Question

MCAT Tutorial: Excretory System Introduction

Answer 1

\({\bf{Steps~of~Excretion:}}\)
- filtration: passing blood over a filter to retain the cells and proteins, while removing water, waste, small molecules into the renal tubule
> filtrate the fluid left in the tubule after filtration
- selective reabsorption: reuptake of glucose, water, and amino acids while leaving waste and water in the tubule
- secretion: adding solutes back to the filtrate
- dilution/concentration: selective reabsorption of water

Answer 2

to go over the steps a bit more carefully
\({\bf{Filtration~Pathway:}}\)
the filtration pathway is --> renal artery --> afferent arteriole --> glomerulus --> efferent arteriole
contraction of efferent arteriole drives movement out of glomerulus

\({\bf{Selective~Reabsorption:}}\)
- water and small hydrophilic molecules returned to the bloodstream via tubule via active transport through the peritubular capillaries
- primarily takes place in proximal and distal convoluted tubules

\({\bf{Concentration~and~Dilution:}}\)
- happens primarily in the distal nephron (DCT and collecting duct)
- two hormones to know: ADH/vasopressin (know both names), and aldosterone

ADH: works under dehydration conditions (low blood V and high solute conc.)
- released by posterior pituitary
- increases water reabsorption by making distal nephron permeable to water

aldosterone: works under low blood pressure
- released by adrenal cortex
- increases Na+ reabsorption in distal nephron
- increases water retention
- decreases sodium concentration in urine

Answer 3

\({\bf{Descending~Limb:}}\)
- thin
- permeable to water
- water exits, making filtrate conc.

\({\bf{Ascending~Limb:}}\)
- thick
- has active transporters to transpoert soidum, potassium, and chlorine ions out of filtrate, and return potassium passively into the filtrate
- increases osmolarity of medulla
- dilutes the fluid inside the tubules

Answer 4

Source material is Chapter 10.2 of MCAT: Biology Review, 2nd edition by Princeton Review