Renal - Renal Physiology

Question 1

Overview?

Answer 1

• 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

Question 2

Control of blood flow?

Answer 2

• 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)

Question 3

Glomerular structure and function?

Answer 3

• 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

Question 4

Substances used to measure GFR have the following features:

Answer 4

1. Inert
2. Free filtration from the plasma at the glomerulus (not protein bound)
3. Not absorbed or secreted at the tubules
4. Plasma concentration constant during urine collection
   examples: inulin, creatinine

Question 5

GFR?

Answer 5

( urine concentration (mmol/l) x urine volume (ml/min) ) / plasma concentration (mmol/l)

• the clearance of a substance is dependent not only on its diffusivity across the basement membrane but also subsequent tubular secretion and / or reabsorption
• so glucose which is freely filtered across the basement membrane is usually reabsorbed from tubules giving a clearance of zero

Question 6

Tubular function?

Answer 6

• reabsorption and secretion of substances occurs in the tubules
• in the proximal tubule substrates such as glucose, amino acids and phosphate are co-transported with sodium across the semi permeable membrane
• up to two thirds of filtered water is reabsorbed in the proximal tubules
• this will lead to increase in urea concentration in the distal tubule allowing for its increased diffusion
• substances to be secreted into the tubules are taken up from the peritubular blood by tubular cells
• solutes such as para-aminohippuric acid are cleared with a single passage through the kidneys and this is why it is used to measure renal plasma flow. Ions such as calcium and phosphate will have a tubular reabsorption that is influenced by plasma PTH levels
• potassium may be both secreted and reabsorbed and is co-exchanged with sodium

Question 7

Loop of Henle?

Answer 7

• approximately 60 litres of water containing 9000mmol sodium enters the descending limb of the loop of Henle in 24 hours
• loops from the juxtamedullary nephrons run deep into the medulla
• the osmolarity of fluid changes and is greatest at the tip of the papilla
• the thin ascending limb is impermeable to water, but highly permeable to sodium and chloride ions
• this loss means that at the beginning of the thick ascending limb the fluid is hypo osmotic compared with adjacent interstitial fluid
• in the thick ascending limb the reabsorption of sodium and chloride ions occurs by both facilitated and passive diffusion pathways

the loops of Henle are co-located with vasa recta, these will have similar solute compositions to the surrounding extracellular fluid so preventing the diffusion and subsequent removal of this hypertonic fluid

the energy dependent reabsorption of sodium and chloride in the thick ascending limb helps to maintain this osmotic gradient