Semester 1: Foundation of Basic Science

Case 1: Chemistry of Life

Obj 10: Volume & Composition of Body Fluid

Basic Info About Body Fluid

- 60% of total body weight

- in 70 kg body weight , about 42 lit body fluid

- ↑ fat % , ↓ water %

- thus,women contain less water (as more fat)

Distribution of Body Fluid

Extra cellular fluid (ECF):-

It is about 14 liters:

Plasma 3 liters
Interstial fluid in spaces between cells 10 L
Transcellular 1-2liters.

↓→Intra ocular space

↓→Synovial , peritoneal , pericardial spaces

↓→Cerebrospinal fluid

Intra cellular fluid (ICF) :-

-Equal 28 liters.

-It is the fluid inside the cells.

-It contains high Concentration of K &less concentration of Na.

-Also it contains large amount of magnesium and phosphate

Homeostasis

-Is to keep the internal environment constant.

-To keep the ECF constant.

-The distribution of fluid between plasma and interstial space is determined by two force across the cell membrane.

-That’s it the hydrostatic pressure and colloid osmotic pressure of plasma protein.

-The distribution of fluid between IC &EC compartment is determined mainly by osmotic effect of solutes e.g. Na, CL and other electrolytes acting on cell membrane

-Osmosis and osmotic pressure:-

It is the diffusion of water through a semi-permeable membrane.
The movement of water molecules from an area of high water conc to area of low water conc
Osmotic pressure is the pressure required to prevent osmosis.
Increased osmotic pressure means excess solutes and less water.
The average extracellular fluid osmolarity is (280-295)meq/l.
It is determined by its water and electrolytes mainly Na

Osmotic equilibrium

Isotonic solution – The water concentration in IC&ECF is equal and solutes can not enter or leave the cell.
Hypotonic solution – There are less solutes less osmotic pressure so water enter the cell and it swell.
Hypertonic solution – High solutes ,high osmotic pressure so water leave the cell and cell shrink. Solution of NaCL 0.9%and glucose 5% are isotonic so it is important in clinical medicine because the cell neither swell nor shrink.

Body water

Daily intake of water (by two main sources):

Ingested in the form of water or liquid which add about 2100ml/day to the body fluid
Synthesized as a result of metabolism and it is about 200ml/day.

Daily loss of body water:

Evaporation 700ml
Urine 1400 ml
Sweat 100 ml
Feces 100ml

Regulation of ECF volume

-The kidney plays a major role in the maintenance of body sodium and water content.

-Na account for more than 90%of all osmoticaly active ECF solutes so the amount of Na is the major determinant of the ECF volume.

-Regulation of water occurs by:

Anti diuretic hormone(ADH)
Thirst sensation

Sodium Regulation

-Normal serum sodium 140 m Eq/L.

-1mEq of Na =23mg.

-Function of sodium:

Regulation of ECF so it essential to maintain blood volume and A.B.P
It is essential for normal tissue excitability.
It forms important buffer salt (NaHCO3).
It is important for glucose absorption.
cause of renal medullary hyperosmolarity.
It is the main extracellular cation

-Factors affecting Na Reabsorption & Excretion:

Glomerular filtration rate (GFR): increased GFR will increase Na filtration and this increase the amount of Na reabsorbed leading to increase in Na excretion.
ECF volume: when it increase it will lead to increase Na excretion (naturiuresis).
Aldosterone hormone: -Secreted from suprarenal gland and helps reabsorption of Na and secretion of K from the distal convoluted tubules and collecting duct.
Atrial naturiuretic peptide: This hormone increases sodium excretion
Angiotensin: Results from angiotensinogen by the action of renin.
Estrogen hormone : leads to salt and water retention in the body.
Diuretic drugs.