Osmotic Relations Between Intracellular Fluid, Interstitial Fluid and Plasma
proteinNa+
protein
Na+
H2O
K+
H2O
K+
Na+
K+
Plasma
Intracellular fluid
Interstitial fluid
Crucial points
• Animal plasma membranes are so delicate that no osmotic gradient between ISF and ICF can exist.
• Only impermeant solutes can act as osmotic effectors• Cytoplasmic protein is the major osmotic effector of the
ICF; its osmotic effect is balanced by the transmembrane Na+ gradient, otherwise cells would swell.
• Plasma proteins are the major osmoeffectors of plasma – they counteract the effect of capillary hydrostatic pressure.
• Na+ is the major osmoeffector of ECF versus ICF. ECF volume closely tracks total body Na+ content.
Characteristics Of ICF and ECF Compartments
30 L total volume
9000 mOsm total solute
Posm = 300 mOsm
15 L total volume
4500 mOsm total solute
2175 mEq total Na+
[Na+] = 145 mEq/L
Posm = 300 mOsm
Intracellular Fluid Extracellular Fluid
The ECF consists of the ISF compartment and the plasma compartment
15 L total volume
4500 mOsm total solute
2175 mEq total Na+
Posm = 300 mOsm
Extracellular Fluid
Interstitial Fluid Plasma
11.25 L total volume
3375 mOsm total solute
Posm = 300 mOsm
3.75 L total volume
1125 mOsm total solute
Posm = 300 mOsm
There are three basic homeostatic challenges
• Gain or loss of isotonic solution– Affects only the ECF volume
• Gain or loss of pure water– Both ICF and ECF compartments change volume
proportionately – osmotic concentration changes in each are equal
• Gain or loss of pure salt– Na+ is confined to the ECF compartment – loss results
in volume shift from ECF to ICF; gain results in volume shift from ICF to ECF.
Regulation of Renal Function
• Intrinsic
• Baroreceptor Reflex
• Three endocrine systems– ADH system– Renin-Angiotensin-Aldosterone System– Atrial Natriuretic Hormone system
Intrinsic regulation + Baroreceptor reflex
Blood Volume
Arterial Blood Pressure
GFRBaroreceptor Reflex
Afferent arteriole dilates
ADH system“Peripheral volume receptors” are stretch receptors located in the right atrium – increased stretch signals a plasma volume increase and exerts an inhibitory effect on ADH secretion
Osmoreceptor cell bodies are in ventromedial hypothalamus – sensitive mainly to [Na+]
ADH = arginine vasopressin – an octapeptide with two major peripheral effects:
Increased water permeability of collecting duct
Vasoconstriction (at high levels)
The Renin-Angiotensin-Aldosterone System – response to loss of pure Na+ or loss of isotonic
solution
• Macula densa (Juxtaglomerular apparatus) secretes renin (a protease) when: – Blood [Na+] falls below normal– Glomerular blood volume flow decreases
Angiotensin cascade
Angiotensinogen
Angiotensin I
Angiotensin II
Aldosterone
Renin
Angiotensin Converting Enzyme (ACE) in lung
Adrenal Cortex
Distal tubule (also sweat glands, salivary glands, colon, etc. Increased Na+ reabsorption
3 Major factors that increase Aldo secretion
Adrenal Cortex
Angiotensin II
Increased Plasma [K+]
Adrenocorticotrophic Hormone (ACTH)
Kidney distal tubule
Na+ reabsorption
K+ secretion
H+ secretion
Aldosterone
Aldosterone effects
• Steroid hormone that increases expression of Na+/K+ ATPase in target epithelia
• Directly regulates total body Na+ - Indirectly regulates ECF volume.
• Also involved in K+ regulation – by a direct effect on the adrenal cortex: increased plasma [K+] increases aldo secretion
Atrial natriuretic peptide – response to gain of isotonic solution
• Stretched atria release 22 aa peptide which – increases GFR by vasodilating renal afferent
arterioles and constricting efferent arterioles– Inhibits Aldo secretion and antagonizes
tubular effect of aldosterone– Inhibits ADH secretion and blocks its action
• Causes marked diuresis (volume loss) and natriuresis (net loss of Na+ )
Study Goals
• Be able to trace the responses of each of the 3 major renal endocrine systems to each of the 3 simple homeostatic challenges.
• Integrate your understanding of these systems with what you know about the baroreceptor reflex and capillary filtration to arrive at a complete picture of whole-body responses to blood loss and plasma volume expansion – i.e. short term and long term regulation of mean arterial pressure.