ILA 2 - water and salt

Question 1

How much of total body weight is water?

Answer 1

60% of body weight is water (42 L in a 70kg man)

Question 2

How much of total body weight is ECF?ICF?

Answer 2

ICF = 65%
ECF = 35%

Question 3

How much of ECF is interstitial fluid/plasma?

Answer 3

interstitial fluid = 28% of total body fluid

plasma = 7% of total body fluid

Question 4

What are the main electrolytes of ECF?

Answer 4

ECF -Na+, C-

Question 5

What are the main electrolytes of ICF?

Answer 5

ICF - K+, PSO4 2-, SO4

Question 6

What is osmolarity?

Answer 6

A measure of dissolved solute molecules per L (osmol/l)

Question 7

What is osmolality?

Answer 7

A measure of dissolved solute molecules per kg (osmol/kg)

Question 8

What is osmosis?

Answer 8

The net movement of water across a semi permeable membrane along a conc grad

Question 9

What is Osmotic pressure?

Answer 9

The pressure that must be applied to a solution to stop the net flow of water

Question 10

What is Oncotic pressure?

Answer 10

The pressure exerted by proteins, notably albumin, in a blood vessel’s plasma that tends to pull water in the circulatory system

Question 11

What are aqua proteins?

Answer 11

Proteins in a cell surface membrane that water molecules diffuse through, number of aqua porins can change due to signalling/hormones

Question 12

What is albumin?

Answer 12

Molecule produced by the liver, important to maintain oncotic pressure aprox 75% related to albumin (encourages water into the circulatory system)

Question 13

Describe the movement of fluid in and out of capillaries in a normal person

Answer 13

Hydrostatic pressure is greater at the arterial end compared with the venous end

• This means that fluid moves out of the capillaries at the arterial end because the hydrostatic pressure is higher than the oncotic/osmotic pressure pulling the fluid in
• At the venous end the fluid returns to the circulatory system because the oncotic/osmotic pressure is now higher than the hydrostatic pressure of the capillary

Question 14

Describe the movement of fluid in and out of capillaries in a person with low albumin levels

Answer 14

• Fluid moves out of the capillaries due to high hydrostatic pressure
• The oncotic/osmotic pressure is low due to the low albumin levels, this means that it is lower than the hydrostatic pressure of the capillaries at the venous end as so fluid is not drawn back into the circulatory system
• The fluid collects in the intercelluar tissue where is forms an odema

Question 15

What are insensible losses? Name the main insensible losses

Answer 15

• Evaporation - sweating
• Respiratory - speaking (?)

For every degree celsius increase in temp there is a 10% increase in insensible losses

Question 16

What is minimal insensible water losses a day for a normal adult?

Answer 16

800ml/day

Question 17

How do hypothalamus and osmoreceptors affect water and sodium homeostasis?

Answer 17

1. Osmoreceptors in the hypothalamus detect changes in osmolality
2. Increased or decreased firing of hypothalamic receptors
3. Posterior pituitary releases
4. Posterior pituitary releases vasopressin (or ADH)
5. Increased/decreased aqua porins
6. Increased/decreased water reabsorption
7. Decreased/increased water reabsorption

Question 18

What is role of the kidneys in water conservation?

Answer 18

1. Changes in vasopressin
2. Vasopressin binds to cell membrane surface receptors
3. inter cellular cascade
4. Increased number of aqua porins at proximal tubule
5. Increased reabsorption of water

Question 19

Explain the RAAS system

Answer 19

1. Renin(enzyme) is secreted by juxtaglomerular apparatuses in afferent arterioles.
2. Renin catalyses the conversion of angiotensinogen (from the liver) to angiotensin I. (10aa)
3. ACE then catalyses the conversion of angiotensin I to angiotensin II. (8aa) ACE is found on the luminal surface of pulmonary capillary endothelial cells
4. Angiotensin II stimulates the secretion of aldosterone from the adrenal cortex, and the constriction of arterioles
5. Aldosterone means that Na+ (and therefore H20) is retained and not excreted leading to an increase in bp
6. Aldosterone stimulates Na+ reabsorption by distal and cortical collecting ducts, it induces the synthesis of all channels and pumps in cortical collecting duct
   no aldosterone = approx 2% of na+ is excreted
   high aldosterone = all na+ reabsorbed
7. Vasoconstriction also leads to an increase in bp

Question 20

What is the normal homeostatic response to excess fluid?

Answer 20

1. Excess fluid detected by osmoreceptors
2. less firing of hypothalamus
3. Less vasopressin from posterior pit.
4. Less aqua porins
5. Less water reabsorbed
6. more water excreted
7. less renin release = less aldosterone = less sodium reabsorption and water coupling
8. less symp firing

Question 21

What is the normal homeostatic response to dehydration?

Answer 21

1. Low osmorality detected by osmoreceptors
2. Increased firing of hypothalamus
3. Posterior pit = vasopressin = more aqua proteins = more water retention
4. juxtaglomerular apparatuses detect reduced plasma volume = RAAS = aldosterone = increased sodium reabsorption (therefore water) and = increased symp firing
5. Signals to brain stimulate thirst

Question 22

What occurs due to aldosterone release?

Answer 22

1. Increased Symp activity
2. SODIUM REABSORPTION = water coupling
3. Aldosterone release = water reabsoption
4. Vasoconstriction = increase in bp
5. acts on pituitary gland = releases ADH