Renal Filtration and Blood Flow Flashcards
What are the 3 general processes that determine the amount of a substance that appears in urine?
1) Glomerular filtration
2) Reabsorption of the substance from tubular fluid back into blood
3) Secretion of the substance from blood into tubule fluid.
What is the first step in the production of urine?
Ultrafiltration of the plasma by the glomerulus
How much of the cardiac output do the kidneys receive and why?
They receive 25% of the CO which far exceeds their metabolic need and this allow for flow to be adjusted with demand as in when working out.
What is the filtration fraction?
Ratio of GFR/RPF
What is the normal value of the filtration fraction?
20%
What is the composition of the ultra filtrate like?
The ultrafiltrate is devoid of cellular elements and is essentially protein free. The salt and organic compound concentrations are similar in the ultrafiltrate and
plasma.
How are GFR and RFP regulated?
Autoregulation
What is the filtration barrier in the renal corpuscle based upon?
Size and Charge
What size particles are freely filtered?
Particles less than 7000 Da and 20 Å are freely filtered
What size molecules are impermeable?
Molecules around 70000 Da and 42 Å are impermeable.
Albumin is 66K Da and it is nearly impermeable
What charge molecules are filtered more readily?
Cationic molecules are filtered more easily than anions
Why are proteins not filtered very easily, or at all?
Most proteins are negatively charged
Why are anions harder to filter?
The filtration barrier contains fixed polyanions that will repel the negatively charged proteins
How does glomerular capillary hydrostatic pressure affect filtration?
Favors
How does glomerular capillary oncotic pressure affect filtration?
Impedes
How does Bowman space hydrostatic pressure affect filtration?
Impedes
How does Bowman space oncotic pressure affect filtration?
Favors
What happens with higher GFR values?
It results in greater filtration and also greater water and salt excretion
What is the Bowman space oncotic pressure normally at?
0 as there is little protein in the BS to extrude water from the glomerulus
What is the equation for the net filtration pressure (NFP)?
NFP = (pGC - πGC) - (pBC - πBC)
What is the equation for the GFR?
GFR = kF(pGC - πGC - pBC)
kF = filtration coefficient
What happens to the πGC along the nephron?
The πGC increases along the nephron towards the end as the water leaves the tubule and the solutes concentrate in it.
What is kF?
It is the product of the intrinsic permeability of the glomerular capillary and the glomerular surface area available for filtration.
How is the GFR regulated normally?
It is regulated by changing the pGC
What are the ways that the pGC can be altered?
- Changes in the afferent arteriolar resistance
- Changes in the efferent arteriolar resistance
- Changes in renal arteriolar pressure
How does an increase in the afferent arteriolar resistance affect the pGC?
Decreases the pGC
How does an increase in the efferent arteriolar resistance affect the pGC?
Increases the pGC
How does an increase in the blood pressure affect the pGC?
Increases the pGC
What are the changes that occur in kidney stones?
Increase the pBS which leads to decreased GFR
What is the ratio of vascularization of the renal cortex to the renal medulla?
Cortex gets 90% of the RBF
Medulla gets 10% of the RBF with 8% in the outer portion and 2% in the inner portion
What is the ration of the cortical nephrons to the juxtamedullary nephrons?
7 to 1
What capillaries are confined to the renal cortex?
Glomerular Capillaries
Peritubular Capillaries
What capillaries dive into the renal medulla?
Vasa Recta Capillaries
What happens to the hydrostatic (P) pressure from the renal artery to the renal vein?
P decreases
Where do the largest decreases in the P occur?
Afferent and the efferent arterioles as they have the highest resistance
What happens to the oncotic pressure (π) in the glomerular capillaries?
π increases in the GC as the concentration of the plasma proteins increases during filtration.
What happens to the oncotic pressure π in the peritubular capillaries?
π decreases in the peritubular capillaries as the dilution of the capillaries occurs during reabsorption
What happens as a result of constriction of the afferent arteriole?
pGC decreases
GFR decreases
RBF decreases
What happens as a result of constriction of the efferent arteriole?
pGC increases
GFR increases
RBF decreases
What happens as a result of dilation of the afferent arteriole?
pGC increases
GFR increases
RBF increases
What happens as a result of dilation of the efferent arteriole?
pGC decreases
GFR decreases
RBF increases
What are the two mechanisms of auto regulation in the kidney?
Myogenic
Tubuloglomerular
What is the auto regulatory range and what happens outside of it?
100 - 180 mmHg is the auto regulation range.
180 leads to increased RBF and GFR
What happens when the arterial pressure falls below 70 mmHg?
Renal shutdown occurs and GFR goes to 0 mL/min but the kidney is still perfused, just no urine is formed.
What happens to the kidney when arterial pressure falls to 0 mmHg (or absolute minimum - mean circulatory pressure)?
Renal death occurs and RBF goes to 0 mL/min and the kidney is no longer perfused.
How does myogenic auto regulation occur?
With increases in pressure, vasoconstriction will occur to increase resistance and decrease flow and with decreases in pressure, vasodilation will occur to decrease resistance and increase flow.
How does tubuloglomerular auto regulation occur?
Increases in GFR will increase the [NaCl] in the loop of Henle which is sensed by the macula densa and turned to a signal to increase the arteriolar resistance and decrease the GFR. Low [NaCl] will cause the opposite.
What is involved in the extrinsic regulation of GFR and RBF?
- Sympathetic innervation of the afferent and efferent arterioles
- Hormones and other substances in blood
What happens to the GFR and RBF in hemorrhage where blood volume decreases?
Decreases
What happens to the GFR and RBF with dehydration where the renin and angiotensin increases?
Decreases
What happens to the GFR and RBF with severe hypoxia at high altitudes?
Decreases
