The body is under the control of two systems: the nervous system and the endocrine system. Co-ordination by the nervous system is usually rapid and precise. Nerve impulses travelling at up to 100 meters per second, are delivered to specific parts of the body and produce an almost immediate response. A different type of co-ordination is bought about by the endocrine system. This system depends on hormone a chemical substance produced by a gland and carried by the blood which alters the activity of one or more specific target organs. They affect cellular metabolism and coordinate the growth, development and activity of an organism.
Glands are classified into two groups: exocrine glands and endocrine glands. Exocrine glands are glands that secrete their products via ducts e.g. sweat glands and salivary glands. Endocrine glands are glands that secrete their products directly into the bloodstream e.g. pituitary gland, thyroid gland, adrenal gland and the gonads.
After being secreted hormones do not remain permanently in the blood but are changed by the liver into inactive compounds and excreted by the kidney. Insulin for example may stay in the bloodstream for just 4-8 hours before being broken down. Some glands are both exocrine and endocrine e.g. the pancreas, which secretes pancreatic juice via the pancreatic duct, and insulin and glucagon from the islets of Langerhans into the blood stream. Table below compares control by the endocrine and nervous system.
Endocrine and nervous control compared
| Feature | Endocrine | Nervous |
|---|---|---|
| Made up of | Secretory cells | Neurones |
| Form of transmission | transmission of chemicals | transmission of electrical impulses |
| Transmission pathway | transmission via blood | transmission in nerves |
| Speed of transmission | slow transmission | rapid transmission |
| Response | hormones dispersed throughout body | impulse sent directly to target organ |
| Duration of effect | long-term effects | short-lived effects |
Many hormones affect long-term changes such as growth-rate, puberty and pregnancy. Nerve impulses often cause a response in a very limited area of the body such as an eyeblink or a finger movement. Hormones often affect many organ systems at once.
Serious deficiencies or excesses of hormone production give rise to illnesses. Small differences in hormone activity between individuals probably contribute to differences of personality and temperament.
Unlike the digestive glands, endocrine glands do not deliver their secretions through ducts (tubes). For this reason, the endocrine glands are sometimes called ‘ductless glands’. The hormones are picked up directly from the glands by the blood circulation. Responses of the body to hormones are much slower than responses to nerve impulses.
Key definition
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity or one or more specific target organs
Types of hormones
| Hormone | Source | Effect |
|---|---|---|
| ADH | Pituitary | Regulated blood osmolarity |
| Adrenaline | Adrenal glands | Increases heart rate and breathing rate during exercise (more O2 for respiration) |
| Insulin | Pancreas | Decreases blood glucose level after a meal |
| Testosterone | Testes | Triggers puberty in boys |
| Progesterone | Ovaries | Maintains uterus lining and causes menstruation |
| Oestrogen | Ovaries | Triggers puberty in girls. Stimulates growth of uterus lining each month and causes ovulation |
The pituitary gland
The hypothalamus is a small area of the brain that monitors hormone levels and indirectly controls functions including body temperature, hunger and sleep. It links the hormonal and nervous systems and secretes releasing hormones that regulate the hormones of the anterior pituitary gland. It has a range of receptors of its own, which allow it to act independently, but it also receives information from other parts of the brain.
The pituitary gland, situated just below the hypothalamus is made up of two different parts – the anterior and posterior lobes. The posterior lobe develops from the brain and has nerurons connecting it directly to the brain. The anterior lobe develops separately and has no direct neural connection with the brain. The hypothalamus has to communicate with each lobe of the pituitary gland in a different way.
- The hypothalamus contains the cell bodies of many neurosecretory cells, which have their terminal ends in the posterior lobe of the pituitary. A neurosecretory cell is simply a neuron that has been modified to secrete and store a large quantity of hormone at the terminal end of the cell body. Surrounding the terminal ends of the neurosecretory cells is a capillary network so that when the cells receive the appropriate information they can release the hormone directly into the blood. Two examples of posterior lobe hormones released in this way are antidiuretic hormone (ADH) and oxytocin.
- Control of the anterior lobe of the pituitary is regulated by another set of neurosecretory cells in the hypothalamus. These cells end in a different capillary bed just above the pituitary gland. The blood from these capillaries flows into a portal vein, which passes into capillaries within the anterior lobe of the pituitary. These neurosecretory cells secrete releasing hormones (RH), which control the release of the hormones from the cells of the anterior lobe. One example is gonadotrophin releasing hormone (GnRH), which controls the release of follicle stimulating hormone (FSH) and luteinising hormone (LH) that induces ovulation.
Adrenal glands
These glands are attached to the back of the abdominal cavity, one above each kidney. One part of the adrenal gland is a zone call the adrenal medulla. The medulla receives nerves from the brain and produces the hormone adrenaline. Adrenaline is quickly converted by the liver to a less active compound, which is excreted by the kidneys. All hormones are similarly altered and excreted, some within minutes, others within days. Thus their effects are not long-lasting. The long-term hormones, such as thyroxine are secreted continuously to maintain a steady level.
Adrenaline has obvious effects on the body:
- In response to a stressful situation, nerve impulses are sent from the brain to the adrenal medulla, which releases adrenaline into the blood.
- It’s presence causes breathing to become faster and deeper. This may be particularly apparent as we pant for breath.
- The heart beats faster, resulting in an increase in pulse rate. This increase in heart rate can be quite alarming, making us feel as if our heart is going to burst out of our chest.
- The pupils of our eyes dilate, making them look much blacker.
These effects all make us more able to react quickly and vigorously in dangerous situations (known as ‘fight or flight situations’) that might require us to run away or put up a struggle. However in many stressful situations, such as taking examinations or giving a public performance, vigorous activity is not called for. So the extra adrenaline in our bodies just make us feel tense and anxious.
Responses to adrenaline
| Target organ | Effects of adrenaline | Biological advantage | Effect of sensation |
|---|---|---|---|
| heart | beats faster | sends more glucose and oxygen to the muscles | thumping heart |
| breathing centre of the brain | faster and deeper breathing | increased oxygenation of the blood; rapid removal of carbon dioxide | panting |
| arterioles of the skin | constricts them | less blood going to the skin means more is available to the muscles | person goes paler |
| arterioles of the digestive system | constricts them | less blood for the digestive system allows more to reach the muscles | dry mouth |
| muscles of alimentary canal | relax | peristalsis and digestion slow down; more energy available for action | hollow feeling in stomach |
| muscles of body | tenses them | ready for immediate action | tense feeling, shivering |
| liver | conversion of glycogen to glucose | more glucose available in blood for energy production, to allow metabolic activity to increase | no sensation |
| fat deposits | conversion of fats to fatty acids | fatty acids available in blood for muscle contraction | no sensation |
The pancreas
The pancreas is a digestive gland that secretes enzymes into the duodenum through the pancreatic duct. It is also an endocrine (ductless) gland. Most of the pancreas cells produce digestive enzymes but some of them produce hormones. The hormone-producing cells are arranged in small isolated groups called islets and secrete their hormones directly into the bloodstream. One of the hormone is called insulin.
Insulin controls the levels of glucose in the blood by instructing the liver to remove the sugars and store them. This happens when levels get too high, such as after a meal rich in carbohydrate.
Reproductive organs
The ovaries and testes produce hormones as well as gametes (sperm and ova). One of the hormones from the ovary, oestrogen, prepares the uterus for the implantation of the embryo, by making its lining thicker and increasing its blood supply.
The hormones testosterone (from the testes) and oestrogen (from the ovaries) play part in the development of the secondary sexual characteristics.
Thyroid gland
Thyroxine is a hormone released by the thyroid gland, which is in the neck. It plays an important role in regulating the basal metabolic rate, the speed at which chemical reactions in the body occur while the body is at rest. Thyroxine is also important for loads of processes in the body, such as stimulating protein synthesis for growth and development. Thyroxine is released in response to thyroid stimulating hormone (TSH), which is released from the pituitary gland.
A negative feedback system keeps the amount of thyroxine in the blood at the right level, when the level of thyroxine in the blood is higher than normal, the secretion of TSH from the pituitary gland is inhibited. This reduces the amount of thyroxine released from the thyroid gland, so the level in the blood falls back towards normal.
Control of blood glucose content
The control of glucose concentration in the blood is a very important part of homeostasis. Two hormones insulin and glucagon control blood glucose levels. Both hormones are secreted by the pancreas and are transported to the liver in the bloodstream.
- Eating foods containing carbohydrates puts glucose (a type of sugar) into the blood from the gut.
- The normal metabolism of cells remove glucose form the blood.
- Vigorous exercise removes much more glucose from the blood.
- Excess glucose can be stored as glycogen in the liver and in the muscles.
- The level of glucose in the blood must be kept steady. Changes are monitored and controlled by the pancreas, using the hormone insulin and glucagon in a negative feedback cycle.
When blood glucose level are too high or too low a person may:
- lose consciousness
- fall into a coma
- die
Too little glucose ⤏ Cells can not release enough energy they need. Brain cells are especially dependent on glucose for respiration, and die quite quickly if they are deprived of it.
Too much glucose in the blood ⤏ water moves out of cells and into the blood by osmosis ⤏ cell has too little water to carry out normal metabolic process.
Diabetes mellitus
Diabetes mellitus is a condition in which the body does not produce sufficient insulin or does not respond to insulin. The excess glucose cannot be completely reabsorbed by the kidneys are excreted in the urine.
Symptoms include:
- A persistent high blood glucose concentration
- Presence of glucose in the urine
- Excessive urination, excessive thirst and weight loss
Diabetes can cause:
- Poor immune response – increased susceptibility to infections
- Damaged blood vessels leading to vision loss and a decreased sensation in the limbs
- Kidney failure and heart failure
Diabetic individuals can control their disease by receiving regular injections and controlling their carbohydrate intake.
Use of hormones in food production
Farmer sometimes use hormones to make their animals grow faster, or to produce more of a particular product. One hormone used in this way is called bovine somatotropin, or BST.
BST is a hormone which is naturally produced by cattle. However, if cows are given extra BS, they make more milk. Some people think it would be a good idea to give cow BST, to get higher milk yields. You would need fewer cows to get the same amount of milk.
Here are arguments against it
- Some people are worried about drinking milk form cows treated with BST. They think BST might damage their health. This is very unlikely because the hormone does not get into the milk in any significant quantity.
- It is difficult to see why we need BST. For example, the European Union already produces more milk than it needs, so milk quotas have to be imposed to stop farmer from producing too much milk.
- There are concerns that the BST might harm the cows. Cows treated with BST make very large amount of milk, far beyond the ‘natural’ levels which they produce. This make them more likely to get infections of their udders (breast), and may make them feel less comfortable.
- There are concerns that the BST might harm the cows. Cows treated with BST make very large amount of milk, far beyond the ‘natural’ levels which they produce. This make them more likely to get infections of their udders (breast), and may make them feel less comfortable.
