Hormonal communication (5.1.4)
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The endocrine system releases hormones into the bloodstream. Hormones are chemical messengers that carry signals to target organs or tissues.
There are different types of hormones:
- Non-steroid hormones are not lipid-soluble; they are first messengers and bind to receptors on a cell’s surface membrane and trigger the release of second messengers.
- Steroid hormones can pass through cell membranes, bind to intracellular receptors, and act as transcription factors that regulate gene expression.
Endocrine glands are ductless glands that secrete hormones (chemical messengers) directly into the bloodstream.
Examples of endocrine glands include the pituitary gland, thyroid, adrenal glands, and pancreas.
Hormonal signalling is typically slow, long-lasting, and can affect the whole body.
After action, hormones are usually broken down by the liver.
Once secreted, hormones enter the bloodstream and are carried to distant parts of the body.
This enables long-distance signalling and coordination between organs (e.g., the pituitary gland and the ovaries). Hormones, therefore, play a role in systemic homeostasis.
The circulation of hormones in the blood means that many tissues are simultaneously exposed to them, but only target cells with specific receptors respond.
Hormones circulate throughout the body, but only affect target cells with the complementary receptors on their cell surface membranes. This ensures specificity, so other cells do not respond.
Target cells may be grouped into target tissues or distributed across the body.
This selective response allows hormones such as adrenaline to trigger effects in multiple tissues, while others, such as prolactin, mainly act on a single tissue.
The adrenal glands are endocrine glands located above each kidney. Each gland has two main regions:
- Adrenal cortex – the outer layer, which produces steroid hormones
- Adrenal medulla – the inner region which produces adrenaline & noradrenaline
Both regions secrete hormones directly into the bloodstream.
Simulated by the pituitary gland, the adrenal cortex produces steroid hormones that regulate gene expression by binding to intracellular receptors. These include:
- Glucocorticoids (e.g., cortisol), which regulate metabolism and help the body respond to stress, stimulating the breakdown of proteins, carbohydrates, and fats into glucose in the liver.
- Mineralocorticoids, which control blood pressure by increasing reabsorption in the kidneys, resulting in water retention.
- Androgens, which help in the production of small amounts of sex hormones.
The adrenal medulla secretes adrenaline and noradrenaline, which are non-steroid (amine) hormones. They help coordinate the fight-or-flight response and have a wide range of effects throughout the body, including changes to:
- heart rate & stroke volume
- blood glucose concentration
- blood flow to muscles, the gut, and skin
- pupil dilation
- dilation of the airway.
The pancreas is located near the liver and has many important endocrine and exocrine functions.
Exocrine tissues are organised into groups of cells called acini, which secrete digestive enzymes into ducts.
These digestive enzymes include: lipase, trypsinogen and amylase.
Pancreatic endocrine tissues are found in the Islets of Langerhans, which are found amongst the acini.
The Islets of Langerhans contain two different types of cells which secrete hormones into the blood:
- cells, which secrete glucagon
- cells, which secrete insulin.
Glucose is a sugar that circulates in the blood and serves as the main respiratory substrate for ATP production. A constant supply is needed, particularly by the brain and muscles.
If blood glucose concentration is too low (hypoglycaemia), insufficient glucose is available for respiration, leading to symptoms such as fatigue, confusion, and, in severe cases, unconsciousness.
If blood glucose concentration is too high (hyperglycaemia), the blood water potential decreases, causing water to leave body cells and enter the blood by osmosis, leading to cellular dehydration.
Blood glucose concentration is the amount of glucose dissolved in the blood. It must be maintained within a narrow range to provide a constant supply of glucose for respiration and to maintain osmotic balance.
Blood glucose concentration regulation is an example of negative feedback:
- When blood glucose concentration rises, insulin is released to lower it.
- When blood glucose concentration falls, glucagon is released to raise it, restoring levels towards the normal range.
An increase in blood glucose concentration causes more glucose to enter pancreatic cells.
The glucose is metabolised in respiration, raising ATP concentration and closing potassium channels. This causes depolarisation of the cell membrane and triggers voltage-gated calcium channels to open.
Calcium ions enter the cell and stimulate exocytosis of insulin–containing vesicles.
Diabetes mellitus is a condition in which the homeostatic control of blood glucose concentration is impaired.
Type I diabetes typically develops in childhood and is caused by an autoimmune response that destroys the cells of the islets of Langerhans in the pancreas. As a result, little or no insulin is produced. Without sufficient insulin, cells take up less glucose from the blood, causing blood glucose concentration to remain abnormally high.
Type II diabetes occurs when body cells become less responsive to insulin due to the reduced sensitivity of insulin receptors – this is called insulin resistance.
As the condition progresses, the pancreas may also produce insufficient insulin to maintain normal blood glucose levels. It usually develops in adults.
Risk factors include obesity, poor diet and lack of exercise.
Without the homeostatic control of blood glucose concentration, blood glucose levels can rise to dangerous levels, potentially causing damage to blood vessels, nerves, kidneys and other organs.
A lack of insulin reduces glucose uptake by muscle and adipose (fat) cells and decreases glycogen synthesis. As a result, cells have less access to glucose for respiration, which can lead to fatigue and weakness.
Treatments for Type I diabetes include:
- Insulin therapy using insulin produced by genetically modified bacteria helps control blood glucose concentration. This may be delivered by injections or an insulin pump.
- Islet cell transplantation replaces damaged cells but is limited by donor availability and the risk of rejection.
- Stem cells may be used in the future to produce new cells or islets of Langerhans, potentially restoring insulin production.
Treatments for Type II diabetes include:
- Lifestyle changes, including a healthy diet, regular exercise and weight loss, can improve insulin sensitivity and may control or reverse early-stage Type II diabetes.
- Medications, such as metformin, improve insulin sensitivity and reduce glucose production by the liver.
- Insulin therapy may be required in later stages if the pancreas can no longer produce sufficient insulin.
Insulin for diabetes treatment can be produced by genetically modified bacteria.
Advantages include:
- Identical to human insulin, making it highly effective.
- Reduced risk of immune reactions compared with animal insulin.
- Lower risk of transmitting animal diseases.
- Cheaper and easier to produce in large quantities.
- Fewer ethical and religious concerns than using insulin extracted from animals.