Homeostasis (BWO)

• What is a Nephron

  Basic structural and functional unit of the kidney

  Associated with each nephron are a network of blood vessels
• Nephron Structure

  A) Bowmans Capsule

  B) Proximal convulated tubule

  C) Loop of Henle

  D) Descending limb

  E) Ascending limb

  F) Distal convulated tubule

  G) Collecting duct

  
• Function of bowmans capsule

  Formation of glomerular filtrate (ultrafiltration)
• Function of proximal convulated tubule 


  Reabsorption of water and glucose (selective reabsorption)
• Function of Loop of Henle


  maintenence of a gradient of sodium ions in the medulla
• Function of distal convulated tubule and collecting duct


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

  High hydrostatic pressure in glomerulus as diameter of afferent arteriole (in) is wider than efferent arteriole (out)

  Small substances e.g. water, glucose, ions, urea forced into glomerular filtrate, filtered by 1. pores between capillary endothelial cells, 2. capillary basement membrane, 3. podocytes

  Large proteins/blood cells remain in blood
• Bowmans Capsule

  A) Afferent arteriole

  B) Efferent arteriole

  C) Glomerulus

  D) Podocyre

  E) Podocyte

  F) Glomerular filtrate

  
• Describe the reabsorption of glucose by the proximal convulated tubule

  Na+ actively transported out of epithelial cells to capillary

  Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against a conc gradient

  Glucose moves into capillary by facilitated diffusion down its conc gradient
• Describe reabsorption of water by the proximal convulated tubule

  Glucose ect in capillaries lower water potential

  Water moves by osmosis down a water potential gradient
• How do features in the PCT allow rapid reabsorption of glucose into the blood

  Microvilli - provide large surface area

  Many channel proteins - for facilitated diffusion

  Many carrier proteins - for active transport/cotransport

  Many mitochondria - produce ATP for active transport

  Many ribosomes - produce carrier/channel proteins
• Why is glucose found in the urine of an untreated diabetic

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

  As glucose carrier / cotransporter proteins are saturated / working at maximum rate
• Importance of maintaining gradient of sodium ions in medulla (conc increases further down)

  So water potential decreases down the medulla (compared to filtrate in the collecting duct)

  So a water potential gradient is maintained between the collecting duct and medulla

  To maximise reabsorption of water by osmosis from filtrate
• Role of loop of Henle in maintaining a gradient of sodium ions in medulla

  In the ascending limb:

  Na+ actively transported out (so filtrate conc decreases)

  Water remains as ascending limb is impermeable to water

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

  In the descending limb

  Water moves out by osmosis then reabsorbed by capillaries (so filtrate conc increases

  Na+ 'recycled' - diffuses back in
• What does the loop of Henle act as

  A counter-current multiplier
• Animals that need to conserve water have long loops of Henle (thick medulla)
• Why do animals needing to conserve water have long loops of Henle

  More Na+ moved out - Na+ gradient is maintained for longer in medulla

  So water potential gradient is maintained for longer

  So more water can be reabsorbed from collecting duct by osmosis
• Describe reabsorption of water by the DCT and Collecting ducts

  Water moves out of DCT and collecting duct by osmosis down a water potential gradient

  Controlled by ADH which increases their permeability
• What is osmoregulation

  Control of water potential of the blood (by negative feedback)
• Describe the role of the hypothalamus in osmoregulation

  Contains osmoreceptors which detect increase or decrease in blood water potential

  Produces more ADH when water potential is low or less ADH when water potential is high
• Describe the role of the posterior pituitary gland in osmoregulation

  Secretes (more / less) ADH into blood due to signals from the hypothalamus
• Describe the role of antidiuretic hormone (ADH) in osmoregulation (in response to an increase in water potential of the blood)


  Attaches to receptors on collecting duct (and DCT)

  Stimulating addition of channel proteins (aquaporins) into cell-surface membranes

  So increases permeability of cells of collecting duct and DCT to water

  So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis

  So decreases volume and increases concentration of urine produced