Human Physiology Lab – Zool 2200L
Excretory Physiology
Name:____________________ Lab Section:_________________
Background:
The kidneys play a role in maintaining homeostasis through excretion. The kidneys are
the major route of excretion for wastes and toxins. The kidneys also play a major role in
regulating the volume and composition of extracellular fluid by regulating the volume
and composition of urine. The nephron is the functional unit of the kidney. The nephron
has a vascular component and a tubular component. The processes of filtration,
reabsorption and secretion occurring in the nephron determine the volume and
composition of urine.
Define in terms of moving from vascular to tubular components in the nephron:
Filtration
Reabsorption
Secretion
The kidney can make hyperosmotic or hyposmotic urine depending on the body’s need.
If the osmolarity of the extracellular fluid increases the kidneys will produce a low
volume highly concentrated urine by reabsorbing most of the water filtered. If the
osmolarity of the extracellular fluid instead decreases the kidneys will produce a large
volume dilute urine by not reabsorbing most of the filtered water. The kidneys can make
hyperosmotic urine because of a vertical osmotic gradient that develops in the renal
medulla due to the action of the juxtamedullary nephrons.
The loop of Henle in the juxtamedullary nephrons extends deep into the renal medulla.
The descending limb of the loop of Henle is permeable to water while the ascending limb
is not. The ascending limb of the loop of Henle actively reabsorbs sodium, potassium
and chloride while the descending limb does not. The flow through the descending limb
runs countercurrent to the ascending limb. The active reabsorption of sodium, potassium
and chloride in the ascending limb makes the extracellular fluid hypertonic which draws
water out of the descending limb which concentrates the fluid in the loop of Henle. This
then increases the amount of sodium, potassium and chloride reabsorbed from the
ascending loop which in turns further concentrates the extracellular fluid drawing yet
