PHYS: Body Fluids

Question 1

body fluid compartments

Answer 1

• ICF (2/3)
• ECF (1/3): lymph, tissue fluid, plasma
• also transcellular fluid: separated from the rest of the ECF by endothelium and epithelium e.g. CSF, pericardial fluid

Question 2

osmolarity

Answer 2

• molarity x number of particles produced in solution (osmoles)
• 1 mole of MgCl2 has an osmolarity of 3 osmoles

Question 3

which body fluid compartment contains Na+ and K+

Answer 3

• ICF: K+, proteins
• ECF: Na+, proteins (plasma only, not tissue fluid)

Question 4

what is water diuresis?

Answer 4

• increased water output in urine but stable solute excretion i.e. removing excess pure H2O from the body

Question 5

response to low water levels (ADH)

Answer 5

• paraventricular and supraoptic nuclei in pituitary gland contain osmoreceptors
• decreased water levels = increased plasma osmolarity so water moves out into ECF, causing osmoreceptors to shrink = increased firing rate
• hering bodies synthesise ADH and secrete it from the posterior pituitary
• ADH inserts more aquaporins into the luminal membrane of DCT/CD epithelial cells = increased permeability and reabsorption of water

Question 6

how does nocturnal enuresis (bed-wetting) work?

Answer 6

• adults secrete more ADH overnight
• theory that nocturnal enuresis is caused by a delay in increased ADH secretion overnight
• Tx: nasal spray of desmopressin

Question 7

counter-current mechanism

Answer 7

• fluid flows in opposite directions in two adjacent limbs of a tube - loop of Henle

Question 8

tonicity of different nephron parts

Answer 8

• glomerulus + PCT + descending loop of henle = isotonic
• distal ascending loop of henle and DCT = hypotonic
• CD = depends how much water gets reabsorbed

Question 9

what is the maximum urine concentration determined by?

Answer 9

• length of loop of henle
• longer = increased potential for concentration

Question 10

normal osmolarity of ECF/ICF

Answer 10

• both approx 280 mOsm/L