physiology: the renal system part 2

what mechanisms control the afferent arteriole radius?

• myogenic response

• tubuloglomerular feedback

myogenic response

• limits blood flow into glomerulus

• contracts in response to stretch when arterial pressure increases

• relaxes and dilates when pressure is low

tubuloglomerular feedback

• regulates filtration of glomerulus by monitoring NaCl levels in fluid

• involves juxtaglomeruler apparatus

juxtaglomerular apparatus

specialized segment of distal tubule that passes through fork formed by arterioles

components of juxtaglomerular apparatus

• granular cells in afferent arteriole

• macula densa cells in distal tubule

• mesangial cells between arterioles

granular cells

releases renin which activates RAAS

macula densa cells

• detect salt delivery in fluid

• increased salt—>ATP and adenosine released—>vasoconstriction of afferent arteriole (release nitrous oxide to limit vasoconstriction)

mesangial cells

• contractile cells

• close off some glomerular capillaries limiting surface area for filtration

• receive SNS innervation

filtration coefficient

• how much glomerular surface area is available for filtration and how permeable it is

• can be altered by mesangial cell contraction—>reduces surface area

podocytes

inner layer of Bowman’s capsule

podocyte contractile activity

• closes off slit openings between foot processes

• reduces glomerular permeability

• less fluid exits plasma

what barriers must reabsorbed substances in the tubules cross (transepithelial transport)?

• luminal cell membrane

• cytosol of epithelial cell

• basolateral cell membrane

• interstitial fluid

• capillary wall

sodium reabsorption

Na+ in lumen—>crosses luminal membrane into tubular cell going down concentration gradient—>Na+-K+ pump in basolateral membrane pumps Na+ into interstitial fluid—>Na+ diffuses down concentration gradient from interstitial fluid into peritubular capillary

RAAS

• renin-angiotensin-aldosterone system

• hormonal system regulating Na+

• long-term mechanism for increasing blood volume

RAAS pathway

renin—>angiotensin I—>angiotensin II—>aldosterone

renin

• secreted by granular cells

• converts angiotensinogen into angiotensin I

angiotensin I

converted to angiotensin II by angiotensin-converting enzyme (ACE) in lungs

angiotensin II

• vasoconstrictor

• stimulates release of vasopressin (constrictor) and aldosterone

aldosterone

• released from adrenal glands

• increases Na+ reabsorption in distal and collecting tubules

what inputs induce renin secretion?

• granular cells (baroreceptors): increase renin release when blood pressure is low and when they receive SNS innervation

• macula densa cells: activated during low salt delivery, stimulate granular cells

most important substances secreted by tubules

• H+—>regulate acid-base balance (proximal, distal, collecting tubules)

• K+—>regulates plasma K+ concentrations (distal and collecting tubules)

• organic ions—> help eliminate foreign organic compounds (proximal tubule)

what is the medullary vertical osmotic gradient established by?

countercurrent multiplication

descending loop vs ascending loop of Henle in countercurrent multiplication

• descending limb: highly permeable to water, doesn’t pump out NaCl, water flows out of tubule into more concentrated interstitial fluid through osmosis, limb becomes more concentrated

• ascending limb: impermeable to water, pumps NaCl into interstitial fluid, water stays in tubule, limb becomes less concentrated

what affects urine concentration?

when tubular fluid undergoes alterations by aldosterone and vasopressin in collecting ducts

vasopressin and osmotic gradient function on water

• vasopressin enables movement of water

• osmotic gradient is driving force

vasopressin

• signals distal tubule and collecting duct to absorb water

• increases H2O permeability in luminal membrane

vasopressin moa

vasopressin—>V2 receptors on basolateral membrane—>cAMP activated—>aquaporins inserted in luminal membrane—>H2O enters cell through aquaporins in luminal membrane—>H2O exits cell through aquaporins in basolateral membrane—>blood

what are some causes of renal failure?

• infectious organisms

• toxic agents

• obstruction of urine flow

azotemia

• build-up of nitrogenous substance in blood

• due to failure of urinary tract to clear them

types of azotemia

• pre-renal—>issue before kidney

• renal—>issue in kidney

• post-renal—>issue after kidney