Week 3 - Clearance and filtration Flashcards
What is the difference between cortical and juxtamedullary nephrons?
- Cortical have shorter loops of henle with peritubular capillaries and a high concentration of renin
- Juxtamedullary had long loop of henle which penetrates deep into medulla and capillaries arranged in vasa recta and no renin
Where are the glomeruli of all nephrons found?
-Cortex only
What is the filtration faction?
-Only 20% of blood passing through afferent arteriole filteres at any one time. 80% exits via efferent arteriole
What is the difference between plasma and ultra-filtrate?
-Identical apart from ultra-filtrate doesnt contain cells or large proteins
What makes up the filtration barrier?
- Fenestrated capillary endotheium
- Shared basement membrane
- Visceral peritoneum forming podocytes
What is the basement layer made of and why is this helpful?
- Acellular gelatinous layer of collagen and glycoproteins
- Glycoproteins have a negative charge which repel proteins which have a negative charge (improves selectivity)
What infers the size limitation of the filtration barrier?
-Filtration slits formed by psuedopodia
What effect does the negative charg of the basement membrane have on protein filtration?
- Negative proteins are repelled and thus less filtered regardless of size
- Positive proteins are pushed through BM which allows slightly bigger proteins to be filtered
What physical forces drive filtration? Which exerts the most force?
- Hydrostatic pressure in capillary (the push) (50mmHg)
- Hydrostatic pressure in bowman’s capsule (15mmHg)
- Oncotic pressure in afferent arteriole(25mmHg)
What is the result of the hydrostatic pressure in the gomeruli capillaries?
-Drives filtrate through the filter
Why is the hydrostatic pressure in glomerular capillaries high?
-Afferent arteriole is wider than efferent arteriole
What effect does oncotic pressure of capillaries have on filtration?
-Draws filtrate back into the capillary
Why is there hydrostatic pressure in the bowman’s capsule and what effect does this have on filtration?
- Generated by the filtrate that is being filtered
- Pushes filtrate back into the capillaries
Describe autoregulation in the kidneys
- Autoregulation is a myogenic response which occurs in response to stretch of the smooth muscle and aims to maintain GFR within normal limits despite small changes in blood pressure
- If bp increase, smooth muscle stretched ->vasoconstriction of afferent occurs in order to maintain bloodflow and thus GFR.
- If bp decreases -> smooth muscle relaxes -> vasodilation occur in afferent arteriole to maintain bloodflow and GFR to remain the same. Vasoconstriction of efferent arteriole to increase hydrostatic pressure
What is tubulo-glomerular feedback?
- Mechanism to reduceGFR
- Occurs when there are changes in tubular flow rate as a result of changes in GFR
- Changes NaCl in DCT due to an change in filtered load
- Detected by macular densa in juxtaglomerular apparatus which stimulates the afferent arteriole to release either adenosine for vasoconstriction or prostaglandins for vasodilation
What is the TGF response if GFR increases?
- Increased GFR
- Increased NaCl in DCT
- Rise in concentration detected by MD cells of JGA
- JGA causes release of adenosine
- vasoconstriction of afferent arteriole
- Decreased GFR
In what limits does autoregulation of the kidneys work?
-80-180mmHg
What is bulk transport?
-Major reabsorption in PCT
What drives reabsorption of solutes and water in PCT?
- NaKATPase on basolat generates Na gradient which cotransports other molecules with it (secondary active transport)
- Water follows down osmotic gradient
What happens to the solutes that move into tubular cells?
-Diffuse into capillaries
Name the Na transporters on the apical membrane of the following segements:
i) PCT
ii) LoH
iii) Early DCT
iv) Late DCT and CD
i) Na:H antiporter or Na:glucose symporter
ii) NKCC2
iii) NCCT
iv) ENaC
Describe glucose reabsorption in PCT
- uses energy from Na gradient to drive reabsorption
- 2 mols of Na per glucose
- glucose passes through interstitium to peritubular capillary down its gradient by facilitated diffusion
What happens if transport maximun of glucose is reached?
-Rest of glucose spills over into urine causing glycosuria and polyuria
Is reabsorption always transcellular?
-No can be paracellular
What happens to the K inside the tubule cells if NaKATPase on basolat?
-Diffuses down its conc gradient via romK back into interstitium
What is meant by secretion in the tubules?
-Solutes secreted from the peritubular capillaries to lumen of PCT
What substances are commonly secreted into tubular fluid?
-H+, K+ and organic cat/anions
Describe organic anion/cation secretion in PCT
- Entry into tubular cell through facilitated diffusion from capillary
- Na:H exchanger generate H gradient from tubule cell to lumen
- Secretion into lumen by H:OC exchanger using H gradient
What is the normal GFR in males and females?
- Males = 115-125ml/min
- Females 90-100ml/min
What characteristics must a substance have in order to be used to calculate GFR
- Freely filtered
- Not secreted
- Not reabsorbed
What is average renal blood flow?
-1.1L/min
What is renal plasma flow?
-Plasma is approx 55% of blood and thus 1.1Lx0.55=605ml/min of plasma
How much of the renal plasma flow is filtered?
-20% (125ml (GFR))
What is renal clearance?How do you calculate renal clearance?
- The volume of plasma that is completely cleaned of the substance by the kidneys per unit time
- (conc in urine x urine volume)/conc in plasma
When are renal clearance tests used?
-Determine GFR
What two substances are used to measure GFR?
- Inulin
- Creatinine
What would a clearance value higher than GFR indicate?
-The substance is secreted
What is eGFR?
-calculation to calculate adjusted GFR which takes into account mass, age, sex and ethnicity
How do you calculate filtered load?
- Work out how many mg/ml in plasma
- Times this by GFR
What is renal threshold?
-The plasma concentration of a substance at which transport maximum is reached and the substance starts spilling into urine
What is Tm for glucose?
- 375 mg/min in males
- 300mg/min in females
Which nephron is most dominant cortical or juxtamedullary?
-Cortical (90%)
What type of reabsorption occurs in pct?
-Isomotic
In addition to glucose, what else uses secondary active transport in PCT for reabsorption?
- Amino acids and water-soluble vitamins (B and C)
- Lactate, ketones and acetate
