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Describe normal renal blood flow and its control, including the concept of autoregulation.
The kidneys receive approximately 25% of cardiac output or 1250ml/min via the renal arteries. The kidneys have a high oxygen consumption of 18ml/oxygen/minute but a low oxygen extraction of 10%. Therefore, the high renal blood flow is due to the need for filtration rather than due to metabolic demand. Each renal artery branches into smaller arteries and eventually into the afferent arterioles. Each afferent arteriole then forms a network of capillaries called the glomerulus which is drained by an efferent arteriole that subsequently forms peritubular capillaries and drain into the venous system. The medullary nephrons have specialised peritubular capillaries that surround the loop of Henle called the vasa recta and participates in counter current exchange of solutes and water. The renal circulation is unique in that it has two arterioles, the efferent and afferent, and two capillary beds; the glomerular and peritubular capillaries. The glomerular capillaries have a high hydrostatic pressure which allows for rapid fluid filtration while the peritubular capillaries have much lower hydrostatic pressures which permits rapid resorption of fluid. The renal cortical nephrons receive over 10 times the blood flow of the medullary nephrons which is one of the factors allowing for maintenance of the medullary osmotic gradient necessary for the countercurrent multiplication system to function.
Renal blood flow is determined by the pressure gradient between the renal artery and the renal vein divided by the total renal vascular resistance. Therefore, blood flow is proportional the difference in pressure between the renal artery and vein, and is inversely proportional to the renal vascular resistance. The main way renal blood flow is controlled is by altering the vascular resistance.
Renal blood flow is kept constant over a wide range of blood pressures in order to maintain a relatively constant glomerular filtration rate and allow precise control of water and electrolyte balance. This is known as autoregulation. Without autoregulation an increase in blood pressure by 25mmHg would result in about a 25% increase in GFR.
Renal blood flow is also controlled by other factors.
What is GFR? Discuss the physiological factors that influence it
Glomerular filtration rate is the amount of plasma filtered into the tubules by all of the glomeruli over time, and is normally approximately 125ml/minute or 180L/day. The factors influencing GFR are Starling forces where GFR = glomerular ultrafiltration coefficient multiplied by the net filtration pressure.
The glomerular ultrafiltration coefficient is the glomerular capillary permeability multiplied by the effective filtration surface area. This coefficient is very high mainly due to high permeability.
Glomerular permeability: this depends on
Effective filtration surface area is the surface area of the capillary bed. This can be altered by mesangial cell contraction which reduces the area or relaxation which increases the area.
Net filtration pressure is the balance between the capillary hydrostatic pressure which moves fluid out of the capillary versus the plasma oncotic pressure in the capillary and the hydrostatic pressure in Bowman’s capsule which opposes this movement. It is equal to the glomerular capillary hydrostatic pressure minus Bowman’s capsule hydrostatic pressure minus glomerular capillary oncotic pressure. The glomerular capillary hydrostatic pressure is determined by:
Bowman’s capsule hydrostatic pressure:
Factors affecting glomerular capillary oncotic pressure:
Describe the renal handling of bicarbonate
The kidneys reabsorb about 4000-5000mmol/day of bicarbonate, most of which occurs within the proximal tubules. Normally, all filtered bicarbonate is reabsorbed.
Proximal tubular mechanism:
Loop of Henle:
Distal tubular mechanism
Factors which increase bicarbonate reabsorption include:
Briefly outline the role of the kidney in body water homeostasis
Water homeostasis is regulated so that there is adequate circulatory volume and so that plasma osmolality is maintained within a narrow range of 275-290 mOsmol, thus preserving normal cellular function. The kidneys role in body water balance is via the excretion of a concentrated or dilute urine. Urine volume can be reduced but the baseline solute load of 600mosmol/day that needs to be excreted means the minimum urine volume is 430ml/day as the maximum concentration capacity of urine is 1400mOsmol/kg. The mechanisms by which the kidneys change urine volume are:
The renal system has a crucial role in the maintenance of homeostasis within the body. The effects of renal dysfunction include effects on:
Maddi is an ICU registrar at St George Hospital who has a keen interest in teaching and education. She wishes that her recollection of medical knowledge was as strong as her recollection of song lyrics or quotes from episodes of ‘The West Wing’. When she’s not thinking of the next unquestionably funny dad joke for this podcast, she enjoys cryptic crosswords, musicals and anything cheese related. She hopes that this podcast helps in some small way to lessen the torment of the ICU primary exam. "
Dr Maddi Anupindi February 28, 2020
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