AlexMccourt The Student Room Biology Notes

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F214 Communication, Homeostasis and Energy Module 2 Excretion 4.2.1 Excretion a. Define the term excretion Excretion-The removal of metabolic waste from the body which are by products or unwanted substances from cell processes. b. Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste from the body Carbon dioxide Excess carbon dioxide is toxic and has three main effects: The majority of carbon dioxide is carried in the blood as hydrogencarbonate ions but in the process hydrogen ions are made inside red blood cells with carbonic anhydrase. These hydrogen ions compete with oxygen for space on the haemoglobin. Therefore too much carbon dioxide can cause a reduction in oxygen…show more content…
6. The glucose and amino acids in the tissue fluid diffuse into the blood and are carried away. 7. Salts, glucose and amino acids are reabsorbed which reduces the water potential in the cells and increases the water potential in the tubule fluid. 8. So water enters the cells. 9. The water is then reabsorbed into the blood by osmosis. 10. Larger molecules like small proteins that may have entered the tubule are reabsorbed by exocytosis. i. Explain, using water potential terminology, the control of the water content of the blood, with reference to the roles of the kidney, osmoreceptors in the hypothalamus and the posterior pituitary gland Roles of the Kidney In the descending limb Loss of water by osmosis to the surrounding tissue fluid. Diffusion of sodium and chloride ions into the tubule from the surrounding tissue. The lower down the descending limb the lower the water potential. In the ascending limb At the base the sodium ions diffuse out. Higher up the sodium and chloride ions are actively transported our into the tissue fluid. The wall is impermeable to water so it cannot leave. The fluid loses salts as it moves up the ascending limb. As the fluid moves up the tubule the water potential increases. Distal convoluted tubule Active transport is used to adjust the concentrations of various salts. Collecting duct Water moves by osmosis from the tubule fluid into the surrounding tissue. It then enters the blood

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