Renal Physiology and Body Fluid Compartments

Functions of the Kidney

excrete metabolic wastes (urea, creatinine)

maintain extracellular volume and osmolar homeostasis

volume, osmolarity, electrolyte concentration

regulate acid-base balance

osmolarity Þ Na, Cl, electrolytes, glucose, urea

regulate extracellular fluid composition (EFC)

osmolarity ECF = osmolarity ICF

if osmolarity ECF increases; water goes from ICF Þ ECF

synthesize and regulate hormones and autacoids

renin-angiotensin-aldosterone (blood pressure and Na regulation)

erythropoietin (RBC synthesis): stimulates bone marrow Þ RBC

vit D metabolites (bone metabolism): helps absorb Ca from diet; helps incorporate Ca into new bone

liver converts 1,25 dihydroxy vit D into its active form

deficiency Þ rickettes

prostaglandins (vasoconstriction and dilation, salt and water regulation

Body Fluid Compartments

	% of body weight	Liters (70 kg man)
Total body water ( = Intracellular + Extracellular)	60 (males have more muscle so hold more water: 65%; women have more fat: 60%)	42
Intracellular fluid	40 (2/3)	28
Extracellular fluid ( = Interstitial fluid + Plasma volume)	20 (1/3)	14
Interstitial fluid	15 (2/3)	10.5
Plasma volume	5 (1/3)	3.5 Þ 5L or 10 units of blood

ways to determine compartment size

total body water

use deuterium oxide ("heavy water")
conc = mass/vol therefore Volume = mass/conc.

extracellular fluid

use inulin (polymer of fructose), radioactive sulfate or chloride

intracellular fluid

must calculate, cannot measure
Vicf = Vtbw - Vecf

Plasma

Use albumin

Interstitial

Interstitial = ECF – plasma

Fluid Composition

Oncotic = balance between vessel capillary (contains proteins and cells) and interstitial fluid

Hydrostatic pressure is exerted by the water
oncotic pressure is exerted by the proteins and cells inside
it’s the size of the particles that’s important

Osmotic = balance between the plasma and intracellular fluid

Plasma = Na⁺, HCO₃^-, Cl^-, organic acids
Intracellular fluid = K⁺, Mg⁺⁺, PO₄^3-, SO₄^2-, protein
Ca⁺⁺ is similar on both sides
It’s the number of particles that’s important

Relationship between Na and extracellular fluid volume

Na is the major determinant of ECFV

Every 150mEQ of Na Þ 1L so important to keep it accurate in ECV

If ECV is normal, you excrete all Na intake

ECFV isnt tightly regulated, but plasma osmolality (Posm) is

Change in Na intake Þ change in ECFV, without a change in Posm

Na intake Posm regulatory mechanisms act to retain water, so the Posm is constant at the expense of ECFV

cannot immediately adapt to increased Na intake; takes about 3-4 days for kidney to adapt and increase excretion for the new steady state – water is retained for this period of adaptation (weight increases)

Fluid Compartment Shifts

addition of isotonic NaCl to ECF

ECFV increases with no change in osmolality

Addition of pure water to ECF

ECFV is diluted (increases) so osmolality decreases
ICFV expands (increases) to equilibriate the osmolality

Addition of hypertonic ( >.9% saline) NaCl to ECF

ECF osmolality increases
To equilibriate, water moves from ICF to ECF
ECF volume increases and ICF volume decreases

Normal Sources of Fluid Intake and Output

Intake = fluids (1200mL/day) + food (1000) + metabolism (350) = 2550

Output = urine (1500mL/day; 300mL per urination X 5 times per day) + insensible loss (skin and lungs) = 900 + stool (100) + sweat (50) = 2550

Renal Mechanisms for Handling Fluid and Electrolytes

glomerular filtration Þ filter about 180L/day

tubular reabsorption Þ 178.5 L/day reabsorbed (99%)

tubular excretion Þ 1.5L/day