Overview

• each nephron is supplied with blood from an afferent arteriole that opens onto the glomerular capillary bed
• blood then flows to an efferent arteriole, supplying the peritubular capillaries and medullary vasa recta
• the kidney receives up to 25% of resting cardiac output

Control of blood flow

• the kidney is able to autoregulate its blood flow between systolic pressures of 80-180mmHg so there is little variation in renal blood flow
• this is achieved by myogenic control of arteriolar tone, both sympathetic input and hormonal signals (e.g. renin) are responsible
• renal cortical blood flow > medullary blood flow (i.e. tubular cells more prone to ischaemia)

Glomerular structure and function

• blood inside the glomerulus has considerable hydrostatic pressure
• the basement membrane has pores that will allow free diffusion of smaller solutes, larger negatively charged molecules such as albumin are unable to cross
• the glomerular filtration rate (GFR) is equal to the concentration of a solute in the urine, times the volume of urine produced per minute, divided by the plasma concentration (assuming that the solute is freely diffused e.g. inulin)
• in clinical practice creatinine is used because it is subjected to very little proximal tubular secretion
• although subject to variability, the typical GFR is 125ml per minute
• glomerular filtration rate = Total volume of plasma per unit time leaving the capillaries and entering the Bowman's capsule
• renal clearance = volume plasma from which a substance is removed per minute by the kidneys

Substances used to measure GFR have the following features:

• 1. Inert
• 2. Free filtration from the plasma at the glomerulus (not protein bound)
• 3. Not absorbed or secreted at the tubules