Hormones in animals

Why do animals need hormones?

Nerves carry messages very fast, but only to the cells that the nerves reach. Some changes must reach every cell of the body, and must last for a while. For these, animals use hormones.

A hormone is a chemical made by a gland and released straight into the blood. The blood carries it to every part of the body. Only certain cells, the target cells, respond to each hormone.

The glands that release hormones into the blood are the endocrine glands. They have no ducts.

Adrenaline: ready for danger

Imagine that a squirrel sees a cat. It must run, and fast. Its adrenal glands, which sit on top of the kidneys, release adrenaline into the blood.

Adrenaline acts on many organs at once:

• the heart beats faster, so more oxygen reaches the muscles;
• blood to the digestive system and the skin is reduced, because the small arteries there contract;
• more blood goes to the muscles of the arms and the legs;
• the breathing rate goes up, because the diaphragm and the rib muscles contract faster.

Together, these changes prepare the body to run or to fight. Your own body does the same in a sudden fright.

The glands and their hormones

• Hypothalamus: releasing hormones, which tell the pituitary when to release its hormones.
• Pituitary: growth hormone, which controls the growth of the body.
• Thyroid: thyroxin, which controls how the body uses carbohydrates, fats and proteins.
• Pancreas: insulin, which lowers the level of sugar in the blood.
• Adrenal glands: adrenaline, which prepares the body for danger.
• Testes: testosterone, which brings the changes in boys at puberty.
• Ovaries: oestrogen, which brings the changes in girls at puberty.

A few of these need a closer look.

• Thyroxin contains iodine. If the food has too little iodine, the thyroid cannot make enough thyroxin, and it swells. The swollen neck is called goitre. This is why we use iodised salt.
• Growth hormone from the pituitary controls growth. Too little in childhood causes dwarfism. Too much causes very great height.
• Insulin helps the cells take in sugar from the blood. If the pancreas makes too little, the sugar in the blood stays high. This is diabetes. Many people with diabetes take insulin injections.

Feedback keeps the levels right

Hormones are made in tiny amounts, and the amount matters. Too much or too little harms the body. So the body controls each hormone by feedback.

Take blood sugar.

1. After a meal, the sugar in the blood rises.
2. Cells of the pancreas detect the rise and release more insulin.
3. Insulin helps the cells take in the sugar, so the level falls.
4. As the level falls, the pancreas releases less insulin.

The result of the hormone controls how much of it is made. Change the numbers below and see how the sugar returns to normal, or does not.

Try it in Python

The program follows the sugar level half an hour at a time. In each step, insulin removes a share of the extra sugar above the normal level.

Practise with new numbers

Common mistakes

• "Hormones are carried by nerves." They are carried by the blood.
• "Every cell responds to every hormone." Only the target cells of a hormone respond to it.
• "Insulin is sugar." Insulin is a hormone that lowers the sugar in the blood.
• "Goitre is caught from other people." It comes from too little iodine in the food.
• "Adrenaline sends more blood to the stomach." It sends less there, and more to the muscles.

Remember these

• Endocrine glands release hormones straight into the blood, which carries them to every part of the body.
• Hormone messages are slower than nerve impulses, but they reach every cell and last longer.
• Adrenaline prepares the body for danger: a faster heart and breathing, and more blood to the muscles.
• Thyroxin needs iodine. Too little iodine causes goitre.
• Growth hormone controls growth. Insulin lowers blood sugar. Too little insulin causes diabetes.
• Feedback control keeps the level of each hormone right.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why can a hormone act on organs far away from its gland?

2. Name two changes that adrenaline causes, and say how each helps in danger.

3. How does iodised salt prevent goitre?

4. After a meal, the sugar in Aman's blood rises. Describe what the pancreas does next.

5. Give one way in which nerve messages and hormone messages differ.