Renal physiology Flashcards
What is the functional unit of the kidney?
nephron
What is the glomerulus?
tuft of glomerular capillaries
What is the normal filtration rate of the kidneys?
~180L/day
How much water is normally excreted via urine, and how much solute?
~1.5L water
600 milliosmoles of solute
Kidney holistic functions
filtration - removal of metabolic products and toxins from the blood for excretion
regulation of fluid balance, electrolyte balance, and acid-base balance
produce or activate hormones involved in erythrogenesis, calcium metabolism and the regulation of blood pressure
(volume, concentration, pH, metabolic, excretory, endocrine)
How can the kidneys produce glucose?
gluconeogenesis
What metabolic waste products do the kidneys excrete?
creatinine
urea
What endocrine actions does the kidney have?
renin production
RAAS system first of 4 pathways to correct low circulating volume
EPO - new red blood cell production
Vit D - controls calcium and phosphorus metabolism
Why is renal blood flow tightly controlled?
blood flow is closely linked to rate of renal filtration and excretion
local renal blood flow must be autoregulated in response to changes in systemic BP to prevent damage
Describe glomerular filtrate
essentially protein-free plasma
produced at rate of 125ml/min (GFR)
contains urea and creatinine
What is GFR determined by?
net filtration pressure across the glomerular capillaries
How is renal blood flow controlled?
vascular bed has 2 major sites of resistance control - afferent and efferent arterioles
selective constriction or relaxation of the afferent and efferent arterioles permit control of blood flow and hydrostatic pressure in the intervening glomerular capillary, and subsequently glomerular filtration
many physiological mediators control arteriolar resistances
Effect of increased afferent arteriolar resistance on renal blood flow and net ultrafiltration pressure
decreased renal blood flow
decreased net ultrafiltration pressure
Effect of decreased afferent arteriolar resistance on renal blood flow and net ultrafiltration pressure
increased renal blood flow
increased net ultrafiltration pressure
Effect of increased efferent arteriolar resistance on renal blood flow and net ultrafiltration pressure
decreased renal blood flow
increased net ultrafiltration pressure
Effect of decreased efferent arteriolar resistance on renal blood flow and net ultrafiltration pressure
increased renal blood flow
decreased net ultrafiltration pressure
General action of sympathetic nerves on afferent and efferent arterioles?
vasoconstriction
both afferent and efferent arteriolar resistances rise
generally decreases both renal blood flow and GFR
General action of prostaglandins on afferent and efferent arterioles?
vasodilation
dampens the renal vasoconstrictor effects of sympathetic nerves or angiotensin II
General action of natriuretic peptides on afferent and efferent arterioles?
vasodilation
dominant action on afferent arterioles
net effect is increased renal blood flow and GFR
Describe autoregulation of the kidneys
maintain renal blood flow and GFR within narrow limits, although MAP may vary
2 mechanisms for renal autoregulation:
- myogenic response of smooth muscle of the afferent arterioles
- tuboglomerular feedback mechanism
Describe the myogenic response in renal autoregulation
afferent arterioles have the ability to respond to changes in vessel circumference by contracting or relaxing
constriction in response to pressure prevents the vessel from being overstretched and, by increasing vascular resistance, helps prevent excessive increases in renal blood flow and GFR when BP rises
Describe the tubuloglomerular feedback in renal autoregulation
increase in arterial pressure increases filtration and ultimately sodium and chloride in proximal tubule
sensed by macula densa cells of the JGA
macula densa cell to release paracrine agents which triggers contraction of nearby vascular smooth muscle cells in afferent arteriole
increased afferent arteriolar resistance decreases GFR, counteracting the initial increase in GFR
Define hydrostatic pressure
force a fluid exerts on walls of compartment (eg. walls of capillaries or Bowman’s capsule)
Define oncotic pressure
pressure exerted by plasma proteins on walls of compartment (encourages fluid to be drawn in)
Define GFR
glomerular filtration rate
total amount of filtrate formed in corpuscles in both kidneys per minute
What factors does GFR take into account?
surface area available for filtration
permeability of glomeruli
What increases the kidneys contribution to whole-body glucose production?
prolonged fasting
Which mediator acts to selectively modulate the sympathetic vasoconstrictive effects on the afferent arterioles to prevent sustained damage?
prostaglandin
How do the kidneys help regulate acid-base balance?
reabsorbing HCO3- into the bloodstream in proximal convoluted tubule
excreting H+ into the urine
What is the enzyme involved in dissolving carbonic acid into bicarb and hydrogen?
carbonic anhydrase type 2
How do the kidneys stop the urine becoming too acidic (pH<4.5)?
ammonia buffer system
phosphate buffer system
