L20 Body Fluid Homeostasis Flashcards
What do hypothalamic osmoreceptors do ?
Located within the hypothalamus of the brain
They maintain the body’s water balance by monitoring the osmolarity of the blood
Stimulated by increase in osmolality
What does plasma osmolality mean?
Refers to the concentraion of solutes in the blood plasma
What would increase the osmolality plasma?
- Excessive sodium intake
- Hyperglycemia (high blood sugar levels - diabetes)
- Stress + drugs - nicotine
What would decrease the osmolality of the plasma?
- Excessive fluid ingestion
- Alcohol (inhibits release of ADH)
Less ADH = Less reabsorption so more excretion of water
What is ADH?
Antidiuretic Hormone - responsible for fluid control of the body through water reabsorption.
What is the role of ADH?
ADH increases the permeability of the collecting ducts in the kidneys to water. This means more water is reabsorbed from the urine back into the bloodstream
As ADH increases what happens to urinary osmolality?
Urinary osmolality also increases because more water absorbed into bloodstream from urine so urine becomes more concentrated with solutes (like sodium and urea)
What happens at the principal water model regarding ADH?
1) When body is dehydrated, pituitary gland releases ADH
2) ADH binds to receptors on principal cells, triggering a signaling cascade that results in the fusion of vesicles containing AQP2
3) The insertion of AQP2 channels significantly increases the water permeability of the apical membrane so water is reabsorbed from the tubular fluid into the bloodstream
What is the net effect of having more AQP2 channels in the apical membrane of principal cells?
- Increased reabsorption water
- Dilution of the plasma
- Fall in body fluid osmolality
What is diabetes insipidus?
Condition that causes your body to produce too much urine
What are the two types of Diabetes Insipidus (DI)?
Central and nephrogenic.
What does Central DI do?
This occurs when the body doesn’t produce enough ADH
What does Nephrogenic DI do?
This occurs when the kidneys are unable to respond to ADH, even if it’s present
Where is Aldosterone released from?
Cortex of the adrenal gland.
What does the Mineralocorticoid do?
Regulates plasma Na+, K+ and body fluid volume.
When is Aldosterone released?
Released when:
- Decreased blood pressure
- Decreased blood volume
- Increased potassium levels
What does the Aldosterone act on?
Aldosterone primarily acts on the kidneys, specifically on the cells of the distal tubules and collecting ducts
What are the causes of Aldosterone release?
- Aldosterone stimulates the kidneys to reabsorb Na+ from the urine back into the bloodstream which leads to increased water retention in the body (reabsorption of water)
- Increases excretion of potassium ions into the urine
Explain the genomic action of Aldosterone in principal cells
1) Aldosterone binds to mineralocorticoid receptors
2) Hormone-receptor complex translocates into the nucleus of the cell
3) RNA transcription & protein synthesis
4) H+ secretion, Na+ reabsorption, K+ secretion
What does pseudohypoaldosteronism do?
Body’s inability to respond to the hormone aldosterone
What causes pseudohypoaldosteronism (PHA)?
- Mineralocorticoid receptor mutations
- Leads to salt-wasting and can cause severe dehydration, hyperkalemia (high K+)
Where is Renin released from?
Juxtaglomerular apparatus (JGA)
What are the three main components of the JGA?
- Macula Densa - Senses changes in NaCl conc in the tubular fluid. When NaCl decrease, macula densa releases renin
- Afferent arteriole stretch receptors - receptors sense changes in blood pressure within the afferent arteriole
- Sympathetic nerve fibres -
How does the Renin-angiotensin cascade occur?
The renin-angiotensin-aldosterone system (RAAS) is primarily activated in response to a decrease in blood pressure or blood flow to the kidneys
What happens in the Renin-angiotensin cascade?
1) When ECF decreases, the JGA releases an enzyme called renin
2) Renin converts angiotensinogen into angiotensin I
3) Angiotensin I is then converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs
What are the effects of Angiotensin II?
- Stimulates the adrenal glands to release aldosterone
- Causes arterioles to constrict to increase blood pressure
- Net effect: increase plasma sodium content + ECFV - increase blood pressure
What are the effects of ingesting salt
- Increase plasma sodium concentration therefore water moves out of ICF
- Increase ECFV and increase osmolalilty plasma
What are the effects of increase osmolality plasma?
- Increased ADH
- Increased rebasorption of water
- Decrease osmolality but increased ECFV
Increase water leads to increase volume in ECFV therefore diluting osmolality
How does the ingestion of salt increase ECFV?
- When you consume salt, Na+ are absorbed from the digestive tract into the bloodstream
- Increased plasma sodium conc
- Osmotic pressure so water moves from ICF (cells) to ECF (blood)]
- Increased ECFV
How would you decrease ECFV?
Decrease aldosterone which increases sodium reabsorption and so increase water loss from ECF so decrease ECFV
