Control and Coordination · Lesson 10 of 11
Hormones in Animals
“Tiny chemical messengers can change heartbeat, growth, metabolism and blood glucose.”
- Explain how endocrine glands coordinate body activities through hormones. - Describe the emergency effects of adrenaline. - Relate thyroxin to iodine, metabolism and goitre. - Explain the roles of growth hormone, testosterone and oestrogen. - Relate insulin to blood-glucose regulation and diabetes. - Explain how feedback mechanisms regulate hormone secretion.
When an animal suddenly faces danger, many organs must prepare together: the heart beats faster, breathing increases and more blood reaches skeletal muscles. A nerve signal alone would be too restricted to organise every tissue involved. Hormones provide a second coordinating system by travelling through the blood and acting on selected target tissues.
Hormonal Coordination
A gland that releases hormones directly into the blood rather than through a duct.
A tissue containing receptors that allow it to detect and respond to a particular hormone.
Although blood carries a hormone widely, not every cell responds in the same way. A response occurs mainly in cells with appropriate receptors. Hormonal effects are often slower than nerve impulses but can be widespread and sustained, making them suitable for metabolism, growth, puberty and long-lasting adjustment.
Adrenaline
Adrenal glands release adrenaline during frightening or stressful situations. Adrenaline prepares the body for vigorous action. The heart beats faster, increasing blood flow and oxygen delivery. Muscles around small arteries supplying the digestive system and skin contract, reducing blood flow to those regions and diverting more towards skeletal muscles. Contraction of the diaphragm and rib muscles increases breathing rate. Together these adjustments prepare the body to fight or run.
Adrenaline does not create a single isolated effect. It coordinates the heart, blood vessels, breathing muscles and skeletal-muscle supply so that the whole body is prepared for rapid action.
Thyroxin
The thyroid gland produces thyroxin using iodine. Thyroxin regulates the metabolism of carbohydrates, proteins and fats so that energy use and material balance support proper growth. If the diet lacks iodine, the thyroid cannot synthesise sufficient thyroxin efficiently. The gland may enlarge, producing the swollen neck associated with goitre. Iodised salt helps provide the iodine required for synthesis.
Growth Hormone
The pituitary gland secretes growth hormone, which regulates growth and development throughout the body. A deficiency during childhood can lead to dwarfism. Growth is not controlled by one body part simply growing faster in a random direction; hormones coordinate when and where tissues grow so that body form is maintained.
Testosterone And Oestrogen
Testosterone is secreted mainly by the testes and oestrogen mainly by the ovaries. Their increased secretion during puberty contributes to changes in body form and reproductive maturity. Oestrogen supports development of female reproductive organs and helps regulate the menstrual cycle. These hormones coordinate changes across several tissues rather than acting at only one visible location.
Insulin
The pancreas produces insulin, which helps regulate blood glucose. After food is digested and glucose enters the blood, pancreatic cells detect the increase and release more insulin. Responsive tissues then remove or store more glucose, helping its level fall. If insulin is not produced in sufficient quantity or does not act effectively, blood glucose can remain abnormally high, producing diabetes and harmful effects. Some people therefore require insulin supplied by injection.
Releasing Hormones
The hypothalamus helps control the release of many hormones. When the level of a hormone such as growth hormone is low, the hypothalamus can release a factor that stimulates the pituitary gland. This illustrates a hierarchy: one endocrine signal can regulate the release of another.
Feedback Mechanism
A control process in which the result of an activity influences further activity, helping regulate the timing and amount of a response.
Hormones must be secreted in precise quantities. In negative feedback, a change triggers a response that reduces the original change. Rising blood glucose promotes insulin release; as glucose falls, the stimulus for high insulin secretion weakens, so secretion decreases. This prevents a correcting response from continuing unchecked.
| Hormone | Source | Main role |
|---|---|---|
| Growth hormone | Pituitary gland | Regulates growth and development |
| Thyroxin | Thyroid gland | Regulates carbohydrate, protein and fat metabolism |
| Insulin | Pancreas | Helps regulate blood glucose |
| Testosterone | Testes | Coordinates changes associated with male puberty |
| Oestrogen | Ovaries | Coordinates female reproductive development and menstrual-cycle regulation |
| Adrenaline | Adrenal glands | Prepares the body for emergency action |
| Releasing hormones | Hypothalamus | Stimulate pituitary hormone release |
Activity
Use the endocrine-gland diagram to locate the hypothalamus, pituitary, thyroid, adrenal glands, pancreas, testes and ovaries. Then cover one column of the hormone table and reconstruct the missing gland or function from memory. Check each answer by reasoning from the hormone's role rather than memorising an isolated pair.
Problem
A person has insufficient iodine in the diet. Which hormone is directly affected and what condition may develop?
- 1.Iodine is required by the thyroid gland.
- 2.The gland uses iodine to synthesise thyroxin.
- 3.Insufficient iodine can reduce thyroxin production and cause thyroid enlargement.
- 4.The associated condition is goitre.
Problem
Explain how adrenaline improves the body's capacity to run from danger.
- 1.Heart rate rises, increasing circulation.
- 2.Breathing rate rises, increasing oxygen intake.
- 3.Blood supply to skin and digestion is reduced.
- 4.More blood and oxygen are directed towards skeletal muscles.
- 5.The coordinated changes support rapid muscular activity.
Problem
Why would constant high insulin secretion be unsuitable after blood glucose has already fallen?
- 1.Insulin promotes processes that lower blood glucose.
- 2.Continued high secretion could lower it beyond a suitable level.
- 3.Negative feedback reduces secretion as the original rise is corrected.
- 4.The system therefore adjusts hormone output rather than behaving like a permanent on-switch.
Hormones travel throughout the blood, but only target cells with suitable receptors respond strongly. Also, diabetes is not simply caused by eating sugar; the central issue discussed here is failure to produce or use insulin properly.
Quiz
Which gland secretes adrenaline?
Why is iodine needed in the diet?
Which hormone helps regulate blood glucose?
What is the purpose of negative feedback?
Why do only certain tissues respond strongly to a circulating hormone?
Practice Problems
- Describe the main changes caused by adrenaline. Model answer: Heart rate and breathing increase, blood supply to skin and digestion decreases, and more blood is directed to skeletal muscles, preparing the body for vigorous action.
- Explain the connection among iodine, thyroxin and goitre. Model answer: Iodine is needed to synthesise thyroxin. Deficiency impairs production and may cause enlargement of the thyroid, visible as goitre.
- Compare growth hormone, testosterone and oestrogen. Model answer: Growth hormone regulates general growth and development; testosterone and oestrogen coordinate reproductive maturation and puberty-related changes in males and females respectively.
- Explain why some people with diabetes receive insulin injections. Model answer: If the pancreas produces insufficient effective insulin, blood glucose remains high. Supplied insulin helps responsive tissues regulate glucose levels.
- Describe negative feedback using insulin. Model answer: Rising blood glucose stimulates pancreatic cells to release more insulin. As glucose falls, the stimulating condition weakens and insulin secretion is reduced.
Key Takeaways
• Endocrine glands release hormones directly into the blood. • Target tissues respond because their cells possess suitable receptors. • Adrenaline coordinates widespread changes that prepare the body for emergency action. • Iodine is essential for thyroxin synthesis; deficiency can cause goitre. • Growth hormone, testosterone and oestrogen coordinate growth and developmental changes. • Insulin helps regulate blood glucose, and insufficient effective insulin can lead to diabetes. • Feedback mechanisms adjust hormone secretion as internal conditions change.