Water Balance (Monday 06/11/17)

Question 1

Control of ADH secretion

Answer 1

ADH produced in posterior pituitary. The supraoptic neurone and the paraventricular neurone store it in themself as vesicles. The sensor/osmoreceptor of osmolality is the 3rd ventricle. When this is stimulated ADH is released. Baroreceptors which measure pressure can also input.

Adding ADH increases urine osmality, due to increased water reabsorption out of the collecting tube.

No ADH secreted below 280 plasma osmalality- which is the normal blood osmolality. Above 280 osmolality, ADH is released. This is osmolar control.

Non-osmolar control: only activated upon greater than 10% decrease in blood volume. The amount of ADH released is much higher, even though only need 12pg/ml for maximum reabsorption of water. Extra ADH is released because it is a vasoconstrictor, important in haemorrage.

Control of blood volume takes precedence over control of osmolality.

Question 2

The Osmostat May be Reset When?

Answer 2

• Pregnancy and Morphine lowers normal osmolality to 270. So release more ADH than normal, more conc urine.
• Alcoholism increases it to 295.

Question 3

Why is plasma sodium used as a measure of osmolality?

Answer 3

1. Osmolality: no. osmotically active particles
2. For each sodium there is an anion
3. So osmolality is 2x sodium level

Question 4

Major Causes of Hypernatremia

Answer 4

• When plasma sodium is greater than 145mM
• So water leaves the cell and they shrink and die (salt balance disturbance). Brain can shrink-coma- children if give too much salt.
• If water intake goes down. EG if unconscious, can’t excess water.
• Uncontrolled loss of water, diahorrhea, vomit, diabetes mellitus. Or no ADH secretion- disease called diabetes insipidus (kidney not responding to ADH).

Question 5

Major causes of Hyponatremia

Answer 5

• Hyponatremia: plasma sodium less then 133mM
• Due to disturbances of salt balance: lose sodium due to hypoaldosteronism and thiazides diuretics
• Due to increased water intake, like due to psychogenic polydipsia
• Due to not being able to lose water, like kidney disease or increased ADH secretion.
• Example, water after marathon. Have extra ADH in body, can die of polydipsia. Instead of peeing it out because urine is very concentrated, rapid gain of water by body overwhelms capacity to lose water – brain cell swell – coma -death
• Patients in pain secrete more ADH, so susceptible to hyponatremia.
• Ectasy/ MDMA is a hepatoxin- kills the liver. Is a promoter of ADH secretion.

Question 6

Diabetes Insipidus

Answer 6

Two types:

1. Central: damage to hypothalmus or osmoreceptors causing impaired secretion od ADH.
2. Nephrogenic: No response to ADH in kidney. Washout of medullary hypertonicity? No functional V2 receptors? Cause: Li therapy, sickle cell anemia, mutations in V2 receptors.

Test for D.I is water restriction test. When a healthy person is restricted from water, they will increase ADH production, and their plasma osmolality will increase. In a person with DI their plasma osmolality won’t increase. If they are central DI, upon giving them ADH their plasma osmolality will increase but not if they are nepjrogenic DI. In polydipsia…