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Human Body^147

HOW DO HORMONES WORK?

Hormone molecules travel in the blood looking for

specific cells which they bind on to. This triggers a

chain of chemical reactions and changes the cell’s job.

Some hormones work by switching genes on or off.

Hormones

PITUITARY GLAND 3

This 3-D CT reconstruction shows

the pituitary gland at the base of

the brain. The pituitary gland

produces hormones and controls

other hormone glands in the body.

Pituitary gland A key gland that releases nine

hormones that control body

functions or trigger other glands.

Pancreas Secretes insulin and glucagon,

which control the level of sugar

in the blood.

Adrenal gland Secretes adrenaline, which helps

the body cope with fear or

excitement, and steroids.

Thyroid gland Secretes thyroxine, which speeds

up the rate of body chemistry.

Parathyroid glands Four small glands that work with

the thyroid gland to balance

levels of calcium in the blood.

Pineal gland Secretes melatonin, a hormone

that controls the daily rhythm of

sleep and waking.

Ovaries Make oestrogen and progesterone,

(in females) which control the development

and function of the sex organs.

Testes Secrete testosterone, which

(in males) controls the development and

function of the male sex organs.

The kidneys are two bean-shaped organs at the

back of the abdomen that filter and clean the blood,

removing chemical wastes and excess water. Wastes

drain out of the kidneys as urine.

Hormones are control chemicals that trigger major

changes in the body. They control many important

functions, including body chemistry, growth and sexual

development, and the body’s response to stress.

WHY DOES BLOOD NEED CLEANING?

If the kidneys did not filter the blood, chemical wastes

would build up in the body and become poisonous.

The kidneys also control the level of water in the

blood by varying how much water is reabsorbed in

the nephrons. If we drink a lot, the kidneys can

excrete the excess to stop the blood becoming diluted.

WHAT IS INSIDE A KIDNEY?

Each kidney contains about a million tiny filtering

units called nephrons. Blood flows through a kind of

filter at the top of each nephron and water and small

molecules pass through it into a long tube. In the tube

useful substances such as glucose and salt are then

reabsorbed, and what is left over forms urine.

WHERE DOES WASTE GO?

Urine from each kidney drains through a tube called a

ureter and collects in the bladder. As the bladder fills,

its muscular wall stretches. When it is full, receptors

in the wall send a signal to the brain and trigger the

urge to urinate. During urination the muscles that

close the bladder exit relax to release the urine.

Kidneys

1 A FULL BLADDER

In this X-ray, dye is used to show

urine from the kidneys draining

through the ureters to a full

bladder. An adult bladder can hold

about 0.5 litre (1 pint) of urine.

The pituitary gland

is about the size

of a pea

HORMONAL GLANDS AND THEIR FUNCTIONS

Left kidney

Stomach

FIND OUT MORE. Brain 139 • Circulation 134

FIND OUT MORE. Cells 73 • Growth 149 • Reproduction 148

hormones

,

kidneys