Control of body-fluid volume and osmolarity

Question 1

how is homeostasis regulated?

Answer 1

mainly of absorption (but GFR can be altered by large changes in blood pressure; extrinsic control)

Question 2

why is there priority of the regulation of ECF osmolarity higher than the ECF volume?

Answer 2

• the need for optimum [ion] & cell volume for cellular function
• ECF volume is also regulated by cardiovascular reflexes

Question 3

what are 3 ways to cope with changes in body fluid volume and composition?

Answer 3

• filtration-size of filtration slits between podocytes
• secretion/reabsorption-changes in solute concentration
• excretion- bladder function under neural control

Question 4

where is ADH produced and where is it stored?

Answer 4

produced by the hypothalamus and stored in the posterior pituitary gland

Question 5

what controls the release of ADH from the pituitary into the blood?

Answer 5

hypothalamus

Question 6

what stimulates ADH secretion?

Answer 6

water deficit e.g. ECF is too concentrated (hypertonic) water must be conserved by the body

Question 7

what inhibits ADH secretion?

Answer 7

water-excess, when the ECF is too dilute (hypotonic) and water must be eliminated in the urine

Question 8

what type of receptor does the ADH (vasopressin) bind to?

Answer 8

type 2 vasopressin receptor

Question 9

what are the two classes of diabetes insipidus?

Answer 9

• central - inable to produce or secrete ADH

- nephrogenic- ADH doesn’t exert correct effect

Question 10

what are the symptoms of diabetes insipidus?

Answer 10

• Large volumes of dilute urine (up to 20 litres per day)

- Constant thirst

Question 11

what is the most important stimulus for ADH-release?

Answer 11

Hypothalamic osmoreceptors

Question 12

what else stimulates ADH release?

Answer 12

activation of left atrial stretch receptors

Question 13

what does decreased atrial pressure lead to?

Answer 13

increased ADH release

Question 14

what effects does nicotine and alcohol have?

Answer 14

Nicotine stimulates ADH release, alcohol inhibits ADH release

Question 15

what regulates the amount of Na+ filtered?

Answer 15

GFR -If Na+ content of plasma decreases, arterial blood pressure falls.
Baroreceptors detect reduced arterial blood pressure and increase
sympathetic nerve activity to afferent arteriole, cause constriction.
GFR reduced

Question 16

what regulates the amount of Na+ reabsorbed?

Answer 16

the renin-angiotensin-aldosterone system

Question 17

what is aldosterone?

Answer 17

Steroid hormone secreted by the adrenal cortex

Question 18

when is aldosterone secreted?

Answer 18

1. In response to rising [K+] or falling [Na+] in the blood
2. activation of the renin-angiotensin system

Question 19

what does aldosterone do?

Answer 19

Stimulates Na+ reabsorption and K+ secretion ( Na+ retention contributes to an increased blood volume & pressure)

Question 20

90% of K+ is reabsorbed in the proximal tubule, what happens when aldosterone is absent?

Answer 20

the rest of K+ is reabsorbed in the distal tubule (therefore no K+ is excreted in the urine

Question 21

what happens when there is an increase in [K+]p?

Answer 21

• This directly stimulates the adrenal cortex

- Aldosterone stimulates the secretion of K+

Question 22

what happens when there is a decrease in plasma [Na+] ?

Answer 22

promotes the indirect secretion of aldosterone by means of the juxtaglomerular apparatus

Question 23

what are the mechanisms that control renin release from granular cells in the JGA?

Answer 23

1) Reduced pressure in afferent arteriole- more renin released, more Na+ reabsorbed, blood vol. increased, blood pressure restored.
2) Macula densa cells sense the amount of NaCl in the distal tubule - If NaCl reduced, more renin released, more Na+ reabsorbed
3) Increased sympathetic activity as a result of reduced arterial blood pressure - granular (renin-secreting) cells directly innervated by sympathetic nervous system, causes renin release.

Question 24

how does aldosterone increase Na+ reabsorption in the distal and collecting tubule?

Answer 24

Increases number of basolateral na/k pumps – get more sodium ions across the basolateral membrane

Question 25

what can abnormal increases in R-A-A system cause?

Answer 25

hypertension

Question 26

what is the course of events that leads to fluid retention association with congestive heart failure?

Answer 26

Failing heart –> decreased CO & BP –> Low BP stimulates R-A-A system –> Increased salt (&water) retention –> failing heart

Question 27

what do ACE inhibitors do?

Answer 27

stop fluid & salt retention and arteriolar constriction

Question 28

where is atrial natriuretic peptide (ANP) stored?

Answer 28

atrial smooth muscle cells

Question 29

when is ANP released?

Answer 29

when atrial smooth muscle cells are mechanically stretched due to an increase in the circulating plasma volume

Question 30

what does ANP promote?

Answer 30

excretion of Na+ and diuresis, thus decreasing plasma volume and exerts effects on the cardiovascular system to lower BP