How the Brain Works The Facts Visually Explained by DK (z-lib.org)

BRAIN FUNCTIONS AND THE SENSES

Hunger and Thirst 94 95

HOW LONG CAN

YOU SURVIVE WITHOUT

FOOD OR WATER?

Three to four days is the

average without water, but

you can go up to two months

without food in certain

circumstances.

The most obvious symptoms

of dehydration are a dry mouth

and eyes, and perhaps a slight

headache. Another good way

to tell is by the color of your

urine. It should be pale yellow

at full hydration. A darker

amber color shows severe

dehydration. Adults should

take in around 3^1 ⁄ 2 –4 pints

(2–2.5 liters) of fluids a day.

ARE YOU DEHYDRATED?

Thirst

When water levels in the body drop, salt

levels in the blood increase. Thirst areas

in the brain detect rising salt levels and

signal to the body to increase water levels

by reducing urine output and taking in

more fluids. After drinking, it takes around

15 minutes before salt concentration levels

in the blood return to normal. It is thought

that the gulping action of the throat when

swallowing liquids sends signals to

stop drinking.

1

Thirst areas of the brain

Two structures, the organum

vasculosum of the lamina

terminalis (OVLT) and the

subfornical organ (SFO)—both

linked to the hypothalamus—help

create the sensation of thirst.

They lack a blood-brain barrier so

are thought to be able to detect

salt levels in the blood.

DANGEROUSLY

DEHYDRATED

VERY

DEHYDRATED

MODERATELY

DEHYDRATED

HYDR ATED

VERY

HYDR ATED

Heart and kidney

receptors detect

decreases in blood

volume and increases in

salt concentration. They

alert the brain.

2

The SFO and

OVLT also

receive signals about

blood volume and salt

concentration. They signal

to the hypothalamus.

The hypothalamus

passes these

signals to the pituitary

gland, which then

produces antidiuretic

hormone (ADH).

3

7

Inhibitory neurons

in the LT are

triggered by gulping

movements in the throat.

These neurons stop

further intake of water.

6

The hypothalamus

creates the

sensation of thirst,

prompting the urge

to drink so as to restore

water levels.

5

The SFO detects

angiotensin II

and stimulates the

hypothalamus to

prompt the formation

of more ADH.

4

High levels of

ADH tell the

kidneys to retain water

and secrete renin. This in

turn forms the hormone

angiotensin II.

Organum vasculosum

of the lamina terminalis (OVLT)

Subfornical organ (SFO)

Pituitary

gland

Lamina

terminalis (LT)

Hypothalamus

US_094_095_Hunger_and_Thirst.indd 95 20/09/2019 12:35