Glomerular Filtration Flashcards
What three process occur in the nephron?
- Filtration (protein free filtrate of plasma)
- Reabsorption (i.e. NaCl and water)
- Secretion
What is filtered at the renal corpuscle?
No red ells and only a fraction of the plasma, the remainder passes via the efferent arterioles –> peritubular capillaries –> renal vein
What is the renal plasma flow?
- Kidney receives blood flow of 1200ml/min
- Plasma is 55% of total BV
55% of 1200 = 660ml/min
What is the normal GFR?
125ml/min
What is the filtration fraction?
- GFR = 125ml/min
- Renal plasma flow = 660ml/min
125/660 x100 = 19%
What is glomerular filtration dependent on?
Balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption - starling’s forces
Primary factor is P(GC) which is dependent on aff/efferent arteriolar diameter
What three characteristics of a substance determine its filterability across glomerular filtration barrier?
Molecular size, electrical charge and shape
- Na highest and serum albumin lowest
Describe the renal arterioles
- Fenestrated capillaries
- Basal lamina of glomerulus
- Slit membrane between pedicels
Why does blood arriving at the glomerulus have a high hydrostatic pressure?
Glomerular capillary pressure P(GC) is higher than in most of the capillaries in the body because the afferent arteriole is short and wide and offers little resistance to flow.
Describe the efferent arteriole
Long and narrow which offers a high post-capillary resistance
What is the relationship between peripheral resistance and hydrostatic pressure?
If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased.
• Compression of an arteriole would increase pressure in front and decreases pressure after
What is the balance between hydrostatic pressure and oncotic pressure which favours filtration?
P(GC) > Oncotic P
Only filtration occurs at the glomerular capillaries
How do extrinsic controls affect afferent and efferent arteriolar diameter (and therefore P(GC))?
- Sympathetic vasoconstrictor nerves -> afferent and efferent constriction
- Circulating catecholamines -> constriction primarily afferent
- Angiotensin II -> constriction, of efferent at [low], both afferent and efferent at [high].
What is the physiological response to increased resistance in afferent arteriole?
Constriction of afferents arterioles; pressure after this decrease and producing resistance -> this causes intra G pressure to decrease
What is the physiological response to increased resistance in efferent arteriole?
Constriction of efferent arterioles; increased pressure in glomerulus as it is before the constriction –> so increases GFR
What is the physiological response to decreased resistance in afferent arteriole?
GFR would increase as the P(GC) would increase (efferent needs to be consitricted more to compensate for decreased resistance in afferent)
What is the autoregulation of renal vasculature?
Intrinsic ability to adjust its resistance in response to changes in arterial BP - dependent on nerves and hormones.
If mean arterial P increases, there is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular P(GC) (prevents too more blood being pumped in due to high pressure and pressure decreases after constriction).
Dilatation occurs if P falls (to allow more blood in).
What are the the percentages of filtration, reabsorption and secretion?
- PV entering afferent arteriole = 100%
- 20% of volume filters
- > 19% of fluid is reabsorbed
- > 99% of plasma entering kidney returning to systemic circulation
- < 1% of volume is excreted to external environment
What vessel is responsible for reabsorption?
Peritubular capillaries
Why is a large pressure drop occur in the peritubular capillaries?
Because it is long and thin so offers resistance along its entire length, there is a large pressure drop so that hydrostatic pressure is very low, ie P(PC) ~ 15mmHg.
What is the balance between P(GC) and P(PC)?
P(GC)»_space; P(PC)
- P(PC) very low because hydrostatic P overcoming frictional resistance in efferent arterioles
- P(Plasma) high compared to normal, loss of 20% plasma concentrates plasma protein
- P(P)»_space; P(PC) - only reabsorption
Since ~20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has a higher concentration of plasma proteins and therefore increase in oncotic pressure.
What is the balancer of starlings forces which favours reabsorption?
Low P(PC) and high oncotic pressure (due to higher concentration of plasma proteins)
How much water, Glc Na and urea that is filtered out is reabsorbed?
99% H2O, 100% glucose, 99.5% Na+ and 50% urea
