L7: renal blood supply Flashcards
How much supply do the kidneys get?
20% of the cardiac output via the renal arteries, since they have important roles in regulating pH, fluid volume, osmolality, electrolytes and blood pressure.
Blood supply to the kidney
- arrives at the kidney via renal artery and branches into smaller arteries
- some divide to become the afferent arteriolar that supply blood to the glomerular capillaries
- from glomerular capillaries blood flows to the efferent arterioles which supply the peritubular capillaries and the vasa recta.
- blood exits the kidney through small venues and veins and finally the renal vein.
Job of the peritubular capillaries
Peritubular capillaries branch off the efferent arterioles and provide nutrients to epithelial and interstitial cells which are present in that region. In addition to serving these cells, these capillaries also supply the blood for reabsorption and secretion in the PCTs.
Job of the vasa recta
Vasa recta are long hairpin-shaped vessels that run alongside LoH. They provide nutrients to the cells in this region but primarily function as countercurrent exchangers which contribute to the concentration of urine by the collecting ducts.
Reabsorbs the water (water does not just sit in interstitium). These blood vessels provides oxygen and nutrients for these regions of the kidneys to work and stay alive.
How is glomerular filtration rate tightly regulated?
Tightly linked to blood pressure.
External mechanism - sympathetic and hormonal mechanisms from the cardiovascular system to alter blood pressure.
Intrinsic mechanism - autoregulation: can help itself by either altering the diameter of afferent and efferent arterioles = myogenic control mechanism or by a tubuloglomerular feedback mechanism.
3 major physiological systems that are activated in response to a drop in blood pressure
- cardiovascular system - baroreceptors detected a decreased BP which activates sympathetic nervous system. detected in the aortic arch and carotid sinus.
- renal system - release of renin from juxtaglomerular apparatus (JGA) and aldosterone from the adrenal cortex.
- neuroendocrine system - release of ADH a from the pituitary gland
Renal auto regulation of GFR
Functions to maintain a constant GFR within a certain BP range:
Diameter (or resistance) of the afferent and or efferent arterioles can be changed to maintain GFR between 80-180mmHg BP. Mechanisms responsible for this are the myogenic mechanism and tubuloglomerular feedback.
Minute 18 graph
What do changes in afferent of efferent arterioles diameters do to GFR?
- dilated afferent: more blood enters the glomerular capillaries so CHP increases, increasing GFR. Can occur if BP is decreased.
- constricted afferent: less blood enters glomerular capillaries so CHP decreases. Can occur if BP is increased, stops damage to kidney.
- efferent dilates: blood leaves glomerular capillaries faster, decreasing CHP. Can occur if BP is increased.
- efferent constricts: blood stays in glomerular capillaries longer, increasing CHP. Can occur if BP is decreased.
What is the most useful mechanism when blood pressure drops?
Constricting of the efferent arterioles
Role of prostaglandins
- prostaglandins E2 and I2 (PGE’2/PGI’2) are the major prostanoids produced by the kidney, especially the glomerulus and play an important role in modulating the renal micro vascular response to a drop in BP.
- PGs protect the renal hemodynamics against excessive vasoconstrictors such as AngII, I.e., they minimise or offset any potent vasoconstriction = vasodilation
- interference with PG synthesis by administration of non-steroidal anti-inflammatory drugs (NSAIDS) can lead to deterioration of renal function and renal blood flow insufficiency in patients who have vasoconstrictor stimuli to the kidney, e.g., in patients losing blood or those with a low BP. so don’t give NSAIDS to low BP patients.
Why is maintaining renal perfusion and GFR critical?
- unlike perfusion of all other organs, perfusion of the kidney is not regulated to maintain organ nutrition but to retain its filtration functions. Glomerular hydrostatic pressure is regulated mainly by balance of vascular tone in afferent and efferent arterioles. The vascular tone of afferent and efferent arterioles is regulated by several systems.
- circulating hormones such as AngII are potent efferent arteriole vasoconstrictors. Paracrine factors released locally by the kidney include NO and PGs. The afferent arteriole is more sensitive to the vasodilator effects of NO. PGs modulate the effects of vasoconstrictors such as AngIi and proctect against potent vasoconstriction. Renal sympathetic nerves also innervate the afferent and efferent arterioles to cause vasoconstriction.
Role of the vasa recta
- loop of henle is designed to ensure as much water as possible can be reasborbed from the collecting ducts
- it creates a corticomedullary interstitial hyperosmotic gradient which allows water to be drawn by osmosis from these regions.
- however for this to happen the hyperosmotic interstitium needs to be maintain and the reabsorbed water needs to be carried away
- job of specialised blood vessels - vasa recta
Why are the vasa recta looped?
- vasa recta are countercurrent loops in which blood flows slowly in opposite directions. Blood flow is also opposite to flow of LoH filtrate.
- they are freely permeable to water and solutes
- the concentration gradient needs to be maintained but oxygen supplied. So it is specially looped to prevent the high concentration of solutes in the medullary interstitium being washed away.
Watch last 10mins of lecture again
3 summary roles of vasa recta
- to provide oxygenated blood to the renal medullary region and carry away metabolic toxins
- to preserve the cortico-medullary osmotic gradient
- to remove reabsorbed water (water entering ascending vasa recta either from descending vasa recta or reabsorbed from descending LoH and CD)
