Homeostasis

Question 1

Homeostasis

Answer 1

The constancy of the internal environment. A state of overall internal chemical and physical stability that is required for survival of cells and the body. The mechanisms of this process help maintain a constant internal environment despite a variable external environment. This can be short term regulation (minute to minute) e.g. blood pressure, body temperature, medium term (hours to days) e.g. food intake and sleep, long term e.g. body weight, blood pressure, growth and sexual maturation. Any deviation from this normalcy indicates a disease.

Question 2

Set Point

Answer 2

The physiological value around which the normal range fluctuates e.g. body temperature at 37C. A normal range is the restricted set of values that is optimal and stable e.g. body temperature from 36.5C - 37.5C. Negative feedback is mechanisms that reverses a deviation from the set point. These can change and maintain new settings e.g. blood pressure can increase due to the continual increase of blood pressure over your life meaning that as you age the homeostatic level for blood pressure is increased.

Question 3

Acclimatization

Answer 3

Changes that occur in organ systems in order to maintain a different set point for homeostasis. An example is going to high altitude to train in a lower oxygen level leading to an increased production of RBCs which ensures the adequate oxygen delivery to tissues.

Question 4

Control of Homeostasis

Answer 4

This typically requires the interaction of sensors, integrators and effectors. Negative feedback is the most common mechanism that reverses the charges to a body system if it exceeds a set point.
stimulus -> sensor -> integrator -> effector -> response -> stops stimulus. This response cancels or counteracts the effect of the original change.

Question 5

Negative Feedback Example

Answer 5

When lying down and quickly standing up a negative feedback will occur.
1. Blood pressure falls (stimulus).
2. Blood pressure receptors respond (sensor).
3. The nerve impulse travels through sensory nerve fibers to the brain (integrating center).
4. Motor nerve fibers bring the impulse to the heart which increases heart rate (effector).
5. The increased heart beating leads to blood pressure rising (loop complete).

Question 6

Positive Feedback Example

Answer 6

When the response to a stimulus increases the effect of the original stimulus. An example is seen in blood clotting.
1. A break in a blood vessel wall causes bleeding (stimulus).
2. Damaged cells release chemicals (clotting factors).
3. Clotting begins.
4. Additional chemicals (clotting factors) are released due to the clotting.
5. These chemicals will then accelerate clotting therefore releasing more chemicals and increasing the speed of clotting even further.
6. The blood clot plugs the break in the vessel wall and bleeding stops (ends feedback).

Question 7

Communication in Homeostasis

Answer 7

This can occur via neurotransmitters or hormones. Neurotransmitters are released at synapses between neurons and target cells. This is a precise and fast form of communication. Hormones are chemicals carried by blood to distant targets which have require a long-lasting reaction communication.

Question 8

Hormones

Answer 8

These are secreted by endocrine glands, endocrine cells and some neurons. They travel through the circulatory cells to target cells. They only effect target cells with specific receptors for the chemicals. These will then change the activity of those target cells. These are produced in the endocrine system and generally regulate slower, long-term changes in growth or functioning of body systems being regulated by negative feedback. There are 2 types being steroid and non-steroid.

Question 9

Steroid Hormones

Answer 9

These are lipids derived from cholesterol which are produced in adrenal glands and reproductive organs e.g. ovaries and testes. They are lipid soluble and hydrophobic. These interact with receptors in the nucleus to change cell activity (increase/decrease protein synthesis or enzyme activity). These diffuse directly across the lipid bilayer, enter the cytoplasm and enter the nucleus. In the nucleus they form a hormone-receptor complex (transcription factor) and bind to promoter regions of genes in order to stimulate or inhibit transcription to change the activity of the target cells. Some of these hormones will bind to cell membrane receptors to change the membrane properties.

Question 10

Protein/Non-Steroid Hormones

Answer 10

These are derived from an amine, a peptide or a protein and are also referred to as peptide hormones. These aren’t soluble in lipids and are hydrophilic. These interact to receptors on the cell membrane to change cell activity (increase/decrease protein synthesis or enzyme activity). These bind to receptors in the plasma membrane and activate an enzyme system to change the target cell activity. In this case the hormone acts as the first messenger.

Question 11

Transport & Action of Hormones

Answer 11

Receptors for hormones can be on the cell membrane or inside the cell. Lipid soluble hormones (steroid) bind to receptors inside the cell by moving through the cell membrane. Water soluble hormones (peptides) bind to receptors on the outside of cells and activate messenger systems (they can’t move through cell membranes).

Question 12

Target Tissues

Answer 12

The location of this tissue is relevant to the delivery method of a hormone. In paracrine cells the chemical released affect nearby cell without entering the blood e.g. neurotransmitters. In autocrine cells the cell stimulates itself by releasing chemicals to which it has receptors e.g. T-lymphocytes and the immune response. Endocrine cells will release hormones which enter the blood and go through the circulatory system until they reach the distance cells with the adequate receptors.

Question 13

The Endocrine System

Answer 13

This is made up of organs and glands which produce hormones allowing for communication throughout the body. The organs and glands are The hypothalamus, pituitary gland, pineal gland, parathyroid gland, thyroid gland, adrenal gland, pancreas, and gonads.

Question 14

Endocrine Organs

Answer 14

The hypothalamus makes and secretes hormones that act on the anterior pituitary (make it secrete hormones). It also makes hormones that are stored in and released from the posterior pituitary. The pituitary gland has an anterior (produces ACTH, TSH, LH, FSH, GH, PRL) and posterior (produces ADH, oxytocin) lobe. The adrenal glands sit above the kidneys having a cortex (produces cortisol, aldosterone, sex hormones) and a medulla (produces adrenaline and noradrenaline). The ovaries and testes make and secrete sex hormones. The pineal gland produces melatonin which deals with sleep/wake cycles. The thyroid gland produces thyroid hormone which increases metabolism and calcitonin which lowers blood Ca2+ ions. The parathyroid glands produces parathyroid hormone which regulates blood calcium. The pancreas produces insulin and glucagon to regulate blood sugar level. All of these structures use negative feedback for regulation.

Question 15

Negative Feedback & Endocrine System

Answer 15

An example of this is seen in normal blood glucose levels (BGL). When fasting (between meals) the level is 4-5.4 mmol/L whereas after eating it is <7.8 mmol/L. After eating the carbohydrates are broken down and absorbed as glucose which increased BGL which causes the pancreas to release insulin telling cells to take in glucose from the blood returning the BGL back to normal. In the other direction between meals when BGL is low the pancreas releases glucagon which causes the liver to release glucose into the blood causing BGL to return to normal.

Question 16

Positive Feedback & Endocrine System

Answer 16

An example of this is seen in the hormonal regulation of childbirth through the hormone oxytocin. When the head of the fetus pushes against the cervix nerve impulses are transmitted to the brain which causes the release of oxytocin from the posterior pituitary due to stimulation from the hypothalamus. The oxytocin stimulates uterine contractions which pushes the fetal head toward the cervix causing a greater pressure on the cervix which continually increase the release of oxytocin which increases contractions. This loop is ended once the fetus has exited the mother.

Question 17

Hypothalamus & Pituitary Gland

Answer 17

These organs interact as a major center of controlling the activity of the other organs. The hypothalamus has secretory neurons which deliver hormones to the pituitary. The posterior section of the pituitary gland stores and releases hormones made in the hypothalamus (ADH and oxytocin). The anterior pituitary however is stimulated by other hormones released by the hypothalamus to produce its own hormones which it secretes (ACTH, TSH, FSH, LH, PRL and GH).

Question 18

Antidiuretic Hormone (ADH) & Oxytocin Release

Answer 18

These hormones are produced by the secretory neurons in the hypothalamus where these hormones are also stored. The secretory neurons will move down in the axons and accumulate and the axon endings. When action potentials occur they cause the release of these hormones which enter the blood capillaries in the posterior lobe of the pituitary gland. The hormones then move into the circulatory system and travel to target cells.

Question 19

Anterior Pituitary Hormone Release

Answer 19

The activity of this lobe of the pituitary gland is controlled by the hypothalamus. The cell bodies of secretory neurons in the hypothalamus secrete hormones. These hormones are picked up by the capillary bed at the base of the hypothalamus. Hormones travel to the capillary bed of this gland and act on it to produce other hormones. These hormones produced by the gland enter circulation. These hormones are adrenocorticotropic hormone (ACTH) acts on the adrenal cortex, thyroid stimulating hormone (TSH) acts on the thyroid, growth hormone (GH), follicle stimulating hormone (FSH), luteinising hormone (LH) and prolactin (PRL) acts in lactation.

Question 20

Development of the Pituitary

Answer 20

At 4 weeks of development the region for the future hypothalamus can be seen with a neurohypophyseal bud (future posterior pituitary) from the brain and hypophyseal pouch (future anterior pituitary) from the developing mouth. These different origins of the lobes of the pituitary is the reason for the different functions of the lobes.

Question 21

Activation Hormones of the Hypothalamus

Answer 21

Growth hormone releasing hormone (GHRH) stimulates the secretion of growth hormone (GH), thyrotropin releasing hormone (TRH) stimulates the secretion of thyroid stimulating hormone (TSH), corticotropin releasing hormone (CRH) stimulates the secretion of adrenocorticotropic hormone (ACTH) and gonadotropin releasing hormone (GnRH) stimulates the secretion of follicle stimulating hormone (FSH) and luteinising hormone (LH).

Question 22

Thyroid Gland

Answer 22

This produces thyroid hormone (TH) (T3 and T4). TH is required for metabolism, growth and development and the nervous system. This sets the basal metabolic rate and enhances the production of GH. It also produces calcitonin to lower the levels of calcium in the blood. The hypothalamus secretes TRH which effects the anterior pituitary to secrete TSH which effects this gland which secretes TH. Excess TH causes a negative feedback loop to the hypothalamus reducing the secretion of TRH which reduces the effects down the chain. In certain cases this gland can expand causing a goiter which stimulates the anterior pituitary to secrete more TSH however this has no effect on the swollen gland causing an abnormal feedback loop.

Question 23

Thyroid Gland Dysfunction

Answer 23

This is the second most common endocrine condition affecting women of reproductive age (after diabetes) and occurs in 2 ways with hypothyroidism and hyperthyroidism. Hypothyroidism is low blood levels of TH which affects 16 million in the US. The symptoms include fatigue, weight gain, intolerance to cold temperatures and dry skin caused commonly by an endemic iodine deficiency. Autoimmune thyroid disease (Hashimoto’s disease) is the most common cause when the iodine intake is adequate. Hyperthyroidism is excess TH in blood. The symptoms include high heart rate, high blood pressure, heavy sweating, heat intolerance and weight loss. This is typically caused by autoimmune or excessive exposure to iodine.

Question 24

Growth Hormone Regulation

Answer 24

GHRH is secreted by the hypothalamus which stimulates the the secretion of GH from the anterior pituitary. This is important for normal body growth and stimulates the growth of bone, cartilage and skeletal muscle. This increases muscle mass and dysfunction of GH causes major abnormalities. Dysfunctions in this hormone can cause pituitary dwarfism where an underproduction of GH during childhood or insensitivity of receptors to GH during childhood. Another dysfunction is gigantism where pituitary overproduction of GH during childhood occurs or acromegaly where pituitary overproduction of GH occurs during adulthood.

Question 25

Adrenal Glands

Answer 25

These glands are involved in the stress response. The cortex secretes steroids known as glucocorticoids when a stressful event occurs a well known hormone in this class being cortisol. This raises blood glucose, promotes the breakdown of muscle protein, stimulates the liver to take up amino acids to synthesise glucose (gluconeogenesis), breaks down fats for energy and reduces inflammation (used in treatments). The cortex also secretes another group of steroids called mineralocorticoids primarily the hormone aldosterone which regulated blood pressure by adjusting the reabsorption of K+ and Na+ in the kidneys. The cortex can also secrete sex hormones in the fetus and early in puberty. The medulla contains neurons that release adrenaline and noradrenaline which activates the sympathetic nervous system.

Question 26

Adrenal Gland Activation

Answer 26

1. Stress (stimulus).
2. A receptor for stress is found in the hypothalamus.
3. The hypothalamus releases CRH into the hypothalamo-hypophyseal portal system.
4. In response to the CRH the anterior pituitary gland secretes ACTH.
5. ACTH stimulates the adrenal cortex to release glucocorticoids e.g. cortisol into the blood.
6. Cortisol stimulates target cells (effectors).
7. This increases the nutrient molecules in the blood.
8. Increasing cortisol levels inhibit the release of CRH and ACTH creating a negative feedback.

Question 27

Pancreas

Answer 27

This organ has both an endocrine (secretion of glucagon and insulin) and an exocrine (secretion of digestive enzymes) function. In the islets of Langerhans there are alpha and beta cells. The alpha cells secrete glucagon which raises blood glucose levels. The beta cells secrete insulin which lowers blood sugar. Glucagon and insulin work antagonistically to maintain blood glucose levels.

Question 28

Pancreas Regulation

Answer 28

Low blood glucose levels can compromise normal brain function whereas high blood glucose levels can cause blood vessel and nerve damage. The homeostatic responses ensure appropriate glucose levels despite intermittent fuel supply and variable rate of glucose utilization.

Question 29

Why Study Nutrition

Answer 29

The study of how living organisms obtain and utilise nutrients needed to grow and sustain life. From an evolutionary perspective is important for fitness (survival + reproduction), craving for sugar, fat and salt and food rituals (unique to humans).

Question 30

Nutrition

Answer 30

All living organisms need nutrients to survive with all animals obtaining nutrients through the consumption of other organisms. Nutrients ( most biomolecules, vitamins and minerals) are required for the synthesis of new molecules, for energy (used for maintenance, growth and repair), obtained through food), levels are regulated during and following meals. One challenge for humans is maintaining a balance between food intake, storage and energy expenditure in which imbalances can have serious health consequences.

Question 31

Nutrients

Answer 31

Any substance in food that is used by the body to promote normal growth, maintenance and repair. These come in 2 categories of macro and micro. Macronutrients must be consumed in relatively large quantities and needed daily some examples are carbohydrates, proteins and lipids. Micronutrients must be consumed in relatively small quantities some examples are vitamins and minerals. These can also be divided into essential (obtained from diet and not synthesised in the body) and non-essential (produced also by biochemical processes in the body and not required in diet). Water is considered one of these.

Question 32

Absorptive State

Answer 32

Also known as the fed state which is the time eating, digesting and absorbing nutrients. This lasts 4 hours after a meal. The concentrations of glucose, triglycerides and amino acids increase as they are absorbed from the GI tract. During this state insulin is the major regulatory hormone released in response to increased BGL. This stimulates liver and muscle cells forming glycogen from blood glucose, adipose tissue increasing the uptake of triglycerides from the blood, stimulation of most cells to increase amino acid uptake and accelerated protein synthesis.

Question 33

Post-Absorptive State

Answer 33

Also known as the fasting state which is the time between meals. During this period the body relies on stores of nutrients with the body working to maintain homeostatic levels of nutrients. During this state glucagon is the major regulatory hormone released in response to decreased BGL. This stimulates the liver to increase the breakdown of glycogen to glucose (glycogenolysis), the formation of glucose from non-carbohydrates (gluconeogenesis) and adipose tissue to break down triglycerides.

Question 34

Carbohydrates

Answer 34

These are polysaccharides (long chain of sugars) some examples being dextrose, cellulose, starch and glycogen. Disaccharides are 2 sugar molecules together some examples being sucrose, lactose and maltose. Monosaccharides are simple sugars with only 1 molecule some examples are glucose, fructose and galactose.

Question 35

Carbohydrate Metabolism

Answer 35

1. Monosaccharides are absorbed from the SI into the blood and then enter hepatocytes through the hepatic portal. All of these monosaccharides are converted to glucose.
2. Non-carbohydrates (glycerol, amino acids) are converted to glucose by gluconeogenesis which is also done in hepatocytes.
3. Glucose molecules are bonded together to form glycogen in a process called glycogenesis in hepatocytes for storage.
4. When extra glucose is required by the body the glycogen can be broken down to produce glucose in a process called glycogenolysis.

Question 36

Endocrine Function of the Pancreas

Answer 36

In the pancreatic islets (PI) there are clusters of 2 types of cells alpha and beta. Beta cells secrete insulin after meals (high blood glucose) which stimulates cells to take up glucose and promotes the synthesis of fats and glycogen which is done to lower BGL. Alpha cells secrete glucagon during low BGL where it works on acts causing glycogen (stored polysaccharide) and amino acids to be converted to glucose which is done to raise BGL. These 2 hormones (glucagon and insulin) work antagonistically to regulate BGL.

Question 37

Negative Feedback of Blood Sugar Regulation

Answer 37

When BGL is high (hyperglycemia) the beta cells secrete insulin to stimulate tissues to uptake glucose and form glycogen and lower the BGL. If BGL is too high then blood vessels and nerves will be damaged by the increased osmotic pressure. When BGL is low (hypoglycemia) the alpha cells secrete glucagon which stimulates the breakdown of glycogen to form glucose (glycogenolysis) to raise the BGL. If BGL is too low then tissues and organs won’t have enough energy causing damage to the brain and other organs.

Question 38

Transport Maximum

Answer 38

The upper limit of the amount of a substance that can be reabsorbed in the kidneys. A low concentration of glucose in the filtrate all of that glucose is reabsorbed back into the blood. At this upper limit all carrier sites that remove glucose from the filtrate are filled and all glucose is reabsorbed into the blood. If the concentration of glucose is high then there won’t be enough carrier sites to reabsorb all of the glucose back to the blood leading to glucose in the urine. This occurs in diabetes mellitus where there is too much glucose in the blood and therefore in the filtrate.

Question 39

Diabetes Mellitus

Answer 39

This comes in 2 types with type 1 and 2. In type 1 there is an inability to produce sufficient insulin (insulin dependent diabetes mellitus (IDDM)) and is referred to as the ‘juvenile’ form as it is seen in many children. This specific variant is 10% of diabetics and is treated by self-monitoring of blood glucose and providing exogenous insulin when required. In type 2 there is a reduced responsiveness to insulin (non-insulin dependent diabetes mellitus (NIDDM)) and has risk factors including genetics, age and obesity. This specific variant is 90% of diabetics and is treated by changes in diet and exercise.

Question 40

Obesity

Answer 40

Having a very high amount of body fat in relation to lean body mass. This is measured through BMI which is a measure of a persons weight in relation to their height.
BMI (kg/m^2) = weight / height^2.
Overweight: BMI > 25.
Obese: BMI > 30.
Worldwide >600 million people are obese and >2 billion are overweight with 1/3 of Australians being obese and another 1/3 are overweight which is a doubling over the last 20 years. This mostly reflects excessive caloric intake (relative to energy expenditure.

Question 41

Drivers of Obesity

Answer 41

Humans evolved to cope with variable food supply (inconsistent eating) where food wasn’t available at all times meaning an evolutionary development of the capacity to store energy was a selective advantage. In the modern day food is widely available and the storage of food is now an evolutionary shortfall causing a higher incidence of obesity in certain human populations. This shows a gene-environment mismatch. There is also evidence that populations with close ancestry to foraging (not agriculture) are more likely to become obese from the large availability of food.

Question 42

Fats & Lipids

Answer 42

Without these nutrients in diet you can’t survive. These are used for fuel (energy reserve), building cell membranes, to cushion organs e.g. kidneys and eyes, provides insulation (hypodermis of skin). These are stored in adipose tissue along with fat-soluble vitamins. The liver can also produce these from proteins and carbohydrates however it can’t make the ‘essential fatty acids’ required.

Question 43

Dietary Lipids

Answer 43

Saturated fatty acids are solid at room temperature which can be found in meat, milk, cheese, coconut oil and palm oil. An excessive intake of these may raise the risk of heart disease. Unsaturated fatty acids are liquid at room temperature which can be found in nuts and certain oils (canola, olive, sunflower etc.). Polyunsaturated fatty acids are liquid at room temperature and can be found in certain oils (soybean and safflower). Cholesterol is a required component of the plasma membrane and acts as the precursor for steroid hormones, bile salts and vitamin D. This can come from the diet or metabolic pathways.

Question 44

Proteins

Answer 44

These are needed to replace worn out structures made of this material. These are broken down into amino acids which are then used for synthesising new forms of these in the body. Of the 20 amino acids 8 are essential and the other 12 can be synthesised in the body. Most animal proteins are complete (contain all 20 amino acids) whereas most plant proteins are incomplete (don’t contain all 20 amino acids). In vegetarian diets a combination of different plants are required. The amount of this nutrient required depends on age (children need more for growth) as well as infection, following injury, stressful conditions and pregnancy which requires a higher intake of this.

Question 45

Protein Metabolism

Answer 45

1. Deamination: when the amine group (NH2) is removed from amino acids and NH2 is converted to urea and urea enters the blood (excreted through kidneys and urine) with the remaining components being oxidised in cellular respiration to generate ATP from the liver.
2. Amino acids are used to form proteins (hormones, enzymes etc.).
3. Transamination: a particular amino acid is converted into another form e.g. tyrosine -> leucine.

Question 46

Vitamins

Answer 46

These are organic substances which have an essential role in metabolism (not fuel source). They act as coenzymes critical for the use of other nutrient and act as antioxidants. Essential ones are necessary from the diet and deficiency occurs if intake or absorption of these is impaired. There are at least 13 essential ones including A, C, D (inadequate UV light exposure), E, K, B group and folic acid. Nonessential ones can be synthesised in the body e.g. D (adequate UV light exposure).

Question 47

Vitamin Deficiency

Answer 47

Most of these are necessary for normal biological process and sustained absence from these is detrimental to health. Without D (important for calcium absorption and bone metabolism) may result in increased risk of osteoporosis. Without K (important blood coagulation factor) results in a decrease in blood clotting leading to higher risk of bleeding.

Question 48

Minerals

Answer 48

Inorganic compounds (ions) obtained from the diet which have essential roles in metabolism and required in daily amounts. Calcium is required for the formation and maintenance of the skeleton, muscle contraction, blood clotting etc. Sodium and potassium maintains the resting membrane potential in excitable cells and is required to generate an action potential. Iodine is required to produce thyroid hormone. Zinc plays a role in protein synthesis and would healing. Iron is required in the electron transport chains, it binds oxygen in hemoglobin and is required for the synthesis of some hormones, neurotransmitters and amino acids. All of these are essential and must be obtained from the diet.

Question 49

Iron Deficiency

Answer 49

This is the most common nutritional deficiency. This is required for haemoglobin and myoglobin which a lack of it causing anaemia. The symptoms are fatigue, weakness, pale skin and sensitivity to cold with the causes being insufficient dietary intake or iron loss from excessive bleeding (periods). This mineral can be obtained for treatment and can be found in meat, poultry, fish, dark leafy vegetables, nuts and whole grains.

Question 50

Iron Overload

Answer 50

This can be caused by hereditary haemochromatosis which is an autosomal recessive condition which is most common for people of Celtic or Northern European descent with it being the most common genetic condition in Caucasian populations with 1/200 at risk. IT is characterised by excessive absorption of this mineral which can lead to liver disease, arthritis, diabetes, cardiac failure etc. It is more common in females (pregnancy and menstruation prevent it). This may have been an evolutionary advantage in populations with a lack of availability of this mineral in their environments however it is disadvantageous in environments with high amounts of this mineral available in food sources.