucose are also transported back into the blood as they are essential for the body. Some Na and Cl ions are also moved back into the capillaries so salt regulation can occur.
Toxins and hydrogen ions are actively secreted into the tubule from the blood. In the loop of henle, the re-absorption of water (by osmosis) occurs – the descending limb allows this. The ascending limb allows the active or passive transport of salt e.g. Na to be re-absorbed. Regulated re-absorption is when hormones control the rate of transport of water and Na depending on certain conditions, this is selective re-absorption. This occurs in the distal tubule. The descending limb of the loop of Henle is permeable to water, water can pass out. The ascending limb is impermeable to water, Na+ and Cl- ions are secreted out which lowers the concentration of water. By osmosis, water moves into the ascending limb from a high concentration to a low concentration. This can be modelled by an experiment with potato slices which retain solutions by different amounts. For example, in an experiment with potatoes it was found that the slices placed in water, there was a gain in mass of 0.12g as water moved from a high concentration to a low concentration, inside the potato by osmosis (gained water). In a weak solution, the mass decreased by 0.14g as water moved out of the potato to a lower con in the solution.
Diffusion is defined to be the spreading of particles, from a high concentration to a low concentration. This occurs in the kidney. Surrounding the tubule are capillaries and by diffusion, useful substances are re-absorbed, which can be used by the body.
Secretion is after filtration. There are still additional substances being secreted into the tubular fluid. This is essential for the regulation of pH and potassium concentration. Excretion is the products that go into urine.
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