Homeostasis, D3.3

• What is a cell's environment, in both plant and animal cells?

  A cell's environment is the immediate surroundings outside the plasma membrane. For a plant cell, this is the cell wall and the fluid that is held within it. For a plant cell, this is the ECM. In animals, there is always tissue fluid- blood- between cells. Regulating the internal environment between cells is known as homeostasis.
• What is homeostasis?


  Homeostasis is the process by which the body maintains a stable internal environment despite changes in the external environment. It involves monitoring and regulating variables within narrow limits so that cells and organs can function properly.

  All life has maintenance of homeostasis; it is a function of life.
• What is the homeostatic range of tolerance?

  Physiological mechanisms of an organism maintain variables in a homeostatic range of tolerance. If an environmental variable goes outside of an organism's range of tolerance, the organism may experience stress and die.
• What are some variables maintained by homeostasis?

  - Heart rate

  - Ventilation rate

  - Blood glucose

  - Body temperature

  - Blood osmolarity

  - Blood pH
• What is feedback control?

  Feedback control uses information about the outcome of a process to make decisions about the future of that process. There are two types: Positive and negative.
• What is positive feedback control?

  Positive feedback control increases the gap between the original and the new level. It amplifies a change.
• What are some examples of where positive feedback control may be used?

  Positive feedback is uncommon in humans. It is used, however, to increase the force of muscle contractions during childbirth, or when normal homeostatic processes have failed (like in blood clotting). As it promotes change, rather than stability, positive feedback is unsuitable for homeostasis.
• What is negative feedback control?

  Negative feedback control decreases the gap between the original and the new level, so that the original level is restored. It counteracts a change.
• What are some examples where negative feedback control may be used?

  Negative feedback mechanisms form the basis of homeostatic control systems that maintain internal conditions. It allows organisms to inhabit hostile environments as their body cells can be kept within stable internal conditions, despite environmental fluctuations.
• What are the steps of a typical negative feedback loop?

  1- A stimulus (change in physiological variable)

  2- A receptor (specialised cells detect the stimulus and send signals to the control centre)

  3- A control centre (processes the information form the specialised cells and determines the response. The hypothalamus controls homeostasis in humans)

  4- An effector (cell, tissue or organ that carries out the response)

  5- A response (counteraction to the stimulus)
• What is blood glucose?

  Glucose is a monosaccharide that is readily broken down to produce ATP through cellular respiration.

  Most carbohydrates consumed are converted to glucose during digestion. Glucose is absorbed into the bloodstream via facilitated diffusion and indirect active transport in the small intestine.

  Once in the bloodstream, glucose can be transported to cells throughout the body easily. This is because glucose is a polar molecule that can dissolve in water.
• Why is the homeostasis of blood glucose concentration essential?

  Blood glucose concentration must be kept within narrow limits in the blood because high blood glucose (hyperglycaemia) and low blood glucose (hypoglycaemia) have serious consequences.

  The set point for blood glucose concentration is 5mmol L^-1 (dm^-3)
• What are the symptoms of hypoglycaemia?

  - Sweating

  - Shaking

  - Extreme hunger

  - Nausea

  - Dizziness

  - Confusion

  - Fast heart rate
• What are the symptoms of hyperglycaemia?

  - Increased thirst

  - Increased urination

  - Fatigue

  - Dry mouth

  - headache

  - Blurred vision

  - Shortness of breath
• What are the receptors that are involved in blood glucose homeostasis?

  Chemoreceptor proteins in the carotid artery sense changes in blood glucose concentration
• What is the control centre involved in blood glucose homeostasis?

  The hypothalamus receives signals of the change and responds by triggering the release of hormones from the pancreas
• What is the role of the pancreas (effector) in blood glucose homeostasis?

  Clusters of endocrine cells within the pancreas (known as Islets of Langerhans) are responsible for insulin and glucagon production and secretion.
• What does the secretion of Insulin via beta cells in the pancreatic Islets of Langerhans do?

  When blood glucose RISES, beta cells in the Islets of Langerhans secrete insulin. Insulin signals cells throughout the body to take up glucose from the bloodstream (through binding with a tyrosine kinase receptor and triggering a sequence of reactions), lowering blood glucose concentration in the blood.
• What does the secretion of glucagon via alpha cells in the pancreatic Islets of Langerhans do?

  When blood glucose DROPS, Alpha cells in the Islets of Langerhans release glucagon. Glucagon signals the liver to link glucose (monosaccharides) to form glycogen (polysaccharide). This process is known as glycogenesis. Glycogen is a polysaccharide used as an energy storage molecule.
• What is the role of the liver (effector) in blood glucose homeostasis?


  The liver is responsible for storing excess glucose when blood glucose concentration is too high, and releasing glucose into the bloodstream when blood glucose concentrations drop.
• What occurs in the liver when blood glucose levels rise?

  Insulin (secreted by the beta cells in the Islets of Langerhans in the pancreas) signals the liver to link glucose (monosaccharides) to form glycogen (polysaccharide). This process is known as glycogenesis. Glycogen is a polysaccharide used as an energy storage molecule.
• What occurs in the liver when blood glucose levels drop?

  Glucagon (secreted by the alpha cells in the Isles of Langerhans in the pancreas) signals the liver to convert glycogen back into glucose (via hydrolysis) and release it into the bloodstream. This is known as glycogenolysis. It also stimulates production of new glucose from fats and amino acids (gluconeogenesis)
• What is diabetes?


  Diabetes is a chronic disease in which a person consistently has high blood glucose levels (hyperglycaemia). This may be identified through the presence of glucose in the urine.
• What are the causes of type I diabetes?

  Type I diabetes is a disease that results from insufficient or no insulin production, due to the autoimmune destruction of the beta cells of the pancreatic Islets of Langerhans. This means that they cannot secrete insulin. Without insulin, cells are not stimulated to take in glucose from the bloodstream (the signal transduction cascade is not activated). Therefore, a high concentration of glucose remains in the blood. The onset of type I diabetes is sudden, and often occurs in childhood.
• What are the treatments for Type I diabetes?

  Type I diabetics must monitor blood glucose levels regularly, and inject insulin when it is too high.

  Scientists are working to differentiate stem cells into insulin-producing beta cells, which could possibly be transported into patients with type I diabetes. This could restore the body's ability to produce insulin on its own.
• What are the causes of Type II diabetes?

  Type II diabetes is a disease resulting from a deficiency of insulin receptors on target cells. Eventually, pancreas may produce less insulin. When blood glucose is high, Insulin is secreted. However, there are insufficient or non-functional insulin receptors on the body cells, so insulin cannot activate the signal transduction cascade and glucose transporter proteins are not activated (so glucose cannot enter the cells and remains in the blood). The onset is gradual, often in overweight adults who live a sedentary lifestyle and have a poor diet.
• What are the treatments for type II diabetes?

  Type II diabetes may be treated by a reduction of weight and modification of dietary habits to reduce high-glucose foods and increase frequency of meals. Sometimes antidiabetic drugs may also be used to stimulate the pancreas to produce more insulin.
• What are the symptoms of both type I and type II diabetes?

  - Increased thirst- to compensate for fluid loss via frequent urination

  - Blurred vision- high blood glucose makes the lens of the eye swell and change shape

  - Vaginal yeast infection- glucose in the blood provides nutrition for yeast

  - Unexplained weight loss- the body cannot use the excess glucose for energy

  - Slow healing of cuts and sores- poor blood circulation and impaired immune function due to high blood glucose

  - Fatigue- cells are deprived of their main energy source (glucose)

  - Frequent urination- Kidneys attempt to filter out the excess glucose
• What is thermoregulation?


  Thermoregulation is the maintenance of a core body temperature despite fluctuations in external temperatures. It occurs by balancing heat generation with heat loss.
• Why is thermoregulation essential?

  Keeps enzymes working optimally.

  Prevents damage from extreme temperatures.

  Maintains overall homeostasis and survival.
• What are the receptors involved in thermoregulation?

  Thermoreceptors are ion channel proteins in the membranes of sensory neurons. When they receive a signal indicating a change in temperature, the channels open, allowing ions to flow into the cell.

  The flow of ions leads to cell membrane depolarisation, which results in action potentials propagated along nerves towards the brain

  Thermoreceptors are located in the skin and the body's core.
• What is the control centre involved in thermoregulation?

  The brain is the central integration organ in humans. It process information from several sensory inputs.

  The hypothalamus is a brain region that receives information from the thermoreceptors and compares it to an internal set point. This internal set point is the homeostatic range of tolerance

  The hypothalamus then sends action potentials to effectors to return body temperature to the internal set point. (around 37 degrees)
• What is the role of the pituitary gland (effector) in thermoregulation?

  Pituitary gland- increases/decreases secretion of TSH (Thyroid stimulating hormone) which stimulates the thyroid to secrete thyroxin.

  - If the hypothalamus detects a drop in body temperature, TSH secretion will increase

  - If the hypothalamus detects an increase in body temperature, TSH secretion will decrease.
• What is the role of the thyroid (effector) in thermoregulation?

  Thyroid- increase/decrease release of thyroxin (stimulated by TSH) to travel to target cells in the body and increase their basal metabolic rate. It does this through its binding to the cell's intranuclear receptors, activating the genes for increasing basal metabolic rate. This is common in liver and muscle cells. Increasing basal metabolic rate results in the production of heat energy, maintaining internal body temperature.
• What is the role of adipose tissue/BAT (effector) in thermoregulation?

  Adipose tissue- brown adipose tissue (in the neck, chest and shoulders) are dense in mitochondria. Stimulated by thyroxin, the brown adipose cells undergo uncoupled cellular respiration to generate heat.
• What is uncoupled cellular respiration?

  Uncoupled cellular respiration is a metabolic process where the mitochondria actively dissipate energy as heat instead of producing ATP.
• What is the role of the blood vessels (effector) in thermoregulation?

  Blood vessels- constrict or dilate to adjust the volume of blood flowing through. Blood distributes heat energy generated through metabolism throughout the body, absorbing heat from active tissues and transferring it to cooler tissues. Through vasodilation/vasoconstriction, the blood vessels can regulate heat exchange with its surroundings.
• What is the role of the skeletal muscles (effector) in thermoregulation?

  Skeletal muscles- rapidly contract and relax, which causes shivering. Muscles produce heat when contracting as the process of muscle contraction relies on the breakdown of ATP molecules. The breaking down of ATP is exothermic, so releases heat.
• What is the role of muscles at the base of hair follicles (effector) in thermoregulation?

  Muscles at the base of hair follicles cause hair to stand up. (piloerection)

  The hair acts as a thermal insulation and traps warm air from dissipating from the body
• What is the body's internal response to a decrease in body temperature?

  - Vasoconstriction

  - Shivering

  - Uncoupled cellular respiration

  - Piloerection