Control of Blood Water Potential

• Describe the structure of a nephron:

  ● Nephron = basic structural and functional unit of the kidney (millions in the kidney)

  ● Associated with each nephron are a network of blood vessels
• What is the role of the Bowman’s / renal capsule?

  Formation of glomerular filtrate (ultrafiltration)
• What is the role of Proximal convoluted tubule:

  Reabsorption of water and glucose (selective reabsorption)
• What is the role of the Loop of Henle?

  Maintenance of a gradient of sodium ions in the medulla
• What is the role of the Distal convoluted tubule:

  Reabsorption of water (permeability controlled by ADH)
• Describe the formation of glomerular filtrate?

  1)High hydrostatic pressure in glomerulus

  ○ As diameter of afferent arteriole (in) is wider than efferent arteriole (out)

  2) Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by:

  a)Fenestrations between capillary endothelial cells

  b) Capillary basement membrane

  c) Podocytes

  3) Large proteins / blood cells remain in blood
• Describe the reabsorption of glucose by the proximal convoluted tubule:

  1)Na+ actively transported out of epithelial cells to capillary

  2) Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient (Via Co-transport protein)

  3) Glucose moves into capillary by facilitated diffusion down its concentration gradient
• Describe the reabsorption of water by the proximal convoluted tubule:

  ● Glucose etc. in capillaries lower water potential

  ● Water moves by osmosis down a water potential gradient
• Describe and explain how features of the cells in the PCT allow the rapid

  reabsorption of glucose into the blood:

  ● Microvilli / folded cell-surface membrane → provides a large surface area

  ● Many channel / carrier proteins → for facilitated diffusion / co-transport

  ● Many carrier proteins → for active transport

  ● Many mitochondria → produce ATP for active transport

  ● Many ribosomes → produce carrier / channel proteins
• Suggest why glucose is found in the urine of an untreated diabetic person:

  ● Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT

  ● As glucose carrier / cotransporter proteins are saturated / working at maximum rate
• Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla:

  1)In the ascending limb:

  ○ Na+ actively transported out (so filtrate concentration decreases)

  ○ Water remains as ascending limb is impermeable to water

  ○ This increases concentration of Na+ in the medulla, lowering water potential

  2) In the descending limb:

  ○ Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases)

  ○ Na+ ‘recycled’ → diffuses back in
• Suggest why animals needing to conserve water have long loops of Henle(thick medulla):

  ● More Na+ moved out → Na+ gradient is maintained for longer in medulla / higher Na+ concentration

  ● So water potential gradient is maintained for longer

  ● So more water can be reabsorbed from collecting duct by osmosis