Kidneys receive 20-25% of cardiac output; are perfused by renal arteries, which come off the abdominal aorta

Enter through hylus --> 3-5 segmental arteries --> interlobar aa, pass alongside medullary pyramids --> interlobular (cortical radial) aa --> microcirculation

Afferent arteriole -->

Glomerular capillaries (HI pressure; large fluid volume filtered into Bowman's capsule) -->

Efferent arteriole -->

Peritubular capillaries (LOW pressure; large amounts of H₂O & solute are reabsorbed)

(note: venous supply runs with the arteries)

Renal clearance

(how is it calculated?)

The volume of plasma from which a substance is completely removed in a given time period (vol/time)

Describes how effectively the kidneys remove a substance from the bloodstream and excrete it in the urine

Clearance of substance X = (conc of X in urine • urine volume)/(conc of X in plasma)

C_x = (U_x • V)/(P_x) = GFR_{for a freely filtered substance}

Inulin: a freely filtered substance not reabsorbed, secreted, nor metabolized in the kidneys; THEREFORE:

GFR = (U_inulin • V)/(P_inulin) = C_inulin

amount filtered = amount secreted (none is reabsorbed)

What is the renal fraction?

What is resting renal blood flow?

Why is this advantageous?

Does renal fraction change during exercise?

Regional blood flow

20-25% of cardiac output

Resting renal blood flow: ~4 ml/g

Enables kidneys to filter plasma, help regulate and control the volume and composition of the plasma.

A drop in BP provokes baroreceptors --> constriction of renal microcirculation, diverts some of the kidney's share to other organs

During exercise, renal blood flow will be pretty well maintained, but total CO will i/c, so fraction will go down

Cortex: 4 ml/g/min

Outer medulla: 0.7-1 ml/g/min

Inner medulla: 0.2-.25 ml/g/min (very low flow in inner medulla helps to conserve water)

Sympathetic stimulation

What's the kidneys' response to a drop in BP? (3)

How do these responses raise blood pressure?

Sympathetic neurons synapse on:

  1. smooth muscle in afferent arterioles --> arteriolar constriction

- arteriolar constriction lowers blood flow through the kidneys, lowers pressure, lowers glomerular filtration

  2. granular cells --> renin secretion

- Renin/A-II: vasoconstriction

3. Stimulates Na⁺ reabsorption in proximal tubule, thick ascending limb of Henle's loop, distal convoluted tubule, collecting duct

- helps expand extracellular fluid volume, helping to maintain BP

1. Glomerular filtration: filtration of plasma from glomerular capillaries into Bowman's capsule

2. Tubular reabsorption: transfer of substances from tubular lumen to peritubular capillaries

3. Tubular secretion: transfer of substances from peritubular capillaries to tubular lumen (removes substances from the body)

4. Excretion: voiding of substances in the urine

Glomerular filtration rate (GFR)

Urinary excretion rate

GFR: Volume of plasma filtered into the nephrons/time

Filtration rate of any *freely filtered* substance (Filtration) = GFR x plasma conc. of substance

- freely filtered substances: glucose, electrolytes

- proteins, blood cells will NOT pass

UER = urine flow rate x conc of substance in urine

Excretion = U_x x V

Tubular reabsorption =

glomerular filtration - urinary excretion

Excretion < Filtration: net reabsorption

Excretion > Filration: tubular secretion

The net rate of reabsorption/secretion of any substance is the difference b/t glomerular filtration and urinary excretion

Creatinine is produced endogenously, so it's used instead of inulin to estimate GFR.

Creatinine secretion in proximal tubule overestimates U_creatinine, but substances in blood also overestimate P_creatinine ==>

C_creatinine = (U_creatinine • V)/P_creatinine) ≈ GFR

P_crt monitors renal function long-term. If a person loses renal function --> d/c GFR --> eventually their creatinine levels will rise. Theoretically GFR is inversely proportional to P_crt, but people with low muscle mass with have lower CRT levels, and there's compensatory increased proximal tubule that will overestimate CRT in a person with failing kidneys.

Renal plasma flow

Filtration fraction

N: 600 ml/min

Inulin clearance = GFR = PAH clearance

Para-amino hippuric acid (PAH) clearance estimates renal flow:

- freely filtered

- avidly secreted in proximal tubule

- it's not produced by the body, but (unlike inulin) it's not cumbersome to administer so it can be used to measure RPF

PAH is completely cleared from peritubular capillaries when plasma PAH conc. is low.

RPF ≈ C_PAH = (U_PAH • V)/(P_PAH)

Filtration fraction = GFR/RPF

About 20% of renal plasma flow is filtered into nephrons = filtration fraction. If you have a renal plasma flow of around 500, you might have a GFR of 100/min. Filtration fraction is the percentage of the plasma entering the glomerulus that is actually cleared (the rest continues on through the microcirculation and is returned to the systemic circulation in the renal vein).