BPK 105 Study Guide - Final Guide: Proximal Tubule, Distal Convoluted Tubule, Descending Limb Of Loop Of Henle
27 Apr 2018
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Module 12 - Review Questions - Part 3
4. Describe the processes of reabsorption that occur as you move through the nephron.
Include the main function, the % of filtrate volume and the main substances reabsorbed
and in each area of the nephron. If we started with 180L of fluid being filtered, what is the
average volume of urine output? [9 marks]
-Tubular reabsorption is the movement of substances from the filtrate across the wall
of the nephron back into the blood of the peritubular capillaries.
- As the filtrate flows from the Bowman capsule through the proximal convoluted tubule,
loop of Henle, distal convoluted tubule, and collecting duct, many of the solutes in the
filtrate are reabsorbed.
- About 99% of the original filtrate volume is reabsorbed and enters the peritubular
capillaries. The reabsorbed filtrate flows through the renal veins to enter the general
circulation. Only 1% of the original filtrate volume becomes urine (figure 18.9).
- Because excess ions and metabolic waste products are not readily reabsorbed, the
small volume of urine produced contains a high concentration of ions and metabolic
waste products.
- The proximal convoluted tubule is the primary site for the reabsorption of solutes
and water.
- The cuboidal cells of the proximal convoluted tubule have numerous microvilli and
mitochondria, and they are well adapted to transport molecules and ions across the
nephron wall by active transport and cotransport.
- Substances transported from the proximal convoluted tubule include proteins, amino
acids, glucose, and fructose molecules, as well as Na+, K+, Ca2+, HCO3−, and Cl−. The
proximal convoluted tubule is permeable to water.
- As solute molecules are transported out of the proximal convoluted tubule into the
interstitial fluid, water moves by osmosis in the same direction.
- The solutes and water then enter the peritubular capillaries.
- Consequently, 65% of the filtrate volume is reabsorbed from the proximal convoluted
tubule (figures 18.9, step 1, and 18.10).
- The descending limb of the loop of Henle further concentrates the filtrate. The renal
medulla contains very concentrated interstitial fluid that has large amounts of Na+, Cl−,
and urea.
- The wall of the thin segment of the descending limb is permeable to water and
moderately permeable to solutes.
- As the filtrate passes through the descending limb of the loop of Henle into the medulla
of the kidney, water moves out of the nephron by osmosis, and some solutes move
