Animal Coordination, control and homeostasis


Human Endocrine System

The endocrine system secretes hormones into the bloodstream from glands throughout the body. Hormones travel in the bloodstream to specific target organs, where they have an effect.

ENDO= into           CRINE= chemicals

Hormones are chemical messengers

To be known as am endocrine gland, they have to secrete hormones into the blood.

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Endocrine Glands

These are the endocrine glands (and the chemicals they secrete) :

-Ovaries --> Oestrogen and Progesterone

-Testes --> Testosterone

-Adrenal Glands --> Adrenaline

-Thyroid Glands --> Thyroxine

-Pituitory Gland --> ACTH, FSH, LH and ADH

-Pancreas --> Glucagon and Insulin

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Hormones are chemical messengers, produced by a gland and carried in the bloodstream, which alters the activity of specific target organs. An example of this is the release of the hormone adrenaline, which is released by the adrenal glands. One of its target organs is the heart, where it increases the heart rate.

Once a hormone has been used, it is destroyed by the liver.

Like the nervous system, hormones can control the body. The effects are much slower than the nervous system, but they last for longer.

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Differences between nervous and hormonal control

                                                                 Nervous                                         Hormonal

Type of signal:      Electical (chemical at synapses)                                          Chemical

Transmission of signal:   By nerve cells (neurones)                          By the bloodstream

Effectors:                                   Muscles or glands        Target cells in particular organs

Type of response: Muscle contraction or secretion                             Chemical change

Speed of response:                               Very rapid                                                Slower

Dur. of response:     Short (til nerve impulses stop)     Long (til hormone  broken down)

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Important Hormones

Hormone --> Source --> Target organ(s) --> Role --> Effects

ADH --> Pituitary gland --> Kidneys --> Controlling the water content of the blood --> Increases reabsorption of water by the collecting ducts

Adrenaline --> Adrenal glands --> Several targets including organs in the respiratory and circulatory systems --> Preperation for 'fight or flight' --> Increases breathing rate, heart rate, flow of blood to muscles, conversion of glycogen to glucose

Insulin --> Pancreas --> Liver --> Controlling blood glucose levels --> Increases conversion of glucose into glycogen for storage

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Master Gland

The pituitary gland in the brain is known as a 'master gland'. It secretes several hormones into the blood in response to the body's condition, such as blood water levels.

The hypothalamous detects changes in hormone levels and will release hormones which control the pituitary gland or other organs. The hormones from the hypothalamus and pituitary can also act on other glands to stimulate the relase of different types of hormones and bring about effects.

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Hormone control of metabolic rate- Thyroxine

One hormone that affects metabolic rate is --> Thyroxine, released by the thyroid gland

Thyroxine is taken into, and affects, many different kinds of cell. 

It causes heart cells to contract more rapidly and strongly, and it also increases the rate at which proteins and carbohydrates are broken down inside cells.


Thyroxine increases metabolic rate by:

  • increasing heart rate
  • increasing the rate of protein/ carbohydrate break down in cells
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Hormone control of metabolic rate- Adrenaline

Adrenaline also increases metabolic rate.

-It causes the liver to break down glycogen releasing glucose.

-It makes the heart beat faster.

-It widens blood vessels to key organs and muscles and contracts blood vessels that are not needed

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Thyroxine and Adrenaline- comparison


Increases metabolic rate----Makes heart beat faster

Unique to Thyroxine:

1. Thyroid gland----increases rate at which proteins + carbohydrates are broken down inside cells----controlled by a negative feedback loop

Unique to Adrenaline:

Adrenal gland----widens blood vessels to key organs and muscles----contracts vessels that are not needed----signals to liver to break down glycogen releasing glucose

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Thyroxine- more detail

Thryoxine is produced from the thyroid gland, which stimulates the metabolic rate. It controls the speed at which oxygen and food products react to release energy for the body to use. Thyroxine also plays an important role in growth and development. Its levels are controlled by negative feedback.

The hypothalamus and pituitary gland have important roles in detecting and controlling thyroxine levels.

  • Low thyroxine levels in the bloodstream stimulate the hypothalamus to release TRH and this causes the pituitary to release TSH so the thyroid releases more thyroxine. So blood levels return to normal.
  • Normal thyroxine levels in the bloodstream inhibit TRH release from the hypothalamus and the production of TSH by the pituitary, so normal blood levels are maintained.

The control of thyroxine levels by the hypothalamus and pituitary gland is an example of negative feedback.

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Adrenaline- more detail

Adrenaline is produced by the adrenal glands in times of fear or stress. It targets vital organs, increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles. Preparing the body for 'fight or flight'. Adrenaline is not controlledby negative feedback.

When adrenaline is released into the bloodstream it creates multiple effects:

  • increases breathing rate, heart rate (therefore blood flow) and blood pressure
  • conversion of glycogen to glucose in liver cells, increasing blood glucose

These effects result in more glucose being delivered to the muscles and more energy being released by respiration in the muscles.

The effects of adrenaline allow the body to prepare for action in situations where a quick response may be needed.

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Regulating blood glucose

Glucose is needed by cells for respiration. It is important that the concentration of glucose in the blood is maintained at a constant level and controlled carefully. Insulin is a hormone produced by the pancreas which regulates glucose concentrations in the blood.

If the blood glucose concentration is too high, the pancreas produces the hormone insulin, this causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage, and will be used at a later date.

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Control of blood sugar levels

If blood sugar levels = too high, insulin is released by the pancreas

If blood sugar levels = too low, glucagon is released by the pancreas

Blood sugar levels being too high may be because you have just eaten

Blood sugar levels being too low may be because you have exercised or not eaten.

Insulin- causes the liver to store excess glucose as glycogen

Glucagon- causes the liver to break down glycogen

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Action of insulin

Low glucose:

  • Effect of pancreas- insulin not secreted into the blood
  • Effect on liver- does not convert glucose into glycogen
  • Effect on blood glucose level- increases

High glucose:

  • Effect on pancreas- insulin secreted into the blood
  • Effect on liver- converts glucose into glycogen
  • Effect on blood glucose level- decreases
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