25. Renal excretory function Flashcards
How is the renal pressure of the glomerulus maintained?
The efferent arteriole is under a greater pressure than the afferent arteriole
What are the vasa recta?
Capillaries that branch off from the efferent arteriole and surround the loop of Henle - provide the blood supply
What is the relevance of the vasa recta?
These each have a hairpin turn and carry blood at a very slow rate
Allows for the countercurrent exchange mechanism at the loop of Henle
What are the structures through which components pass from the Bowman’s capsule to be filtered out in the urine?
Podocytes
Glomerular basement membrane
Fenestrations in the endothelium of the Bowman’s capsule
What are the factors that determine the filtrate that is produced by the kidney?
Net filtration pressure
The fenestrations of the endothelium, glomerular basement membrane and podocytes
Size of the molecule
Charge of the molecule
Why does the charge of the molecule determine the filtrate produced by the kidney?
The glomerular basement membrane has a negative charge due to the presence of glycoproteins
Restricts large, negatively charged proteins
What is filtered through the glomerulus?
Free movement of small molecules e.g. water, electrolytes (Na+, K+, Cl-, phosphate, glucose), urea and amino acids
Restriction of larger solutes e.g. proteins and negatively charged proteins
At what molecular mass are proteins no longer filtered through the glomerulus?
55,000
What is meant by the glomerular filtration rate (GFR)?
This is the total amount of fluid that is filtered through the glomerulus - the clearance rate
What is creatinine?
Metabolic waste produced by muscle metabolism that is excreted in the urine?
What is the effect on creatinine of kidney failure?
This will result in a reduced excretion of creatinine and so the levels of creatinine in the blood will build up
What is the relevance of creatinine in assessing renal function?
Creatinine is a naturally occurring substance which is completely filtered out by the kidney and not reabsorbed - so you can compared other components to this to measure the GFR
A raised creatinine level signifies damage to the kidney
How can the creatinine levels be measured?
(urine concentration x urine volume)/plasma concentration
What is the glomerular filtration rate?
The rate of fluid being filtered through the kidney
What is the creatinine clearance rate and how does this relate to GFR?
This is the volume of blood that is cleared of creatinine per unit time
Can be used to estimate the GFR - eGFR
How can the GFR be estimated for an individual? (eGFR)
Using the MDMR calculation
What are the four variables used in the MDMR formula?
Serum creatinine, age, ethnicity and sex
What is considered to be more useful for assessing kidney function out of the GFR and the serum creatinine?
GFR
Why is GFR considered to be more useful that creatinine when assessing muscle function?
Creatinine is produced by muscle - muscular individuals will have a naturally raised serum creatinine so the eGFR will underestimate the true GFR
AND malnourished individuals will have a low serum creatinine so the eGFR will overestimate the true GFR
Some drugs inhibit the natural tubular secretion of creatinine
What is the apical surface?
Surface facing the tubule lumen
What is the baso-lateral surface?
Surface facing the vascular compartment’/interstitium (outside of the lumen)
What is the transport mechanism at the apical surface of the glomerular tubules?
Carrier proteins
Tight junctions between cells
What is the transport mechanism at the baso-lateral surface of the glomerular tubules?
Primary active transport - Na+/K+ ATPase pumps 3 sodiums out of the tubule for 2 potassiums entering the tubule
Creates an electrochemical gradient to allow sodium, phosphate, glucose and amino acids to enter the tubule cell via the apical membrane
What is the function of the proximal convoluted tubule?
Bulk of reabsorption of solutes - 80%
Reabsorption of 65% of water
100% reabsorption of amino acids and low molecular weight proteins -
How does the fluid in the descending and the ascending limbs of the loop of Henle differ to each other?
Descending limb - isotonic
Ascending limb - hypotonic
What enters/leaves the descending loop of Henle?
Passive entry of water and NaCl into the descending loop of Henle
What enters/leaves the ascending loop of Henle?
Active sodium and chloride transport out of the ascending loop - controlled
