Control of body-fluid volume and osmolarity Flashcards
how is homeostasis regulated?
mainly of absorption (but GFR can be altered by large changes in blood pressure; extrinsic control)
why is there priority of the regulation of ECF osmolarity higher than the ECF volume?
- the need for optimum [ion] & cell volume for cellular function
- ECF volume is also regulated by cardiovascular reflexes
what are 3 ways to cope with changes in body fluid volume and composition?
- filtration-size of filtration slits between podocytes
- secretion/reabsorption-changes in solute concentration
- excretion- bladder function under neural control
where is ADH produced and where is it stored?
produced by the hypothalamus and stored in the posterior pituitary gland
what controls the release of ADH from the pituitary into the blood?
hypothalamus
what stimulates ADH secretion?
water deficit e.g. ECF is too concentrated (hypertonic) water must be conserved by the body
what inhibits ADH secretion?
water-excess, when the ECF is too dilute (hypotonic) and water must be eliminated in the urine
what type of receptor does the ADH (vasopressin) bind to?
type 2 vasopressin receptor
what are the two classes of diabetes insipidus?
- central - inable to produce or secrete ADH
- nephrogenic- ADH doesn’t exert correct effect
what are the symptoms of diabetes insipidus?
- Large volumes of dilute urine (up to 20 litres per day)
- Constant thirst
what is the most important stimulus for ADH-release?
Hypothalamic osmoreceptors
what else stimulates ADH release?
activation of left atrial stretch receptors
what does decreased atrial pressure lead to?
increased ADH release
what effects does nicotine and alcohol have?
Nicotine stimulates ADH release, alcohol inhibits ADH release
what regulates the amount of Na+ filtered?
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
what regulates the amount of Na+ reabsorbed?
the renin-angiotensin-aldosterone system
what is aldosterone?
Steroid hormone secreted by the adrenal cortex
when is aldosterone secreted?
- In response to rising [K+] or falling [Na+] in the blood
- activation of the renin-angiotensin system
what does aldosterone do?
Stimulates Na+ reabsorption and K+ secretion ( Na+ retention contributes to an increased blood volume & pressure)
90% of K+ is reabsorbed in the proximal tubule, what happens when aldosterone is absent?
the rest of K+ is reabsorbed in the distal tubule (therefore no K+ is excreted in the urine
what happens when there is an increase in [K+]p?
- This directly stimulates the adrenal cortex
- Aldosterone stimulates the secretion of K+
what happens when there is a decrease in plasma [Na+] ?
promotes the indirect secretion of aldosterone by means of the juxtaglomerular apparatus
what are the mechanisms that control renin release from granular cells in the JGA?
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.
how does aldosterone increase Na+ reabsorption in the distal and collecting tubule?
Increases number of basolateral na/k pumps – get more sodium ions across the basolateral membrane
what can abnormal increases in R-A-A system cause?
hypertension
what is the course of events that leads to fluid retention association with congestive heart failure?
Failing heart –> decreased CO & BP –> Low BP stimulates R-A-A system –> Increased salt (&water) retention –> failing heart
what do ACE inhibitors do?
stop fluid & salt retention and arteriolar constriction
where is atrial natriuretic peptide (ANP) stored?
atrial smooth muscle cells
when is ANP released?
when atrial smooth muscle cells are mechanically stretched due to an increase in the circulating plasma volume
what does ANP promote?
excretion of Na+ and diuresis, thus decreasing plasma volume and exerts effects on the cardiovascular system to lower BP
