Topic 11.3 The Kidney

Question 1

11.3.1 Define excretion.

Answer 1

Excretion is the removal from the body of the waste products of metabolic pathways.

Question 2

11.3.2 Draw and label a diagram of the kidney.

Answer 2

• renal artery
• renal vein
• renal pelvis
• ureter
• medulla
• cortex

Question 3

11.3.3 Annotate a diagram of a glomerulus and associated nephron to show the function of each part.

Answer 3

• Afferent arteriole: carries blood to glomerulus
• Efferent arteriole (thinner): carries blood from glomerulus (+ RBC, WBC, platelets and proteins)
• Glomerulus: capillary tuft between afferent and efferent arterioles. Surrounded by Bowman’s capsule. Small particles are pushed from glomerulus into Bowman’s capsule. Larger particles continue into efferent arteriole.
• Bowman’s capsule: cup-like end of nephron where wastes are forced out of the blood and into the nephron (i.e. H₂O, amino acids, glucose, NaCl, urea)
• Proximal convoluted tubule: where H₂O, amino acids, glucose, Na⁺, Cl^-, and K⁺ are filtered out to the peritubular capillary network
• Loop of Henle: where H₂O, Na⁺, Cl^- and K⁺ are filtered out to the peritubular capillary network
  
  • descending limb
  • ascending limb
• Distal convoluted tubule: where materials from Loop of Henle move through; collects penicillin, histamine, and H⁺ from renal vein
• Collecting duct: where last chance for H₂O to be filtered out to peritubular capillaries takes place

Question 4

11.3.4 Explain the process of ultrafiltration, including blood pressure, fenestrated blood capillaries and basement membrane.

Answer 4

• Ultrafiltration occurs when blood pressure is high in glomerulus; forces small molecules (*H₂O, nitrogenous wastes, *nutrients, *ions (salts)) into Bowman’s capsule
  
  • large molecules are unable to pass (RBC, WBC, platelets, proteins) → remain in blood and leave glomerulus via efferent arteriole
• small, filterable molecules are forced into Bowman’s capsule to form filtrate; three things involved in forming filtrate:

High Blood Pressure

• glomerulus increases blood pressure by forming narrow branches (also increases SA for filtration)
• efferent arteriole has smaller diameter than afferent arteriole ∴ restricts flow of blood and keeps pressure high
• net pressure gradient in glomerulus forces blood into capsule space (↑ pressure to ↓ pressure)

Fenestrated Blood Capillaries

• walls of glomerulus capillaries have fenestrations (small slits) that open when blood pressure is increased
  
  • lets small molecules through and keeps large molecules out
  • blood exits glomerulus directly through pores as capillaries are fenestrated

Basement Membrane

• lies between glomerulus and Bowman’s capsule
• is a fine mesh that restricts passage of large molecules → the sole filtration barrier

Question 5

11.3.5 Define osmoregulation.

Answer 5

Osmoregulation is the the control of the water balance of the blood, tissue or cytoplasm of a living organism.

Question 6

11.3.6 Explain the reabsorption of glucose, water and salts in the proximal convoluted tubule, including the roles of microvilli, osmosis and active transport.

Answer 6

• molecules from filtrate move from proximal convoluted tubule to the peritubular capillary network
  
  • most water, nutrients and some salts are reabsorbed here (not wastes and excess salts)
• one area of lumen of proximal convoluted tubule has microvilli cell lining to increase SA for absorption of materials from filtrate
• reabsorption is both active and passive:
  
  • Active: requires ATP and carrier molecule (e.g. glucose, Na⁺)
    
    • reabsorption is “selective” because only molecules with carriers are reabsorbed
    • reabsorption continues until threshold is met (e.g. 0.15 g glucose/100 mL blood)
    • any excess is left in nephron and excreted in urine
    • Note: urea has low threshold level → very little reabsorbed into bloodstream, most is excreted
  • Passive: e.g. Cl^- and H₂O
    
    • water moves by osmosis from nephron to capillary network (↑ conc’n to ↓ conc’n)
    • Cl^- passively follows Na⁺ out when it is actively transported
      
      • this increases [ion] in bloodstream, further encouragin water to be reabsorbed

Question 7

11.3.7 Explain the roles of the loop of Henle, medulla, collecting duct and ADH (vasopressin) in maintaining the water balance of the blood.

Answer 7

Loop of Henle and Medulla

• Loop of Henle creates hypertonic conditions in medulla
• Primary role of Loop of Henle reabsorption of water → over 99% of water in original filtrate is reabsorbed during urine formation
• This concentrates urine, allowing it to be hypertonic to plasma
  
  • ​inner medulla tissue has high [salt]
    
    • maintained by Na⁺ and Cl^- passively diffusing out at descending limb and being pumped out at ascending limb
    • salty concentration of medulla during descending limb causes water to diffuse out of nephron
    • ascending limb is impermeable​ to water so water cannot diffuse out here → keeps medulla area concentrated in salts

Collecting Duct and ADH (Antidiuretic Hormone)

• in cortex, fluid in duct is isotonic to surrounding cells → no net movement of water
• in medulla, fluid is hypotonic to cells of medulla → H2O passively diffuses out of collecting duct back into capillary network
• Hypothalamus monitors water balance in blood and detects dehydration → causes pituitary to release ADH in response to dehydration
  
  • ADH promotes reabsorption of water from collecting duct
    
    1. Cells in hypothalamus detect low water content of blood
    2. If blood is too hypertonic:
  • ADH released into blood, acts on collecting duct, aquaporins are opened (increases permeability of collecting ducts to water) → more water reabsorbed, blood volume + pressure increases, & less water in urine
    
    3. If blood is hypotonic:
  • as blood becomes dilute, detected by hypothalamus, ADH secretion stops (neg. feedback), aquaporins close, collecting duct = less permeable, less water reabsorbed, more water in urine
• alcohol and diuretic drugs (for high blood pressure, but lowers blood volume) inhibit ADH secretion

Question 8

11.3.8 Explain the differences in the concentration of proteins, glucose and urea between blood plasma, glomerular filtrate and urine.

Answer 8

Proteins

• present in blood plasma, not in glumerular filtrate or urine
  
  • b/c proteins can’t pass basement membrane during ultrafiltration ∴ can’t become part of filtrate

Glucose

• present in blood plasma and glomerular filtrate, not usually present in urine (unless diabetic, which is bad)
  
  • b/c glucose is selectively reabsorbed in proximal convoluted tubule by active transport

Urea

• present in blood plasma, glomerular filtrate and urine
  
  • b/c water is reabsorbed from filtrate (by osmosis due to hypertonicity of medulla), urea becomes more concentrated in urine
  • conc’n of urea in urine depends on amount of H₂O in urine

Question 9

11.3.9 Explain the presence of glucose in the urine of untreated diabetic patients.

Answer 9

• lack of insulin/sensitivity to insulin in diabetics leads to high amounts of glucose in blood
• normally, all glucose is reabsorbed in proximal convoluted tubule, but for diabetics, blood glucose level is too high and not all glucose is reabsorbed
  
  • glucose is actively transported so needs carrier and energy → not enough carriers in PCT to reabsorb all of the glucose