Download Acid/Base, Fluid Physiology

Lecture: Fluid, Electrolyte, Acid/Base Balance
I.
Fluids of the Body
A.
Fluid Compartments
1.
intracellular fluid compartment - within the cells themselves (about 25L or 40% of body
weight)
2.
extracellular fluid compartment outside the cells (about 15L or 20% of body weight)
a.
plasma - fluid portion of the blood (3 L)
b.
interstitial fluid - fluid bathing all cells and tissues of body (12 L)
Total body water volume = 40L or about 60% of body weight
B.
Fluid Composition
1.
2.
3.
nonelectrolytes - no electrical charge (glucose)
electrolytes - dissociate into ions (NaCl)
milliequivalents per liter (mEg/L) - measure of number of charges in 1 liter of solution
concentration of ion (mg/L)
mEq/L =
x
# charges per ion
atomic weight of ion
C.
D.
Extracellular vs. Intracellular Fluids
1.
extracellular - high Na+ and high Cl-
2.
intracellular - low Na+ and low Cl-
Fluid Movement between Compartments
1.
2.
3.
4.
II.
plasma -> interstitial -> plasma & lymphatics
Oxygen, glucose -> into cells
Carbon Dioxide, nitrogenous wastes-> out of cells
ECF <====> ICF depends on NaCl in the ECF
Water Balance
A.
Overview of Water Balance
1.
2.
3.
B.
intake - 90% ingested water; 10% metabolic water
output - 60% urine; 28% lungs/skin (mucosa); 12% sweat/feces
water need - trigger thirst & release of ADH
The Thirst Mechanism
decrease in plasma volume OR increase in osmolarity 
excitation of hypothalamic thirst center 
1
sensation of thirst
C.
Regulation of Water Output
1.
2.
D.
Disorders of Water Balance
1.
2.
3.
III.
obligatory water loss - lungs, sweat, feces
regulation of water - kidneys (ADH -> Na+)
dehydration - water loss -> water intake
a.
bleeding, burns, sweating, diuretics
hypotonic hydration - too much water or Na+
edema - accumulation of water in interstitial space
Regulation of Sodium (Na+) Balance
A.
B.
Sodium (Na+) - 90% of solutes in the ECF; most important and prevalent of all electrolytes
Aldosterone - released by adrenal cortex (renin-angiotensin)
1.
released in response to:
a.
b.
c.
2.
function - increase Na+ reabsorption at distal tubule
a.
C.
D.
decrease in blood pressure
decreased osmolality of filtrate
sympathetic stimulation of juxtoglomerular cells
water will follow if ADH makes the distal tubule permeable to water
Baroreceptors
1.
located in carotid arteries and aorta
2.
respond to changes in stretch due to blood pressure
a.
blood pressure increases 
hypothalamic stimulation 
sympathetics to kidneys decrease 
increased GFR (water removed)
b.
blood pressure decreases (same path as above) ----> ----> ----> lower GFR
(water retained)
Antidiuretic Hormone (ADH)
1.
released from the posterior pituitary
2.
responds to osmoreceptors in the hypothalamus
a.
decrease in osmo of ECF 
2
decreased release of ADH 
less permeability of distal tubule to water 
more water released into urine
b.
E.
Atrial Natriuretic Factor (ANF)
1.
2.
F.
released by cell of heart atria under high B.P.
reduces blood pressure and blood volume by INHIBITING nearly all events that
promote vasoconstriction and Na+/water retention.
Steroid Hormones
1.
2.
IV.
estrogen - increases resorption of Na+ in distal convoluted tubule
glucocorticoids (cortisol) - increases resorption of Na+ in the distal tubules
Regulation of Potassium (K+) Balance
A.
Importance of K+
1.
2.
B.
K+ is primary CATION (+) within cells
K+ abnormalities cause changes in Resting Mem Pot (neurons, cardiac muscle
especially)
Renal Control of K+ Levels
1.
2.
3.
10-15% constantly lost in urine
most resorption occurs in Proximal Tubule
regulation - changing amount SECRETED into urine in the collecting tubules
a.
b.
C.
low K+ ----> less secretion (intercalated cells in collecting tube can reabsorb
more)
high K+ -----> more secretion
Factors Controlling K+ Secretion
1.
2.
3.
V.
increase in osmo of ECF 
increased release of ADH 
more permeability of distal tubule to water 
less water released into urine
tubule cell intracellular K+ level - when low, secrete less; when high, secrete more
aldosterone level - K+ secreted: Na+ reabsorbed
a.
increase aldosterone -> more K+ secretion
b.
decrease aldosterone -> less K+ secretion
+
pH - K and H+ compete for antiport with Na+
a.
lower pH (high H+) -> less K+ secretion
b.
higher pH (low H+) -> more K+ secretion
Regulation of Calcium (Ca++) Balance
3
A.
Importance of Ca++
1.
bone is the "reservoir" of most Ca++ in the form of Calcium Phosphate (Ca++ with PO4-2)
2.
primary physiological roles (already covered):
a.
b.
c.
B.
at the muscle and neuron synapse
essential for blood clotting process
very critical in heart rhythms
Ca++ Regulation: Parathyroid Hormone (PTH) & Calcitonin
1.
parathyroid hormone (PTH) - chief control when Ca++ begins to DECREASE too low
a.
b.
c.
2.
bones - Ca++ and PO4-2 release to blood
small intestine - activates Vitamin D which is essential for Ca++ resorption
kidneys - Ca++ reabsorbed (but PO4-2 excreted)
calcitonin (thyroid gland) - released when Ca++ begins to INCREASE too high
a.
inhibits Ca++ release from bone, increases actual formation of Calcium
Phosphate in bone
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