Renal phys. Flashcards
(23 cards)
What 3 layers make up the glomerular filtration barrier?
- Fenestrated endothelium of glomerular capillaries
- Inner epithelial layer of Bowmans capsule
- Intervening basal lamina
What molecules are/aren’t able to pass through the glomerular filtration barrier?
- Water and small molecules can pass through
- Plasma proteins and blood cells cannot
What are the 5 components of the filtration pathway?
- Capillary endothelium
- Fenestrae of glomerular capillaries
- Basal lamina
- Slits between podocytes
- PCT lumen
Which one plasma protein is normally filtered by the glomerulus?
Albumin
What are 4 factors affecting glomerular filtration?
- Back pressure - hydrostatic prssure in Bowman’s space forcing material back towards the capillary; not significant though
- Capillary oncotic pressure - exceeds back pressure, material moves into Bownman’s space
- Bowman’s space oncotic pressure - negligible
- Capillary pressure - high
What is the net filtrate pressure?
cap. pressure - back pressure - oncotic pressure
= 50 - 10 - 30
= 10mmHg
Define the glomerular filtration rate
Volume of fluid filtered per minute from glomerular capillaries into Bowman’s space.
It is the product of the net filtration pressure across the glomerular filter ad filtration coefficient (Kp).
What are 3 factors regulating GFR?
- Autoregulation
- Angiotensin II
- Sympathetic input to kidneys
Explain the effect of autoregulation on GFR
- Resistance in afferent arterioles can be altered in response to changes in blood pressure
- Smooth muscle in arteriole walls automatically constrict when arterial BP is high & dilate when it is low - stabilises perfusion & filtration
- Increased arterial BP temporarily increases blood flow and GFR - fluid flow in nephron monitored by chemicals in macula densa which signal to SM in arterioles to constrict (decreased blood pressure has opposite effect)
- Prevents potentially large changes in GFR and urine output
Explain the effect of angiotensin II
- Acts on efferent arteriole
- Decreased arterial BP, juxtaglomerular cells release renin
- Renin converts angiotensinogen to angiotensin I
- Angiotensin I converted to angiotensin II in lungs
- Angiotensin II constricts all arterioles in body, acts on adrenal gland to increase aldosterone secretion
- Efferent arteriole in kidney constricts, increasing glomerular HP, allowing filtration to continue
* aff. & eff. constriction, decrease HP peritubular capillaries, promote tubular reabsorption, normalise ECV & arterial BP in hypotension
Explain the sympathetic effects on GFR
Similar to action of angiotensin II
What characterises PCT activity?
- Iso-osmotic reabsorption
- Delivery of iso-osmotic product to loop of Henle
- 66% reabsorption
Describe the reabsorption of Na in PCT
- Na simply diffuses into tubular epithelium from tubular lumen
- Na exchanged for K in an E independant process on the basolateral membrane. Na enters interstitial fluid
- Water follows Na reabsorption, an inreased oncotic pressure in peritubular capillaries enables water to be easily reabsorbed
Describe the reabsorption of glucose from the PCT
- Glucose cotransported with Na from lumen into epithelial cells using E released when Na is transported across apical membrane
- Glucose transported by facilitated diffusion across basolateral membrane into interstitial fluid
- From here diffuses into capillary lumen
- 100% glucose normally reabsorbed
What is ‘Tm’?
Tubular maximum - max. amount of glucose that can be reabsorbed
What happens to protein and phosphate in the PCT?
- Protein reabsorbed by endocytosis
- PTH inhibits reabsorption of phosphate so is excreted in urine
What characterises the activity of the loop of Henle?
- Recieves iso-osmotic filtrate from PCT
- Only the thick limb is metabolically active
Describe the processes that occur in the loop of Henle
- Ascending thick limb impermeable to water, pumps out 25% Na, Cl, K, Mg
- Causes water to be drawn out of thin descending limb - 25% - & CD
- Descending limb becomes hyperosmolar
What characterises DCT activity?
- Receives 9% of total glomerular filtrate
- All activities are hormonally regulated
Describe the processes that occur in the DCT
- Receives hypo-osmotic filtrate from loop of Henle
- Aldosterone causes selective Na reabsorption
- Na exchanged for K, urine Na content reduced to < 1%
- Water reabsorbed
- PTH stimulates final Ca reabsorption
- CT inhibits Ca reabsorption
Describe the processes that occur in the CD
Initial section:
- Sensitive to aldosterone, maintains Na reabsorption via Na/K exchange
- Allows for variable amounts of K to be excreted in urine
Deep medullary section:
- Water reabsorption - satisfy Na:water in ECF
- Hypothalamus controls this by:
- thirst - (dehydration) - Na conc. increases in ECF as water is lost, water moves out of osmoreceptor cells, shrink, hypothalamus stimulates thirst & ADH secretion
- ADH - increases CD water permeability, water drawn into medulla, returns to ECF, Na conc. decreases, osmoreceptor cells return to original turgor
- Reabsorption of water causes reabsorption of urea; contributes to osmotic gradient drawing more water out
What happens during hypovolaemia?
- Low arterial BP acts on arterial baroreceptors to activate sympathetic NS
- Juxtaglomerular apparatus stimulated to secrete renin
- Directly overrides autoregulation to cut GFR to preserve plasma volume
- Renin activates angiotensin pathway
- Thirst and ADH stimulated
- Water and Na conserved to increase ECV and increase BP
What happens during hypervolaemia?
- Sympathetic NS low tone, JG low activity, low renin activity
- High blood volume detected by baroreceptors in R atrium, trigger atrial natriuretic hormone, inhibits aldosterone
- Low PCT reabsorption
- ADH inhibited
- Urine has low specific gravity
