1. Water
Dr;
Bashardoust

2. Body Water Content
Healthy males are about 60% water; healthy females are around 50%
Infants have low body fat, low bone mass, and are 73% or more water
This difference reflects females
Higher body fat
Smaller amount of skeletal muscle
Total water content declines throughout life
In old age, only about 45% of body weight is water

3. Fluid Compartments Water occupies two main fluid compartments
Intracellular fluid (ICF) – about two thirds by volume, contained in cells
Extracellular fluid (ECF) – consists of two major subdivisions
Plasma – the fluid portion of the blood
Interstitial fluid (IF) – fluid in spaces between cells
Other ECF – lymph, cerebrospinal fluid, eye, synovial fluid, serous fluid, and gastrointestinal secretions

4. Composition of Body Fluids
Water is the universal solvent
Solutes are broadly classified into:
Electrolytes – inorganic salts, all acids and bases, and some proteins
Nonelectrolytes – examples include glucose, lipids, creatinine, and urea
Electrolytes have greater osmotic power than nonelectrolytes
Water moves according to osmotic gradients

5. WATER BALANCE Normal plasma osmolality is 275 to 290 mosmol/kg
Sensing a 1 to 2% change in tonicity
Disorders of water homeostasis result in hypo- or hypernatremia.
Obligate water loss consisting of urine, stool, and evaporation from the skin and respiratory tract
Gastrointestinal excretion is usually a minor component
Insensitive water losses are important in the regulation of core body temperature

6. WATER BALANCE Water Intake
Obligatory renal water loss is caused by the minimum solute excretion required to maintain a steady state
Thirst mediated either by an increase in effective osmolality or a decrease in ECF volume or blood pressure
Located in the anterolateral hypothalamus
The average osmotic threshold for thirst is approximately 295 mosmol/kg

7. WATER BALANCE Water Excretion
principal determinant of renal water excretion is arginine vasopressin
Polypeptide synthesized in the supraoptic and paraventricular nuclei of the hypothalamus
AVP to V2 receptors on the basolateral membrane of principal cells in the collecting duct
Adenylyl cyclase and insertion of water channels into the luminal membrane
The major stimulus for AVP secretion is hypertonicity
Hemodynamic response is mediated by baroreceptors in the carotid sinus
Nonosmotic factors volume, nausea, pain, stress, hypoglycemia, pregnancy, and numerous drugs

8. conservation

9. Excretion

10. HYPOVOLEMIA

11. Causes of Hypovolemia ECF volume contracted A. Extrarenal Na+ loss
1. Gastrointestinal (vomiting, nasogastric suction,
drainage, fistula, diarrhea)
2. Skin/respiratory (insensible losses, sweat, burns)
3. Hemorrhage
B. Renal Na+ and water loss
1. Diuretics
2. Osmotic diuresis
3. Hypoaldosteronism
4. Salt-wasting nephropathies
C. Renal water loss
1. Diabetes insipidus (central or nephrogenic)

12. Causes of Hypovolemia ECF volume normal or expanded
A. Decreased cardiac output
1. Myocardial, valvular, or pericardial disease
B. Redistribution
1. Hypoalbuminemia (hepatic cirrhosis, nephrotic syndrome)
2. Capillary leak (acute pancreatitis, ischemic bowel, rhabdomyolysis)
C. Increased venous capacitance
1. Sepsis

13. PATHOPHYSIOLOGY
ECF volume contraction is manifest as a decreased plasma volume and hypotension
Decreased venous return (preload) and diminished cardiac output
Triggers baroreceptors in the carotid sinus and aortic arch
Leads to activation of the sympathetic nervous system and the renin-angiotensin system
Maintain mean arterial pressure and cerebral and coronary perfusion

14. Renal response Decreasing the GFR and filtered load of Na+
Tubular reabsorption of Na+
Increased sympathetic tone increases proximal tubular Na+ reabsorption
Decreases GFR by causing preferential afferent arteriolar vasoconstriction
Sodium reabsorbed
Angiotensin II
Decreased hydraulic and increased oncotic pressure
Aldosterone and AVP secretion
Suppressed atrial natriuretic peptide secretion.

15. Electrolyte Concentration
Expressed in milliequivalents per liter (mEq/L), a measure of the number of electrical charges in one liter of solution
mEq/L = (concentration of ion in [mg/L]/the atomic weight of ion)  number of electrical charges on one ion
For single charged ions, 1 mEq = 1 mOsm
For bivalent ions, 1 mEq = 1/2 mOsm

16. Extracellular and Intracellular Fluids
Each fluid compartment of the body has a distinctive pattern of electrolytes
Extracellular fluids are similar (except for the high protein content of plasma)
Sodium is the chief cation
Chloride is the major anion
Intracellular fluids have low sodium and chloride
Potassium is the chief cation
Phosphate is the chief anion

17. Extracellular and Intracellular Fluids
Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites
This reflects the activity of cellular ATP-dependent sodium-potassium pumps
Electrolytes determine the chemical and physical reactions of fluids

18. Extracellular and Intracellular Fluids
Proteins, phospholipids, cholesterol, and neutral fats account for:
90% of the mass of solutes in plasma
60% of the mass of solutes in interstitial fluid
97% of the mass of solutes in the intracellular compartment

19. Fluid Movement Among Compartments
Compartmental exchange is regulated by osmotic and hydrostatic pressures
Net leakage of fluid from the blood is up by lymphatic vessels and returned to the bloodstream
Exchanges between interstitial and intracellular fluids are complex due to the selective permeability of the cellular membrane

20. Extracellular and Intracellular Fluids
Ion fluxes are restricted and move selectively by active transport
Nutrients, respiratory gases, and wastes move unidirectional
Plasma is the only fluid that circulates throughout the body and links external and internal environments

21. Water Balance water intake must equal water output
Water intake sources
Ingested fluid (60%) and solid food (30%)
Metabolic water or water of oxidation (10%)
Water output:
Urine (60%) and feces (4%)
Insensible losses (28%), sweat (8%)
Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH)

22. Regulation of Water Intake
The hypothalamic thirst center is stimulated by:
Decreases in plasma volume of 10%
Increases in plasma osmolality of 1–2%
Thirst is subside as soon as we begin to drink water
Feedback signals that inhibit the thirst centers include:
Damping of mucosa of the mouth
Moistening of the throat
Activation of stomach and intestinal stretch receptors

23. Atrial Natriuretic Peptide (ANP)
Figure 25.10