5.2-5.3

5.2 Homeostatic mechanisms in plants

Maintaining Equilibrium

An animal is able to bring out balance or equilibrium in its internal environment by coordinating a number of systems, such as circulatory, respiratory, immune, digestive, and excretion systems, and by changing its behaviour.

HOMEOSTASIS

Homeostasis is the maintenance of a stable internal environment within an organism.

When an organism is healthy and functioning well, its systems are in homeostasis. Homeostasis is achieved by a variety of mechanisms that respond to keep internal environments within certain limits. This maintains conditions to an optimum level when the internal or external environment changes.

Animals coordinate the activities of their cells, tissues and organs so that responses happen in an integrated, and controlled manner.

Detecting and responding to a stimulus requires an effective internal communication system. Communication in animals is achieved by hormonal and nervous system mechanisms, which transmit information between different parts of the organism which transmit information between different parts of the organism and translate environmental disturbances into signals that can be interpreted and responded to. E.g, the iris of the human eye detects light and responds by dilating or constricting to regulate the amount of light that enters the eye.

Two most important systems in maintaining homeostasis in animals are:

• The endocrine system (produces hormones)

• The nervous system

Physiological and behavioural responses

• Short-term and long-term regulation of growth

• Maturation and reproduction

• Homeostatic regulation of the internal environment

Nervous system provides rapid response to:

• Produce efficient coordinated movement to detect and avoid predators.

• Find and capture prey

Although in many ways hormonal systems and nervous systems appear distinctly different, they share one common feature: they both involve chemical communication. Chemical communication involves signals being passed from one cell to the next by the release of specific signalling molecules, known as hormones and neurotransmitters. Hormones are released from glands or other tissues, and neurotransmitters are released from nerve endings. These molecules exert their effects by highly specific interactions with a receptor on, or within, the responding or target cell. Receptors are specialised structures that can detect a specific stimulus and initiate a response.

A response is a physiological or behavioural change in an organism as a result of receiving a stimulus.

Feedback loops

Feedback loops may involve the endocrine and nervous systems working together to regulate the internal environment.

Negative feedback loops promote stability in the internal environment and maintain homeostasis by responding to changes in the body and adjusting the variables to their original or optimal state. They are stimulus-response mechanisms in which the response produced reduces the effect of the original stimulus by reversing its direction. For example, if the concentration of a substance in the blood is too high, a negative feedback loop will lower the concentration. If the concentration is too low, a negative feedback loop will increase the concentration.

Most feedback loops in biological systems are negative.

Negative feedback loops are called negative because the information produced by the feedback causes a reversal of the size or effect of the stimulus. Negative feedback loops maintain stability through the action of the nervous or hormonal systems, or both acting together.

An example of a negative feedback loop is the regulation of blood glucose levels by the hormone insulin. When blood glucose levels are high, receptors detect the change and the pancreas secretes insulin. This lowers the blood glucose levels until homeostasis is reached, at which point the pancreas stops releasing insulin.

A negative feedback loop acts as follows:

1 The system is in a stable state. A change (stimulus) occurs.

2 The change is detected by an appropriate receptor.

The receptor sends a signal to a control centre (hypothalamus or transmission molecules) via the efferent pathway (carries signals to the central nervous system).

4 The control centre sends a signal to an appropriate effector or a specific effector cell, tissue or organ via the efferent pathway (carries signals away from the central nervous system).

The effector responds to the signal, and the original state is restored.

The hypothalamus is known as the control centre of the brain—it receives information from all parts of the body and regulates the internal environment through the secretion of hormones.

An effector is a cell or tissue that responds to a stimulus.

In the control centre, information from sensory receptors is received and compared with a set-point (the optimal value for the functioning of that organism).

This information is processed with other information about the state of the organism, and an appropriate response is initiated.

Regulation therefore involves fluctuations around the set-point. The size of the fluctuations depends on:

• the sensitivity of the receptor

• the tolerance of the control centre to variation from the set-point

• the efficiency of the effector.

Some features of the internal environment, such as blood glucose levels, can vary considerably; others, such as body temperature in mammals, are tightly controlled.

In contrast to negative feedback loops, positive feedback loops force an organism out of homeostasis by maintaining the direction of the stimulus, and sometimes increasing the stimulus.

An example of a positive feedback loop is uterine contractions during childbirth. The hormone oxytocin stimulates the uterus to contract, causing pain. Rather than the nervous system signalling the endocrine system to lower the oxytocin and reduce the pain, more oxytocin is produced to stimulate stronger contractions. The contractions work to push the baby into the birth canal and continue until the baby is born.

Receptors—detecting external and internal stimuli

Animals have sensory receptors to detect aspects of their environment that may affect their ability to survive and reproduce. The types of sensory receptors present and their sensitivity differ substantially between animals, and are related to the way animals have adapted to their environments. For example: a wombat has less visual acuity for distinguishing small objects than an eagle; dogs use chemical scents much more than humans; some moths have chemoreceptors that can detect a single molecule of pheromone; and platypuses can detect weak electric currents. Some animals respond to different parts of the electromagnetic spectrum-for example, snakes can detect infrared radiation and bees see ultraviolet light.

In humans, the five senses (vision, hearing, taste, smell and touch) are perceived through sense organs (eyes, ears, tongue, nose and skin) that collect and process sensory information. Receptors that detect external stimuli are known as exteroreceptors. These are usually located close to the surface of the body and detect stimuli such as pain and pressure. Some receptors detect internal states, such as blood pressure and blood chemistry (e.g. oxygen and carbon dioxide levels), and are known as interoceptors or visceral receptors. From a functional point of view, the types of sensory receptors can be classified as photoreceptors (vision), chemoreceptors (taste, smell, communication), mechanoreceptors (hearing, balance, pressure, touch) and thermoreceptors (temperature)

REGULATION OF BODY TEMPERATURE

The maintenance of core body temperature within a specific range is called thermoregulation.

Regulation of body temperature in humans involves a complex negative feedback pathway with several sensory inputs and many effector responses that act together to maintain a stable body temperature.

The control centre for measuring the body temperature set-point (37°C) is in the hypothalamus.

A change in the temperature of the hypothalamus initiates regulatory responses that can reduce heat loss or initiate heat production or heat exchange.

REGULATION OF BLOOD GLUCOSE

Blood Glucose Level, also known as blood sugar level, is the concentration of glucose in the blood of mammals, incl. humans. This level is constantly changing in your body, and is tightly regulated by homeostatic mechanisms. Glucose is the main source of energy for your body’s cells. Eat carbs and physical exercise changes the levels.

More carbs = more insulin produced

Glucose is stored in the body in the form of a polymer called glycogen.

When the body needs glucose, glycogen is broken back into usable glucose for cellular respiration, which is the energy-producing reaction in cells.

If BGL is not maintained within optimum range, hyperglycaemia (BGL too high) or hypoglycemia (BGL too low) can develop. Lots of helt problem - can die.

Detecting temperature change

Regulation of temperature in humans is an example of the way different sensory receptors work together to produce an integrated response. Arterial blood has the most constant temperature. The relatively constant temperature of many other parts of the body indicates that they are well-supplied with arterial blood.

In endotherms (e.g. mammals and birds), a group of temperature-sensitive cells in the hypothalamus act as misalignment detectors, triggering homeostatic responses if blood temperature deviates from the optimal temperature range, or set-point. Lowering or raising the temperature of the hypothalamus initiates regulatory changes in heat production or heat exchange.

Temperature receptors are also found in the skin. A decrease in environmental temperature detected by these receptors will initiate regulatory responses such as decreased blood flow to the skin to reduce heat loss, and behavioural changes such as moving into a warmer or more sheltered environment. These responses take place long before there has been any change in the internal temperature of the body.

Skin temperature receptors act as disturbance detectors, detecting changes in the external environment and triggering responses before there is a change in core body temperature.

As the environmental temperature falls, disturbance detectors stimulate responses that reduce heat loss and increase heat production.

The reverse occurs as environmental temperature increases. If the arterial temperature falls despite the regulatory responses that have been initiated, or if it rises because the responses made have been too effective, these changes will be detected by the misalignment receptors in the brain, which will fine-tune the temperature-regulating mechanisms.

The value of the disturbance detectors in the skin is to reduce fluctuations in arterial blood temperature, providing a more precise control around the set-point level than there would be if misalignment detectors (the brain's temperature receptors) alone were involved.

Heat loss

Organisms are constantly exchanging heat with their environment. This heat exchange occurs through four mechanisms:

• conduction-Occurs when the temperature of the organism and the environment are different. Heat exchange is a result of direct contact (e.g. a lizard basking on a warm rock).

• convection—Transmission of heat from a warmer region to a colder region, resulting from the movement of liquid or gas (e.g. heat is lost from the body surface by convection, which transfers heat through the movement of liquid or air).

• radiation— Occurs all the time, without direct contact, regardless of temperature differences between the organism and their environment (e.g. heat radiating from dark coloured surfaces).

• evaporation-Heat loss by water evaporation. This occurs most rapidly when the air is hot and dry (e.g. sweating).

Responding to cold

A number of nervous and endocrine responses occur rapidly to reduce heat loss from the body and increase heat production when the body becomes too cold.

The following involuntary responses reduce heat loss from the body:

• vasoconstriction (constriction of the blood vessels in the skin)—This reduces heat loss from the skin, as the amount of blood moving close to the exposed surface is reduced

piloerection-The constriction of the piloerector muscles around hair follicles ('goose bumps"), which increases the insulating effect of the hairs (Figure 5.2.8).

This response has a minimal effect in humans but in animals with thick fur, the layer of trapped air increases significantly and reduces heat loss from the body. shivering thermogenesis The production of metabolic heat is increased through shivering. This involuntary movement of the muscles generates large amounts of heat. Shivering thermogenesis is stimulated by adrenaline.

• non-shivering thermogenesis in brown fat (brown adipose tissue or BAT)—

Increased cellular activity in BAT, a tissue specialised for heat production, causes the tissues to warm up. The heat produced is carried to other parts of the body in the blood. Brown fat contains many mitochondria (which give it its brown colour), fat-metabolising enzymes and an extensive vascular network. Brown fat is capable of high rates of aerobic metabolism using a pathway that breaks down fats to produce large amounts of heat, but little ATP (energy). This mechanism is crucial in many baby animals and hibernators. increasing metabolism (the rate of cellular respiration)—Metabolic processes in the internal organs are the main source of heat when the organism is at rest (Table 5.2.3). In humans, around 60% of the energy released during cellular respiration is transformed into thermal energy. In humans, the overall metabolic rate, and therefore the rate of heat production, is controlled by hormones. thyrotropin releasing hormone (TRH) secretion by the hypothalamus— TRH acts on the anterior pituitary to secrete thyroid stimulating hormone (TSH). As the name suggests, TSH acts on the thyroid gland to release thyroid hormones, triiodothyronine (T3) and thyroxine (T4) (Figure 5.2.9). T3 and T4 hormones regulate metabolic processes, increasing heat production and body temperature. The amount of T3 and T4 in the bloodstream is regulated by the pituitary gland via a negative feedback loop; if there is too much or too little T3 or T4, the pituitary gland reduces or increases the amount of TSH it secretes. This mechanism allows a very delicate regulation of the level of thyroid hormones in the blood.

Responding to heat

As well as responding to cooler temperatures, animals can adjust their internal environment in response to high external temperatures. Many of the responses to heat work in the opposite way to the responses to cold temperatures. Some of the processes that take place in animals when it is hot include:

• evaporative coolingThis is a very effective way of losing heat energy from the body as water (sweat) changes to water vapour. Sweat glands are distributed over much of the human body and release sweat onto the skin surface via pores when your body temperature rises. Most animals do not sweat in the same way as humans, but many use panting, where their breathing is fast but shallow, to increase evaporative cooling. Spraying water on your skin produces the same evaporative cooling effect as sweating.

• vasodilation (dilation of the blood vessels in the skin)-This means more blood is sent to the extremities. Heat is lost to the environment by radiation and convection, especially if it is windy. Furry animals often have areas of bare skin that are rich in blood vessels in order to allow vasodilation to take place.

Animals may also change their behaviour to release excess heat or to avoid increasing their body temperature. Some examples include changing body shape or increasing surface area (e.g. standing with your arms outstretched), swimming or bathing in cool water, seeking shade during the hottest parts of the day and decreasing physical activity)

REGULATION OF BLOOD GLUCOSE

Blood glucose level (BGL), sometimes called blood sugar level, is the concentration of glucose in the blood of mammals, including humans. This level is constantly changing in your body and is tightly regulated by homeostatic mechanisms. Glucose is the main source of energy for your body's cells. Eating carbohydrates and doing physical activity will change blood glucose levels throughout the day. The more carbohydrates you eat, the more insulin you will produce. Glucose is stored in the body in the form of a polymer called glycogen.

When the body needs glucose, the glycogen is broken back down into usable glucose for cellular respiration, which is the energy-producing reaction in cells. If BGL is not maintained within its optimal range, hyperglycaemia (BGL too high) or hypoglycaemia (BGL too low) can develop, leading to a range of long-term health problems, such as diabetes.

Detecting blood glucose level change

Cells in the pancreas detect changes in BGL. Blood glucose concentration is regulated so it remains within a range of about 3.5-8 mmol/L. A deviation from these levels in either direction will result in a response by clusters of specialised cells in the pancreas, called the islets of Langerhans. These cells detect blood glucose levels and release insulin and glucagon to maintain blood glucose levels within the normal range. Glucagon is a hormone that stimulates the conversion of glycogen to glucose, which raises BGL.

There are also glucose-sensing neurons in the hypothalamus in the brain.

Scientists believe that these glucose-sensing neurons play a key role in food intake, thus helping to regulate blood glucose concentrations.

Responding to high blood glucose levels

When glucose levels rise above about 5 mmol/L, the islets of Langerhans in the pancreas release insulin. Insulin increases the conversion of glucose to glycogen, fats or fatty acids for storage in the liver and skeletal muscles. The overall effect of insulin is to lower BGL

Responding to low blood glucose levels

When glucose levels fall below about 5 mmol/L, the islets of Langerhans in the pancreas release glucagon. Adrenaline also raises BGL by its actions on fat cells and the liver.

REGULATION OF WATER BALANCE

The maintenance of an internal balance between water and solute concentrations is called osmoregulation.

Animals that control their cells’ solute concentrations and maintain a stable environment are known as osmoregulators.

Most animals are osmoregulators

Animals that conform to changes in their external environment matching their solute concentrations in their cells to their surroundings, are known as osmoconformers.

Marine invertebrates e.g jellyfish

Maintaining a water balance is necessary to control salt concentrations.

Salts form ions in solutions, and cells require the concentration of ions to be within tight limits to ensure biochemical processes can occur efficiently.

Some ions (e.g hydrogen carbonate ion) are also important for regulating the pH of body fluids. Should be at a suitable level for enzymes and molecules to function efficiently.

Maintaining the correct concentration of ions is done by regulating water and salt balance.

Water balance involves regulating the loss and intake of water and salts.

In organisms, net movement of water occurs as a result of osmosis which is regulated by solute concentrations

Water moves across a semipermeable membrane from regions of low solute concentration (high concentration of free water molecules ) to an area of a high solute concentration (low concentration of free water molecules).

Amount of water lost and gained differs in individuals and depends on exercise.

Urination rather than water intake is a better indicator of whether an individual has good water balance.

Water gain and water loss

Total volume of water and fluid taken into the body depends on diet and activity levels.

• Varies from from 2 - 16 L

Minimum water requirement for fluid replacement in a 70kg person is about 3000 mL per day.

• Some obtained from eating (400-600mL), and cellular respiration(400mL)

• Other obtained from drinking water (2000-2200mL)

Salt Gain and Loss

Salt intake varies on diet.

Three major groups of salt in human diet

• Sodium salts

• Potassium salts

• Calcium salts

• Daily sodium intake (1-10g per day)

• Daily potassium intake (2-4g per day)

• Calcium salt intake (up to 2.4 g per day)

Hormonal Control of Water balance

Water and solute concentration is monitored by osmoreceptors in the hypothalamus and baroreceptors in the atria of the heart.

Osmoreceptors are sensitive to blood solute concentrations, while baroreceptors detect changes in blood pressure, which is an indicator of the volume of blood.

Collectively, these receptors detect the solute concentrations in blood and extracellular fluid. The unit of measurement for these blood concentrations is osmolarity, because they contribute to osmotic effects on cells.

Osmolarity is a measure of the concentration of particles (e.g sodium and chloride ions) that affect osmosis.

Because plasma membranes are permeable to water, the osmolarity in the extracellular fluid is about the same as it is in the intracellular fluid (cytosol).

Changes in the osmolarity of the extracellular fluid will therefore affect cytosol concentrations, which can cause problems with cellular metabolic rates. Compared to extracellular fluid, the cytosol of cells is high in potassium and magnesium and low in sodium and chloride ions.

Antidiuretic hormone (ADH) also called vasopressin, regulates water reabsorption. It is synthesised in the hypothalamus and transported to the posterior pituitary gland, where it is stored. When osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood, a signal is sent to the posterior pituitary gland and ADH is released.

ADH acts on the kidneys to increase the permeability to water of the distal tubules and collecting ducts. The collecting duct runs through the medulla of the kidney, which has high salt levels (and therefore a higher osmotic potential). This causes the absorption of water from the tubules back into the blood by osmosis, decreasing urine output; the urine becomes more concentrated and has a darker yellow colour. As the blood returns to a normal concentration, negative feedback stops the production of ADH.

Conversely, if the osmoreceptors detect an decrease in osmolarity (e.g if too much water is taken into the body), ADH release will be stopped. This reduces the reabsorption of water and consequently increases urine volume; the urine becomes more dilute and has a paler yellow colour.

A number of substances such as, nicotine, alcohol and narcotics can interrupt the feedback control of water balance in the body. This can also occur because of pain, stress or hypothermia (lowered body temperature)

Changes in blood osmolality or blood pressure can also stimulate counteracting response. Initially an enzyme called renin is secreted from the kidneys in response to these changes Renin then triggers a series of reactions involving other hormones that result in the release of aldosterone from the adrenal glands located above the kidney. Aldosterone simultaneously regulates sodium and potassium levels by increasing potassium excretion into the urine causing sodium reabsorption into the blood. This causes more water to be drawn into the blood by osmosis, thus increasing blood volume and pressure.

A lack of aldosterone can result in low sodium levels, high potassium levels, and high acid levels in the blood. These are potentially dangerous conditions. People with aldosterone suffer from Addison’s disease and must take a synthesised hormone called fludrocortisone acetate.

Homeostasis

Regulation of blood glucose

• Homeostasis is the maintenance of a stable internal environment within an organism.

• Regulation in animals involves internall communication by the endocrine (hormone) and nervous systems to integrate and coordinate the activities of cells, tissues, organs and systems.

• In both endocrine and nervous systems, signals are passed from one cell to the next by chemical communication —the release of signalling molecules (hormones and neurotransmitters) and their detection by matching receptors on the target cells.

• The nervous system provides rapid responses to produce efficient coordinated movement.

• Hormones are specific, effective in low concentrations • Adrenaline acts on: and they produce responses that are generally slower and more indirect than nervous responses.

Feedback systems

• Negative feedback loops are stimulus-response

mechanisms that respond to changes in the body by • Water enters body cells throughout the day from adjusting variables back to their original or optimal state, reversing the direction of the stimulus.

• Positive feedback loops are the opposite of negative

feedback loops. They promote a process rather than • Osmoreceptors in the hypothalamus and reversing the effect of the stimulus.

Regulation of body temperature

• A change in the temperature of the hypothalamus initiates regulatory responses that can involve heat production or heat exchange.

• Temperature receptors are found in the skin and the hypothalamus.

• Heat is lost from the body by conduction, convection, radiation and evaporation.

• Responses to cold environmental temperatures

Include:

- vasoconstriction

- piloerection

- shivering thermogenesis

- non-shivering thermogenesis

- increasing metabolism

- TRH secretion by the hypothalamus.

• Responses to hot environmental temperatures include evaporative cooling and vasodilation.

• Blood glucose levels are detected by receptor cells in the pancreas and neurons in the hypothalamus.

• When glucose levels rise, insulin is released from the beta cells in the islets of Langerhans in the pancreas.

Insulin causes a decrease in BGL by acting on a number of tissues to:

- increase conversion of glucose to fat in fat cells

- increase uptake of glucose in muscle and fat cells

- increase conversion of glucose to the storage compound glycogen for storage in the liver.

• When glucose levels decrease, glucagon is released from alpha cells in the islets of Langerhans and stimulates the conversion of glycogen to glucose.

- skeletal muscle and the liver to increase breakdown of glycogen to glucose

- fat cells to increase fat breakdown for energy.

Regulation of water balance

drinking, eating and cellular respiration.

Water is mainly lost from the body in urine, faeces,

sweat and from the lungs.

baroreceptors in the atria of the heart detect the osmolality of the blood.

• An increase in blood osmolality causes:

- release of ADH from the pituitary; ADH acts on the kidney to increase water absorption back into the blood

- increase in urine concentration and decrease in urine output.

• A decrease in blood osmolality causes:

- a decrease in ADH levels

- an increase in urine volume.

• Low blood volume stimulates the secretion of aldosterone, involving the following steps:

- renin is secreted from the kidneys

- renin causes release of aldosterone

- aldosterone causes absorption of sodium into the blood

- aldosterone causes potassium excretion into the urine

- blood volume and blood pressure increase.

5.3 Malfunctions in homeostatic mechanisms

The regulation of the internal environment, and thus the functionality and health of your body, is maintained by homeostatic mechanisms. These mechanisms are extremely sensitive to changes in internal conditions. However, homeostatic mechanisms can malfunction because of factors such as genetic disorders, ageing, poor nutrition, insufficient physical activity or exposure to harmful substances.

A malfunction in a homeostatic mechanism causes an imbalance and a subsequent oversupply or undersupply of substances to cells. Many diseases are associated with malfunctions in homeostatic mechanisms.

Hormonal balance plays a key role in regulating the internal environment. The endocrine system makes, stores and releases hormones, which act as chemical messengers that trigger and direct functions in the body. The endocrine system controls vital functions in growth and development, metabolism, reproduction and tissue repair, among many others. Malfunctions in the endocrine system often lead to a disruption in a homeostatic mechanism, which can have an adverse effect on the body

Diseases of the endocrine system fall into three groups

• Hypersecretion (oversupply of hormones)

• Hyposecretion (undersupply of hormones)

• Cancers of endocrine glands

An autoimmune disease occurs when a person's immune system mistakenly targets the body's own cells.

Examples of autoimmune diseases are type 1 diabetes, Graves' disease and multiple sclerosis.

TYPE 1 DIABETES

Type 1 diabetes is caused by a malfunction of the pancreas, which leads to a deficiency in insulin secretion. It is an autoimmune disease in which the body’s immune system destroys the insulin-producing beta cells in the islets of Langerhans in the pancreas. Insulin allows the muscles, liver, and fat cells to absorb glucose from the blood and store it until the body needs energy, for example, in between meals. Treatment of type 1 diabetes involves artificially increasing the insulin supply by injections or an insulin pump.

The onset of type 1 diabetes symptoms often occurs in childhood to early adulthood, and there is currently no cure or way of preventing the disease. People with type 1 diabetes must monitor their blood glucose levels (BGL) every day, and inject, or pump insulin into their bodies every day. Type 2 diabetes is also a disturbance of glucose homeostasis. In this disorder the pancreas still makes insulin, but the body’s cells do not respond. This form of diabetes often appears later on in life, but is increasingly common in young people, and is more common in those who are overweight or obese and live a sedentary lifestyle. Can be managed by diet, exercise and medication.

Causes of type 1 diabetes

Scientists are unsure of what causes the beta cells in the islets of Langerhans in the pancreas to be destroyed in type 1 diabetes. There is some evidence for a link between the Coxsackie A and B4 viruses (which are common in children) and the onset of the autoimmune disease. Other childhood viruses, including enterovirus, mumps,polie, and rubella have also been suggested as triggers for type 1 diabetes. Without functioning beta cells, the body cannot secrete the insulin into muscle and fat. This causes blood glucose levels to increase to dangerously high levels.

Symptoms of type 1 diabetes

Insulin deficiency results in hyperglycemia (high blood glucose levels) and accelerates the breakdown of fat for the body to use as energy. Symptoms of the disease include

• Glucose in the urine

• Increased urine production

• Excessive thirst

• Excessive hunger

• Ketossi

• Weight loss

• Fatigue

• Blurred vision

• Irritability

• Muscle cramps

• Skin infections

• Delayed wound healing

• Tingling or numbness in the feet

Longer term consequences are kidney and eye disease. All of these symptoms occur because of the elevated levels in the blood, Glucose is excreted in the urine because the raised blood glucose levels exceed the filtration capacity of the kidneys (normally the kidneys prevent glucose from entering the urine). Glucose escaping the nephron tubules draw in more water, by osmosis, increasing the volume of urine produced. As a result, more frequent urination leaves the body dehydrated and feeling thirsty. The presence of glucose in the urine is a simple test for diabetes.

Dehydration can lead to blurred vision as the lens loses moisture and the blood vessels are damaged. This can result in blindness if left untreated. The raised glucose levels in blood cause chemical reactions with molecules on the surface of neurons and cells lining the small blood vessels. The resulting damage to the body’s nerves can result in loss of sensation in limbs, while damage to capillaries contributes to kidney malfunction and eye diseas (diabetic retinopathy).

Management of type 1 diabetes

Artificial insulin

As well as managing their diet, people with type 1 diabetes must receive insulin artificially. This is usually by injection. Blood glucose levels are monitored by pricking a finger and testing a small drop of blood with a blood glucose metre or a chemical strip.

Alternatively, an electrode placed under the skin and connected to a continuous glucose monitoring device warns a person when their glucose level is reaching a high (or low) level The monitor can be coupled to an electronic pump that delivers insulin when blood glucose levels reach a predetermined level.

An improved system called an 'artificial pancreas' is currently being tested in several countries, including Australia. This system uses a monitoring and feedback system to deliver insulin as the body requires it, in the same way that the pancreas produces insulin.

Transplants

Transplants

Pancreas transplants from deceased donors are usually given to patients with serious complications from diabetes. Human pancreas cells can also be transplanted into a patient's liver, where they begin to produce insulin. This process is called pancreatic islet transplantation. Although it is still in the experimental stage, it may become widely available in the next few years. Recipients of pancreas or pancreatic islet transplants must take immuno-suppressant drugs for the rest of their lives to prevent their bodies from rejecting the transplanted organ. These drugs can have side effects such as high blood pressure, fatigue, and increased risk of bacterial and viral infections.

Gene therapy

Gene therapy, in which the gene that codes for insulin is inserted into the patient's cells, is a potential future treatment for diabetes. Trials in the USA have been successful in diabetic rats, targeting the liver because of the organ's regenerating ability. A major benefit of gene therapy is that patients would not require immunosuppressant drugs.

HYPOGLYCAEMIA

Hypoglycaemia is a lower than normal blood glucose level (BGL) i.e less than 4 mmol/L. It may develop in non-diabetic people. Hypoglycemia may be caused by an overproduction of insulin or an underlying reason such as, anorexia, excessive alcohol consumption, or even pregnancy.

Causes of hypoglycemia

Hypoglycemia is divided into two main categories, reactive hypoglycemia and fasting (non-reactive) hypoglycemia.

Reactive hypoglycemia is the most common form and occurs 3-5 hours after a meal. The cause of reactive hypoglycemia is an overproduction of insulin. Insulin released from the pancreas stimulates glucose uptake by tissue cells and glycogen formation in the liver, which in turn leads to a decrease in blood glucose levels. Fasting hypoglycemia appears in people suffering from severe diseases such as pancreatic tumours, extensive liver damage (e.g from severe alcoholism), prolonged starvation (e.g anorexia) and various cancers. It can also occur during pregnancy..

Symptoms of hypoglycemia

• Shaking

• Confusion

• Headaches

• Dizziness

• Moodiness

• Drowsiness

• Seizures or convulsions

• Sweating

• Anxiety and nervousness

• Nausea

• Hunger

• Fatigue

• faster heartbeat

• Pale skin

Management of hypoglycemia

It is important to identify the cause to manage hypoglycemia. In some cases, eating sugar food may help raise the BGL, other more severe cases may be managed by an injection of glucagon. Glucagon is a hormone that stimulates the conversion of glycogen to glucose, which raises blood glucose levels.

HYPERTHYROIDISM

Hormones secreted by the thyroid gland interact with cells throughout the body. They are responsible for regulating growth, development, and metabolic rate, along with many other vital functions. Malfunction of the thyroid can therefore have widespread and serious effects on a range of organs and bodily functions.

Hyperthyroidism is a condition in which excess amounts of the hormones triiodothyronine (T3) and thyroxine (T4) are secreted by the thyroid gland. T3 and t4 are made from an amino acid (tyrosine) and contain iodine, blood tests for these hormones and thyroid stimulating hormones (TSH) are used to diagnose the condition. When t3 and t4 are oversupplied by the thyroid, a negative feedback message is sent to the hypothalamus to decrease the release of thyrotropin releasing hormone (TRH). This in turn decreased the synthesis of TSH from the pituitary gland. Increased metabolic rate activity is only one of the effects of hyperthyroidism which causes the body to work harder and faster.

A positive blood test for hyperthyroidism shows elevated levels of t3 and t4 and decreased TSH levels. Thyroid malfunction affects about 6-7% of the population Hyperthyroidism occurs in about 2% of the Australian population. The disease most commonly affects people over the age of 60 and women are 5-8 times more likely to develop hyperthyroidism,

In hypothyroidism the thyroid produces less t3 and t4 than the body needs. Hypothyroidism can also have serious effects on health, including decreased glucose metabolism, low heart rate and blood pressure, sluggish muscle actions and decreased ovarian function.

Causes of hyperthyroidism

Most common cause of hyperthyroidism is an autoimmune disease called Graves’ disease. In patients with Graves’ disease, the immune system makes an antibody called the thyroid stimulating immunoglobulin (TSI), which mimics TSH, stimulating the thyroid to make more t3 and t4 hormones than the body needs. Trigger for the production of the antibody is unknown but a combination of environmental and genetic factors are thought to contribute. Infections caused by some viruses and bacteria, stress, childbirth, excess iodine (through food or contrast dyes used for imaging) and some medications have been linked to the onset of Graves disease. People with other autoimmune diseases such as type 1 diabetes, as well as smokers and those with tumours of the testes or ovaries, have a higher risk of developing this disease.

Symptoms of hyperthyroidism

The symptoms of hyperthyroidism can include some or all of the following:

• goitre (a visibly enlarged thyroid) or thyroid nodules (Figure 5.3.9a)

• weight loss

• rapid heartbeat (tachycardia)

• irregular heartbeat (arrhythmia)

• pounding heart (palpitations)

• increased appetite

• nervousness, anxiety and irritability

• changes in bowel patterns (more frequent)

• fatigue, muscle weakness

• tremor—usually a fine trembling in hands and fingers

• breast development in men

• bulging eyes (exophthalmos) (Figure 5.3.9b)

• nausea and diarrhoea

• sweating and heat intolerance

• changes in menstrual patterns, possibly including light or absent menstrual periods

• trouble sleeping

• skin thinning, blushing, flushing or being itchy or clammy

• fine, brittle hair or hair loss.

Management of hyperthyroidism

The symptoms of hyperthyroidism can be treated with drugs called beta-blockers. Which act on the circulatory and nervous systems to slow down the increased heartbeats and tremors associated with the disease., However, these drugs do not have an effect on the thyroid itself,

Sometimes antithyroid drugs are prescribed to interfere with the thyroid's ability to make hormones. These drugs act on the thyroid gland to slow the production of hormones to normal levels and reduce or eliminate the symptoms. Only 20-30% of patients have long-term success in treating hyperthyroidism with anti-thyroid grudges. Radioactive iodine treatment is the most widely used permanent treatment for hyperthyroidism. (iodine is needed to make T3 and T4 hormones.) Thyroid cells are the only cells in the body that absorb iodine. Taken orally, the radioactive iodine (I-131) is absorbed by the thyroid cells. I-131 emits beta radiation, which kills thyroid cells.

Surgery is another permanent cure and involves the removal of all or part of the thyroid. Removal or destruction of the thyroid means the patient will suffer from hypothyroidism, and must take thyroid pills such as levothyroxine for the rest of their life.