fluid compartments

what percentage of body weight is water?

60%

what percentage of body weight is extracellular fluid?

20%

what percentage of body weight is intracellular fluid?

40%

what percentage of total body mass is blood plasma or intravascular fluid?

5%

what percentage of total body mass is interstitial fluid?

15%

what are the 2 driving forces of the movement of water between fluid compartments?

- hydrostatic pressure

- oncontic pressure

what is hydrostatic pressure caused by?

pressure on the capillary walls by the pumping of the heart

what happens with high hydrostatic pressure? (3)

- high hydrostatic pressure (hp) in capillaries, lower hp in interstitial fluid

- water moves from high hp to low hp

- water moves out of capillaries into interstitial fluid

what is oncotic pressure caused by?

large proteins/colloids in the capillaries that cannot pass through membrane - which leads to osmotic pressure

what does high oncotic, and hence osmotic pressure cause? (3)

- higher oncotic pressure in capillaries due to proteins/colloids

- water moves by osmosis from an area of low oncotic pressure to high

- water is drawn back into capillaries

are cell membranes permeable to Na+?

no, so Na+ cannot move into cells by simple diffusion

are cell membranes permeable to K+?

yes, so K+ can diffuse out of cells

where are most Na+ and K+ ions?

- Na+ mostly outside cells

- K+ mostly inside cells

what are the 2 ways the body achieves a homeostatic fluid balance?

- hormonal control

- behavioural control

example of hormonal control of fluid balance?

reabsorption in kidneys, depending on ADH

example of behavioural control of fluid balance?

drinking water

what are the 2 basic steps for intestinal fluid absorption?

1) absorb the solute

2) absorb the water

which is the major site for Na+ absorption?

jejunum

how is Na+ absorbed?

via Na-dependent coupled transporters

what are 4 examples of Na-dependent coupled transporters?

- Na-glucose

- Na-galactose

- Na-amino acid

- Na-H+

how does Na+ enter the bloodstream?

leaves basolateral membrane via Na+/K+ ATPase

what is different to ion absorption in the ileum compared to jejunum?

ileum has Cl-HCO3 exchanger in the apical membrane and Cl- channels in the basolateral membrane

what electrolytes are absorbed and secreted in the colon?

- Na+ absorbed

- K+ secreted

what stimulates Na+ channels to be made?

aldosterone

how is Na+ absorbed and K+ secreted in colon?

- Na+ diffuses through Na+ channels in apical membrane

- Na+ pumped out basolateral membrane by Na-K ATPase

- hence K+ pumped into cell

- K+ secreted into intestinal lumen via K+ channels in apical membrane