Lecture #19: Urea Excretion and Thermoregulation
I. INTRODUCTORY
REMARKS
1. Osmolarity
is the total solute concn expressed as molarity, or m/L of soln.
2. Osmolarity of
human blood is about 300 mOSM/L. (sea water = 500 mOSM/L)
3. Osmoconformers
are isosmotic with their aqueous surroundings.
4. Osmoregulators
inhabit hypo-, or hyper-osmotic, environments.
5. Humans must
compensate for water loss (= hyperosmotic).
6. Humans die if
they lose about 12% of their body water. (diarrhea)
7. The nephron
is the functional unit for human osmoregulation (collected into kidneys).
8. The nephron
regulates water, salts, pH, and removes nitrogenous wastes (urea).
9. About 1
million nephrons/kidney. (80 km
of tubules).
10. 1100-2000
L blood thru kidneys gives 180 L filtrate, gives 1.5 L of
urine.
II. ANATOMY OF A
NEPHRON (Fig. 44.21)
1. Afferent arteriole, glomerulus, Bowman's capsule,
proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting
duct, renal pelvis, ureter.
2. Renal
cortex versus renal medulla (cortical nephrons vs juxtamedullar
nephrons).
3. Peritubular
capillaries and the vasa recta.
III. PHYSIOLOGY
OF A NEPHRON (book compares
processes to cleaning out a drawer)
1. Filtration in glomerulus
has capillaries (with fenestrations) plus podocytes for filtering.
2. Secretion
is mostly in tubules (both active and passive). (Fig.
44.17)
3. Reabsorption
in tubules and loop target sugar, vitamins, other nutrients, H20.
4. Transport
properties of the renal tubule: (Fig. 44.22)
a. proximal
tubule actively transports glucose, AAs, H+, NaCl (75%).
b. descending
limb freely reabsorbs water (but not very permeable to salts).
c. ascending
limb actively transports NaCl along outer medulla.
d. distal
tubule actively transports Na, K, H, and HCO3.
e. collecting
duct is permeable to H2O and urea.
5. Countercurrent
mechanism and the conservation of osmotic gradient. (Fig 44.23)
IV. REGULATION OF
THE KIDNEYS (Fig 44.24a & 44.24b)
1. Kidneys can
excrete hypo-osmotic urine as dilute as 70 mOSM/L, or 1200 mOSM/L.
2. ADH
from osmoreceptor cells increases permeability of collecting ducts.
(Alcohol can inhibit ADH release, and cause excess water
loss.) (hangover)
3. Juxtaglomerular
apparatus (JGA) produces renin under low Na+ or blood pressure.
4. Urea is 100,000
times less toxic than ammonia, and allows conservation of water.
V.
THERMOREGULATION (Fig. 44.10)
1. Hyperthermia
(capillary dilation) (sweat gland activation) (panting)
2. Hypothermia
(vasoconstriction) (shivering)