L20 Body Fluid Homeostasis

Question 1

What do hypothalamic osmoreceptors do ?

Answer 1

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

Question 2

What does plasma osmolality mean?

Answer 2

Refers to the concentraion of solutes in the blood plasma

Question 3

What would increase the osmolality plasma?

Answer 3

• Excessive sodium intake
• Hyperglycemia (high blood sugar levels - diabetes)
• Stress + drugs - nicotine

Question 4

What would decrease the osmolality of the plasma?

Answer 4

• Excessive fluid ingestion
• Alcohol (inhibits release of ADH)

Less ADH = Less reabsorption so more excretion of water

Question 5

What is ADH?

Answer 5

Antidiuretic Hormone - responsible for fluid control of the body through water reabsorption.

Question 6

What is the role of ADH?

Answer 6

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

Question 7

As ADH increases what happens to urinary osmolality?

Answer 7

Urinary osmolality also increases because more water absorbed into bloodstream from urine so urine becomes more concentrated with solutes (like sodium and urea)

Question 8

What happens at the principal water model regarding ADH?

Answer 8

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

Question 9

What is the net effect of having more AQP2 channels in the apical membrane of principal cells?

Answer 9

• Increased reabsorption water
• Dilution of the plasma
• Fall in body fluid osmolality

Question 10

What is diabetes insipidus?

Answer 10

Condition that causes your body to produce too much urine

Question 11

What are the two types of Diabetes Insipidus (DI)?

Answer 11

Central and nephrogenic.

Question 12

What does Central DI do?

Answer 12

This occurs when the body doesn’t produce enough ADH

Question 13

What does Nephrogenic DI do?

Answer 13

This occurs when the kidneys are unable to respond to ADH, even if it’s present

Question 14

Where is Aldosterone released from?

Answer 14

Cortex of the adrenal gland.

Question 15

What does the Mineralocorticoid do?

Answer 15

Regulates plasma Na+, K+ and body fluid volume.

Question 16

When is Aldosterone released?

Answer 16

Released when:
- Decreased blood pressure
- Decreased blood volume
- Increased potassium levels

Question 17

What does the Aldosterone act on?

Answer 17

Aldosterone primarily acts on the kidneys, specifically on the cells of the distal tubules and collecting ducts

Question 18

What are the causes of Aldosterone release?

Answer 18

• 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

Question 19

Explain the genomic action of Aldosterone in principal cells

Answer 19

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

Question 20

What does pseudohypoaldosteronism do?

Answer 20

Body’s inability to respond to the hormone aldosterone

Question 21

What causes pseudohypoaldosteronism (PHA)?

Answer 21

• Mineralocorticoid receptor mutations
• Leads to salt-wasting and can cause severe dehydration, hyperkalemia (high K+)

Question 22

Where is Renin released from?

Answer 22

Juxtaglomerular apparatus (JGA)

Question 23

What are the three main components of the JGA?

Answer 23

• 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 -

Question 24

How does the Renin-angiotensin cascade occur?

Answer 24

The renin-angiotensin-aldosterone system (RAAS) is primarily activated in response to a decrease in blood pressure or blood flow to the kidneys

Question 25

What happens in the Renin-angiotensin cascade?

Answer 25

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

Question 26

What are the effects of Angiotensin II?

Answer 26

• 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

Question 27

What are the effects of ingesting salt

Answer 27

• Increase plasma sodium concentration therefore water moves out of ICF
• Increase ECFV and increase osmolalilty plasma

Question 28

What are the effects of increase osmolality plasma?

Answer 28

• Increased ADH
• Increased rebasorption of water
• Decrease osmolality but increased ECFV

Increase water leads to increase volume in ECFV therefore diluting osmolality

Question 29

How does the ingestion of salt increase ECFV?

Answer 29

• 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

Question 30

How would you decrease ECFV?

Answer 30

Decrease aldosterone which increases sodium reabsorption and so increase water loss from ECF so decrease ECFV