Renal Physiology - Blood Flow and GFR

Question 1

Discuss the blood supply to the Kidney

Answer 1

Receives about 1/4 of cardiac output, about 1.25L/min (400-500ml/100g/minute) - necessary for efficient filtration, rather than because of high metabolic demand. The renal artery feeds the high-pressure glomerular network, continuing to the low-pressure peritubular network.

90% to the cortex, 5% to the medulla (needed to keep the urea concentration gradient steady)

Unlike other organs, the metabolic demand of the kidney is determined by blood flow - it varies according to the amount of sodium to reabsorb, which is dependent on GFR. Therefore, oxygen extraction is relatively static across a range of blood flow.

Question 2

Explain renal blood flow and GFR autoregulation using a graph

IMAGE - Graph of renal blood flow

Answer 2

Stable for MAPs between 75-160mmHg

X axis: MAP (mmHg)
Y axis: GFR (ml/g/min) and Renal blood flow (ml/g/min)
Two curves:
Renal blood flow starts at 0,0, and is sigmoid, rising sharply until 50mmHg, plateauing until 150mmHg, and rising sharply again.
GFR: Rises more sharply than bloodflow, plateauing at 50mmHg, higher than the plateau of the renal blood flow curve, and not rising any further

Maintained via four mechanisms:
Tubuloglomerular feedback (-ve feedback loop, increased Na at macula densa causes afferent arteriolar constriction)
Myogenic response (If the blood vessel wall is stretched by increased BP, it vasoconstricts)
Humoral influence
Vasoconstrictors: Angiotensin II (Constricts efferent to decrease renal blood flow, but maintain GFR), Endothelin
Vasodilators: Prostaglandins, Bradykinin, Amino acids, Blood glucose
Autonomic nervous system (Mostly via sympathetic tone, which vasoconstricts afferent slightly more than efferent, reducing renal blood flow)

Question 3

What is the course of renal blood vessels through the Kidneys?

Answer 3

Aorta
Renal artery
Segmental artery
Interlobar artery
Acuate artery
Cortical radial artery
Afferent arteriole
Glomerulus
Efferent arteriole
Peritubular capillary
Vasa recta
Veins

Around 30 vasa recta branch off from each efferent arteriole.

Question 4

What is the role of the vasa recta and peritubular capillaries?

Answer 4

Vasa recta:
Blood supply to the medulla
Reabsorption of water from the meedulla
Countercurrent exchange to maintain hypertonic environment in the medullary interstitium

Peritubular capillaries:
Solute and ion exchange in the cortex

Question 5

Discuss GFR

Answer 5

Ultrafiltration process occurs around 125ml/min
Roughly 180L/day - main determining factor is particle size
Molecules over 70,000 daltons are not filtered, below 7,000 daltons are freely filtered
Negatively charged molecules are repelled by basement membrane and therefore have reduced filtration fraction.

Glomerular filtration and tubular reabsorption of fluid/solutes can be altered by the kidneys to maintain homeostasis and remove waste products/drugs - the principle being the same as of starling’s forces.

GFR = Kf (PG - PB - πG)

Kf = glomerular ultrafiltration coefficient
PG = Glomerular hydrostatic pressure
PB = Bowman’s capsule hydrostatic pressure
πG = Glomerular oncotic pressure

Other forces affect glomerular filtration and tubular re-absorption but are generally ignored, including the viscosity of the fluid, friction to the flow of the fluid, and interstitial pressure compressing the renal tubules

Question 6

What is filtration fraction?

Answer 6

The proportion of plasma supplied to the kidney that is filtered, most of which is then reabsorbed.

Renal plasma flow can be measured using p-aminohippuric acid
Renal blood flow can then be estimated from the equation

RBF = RPF/(1 - Haematocrit)

Renal plasma flow is around 600ml/min, so if GFR is 125ml/min, then the filtration fraction is around 20%