Lecture 37 Flashcards
What molecule is used to calculate glomeruler filtration?
Glomerular filtration can be calculated by the clearance for creatinine.
Why is reabsorption important? where does it occur and what are some examples of reabsorption/secretion relationships?
Filtration at the glomerulus is not very selective and hence we need ways to get some substances back and leave the waste behind. It occurs via increasing or decreasing secretions and occurs in the tubules either passively or actively. Some solutes, like glucose, water or sodium are only re-absorbed (not secreted). Some solutes are only secreted (e.g orgainc cations and organic anions. Some solutes are secreted and re-absorbed by the renal tubule (e.g K+, NH3, H+, HCO3- or urea).
What do the proximal tubules do?
The proximal tubule is the site where most reabsorption (bulk reabsorption) occurs. This is 2/3rds of sodium, water and chloride, all glucose, all amino acids, most K+, phosphate, calcium etc.
What are the tubular epithelia held together by and what are the pathways for molecules
The tubular epithelia are held together by tight junctions and have brush border membranes (microvilli) which act to increase surface area. The water and substances can move between the cells (paracellularly, leaky epithelia for bulk transport, it is a single barrier which connects the tubular lumen and lateral interstitial space, it doesnt use transport proteins and has limited selectivity. The ability for molecules to get through depends on the tightness of the tight junctions) or through the cells (trans-cellular, this has two barriers, the apical (mucosal) and basolateral (back), typically uses transport proteins).
How is sodium re-absorbed andd what is coupled with it?
Sodium is absorbed 66% in PCT, 25% in the TAL. It works via Na+/K+ ATPase (3NA+ ions out for one potassium), this makes the Na+ inside the cell low and hence creates a concentration gradient which brings Na+ into the cell which is then released on the other side. Glucose is coupled in symport with the Na+ against its concentration gradient, it can then flow out via channels on the baso-lateral side, this is known as secondary active as it moves passively via sodiums gradient.
Why does diabetes lead to glucose in the urine?
Glucose is found in urine from people with diabetes because the glucose concentration in the plasma is so high that the filtered load exceeds the capacity of the transporters.
How is water reabsorbed?
Water can move paracellularly through leaky epithelium as a consequence of the sodium absorption, which also drives trans-cellular movement by special channels.
