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Flashcards in Physiology Deck (21):

What does a nephron comprise of?

1 glomerulus and 1 tubule


What types of nephrons are there?

Cortical - 80%
Juxtaglomerulus - 20% - loop of Henle extends down to medulla giving the kidneys ability to concentrate


Are there new nephrons formed after birth?



What is the juxtaglomerular apparatus and what is its function?

Made up of
- Macula densa (epithelia cells near renal corpuscle)
- Juxtaglomerular cells (smooth muscle fibres in walls of afferent arterioles)

Secretes renin and angiotensin when
- arteriole pressure in afferent arteriole falls
- osmotic concentration of tubular fluid reduces (occurs when the rate of glomerular filtration reduces and tubular fluid spends more time in ascending limb of loop of Henle and conc of Na / Cl becomes abnormally low


What does filtration of a molecule depend on?

Molecular size
- <4nm passes freely
Molecular charge
- all layers are negative charged: repel anions

** Albumin is barely filtered because it is too big and negatively charged **


What is the glomerulus made up of?

Fenestrated endotherpium, basement membrane (homogenous glycoprotein) and podocytes

Mesangial cells are located between endothelium and BM to allow striation support. Also contractile and helps regular GBF and filtration


What is ultrafiltration dependent on?

Renal flood flow, hydrostatic pressure and plasma oncotic pressure


How do you measure GFR?

GFR x plasma [Cr] = urine flow rate x urine [Cr]
- slightly overestimates GFR due to small amount o tubular secretion
- inc Cr correlates with reduced GFR in steady state
- 50% nephrons lost, reduces GFR down to 80%
- 80% nephrons lost, reduces GFR down to 50%

True measurement of GFR
- excretion of inulin
- filtered, not secreted nor reabsorbed


What are tubular components?

Bowman's capsule
- collects glomerular filtrate

Proximal tubule
- uncontrolled secretion of H+ and organic acids
- resorption of NaCl, K, HCO3, PO4, Ca, urine acid, glucose, AA, water (passively), urea

Loop of Henle
- establishes osmotic gradient

Distal tubule
- Controlled secretion of H, K, NH3, drugs
- Reabsorbs NaCl, water, HCO3

Collecting tubules
- variable water reabsorption


What occurs in the proximal tubule?

67% ultrafiltrate is reabsorbed

Active reabsorption of Na+ (Na/K ATPase)
- followed by water (osmosis)
- Also important in reabsorption of glucose, amino acids, chloride and urea

H+ excreted into urine in exchange for Na+
PO4 and HCO3 also absorbed

Osmolality doesn't change as both solute and water reabsorbed at same rate


What occurs at Loop of Henle?

20-25% water and sodium reabsorbed

- permeable to water, NOT solutes
- permeable to solutes, NOT WATER
- Fluid leaving always hypo osmotic

Urea contributes to establishment of osmotic gradient --> high protein diet facilitates ability to concentrate urine


What happens in the distal tubule?

NaCl reabsorption and K excretion
Urine acidification

Aldosterone controls active reabsorption of Na, in exchange for K. Also acidifies urine by HCO3 reabsorption and H+ secretion.


What happens at the collecting ducts?

Concentrate urine under control of ADH


What's the function of renin?

Activates conversion of angiotensinogen (liver) to angiotensin I.


What is angiotensin II?

Angiotensin I is converted to angiotensin II in the lungs.
- Causes thirst
- potent vasoconstrictor of efferent arterioles to increase GFR (whereas vasodilation of afferent arterioles is caused by prostaglandin 2)
- Increases aldosterone from zona glomerulosa in adrenal cortex


What are the actions of aldosterone?

Acts on Na/K ATPase in the distal tubule & collecting duct
- Promotes resorption of Na and water and in exchange, excreting K / H
- Stimulated by increased K+ and RAS


What is the ANP?

Atrial natriuretic peptide is released from atrial tissue when when it stretched (fluid overloaded)
- causes increased GFR (afferent dilation, efferent constriction) --> Increasing Na / H2O excretion
- reduces tubular re-absorption of Na
- reduces renin


At birth, what does Cr reflects?

Maternal level.
Cr reduces to neonatal levels at 5 days of age


What happens to GFR at birth?

Rapidly increases in first 72 hours and reaches adult levels in 1st two years


What's the significance of casts in urine?

- transparent, don't indicate disease on their own
- precipitated protein on surface seen in glomerular and tubular disease
White cell cast
- acute pyelo and interstitial nephritis
Red cell casts
- always pathological due to GN


How do you calculate FeNa?

FeNa = 100 x (urinary Na x serum Cr) / (serum Na x urinary Cr)

- Below 1
- the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hyponatremia

Acute tubular necrosis
- 2-3%
- Either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypovolemia resulting in the normal response of sodium wasting