Control of plasma osmolality

Question 1

What is normal body fluid osmolality? What is the exception to this?

Answer 1

285-295 mOsm/kg

Exception is urine

Question 2

How is water taken into the body?

Answer 2

Drinking

Question 3

How is water lost from the body?

Answer 3

Urine by the kidneys

Faeces from the GI tract

Expired air by the lungs

Sweat from the skin

Question 4

How are water intake and water loss related to each other ideally?

Answer 4

Water intake = water loss

Question 5

What happens to plasma osmolality when water intake = water loss? Why?

Answer 5

Remains the same

because have same amount of water per solute

Question 6

What happens to plasma osmolality when water intake is more than water loss? Why?

Answer 6

Decreases

because have more water per solute

Question 7

What happens to plasma osmolality when water intake is less than water loss? Why?

Answer 7

Increases

because have less water per solute

Question 8

What is the relative size of the range of urine osmolality?

Answer 8

Large range

Question 9

How is the osmolality of urine changed?

Answer 9

By changing the volume of urine

Question 10

How does changing the volume of urine change its osmolality?

Answer 10

Different amounts of water to same amount of solute

Question 11

What are the mechanisms that regulate plasma osmolality?

Answer 11

ADH

Thirst

Question 12

What detects plasma osmolality?

Answer 12

Osmoreceptors

Question 13

Where are osmoreceptors located?

Answer 13

OVLT in the hypothalamus

Question 14

How do osmoreceptors in the OVLT in the hypothalamus get into contact with plasma?

Answer 14

The capillary endothelium supplying them is fenestrated

plasma leaks out of capillaries

Question 15

What produces ADH?

Answer 15

Neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus

Question 16

What happens to ADH after it is produced?

Answer 16

It is transported down the axon of the neurosecretory cell

stored in vesicles in its tip

Question 17

Where is the tip of the neurosecretory cell located?

Answer 17

Posterior pituitary gland

Question 18

What causes ADH to be released?

Answer 18

Osmoreceptors detect an increase in plasma osmolality

action potentials are fired down neurosecretory cells that produce ADH

Question 19

How much does the plasma osmolality have to increase by for ADH to be released?

Answer 19

1%

Question 20

How does the amount of ADH released change with plasma osmolality?

Answer 20

Below 285mOsm/kg
amount of ADH released is constant at above 0

As plasma osmolality increases above 285mOsm/kg
amount of ADH released increases proportionately

Question 21

How is ADH released?

Answer 21

Vesicles containing ADH in the tip of neurosecretory cells exocytose
release ADH into the blood

Question 22

Where does ADH travel to after it has been released?

Answer 22

Kidneys

Question 23

What does ADH do in the kidneys?

Answer 23

Increases the expression of aquaporin2 channels on the apical domain of epithelial cells in the later distal convoluted tubule and the collecting ducts

Question 24

What is the effect of increased expression of aquaporin 2 channels on the apical domain of epithelial cells in the later distal convoluted tubule and the collecting ducts?

Answer 24

More water molecules move through it from the tubular lumen into the epithelial cell
so more water is reabsorbed

Aquaporin3 and aquaporin4 channels on basolateral domain of epithelial cell
water molecules move through them from the epithelial cell into the interstitial fluid

Question 25

What is the effect of increased water reabsorption on the urine?

Answer 25

Smaller volume of more concentrated urine produced

Question 26

What is the effect of increased water reabsorption on plasma osmolality?

Answer 26

Plasma osmolality decreases

Question 27

What stimulates thirst?

Answer 27

Osmoreceptors detect an increase in plasma osmolality | action potentials are fired down neurones

Question 28

How much does the plasma osmolality have to increase by for thirst to be stimulated?

Answer 28

10%

Question 29

When does the stimulation for thirst stop?

Answer 29

After a sufficient volume of water has been ingested | so before it has been absorbed into the blood

Question 30

What does thirst result in?

Answer 30

Increased water intake

Question 31

What is the importance of the stimulation for thirst stopping after water has been consumed, before it has been absorbed into the blood?

Answer 31

It takes a while for water to be absorbed into the blood so prevent ingestion of water in the meantime as this would lead to overhydration

Question 32

How do the ADH and thirst mechanisms regulating plasma osmolality compare to each other?

Answer 32

ADH mechanism occurs before thirst so ADH mechanism is faster ADH mechanism gives short-term solution whereas thirst gives long-term solution

Question 33

What else can stimulate ADH release and increased thirst apart from increased plasma osmolality?

Answer 33

Decreased blood volume and blood pressure

Question 34

How does a decreased blood volume and pressure affect ADH release?

Answer 34

ADH released at lower plasma osmolalities than normal More ADH released per unit increase in plasma osmolality

Question 35

What are ADH and the thirst mechanism more sensitive to: changes in plasma osmolarity or changes in blood volume and pressure? How is this shown?

Answer 35

More sensitive to changes in plasma osmolarity Smaller changes in plasma osmolarity than blood volume and pressure can stimulate ADH and the thirst mechanism

Question 36

What is more important to control: plasma osmolality or blood volume and blood pressure? How is this shown?

Answer 36

Blood volume and blood pressure Plasma osmolality will be compromised to maintain normal blood volume and blood pressure

Question 37

How does an increased blood volume and pressure affect ADH release?

Answer 37

ADH released at higher plasma osmolalities than normal Less ADH released per unit increase in plasma osmolality

Question 38

How can ADH cause disease?

Answer 38

Deficiency of ADH Excess of ADH

Question 39

What is the disease of ADH deficiency called?

Answer 39

Diabetes insipidus

Question 40

What are the types of diabetes insipidus?

Answer 40

Central Nephrogenic

Question 41

What is central diabetes insipidus?

Answer 41

Low levels of ADH in the plasma | because the hypothalamus and posterior pituitary gland don't produce and release enough

Question 42

What are the causes of central diabetes insipidus?

Answer 42

Damage to hypothalamus and posterior pituitary gland - tumours - trauma - infection

Question 43

What is nephrogenic diabetes insipidus?

Answer 43

Normal levels of ADH in the plasma | but the kidneys are insensitive to it

Question 44

What are the signs symptoms of diabetes insipidus?

Answer 44

Polyuria - large volume of dilute urine produced Polydipsia

Question 45

How is diabetes insipidus treated?

Answer 45

Giving ADH

Question 47

How can ADH be given?

Answer 47

Injections Nasal spray

Question 47

What is the disease of excess ADH called?

Answer 47

Syndrome of inappropriate ADH secretion (SIADH)

Question 49

What causes SIADH?

Answer 49

High levels of ADH in the blood | excessively produced by the hypothalamus, or other sources

Question 49

What are the signs and symptoms of SIADH?

Answer 49

Small volume of concentrated urine produced always High total body fluid Dilutional hyponatraemia