Kidneys

• Kidney Structure

  A) Cortex

  B) Renal Pelvis

  C) Medulla

  D) Fibrous capsule

  E) Renal Artery

  F) Renal Vein

  G) Ureter

  
• Nephron Structure

  A) Bowman's Capsule

  B) Proximal Convoluted Tubule

  C) Glomerulus

  D) Loop of Henle

  E) Renal Artery

  F) Renal Vein

  G) Distal Convoluted Tubule

  H) Collecting Duct

  
• Importance of Kidneys

  • Water - cells function in isotonic solutions (for metabolic reactions)
  • Ions - cellular processes
  • Urea - toxic product of a breakdown, could damage cells
• Blood vessels in the nephron

  • Afferent arteriole - supplies glomerulus with blood from renal artery
  • Efferent arteriole - carries blood from the glomerulus to capillaries surrounding the loop of Henle
  • Capillaries - supply blood to the renal vein
• What does urine not contain ?

  • Proteins - too large to be filtered out of the blood
  • Glucose - all glucose is absorbed at the selective reabsorption stage in the PCT
• Process of Ultrafiltration

  1. Blood in the afferent arteriole enters the glomerulus
  2. This causes a high hydrostatic pressure of the blood
  3. Water and small molecules (glucose & ions) from the blood is forced into the Bowman's Capsule and forms glomerular filtrate
  4. Large proteins and blood cells are too big to fit through the gaps of the capillary endothelium so remain in the blood
  5. Blood leaves through the efferent arteriole
• To allow glomerular filtrate to flow:

  • Endothelium of capillaries have narrow gaps between cells to allow filtrate through
  • Basement membrane - fine mesh of collagen fibres & glycoprotein that filter the filtrate (stops proteins and blood cells from passing)
  • Podocytes - epithelial cells of Bowman's Capsule that contain gaps so filtrate can pass through
• Describe how ultrafiltration produces glomerular filtrate (5)

  • Hydrostatic pressure 
  • Small molecules
  • Pass through basement membrane
  • Protein too large to go through
  • Presence of pores in capillaries /presence of podocytes 
• How are PCT cells specialised for reabsorption ?

  • Microvilli so large surface area
  • Cotransporter proteins for transport of glucose and amino acids in association with sodium ions by facilitated diffusion
  • Many channel proteins so more diffusion
  • The opposite membrane is folded so increased surface area
  • Cell cytoplasm has many mitochondria so more ATP for active transport
• Process that occurs at the PCT

  1. Na+ ions are actively transported out of cells lining the PCT and diffuse into blood. Mitochondria supply ATP.
  2. Na+ ion conc in the cell is lowered so more Na+ ions in the PCT lumen than in the cell
  3. Na+ ions diffuse into the cell from the PCT lumen down conc gradient bringing along glucose
  4. Glucose moves from cell lining PCT into blood via facilitated diffusion (reabsorbed)
• Descending limb: Thinner walls - permeable to water
• Maintaining a Sodium ion Gradient by the Loop of Henle

  1. Mitochondria releases energy for the active transport of Na+ out of the ascending limb into the interstitial space
  2. Conc of Na+ in the interstitial space increases, lowers water potential
  3. Causing the osmosis of water into the interstitial space from descending limb, then reabsorbed into blood capillaries
  4. Filtrate is at its lowest water potential at the base of the loop of Henle so some Na+ diffuses out
• DCT and the Collecting Duct

  • Due to all the Na+ ions being actively transported out of the loop of Henle when filtrate reaches the DCT it is very dilute
  • The loop of Henle created a very low water potential in medulla (by releasing Na+)
  • Meaning more water is reabsorbed from the filtrate to the blood
  • Making concentrated urine
• Explain the role of the loop of Henle in producing concentrated urine. (5)

  1. Sodium ions diffuse into descending limb 
  2. Water moves out of the descending limb 
  3. Sodium ions actively removed from ascending limb 
  4. Low water potential in medulla
  5. Water leaves collecting duct 
  6. By osmosis
• • Osmoreceptors in the hypothalamus can detect water potential of the blood 
  • The axons of these receptors extend to the posterior pituitary that release ADH when blood water potential is low
• • Water potential of blood too low - water leaves the osmoreceptors by osmosis and they shrivel. This stimulates the hypothalamus to produce more ADH
• • Water potential of blood too high - water enters the osmoreceptors by osmosis. This stimulates the hypothalamus to produce less ADH
• Role of ADH (Antidiuretic Hormone)

  1. ADH binds to receptors on cells lining the collecting duct
  2. This causes the synthesis of phosphorylase in the cells
  3. Phosphorylase causes vesicles with aquaporins in the membrane to fuse with the cell membrane
  4. Aquaporins are channel proteins that allow the movement of water, thus increasing the permeability of the duct
  5. More ADH in the blood means more aquaporins are inserted allowing more water to be reabsorbed
• Water potential of the blood decreases:

  • Detected by osmoreceptors in the hypothalamus 
  • Pituitary gland releases more ADH which is released into the blood by the posterior pituitary gland 
  • DCT and collecting duct walls become more permeable to water 
  • More water is reabsorbed into the blood and less is lost in urine
• Explain how ADH increases the movement of water from the lumen of the collecting duct into the blood (4)

  • ADH causes vesicles containing aquaporins to be inserted into membrane
  • Water enters cell through aquaporins 
  • By osmosis 
  • From cell to capillary
  • Via interstitial fluid 
• The gene for the ADH receptor proteins is found on the X chromosome. One allele of this gene causes a non-functioning receptor protein to be made. This allele is recessive and it is one cause of the condition called diabetes insipidus.

  Suggest why diabetes insipidus is more common in males (2)

  • Males only have one X chromosome 
  • A single copy of the recessive allele will be expressed 
• Ascending limb: Thicker walls - impermeable to water. Na+ ions are actively transported out
• Thicker medulla means longer loop of Henle
• Why is glucose found in the urine of a person with untreated diabetes ? (3)

  • High concentration of glucose in the blood
  • Not all of the glucose is reabsorbed at the proximal convoluted tubule
  • Co-transport proteins are saturated