M07_REEC5174_09_IE_44-56

M07_REEC5174_09_IE_44-56 - Notes to Instructors Chapter 44...

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Notes to Instructors Chapter 44 Osmoregulation and Excretion What is the focus of this activity? The digestive system and the respiratory system function in the bulk flow of nutrients and gases into the body. The excretory system functions to maintain the body’s water balance and to remove metabolic wastes from the body—in particular, nitrogenous wastes. All three of these systems involve the organism internalizing a part of the external environment to gain control over these functions. What is this particular activity designed to do? Activity 44.1 What is nitrogenous waste, and how is it removed from the body? This activity is designed to help students understand the structure and function of the mammalian kidney. What misconceptions or difficulties can this activity reveal? Activity 44.1 Most students can work through how the filtrate is formed and how various substances are later removed from the filtrate. Many don’t understand what happens to these substances after they are removed from the filtrate, however. It helps to remind students that the efferent arterioles subdivide to form a capillary network around the loops of Henle. These drain into the renal vein. The substances removed from the filtrate either diffuse back into the capillaries or are actively transported into them. As students study how the concentration gradient of NaCl is set up in the medulla, some wonder why this region doesn’t become a solid block of NaCl. After all, the diagrams all show NaCl being actively pumped out of the filtrate in the medullary regions. Again, it helps to remind students that the loops of Henle are surrounded by a network of capillaries. Substances in the interstitial fluid of the kidney can diffuse into the capillary network. Blood flowing from the afferent arteriole and into the capillary network picks up salt and loses water as it moves into the medulla. As it flows from the capillary network to the renal vein, the blood reverses direction. As it encounters progressively lower extracellular salt concentrations, the blood in the capillaries loses salt and picks up water. A type of countercurrent exchange system is set up. In association with the active transport of NaCl out of the tubules, this sets up a gradient of salt concentration within the medulla. Answers Activity 44.1 What is nitrogenous waste, and how is it removed from the body? In the space below, draw a longitudinal section of a mammalian kidney. Next to this, draw a blowup of a nephon (including Bowman’s capsule and the loop of Henle) and its associated collecting duct. Be sure to include the afferent arteriole, glomerulus, and efferent arteriole that are associated with this nephron. You may do your drawing in chalk on a tabletop or blackboard if they are available.
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M07_REEC5174_09_IE_44-56 - Notes to Instructors Chapter 44...

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