2. Plasma volume and electrolyte homeostasis Flashcards
(23 cards)
What is obligatory reabsorption in the PCT?
Approximately 65% of water is reabsorbed in the proximal convoluted tubule
- not regulated independently of resorption of solutes
What substances do cells of the PCT secrete into the lumen?
- metabolites (bile salts, urate, prostaglandins)
- xenobiotics (externally administered molecules) e.g., penicillin, aspirin and morphine
Why is penicillin effective in the treatment of UTI’s?
because it achieves high concentrations in the filtrate
What events occur in the Descending loop of Henle?
Na+ and Cl- are not reabsorbed - region is impermeable to electrolytes
- concentrated in the lumen as water is pulled osmotically into the surrounding medullary interstitium
What events occur in the ascending loop of Henle?
Na+, K+ and Cl- are actively transported out of the lumen
- water is not reabsorbed, region is impermeable to water
How does the osmotic gradient change from the superficial to deep medulla?
Increasing which is used to reabsorb water from the collecting duct
What ions are reabsorbed in the distal convoluted tubule?
Small amount of Na+
Cl- is co-transported
Impermeable to water
Ca2+ reabsorption occurs under the control of parathyroid hormone
What ions and hormones are reabsorbed in the late distal tubule and collecting ducts?
- approx. 4% of Na+ is actively reabsorbed in this segment
- H+ and K+ are actively secreted
- aldosterone acts on the segment to increase Na+ absorption and K+ excretion
- water is only reabsorbed if ADH is present (vasopressin)
Describe the net movement of water when plasma osmolality is low
Net movement of water into cells
Describe the net movement of water when plasma osmolality is too high
Net movement of water out of cells
Where is ADH/Vasopressin excreted from?
Posterior pituitary gland in response to increased plasma osmolality
How is secretion of ADH regulated?
Osmoreceptors in supraoptic and paraventricular nuclei of hypothalamus
How does ADH mediate an increase in extracellular fluid osmolality?
ADH released
- water is allowed to diffuse out of distal and collecting tubules
- increasing retention of water and makes urine more concentrated
How does ADH make the collecting duct permeable to water?
Causes the insertion of water permeable channels into late distal convoluted tubule and collecting duct
- allows penetration of water
Describe the haemodynamic control of ADH
Decrease in blood pressure or volume also stimulates ADH secretion
- blood pressure changes are sensed by baroreceptors in the aortic arch and carotid sinus which respond to stretch
What cells release renin and why?
When sodium levels are low in the tubular fluid of the distal convoluted tubule, macula densa cells cause juxtaglomerular cells (granula cells) of afferent arteriole to secrete renin into blood
What does renin mediate the conversion of?
Converts angiotensinogen into angiotensin I
How is angiotensin I converted to angiotensin II?
ACE enzyme
- angiotensin converting enzyme
What is the effect of angiotensin II on the adrenal cortex?
Acts on Zona glomerulosa cells of the adrenal cortex which secretes aldosterone into the blood
What does aldosterone stimulate the synthesis of?
Synthesis and insertion of epithelial sodium channel (ENaC) and a sodium pump in distal tubule and collecting duct cells
- promotes Na+ reabsorption from tubular fluid and K+ secretion to tubular fluid
How does the renin-angiotensin-aldosterone system contribute to the regulation of sodium balance?
All of the mechanisms work independently of each other to regulate osmolality of bodily fluids and extracellular fluid volume
What is the atrial natriuretic peptide (ANP)?
Works opposite to the ADH system + RAA system
How does ANP work?
- Increase in total body Na+ increases plasma volume
- Increased blood volume is sensed by atrial myocytes and stimulates release of ANP
- Inhibits secretion of renin, aldosterone and ADH
- Causes vasodilation thereby decreasing vascular resistance
