Week 11: Renal System

Question 1

what is the renal system

Answer 1

• integrative system
• excretion of waste
• regulation of H20 and electrolyte balance
• hormones

Question 2

what is the function of the renal system

Answer 2

• regulation of water, inorganic ion balance and acid base balance
• removal of metabolic waste products from the blood and their excretion in the urine
• removal of foreign chemicals from the blood and their excretion in the urine
• gluconeogenesis
• production of hormones / enzymes
  
  • erythropoietin which controls erythrocyte production
  • renin, an enzyme that controls the formation of angiotensin, which influences blood pressure and sodium balance
  • conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, which influences calcium balance

Question 3

what is the structure of the kidney

Answer 3

• nephron
• renal pelvis
• ureter
• renal cortex
• renal medulla

Question 4

what is a nephron

Answer 4

• nephrons are the structural and functional units of the kidney. Each kidney has over 1 million of these units
• each nephrons consists of a renal corpuscle which contains the glomerulus (which is a tuft of capillaries) and a renal tube
• the tubule forms a cup shape around the glomerulus called the glomerular capsule (Bowman’s capsule)

Question 5

what is juxtaglomerular apparatus

Answer 5

• the juxtaglomerular cells are mechanoreceptors (they sense blood pressure) in the afferent arteriole
• the macula densa cells are chemoreceptors that respond to changes in the NaCl content of the filtrate

Question 6

what is the basic renal process

Answer 6

• glomerular filtration
  
  • ~20% of the plasma entering Glomerular Capillary (GC) is filtered into BC
• tubular secretion
• tubular reabsorption

Question 7

what is differential handling of the kidney

Answer 7

• the excretion o a substance is equal to the amount filtered plus the amount secreted minus the amount reabsorbed

amount excreted = amount filtered + amount secreted - amount reabsorbed

Question 8

what is glomerular filtration (GFR)

Answer 8

• the GFR is the amount of blood filtered by the glomeruli each minute
• factors influencing GFR:
  
  • capillary permeability [+]
  • surface area (the size of the capillary bed)
  • hydrostatic pressure that drives fluid out of the capillaries
  • osmotic forces within the capillaries, which oppose the exit fluid
• net filtration pressure = change in trans capillary hydrostatic pressure gradient - change in colloid osmotic pressure gradient
• governed by starlings forces:
  
  • hydrostatic pressure difference across capillary wall favouring filtration (Pgc - Pbs)
  • protein concentration difference across the wall that creates an osmotic force
• therefore GFR controlled by vascular changes
• the kidneys glomeruli are far more efficient filler than other capillary beds in the body because
  
  • filtration membrane is a large surface area and very permeable to water and solutes
  • glomerular pressure is higher, so they produce 180L/day vs 3-4L/day formed by other capillary beds
  • that’s 125mL/min of total plasma volume
  • thus kidneys filter the entire PV - 60 times a day
• during filtration, plasma proteins remain in plasma to maintain osmotic pressure
• blood cells or protein in the urine (proteinuria) indicates a problem with the filtration membrane
  
  • common finding during diabetes and hypertension and signals that kidney damage has occurred. If untreated, will progress to end stage renal disease and renal failure

Question 9

what is tubular reabsorption

Answer 9

• tubular reabsorption begins as soon as filtrate enters the tubule cells
• paracellular transport occurs between cells (even though they have tight junctions) and is seen mainly with ions)
• transport can be active (requires ATP) or passive (no ATP)

Question 10

what is filtration fraction

Answer 10

• the filtration fraction is the proportion of the plasma that enters the kidneys that is subsequently filtered at the glomerulus and passes into the renal tubules
• it is calculated from the ratio of GFR : Renal plasma flow (RPF)
• filtration fraction is typically 0.16-0.20
• meaning 20% of the blood that enters the kidneys is filtered

Question 11

what is tubular secretion

Answer 11

• substances such as hydrogen ion, potassium, and organic anions move from the peritubular capillaries into the tubular lumen
• tubular secretion is an important mechanism for:
  
  • disposing of drugs and drug metabolites
  • eliminating undesired substances or end products that have reabsorbed by passive processes (urea and urea acid)
  • removing excess K+
  • controlling blood pH

Question 12

what is the ‘division of labour’ in the tubules

Answer 12

• the majority of the reabsorption is accomplished by the proximal tubule and the loop of henle
• extensive reabsorption by the proximal tubule and henle’s loop ensures that the masses of solutes and the olume of water entering the tubular segments beyond henle’s loop are relatively small
• these distal segments then do the fine-tuning for most substances, determining the final amounts excreted in the urine by adjusting their rates of reabsorption and in a few cases, secretion

Question 13

what is renal clearance (RC)

Answer 13

• renal clearance (mL/min) is the amount of a substance filtered per minute, divided by its plasma concentration. The clearance for any substance can be calculated
• RC = (UxV)/P
  
  • U = concentration of the substance in the urine (mg/mL)
  • V = flow rate of urine formation (mL/min)
  • P = concentration of substance in the plasma (mg/mL)
• if the substance is freely filtered and not reabsorbed or secreted, this ration allows us to estimate the rate at which fluid is filtered at the glomerulus
  
  • to determine RC, we use inulin (it is freely filtered and neither reabsorbed nor secreted)
  • creatine can also be used but is less accurate