The Glomerulus: blood flow and filtration

Question 1

explain how blood flows through the kidneys and identify the vessels that offer greater resistance to renal blood flow (8)

Answer 1

1. renal artery (20% cardiac output) gives off progressively smaller arteries including
2. arcuate arteries (parallel to the corticomedullary junction) which gives off
3. interlobar arteries that move toward the surface in parallel series reducing resistance and give off
4. afferent arterioles (increase or decrease resistance) that give off
5. glomerular capillaries then lead to
6. efferent arterioles (increase of decrease resistance) that lead to
7. peritubular capillaries that lead to
8. venous circulation

Question 2

what aspect of the renal vasculature decreases resistance to blood flow? how?

Answer 2

each interlobular artery radiates toward the surface before branching into multiple afferent arterioles in parallel connections that decrease resistance to blood flow

Question 3

describe cortical blood flow versus medullary blood flow

Answer 3

the cortical nephrons have short loops of henle and short peritubular capillaries that do not reach the inner medulla, cortex is very vascular

the juxtamedullary/deep nephrons have specialized peritubular capillaries called vasa recta that lie side by side with long loops of henle

only the longest centermost vasa recta supple capillaries in the medulla; so very little blood flows into the papilla

Question 4

describe 2 special attributes of glomerular capillaries that help them accomplish their function of massive filtration of small molecules

Answer 4

1. high capillary hydrostatic pressure favors filtration
2. highly permeable, but highly selective barrier mostly allows only small molecules to be filtered

Question 5

describe the factors that influence whether a substance in the plasma is freely filtered through or restricted by the glomerular filtration barrier (4)

Answer 5

1. size (small molecules freely filtered)
2. shape
3. charge
4. radius

Question 6

provided with the size of a molecule relative to the size of albumin, predict whether the molecule will be filtered by the glomerulus

Answer 6

albumin is approx 66,000-69,000 g/mol and is nearly 100% excluded from filtration, so anything larger than albumin is highly unlikely to be filtered

if a substance is between 7,000-60-70,000 g/mol is freely filtered

if between 60-70,000 g/mol, the amount filtered is inverse to its size

Question 7

describe the composition of the glomerular filtrate relative to that of plasma

Answer 7

glomerular filtrate is similar to plasma but has very little protein, so has

water, Na+, K+, Cl-, HCO3-, glucose, urea, amino acids, and peptides (freely filtered substances) plus a teeny tiny bit of albumin

Question 8

identify the components of the glomerular filtration barrier

Answer 8

1. capillary endothelial cells
2. shared glomerular basement membrane (like a gel)
3. podocytes/glomerular epithelial cells

Question 9

define glomerular filtration rate (3)

Answer 9

1. tells how much the kidneys are filtering, in terms of how many mL of filtrate per unit of time (minute) per kg of body weight the animal is producing (mL/min/kg)
2. represents the flow of plasma filtrate from the glomerulus into urinary space over a specified period
3. THE MAIN DETERMINANT OF RENAL FUNCTION because represents kidney ability to excrete large quantities of waste product and regulate constituents of the internal environment very precisely (fine tune)

Question 10

list the main determinants of glomerular filtration rate

Answer 10

starling forces! filtration coefficient, Kf (hydraulic permeability x surface area) x net filtration pressure

so GFR = Kf x (glomerular capillary hydrostatic pressure - bowman/urinary space hydrostatic pressure - glomerular capillary oncotic pressure)

bowman/urinary space oncotic pressure is negligible, so just sum the forces favoring filtration and the forces favoring reabsorption to get GFR

Question 11

explain how capillary and Bowman’s capsule hydrostatic and oncotic pressure affect the glomerular filtration rate

Answer 11

capillary hydrostatic pressure is the main force driving filtration, so changes in PGC will significantly impact GFR

capillary oncotic pressure is the main force driving reabsorption

bowman capsule hydrostatic pressure works in favor of reabsorption

bowman capsule oncotic pressure favors filtration but is so small it is negligible

Question 12

predict how changes in afferent and arteriolar resistances affect renal blood flow and glomerular filtration rate

Answer 12

dilate afferent arterioles: increase both renal blood flow and GFR

constrict afferent arterioles: decrease both renal blood flow and GFR

constrict efferent arterioles: decrease renal blood flow, increase GFR

dilate efferent arterioles: increase renal blood flow and decrease GFR