Fluid Volume and Regulation

Question 1

Describe the distribution of body fluids

Answer 1

• Total body water- ~42L
• Intracellular fluid- ~26L
• Extracellular- ~16L
• • Interstitial - 13L
• • Plasma - 3L

Question 2

Describe solute distribution

Answer 2

• Iso-osmotic distribution across the membrane
• Composition of ions is different
• High sodium out, low sodium in
• High potassium in, low potassium out

Question 3

What is osmolality?

Answer 3

• Osmolality of fluids is dependent of solutes within each compartment- each solute will have osmotic action
• Sodium (~140mosmol/kg) largest determinant of plasma osmolality
• • Approx. 50% of normal plasma osmolality
• Fluid regulation is intrinsically linked to electrolyte regulation

Question 4

Describe sodium and water balance

Answer 4

• Typical daily water intake- 2500 ml
• • Dietary sources (2300ml) and metabolism (200ml)
• Balanced daily water loss- around 1500ml, faeces, sweat, skin and respiratory tract
• Typical Na+ intake- 100-200mmoles/day
• Kidneys account for 90% of Na+ excretion- remaining 10% from sweat, faeces

Question 5

Describe the role of the kidneys in fluid volume and regulation

Answer 5

• Water and electrolyte balance
• 20-25% of resting CO (0.5% body mass)
• Functional unit is the nephron- 1mn per kidney
• Plasma filtered into nephron across glomerulus
• • Electrolytes are reabsorbed and waste products excreted

Question 6

What is a nephron and what happens there?

Answer 6

• Filtration by glomerulus, filtrate enters proximal tubule
• Obligatory absorption and secretion by proximal tubule
• Generation of osmotic gradient in loop of Henle
• Reabsorption and secretion in distal tubule
• Regulation of water uptake by collecting duct, osmotic gradient draws water out

Question 7

Describe plasma osmolality and ADH

Answer 7

• Maintenance of plasma osmolality in what prevents cell shrinkage/growth
• • Approx. 285 mosmol/kg
• Changes in plasma lead to changes in ADH release
• • In periods of high electrolyte, we secrete ADH which inhibits diuresis- passing of fluid
• ASH controls absorption of water from collecting duct

Question 8

What is ADH?

Answer 8

• Anti-diuretic hormone
• Increases permeability of cells to water via insertion of aquaporins in collecting duct
• Secreted from posterior pituitary glands
• In response to changes is plasma osmolality or volume