Anatomy and Physiology of Animals/Endocrine System

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After completing this section, you should know:

  • The characteristics of endocrine glands and hormones
  • The position of the main endocrine glands in the body
  • The relationship between the pituitary gland and the hypothalamus
  • The main hormones produced by the two parts of the pituitary gland and their effects on the body
  • The main hormones produced by the pineal, thyroid, parathyroid and adrenal glands, the pancreas, ovary and testicle in regard to their effects on the body
  • What is meant by homeostasis and feedback control
  • The homeostatic mechanisms that allow an animal to control its body temperature, water balance, blood volume and acid/base balance

The Endocrine System[edit]

In order to survive, animals must constantly adapt to changes in the environment. The nervous and endocrine systems both work together to bring about this adaptation. In general the nervous system responds rapidly to short-term changes by sending electrical impulses along nerves and the endocrine system brings about longer-term adaptations by sending out chemical messengers called hormones into the blood stream.

For example, think about what happens when a male and female cat meet under your bedroom window at night. The initial response of both cats may include spitting, fighting and spine tingling yowling - all brought about by the nervous system. Fear and stress then activates the adrenal glands to secrete the hormone adrenaline which increases the heart and respiratory rates. If mating occurs, other hormones stimulate the release of ova from the ovary of the female and a range of different hormones maintains pregnancy, delivery of the kittens and lactation.

Endocrine Glands And Hormones[edit]

Hormones are chemicals that are secreted by endocrine glands. Unlike exocrine glands (see chapter 5), endocrine glands have no ducts, but release their secretions directly into the blood system, which carries them throughout the body. However, hormones only affect the specific target organs that recognize them. For example, although it is carried to virtually every cell in the body, follicle stimulating hormone (FSH), released from the anterior pituitary gland, only acts on the follicle cells of the ovaries causing them to develop.

A nerve impulse travels rapidly and produces an almost instantaneous response but one that lasts only briefly. In contrast, hormones act more slowly and their effects may be long lasting. Target cells respond to minute quantities of hormones and the concentration in the blood is always extremely low. However, target cells are sensitive to subtle changes in hormone concentration and the endocrine system regulates processes by changing the rate of hormone secretion.

The main endocrine glands in the body are the pituitary, pineal, thyroid, parathyroid, and adrenal glands, the pancreas, ovaries and testes. Their positions in the body are shown in diagram 16.1.

Anatomy and physiology of animals Main endocrine organs of the body.jpg

Diagram 16.1 - The main endocrine organs of the body

The Pituitary Gland And Hypothalamus[edit]

The pituitary gland is a pea-sized structure that is attached by a stalk to the underside of the cerebrum of the brain (see diagram 16.2). It is often called the “master” endocrine gland because it controls many of the other endocrine glands in the body. However, we now know that the pituitary gland is itself controlled by the hypothalamus. This small but vital region of the brain lies just above the pituitary and provides the link between the nervous and endocrine systems. It controls the autonomic nervous system, produces a range of hormones and regulates the secretion of many others from the pituitary gland (see Chapter 7 for more information on the hypothalamus).

The pituitary gland is divided into two parts with different functions - the anterior and posterior pituitary (see diagram 16.3).

Anatomy and physiology of animals Position of the pituitary gland and hypothalamus.jpg

Diagram 16.2 - The position of the pituitary gland and hypothalamus

Anterior and posterior pituitary.jpg

Diagram 16.3 - The anterior and posterior pituitary

The anterior pituitary gland secretes hormones that regulate a wide range of activities in the body. These include:

1. Growth hormone that stimulates body growth.
2. Prolactin that initiates milk production.
3. Follicle stimulating hormone (FSH) that stimulates the development of the follicles of the ovaries. These then secrete oestrogen (see chapter 6).
4. melanocyte stimulating hormone (MSH) that causes darkening of skin by producing melanin
5. lutenizing hormone (LH) that stimulates ovulation and production of progesterone and testosterone

The posterior pituitary gland

1. Antidiuretic Hormone (ADH), regulates water loss and increases blood pressure
2. Oxytocin, milk "let down"

The Pineal Gland[edit]

The pineal gland is found deep within the brain (see diagram 16.4). It is sometimes known as the ‘third eye” as it responds to light and day length. It produces the hormone melatonin, which influences the development of sexual maturity and the seasonality of breeding and hibernation. Bright light inhibits melatonin secretion Low level of melatonin in bright light makes one feel good and this increases fertility. High level of melatonin in dim light makes an animal tired and depressed and therefore causes low fertility in animals.

Anatomy and physiology of animals Pineal gland.jpg

Diagram 16.4 - The pineal gland

The Thyroid Gland[edit]

The thyroid gland is situated in the neck, just in front of the windpipe or trachea (see diagram 16.5). It produces the hormone thyroxine, which influences the rate of growth and development of young animals. In mature animals it increases the rate of chemical reactions in the body.

Thyroxine consists of 60% iodine and too little in the diet can cause goitre, an enlargement of the thyroid gland. Many inland soils in New Zealand contain almost no iodine so goitre can be common in stock when iodine supplements are not given. To add to the problem, chemicals called goitrogens that occur naturally in plants like kale that belong to the cabbage family, can also cause goitre even when there is adequate iodine available.

Anatomy and physiology of animals Thyroid & parathyroid glands.jpg

Diagram 16.5 - The thyroid and parathyroid glands

The Parathyroid Glands[edit]

The parathyroid glands are also found in the neck just behind the thyroid glands (see diagram 16.5). They produce the hormone parathormone that regulates the amount of calcium in the blood and influences the excretion of phosphates in the urine.

The Adrenal Gland[edit]

The adrenal glands are situated on the cranial surface of the kidneys (see diagram 16.6). There are two parts to this endocrine gland, an outer cortex and an inner medulla.

Anatomy and physiology of animals Adrenal glands.jpg

Diagram 16.6 - The adrenal glands

The adrenal cortex produces several hormones. These include:

1. Aldosterone that regulates the concentration of sodium and potassium in the blood by controlling the amounts that are secreted or reabsorbed in the kidney tubules.
2. Cortisone and hydrocortisone (cortisol) that have complex effects on glucose, protein and fat metabolism. In general they increase metabolism. They are also often administered to animals to counteract allergies and for treating arthritic and rheumatic conditions. However, prolonged use should be avoided if possible as they can increase weight and reduce the ability to heal.
3. Male and female sex hormones similar to those secreted by the ovaries and testes.

The hormones secreted by the adrenal cortex also play a part in “general adaptation syndrome” which occurs in situations of prolonged stress.

The adrenal medulla secretes adrenalin (also called epinephrine). Adrenalin is responsible for the so-called flight fight, fright response that prepares the animal for emergencies. Faced with a perilous situation the animal needs to either fight or make a rapid escape. To do either requires instant energy, particularly in the skeletal muscles. Adrenaline increases the amount of blood reaching them by causing their blood vessels to dilate and the heart to beat faster. An increased rate of breathing increases the amount of oxygen in the blood and glucose is released from the liver to provide the fuel for energy production. Sweating increases to keep the muscles cool and the pupils of the eye dilate so the animal has a wide field of view. Functions like digestion and urine production that are not critical to immediate survival slow down as blood vessels to these parts constrict.

Note that the effects of adrenalin are similar to those of the sympathetic nervous system.

The Pancreas[edit]

In most animals the pancreas is an oblong, pinkish organ that lies in the first bend of the small intestine (see diagram 16.7). In rodents and rabbits, however, it is spread thinly through the mesentery and is sometimes difficult to see.

Anatomy and physiology of animals The pancreas.jpg

Diagram 16.7 - The pancreas

Most of the pancreas acts as an exocrine gland producing digestive enzymes that are secreted into the small intestine. The endocrine part of the organ consists of small clusters of cells (called Islets of Langerhans) that secrete the hormone insulin. This hormone regulates the amount of glucose in the blood by increasing the rate at which glucose is converted to glycogen in the liver and the movement of glucose from the blood into cells.

In diabetes mellitus the pancreas produces insufficient insulin and glucose levels in the blood can increase to a dangerous level. A major symptom of this condition is glucose in the urine.

The Ovaries[edit]

The ovaries, located in the lower abdomen, produce two important sex hormones.

1. The follicle cells, under the influence of FSH (see the pituitary gland above), produce oestrogen, which stimulates the development of female sexual characteristics - the mammary glands, generally smaller build of female animals etc. It also stimulates the thickening of the lining of the uterus in preparation for pregnancy (see chapter 13).
2. Progesterone is produced by the corpus luteum, the endocrine gland that develops in the empty follicle following ovulation (see chapter 13). It promotes the further preparation of the uterine lining for pregnancy and prevents the uterus contracting until the baby is born.

The Testes[edit]

Cells around the sperm producing ducts of the testis produce the hormone testosterone. This stimulates the development of the male reproductive system and the male sexual characteristics - generally larger body of male animals, mane in lions, tusks in boars, etc


  • Hormones are chemicals that are released into the blood by endocrine glands i.e. Glands with no ducts. Hormones act on specific target organs that recognize them.
  • The main endocrine glands in the body are the hypothalamus, pituitary, pineal, thyroid, parathyroid and adrenal glands, the pancreas, ovaries and testes.
  • The hypothalamus is situated under the cerebrum of the brain. It produces or controls many of the hormones released by the pituitary gland lying adjacent to it.
  • The pituitary gland is divided into two parts: the anterior pituitary and the posterior pituitary.
  • The anterior pituitary produces:
  • Growth hormone that stimulates body growth
  • Prolactin that initiates milk production
  • Follicle stimulating hormone (FSH) that stimulates the development of ova
  • Luteinising hormone (LH) that stimulates the development of the corpus luteum
  • Plus several other hormones
  • The posterior pituitary releases:
  • Antidiuretic hormone (ADH) that regulates water loss and raises blood pressure
  • Oxytocin that stimulates milk “let down”.
  • The pineal gland in the brain produces melatonin that influences sexual development and breeding cycles.
  • The thyroid gland located in the neck, produces thyroxine, which influences the rate of growth and development of young animals. Thyroxine consists of 60% iodine. Lack of iodine leads to goitre.
  • The parathyroid glands situated adjacent to the thyroid glands in the neck produce parathormone that regulates blood calcium levels and the excretion of phosphates.
  • The adrenal gland located adjacent to the kidneys is divided into the outer cortex and the inner medulla.
  • The adrenal cortex produces:
  • Aldosterone that regulates the blood concentration of sodium and potassium
  • Cortisone and hydrocortisone that affect glucose, protein and fat metabolism
  • Male and female sex hormones
  • The adrenal medulla produces adrenalin responsible for the flight, fright, fight response that prepares animals for emergencies.
  • The pancreas that lies in the first bend of the small intestine produces insulin that regulates blood glucose levels.
  • The ovaries are located in the lower abdomen produce 2 important sex hormones:
  • The follicle cells of the developing ova produce oestrogen, which controls the development of the mammary glands and prepares the uterus for pregnancy.
  • The corpus luteum that develops in the empty follicle after ovulation produces progesterone. This hormone further prepares the uterus for pregnancy and maintains the pregnancy.
  • The testes produce testosterone that stimulates the development of the male reproductive system and sexual characteristics.

Homeostasis and Feedback Control[edit]

Animals can only survive if the environment within their bodies and their cells is kept constant and independent of the changing conditions in the external environment. As mentioned in module 1.6, the process by which this stability is maintained is called homeostasis. The body achieves this stability by constantly monitoring the internal conditions and if they deviate from the norm initiating processes that bring them back to it. This mechanism is called feedback control. For example, to maintain a constant body temperature the hypothalamus monitors the blood temperature and initiates processes that increase or decrease heat production by the body and loss from the skin so the optimum temperature is always maintained. The processes involved in the control of body temperature, water balance, blood loss and acid/base balance are summarized below.

Summary of Homeostatic Mechanisms[edit]

1. Temperature control[edit]

The biochemical and physiological processes in the cell are sensitive to temperature. The optimum body temperature is about 37 C [99 F] for mammals, and about 40 C [104 F] for birds. Biochemical processes in the cells, particularly in muscles and the liver, produce heat. The heat is distributed through the body by the blood and is lost mainly through the skin surface. The production of this heat and its loss through the skin is controlled by the hypothalamus in the brain which acts rather like a thermostat on an electric heater. .

(a) When the body temperature rises above the optimum, a decrease in temperature is achieved by:

  • Sweating and panting to increase heat loss by evaporation.
  • Expansion of the blood vessels near the skin surface so heat is lost to the air.
  • Reducing muscle exertion to the minimum.

(b) When the body temperature falls below the optimum, an increase in temperature can be achieved by:

  • Moving to a heat source e.g. in the sun, out of the wind.
  • Increasing muscular activity
  • Shivering
  • Making the hair stand on end by contraction of the hair erector muscles or fluffing of the feathers so there is an insulating layer of air around the body
  • Constricting the blood vessels near the skin surface so heat loss to the air is decreased

2. Water balance[edit]

The concentration of the body fluids remains relatively constant irrespective of the diet or the quantity of water taken into the body by the animal. Water is lost from the body by many routes (see module 1.6) but the kidney is the main organ that influences the quantity that is lost. Again it is the hypothalamus that monitors the concentration of the blood and initiates the release of hormones from the posterior pituitary gland. These act on the kidney tubules to influence the amount of water (and sodium ions) absorbed from the fluid flowing along them.

(a) When the body fluids become too concentrated and the osmotic pressure too high, water retention in the kidney tubules can be achieved by:

  • An increased production of antidiuretic hormone (ADH) from the posterior pituitary gland, which causes more water to be reabsorbed from the kidney tubules.
  • A decreased blood pressure in the glomerulus of the kidney results in less fluid filtering through into the kidney tubules so less urine is produced.

(b) When the body fluids become too dilute and the osmotic pressure too low, water loss in the urine can be achieved by:

  • A decrease in the secretion of ADH, so less water is reabsorbed from the kidney tubules and more diluted urine is produced.
  • An increase in the blood pressure in the glomerulus so more fluid filters into the kidney tubule and more urine is produced.
  • An increase in sweating or panting that also increases the amount of water lost.

Another hormone, aldosterone, secreted by the cortex of the adrenal gland, also affects water balance indirectly. It does this by increasing the absorption of sodium ions (Na-) from the kidney tubules. This increases water retention since it increases the osmotic pressure of the fluids around the tubules and water therefore flows out of them by osmosis.

3. Maintenance of blood volume after moderate blood loss[edit]

Loss of blood or body fluids leads to decreased blood volume and hence decreased blood pressure. The result is that the blood system fails to deliver enough oxygen and nutrients to the cells, which stop functioning properly and may die. Cells of the brain are particularly vulnerable. This condition is known as shock.

If blood loss is not extreme, various mechanisms come into play to compensate and ensure permanent tissue damage does not occur. These mechanisms include:

  • Increased thirst and drinking increases blood volume.
  • Blood vessels in the skin and kidneys constrict to reduce the total volume of the blood system and hence retain blood pressure.
  • Heart rate increases. This also increases blood pressure.
  • Antidiuretic hormone (ADH) is released by the posterior pituitary gland. This increases water re-absorption in the collecting ducts of the kidney tubules so concentrated urine is produced and water loss is reduced. This helps maintain blood volume.
  • Loss of fluid causes an increase in osmotic pressure of the blood. Proteins, mainly albumin, released into the blood by the liver further increase the osmotic pressure causing fluid from the tissues to be drawn into the blood by osmosis. This increases blood volume.
  • Aldosterone, secreted by the adrenal cortex, increases the absorption of sodium ions (Na+) and water from the kidney tubules. This increases urine concentration and helps retain blood volume.

If blood or fluid loss is extreme and the blood volume falls by more than 15-25%, the above mechanisms are unable to compensate and the condition of the animal progressively deteriorates. The animal will die unless a vet administers fluid or blood.

4. Acid/ base balance[edit]

Biochemical reactions within the body are very sensitive to even small changes in acidity or alkalinity (i.e. pH) and any departure from the narrow limits disrupts the functioning of the cells. It is therefore important that the blood contains balanced quantities of acids and bases.

The normal pH of blood is in the range 7.35 to 7.45 and there are a number of mechanisms that operate to maintain the pH in this range. Breathing is one of these mechanisms.

Much of the carbon dioxide produced by respiration in cells is carried in the blood as carbonic acid. As the amount of carbon dioxide in the blood increases the blood becomes more acidic and the pH decreases. This is called acidosis and when severe can cause coma and death. On the other hand, alkalosis (blood that is too alkaline) causes over stimulation of the nervous system and when severe can lead to convulsions and death.

(a) When vigorous activity generating large quantities of carbon dioxide causes the blood to becomes too acidic it can be counteracted in two ways:

  • By the rapid removal of carbon dioxide from the blood by deep, panting breaths

By the secretion of hydrogen ions (H+) into the urine by the kidney tubules.

(b) When over breathing or hyperventilation results in low levels of carbon dioxide in the blood and the blood is too alkaline, various mechanisms come into play to bring the pH back to within the normal range. These include:

  • A slower rate of breathing
  • A reduction in the amount of hydrogen ions (H+) secreted into the urine.


Homeostasis is the maintenance of constant conditions within a cell or animal’s body despite changes in the external environment.

The body temperature of mammals and birds is maintained at an optimum level by a variety of heat regulation mechanisms. These include:

  • Seeking out warm areas,
  • Adjusting activity levels,

blood vessels on the body surface,

  • Contraction of the erector muscles so hairs and feathers stand up to form an insulating layer,
  • Shivering,
  • Sweating and panting in dogs.

Animals maintain water balance by:

  • adjusting level of antidiuretic hormone(ADH)
  • adjusting level of aldosterone,
  • adjusting blood flow to the kidneys
  • adjusting the amount of water lost through sweating or panting.

Animals maintain blood volume after moderate blood loss by:

  • Drinking,
  • Constriction of blood vessels in the skin and kidneys,
  • increasing heart rate,
  • secretion of antidiuretic hormone
  • secretion of aldosterone
  • drawing fluid from the tissues into the blood by increasing the osmotic pressure of the blood.

Animals maintain the acid/base balance or pH of the blood by:

  • Adjusting the rate of breathing and hence the amount of CO2 removed from the blood.
  • Adjusting the secretion of hydrogen ionsinto the urine.


Endocrine System Worksheet

Test Yourself[edit]

1. What is Homeostasis?

2. Give 2 examples of homeostasis

3. List 3 ways in which animals keep their body temperature constant when the weather is hot

4. How does the kidney compensate when an animal is deprived of water to drink

5. After moderate blood loss, several mechanisms come into play to increase blood pressure and make up blood volume. 3 of these mechanisms are:

6. Describe how panting helps to reduce the acidity of the blood

Test Yourself Answers


  • Wikipedia. Much, much more than you ever need to know about hormones and the endocrine system but with a bit of discipline you can glean lots of useful information from this site.