Biology Homeostasis and Feedback Systems

Homeostasis

• The ability of an organism to maintain a constant internal environment within small tolerance limits necessary for life.
• Important internal environmental factors that the body regulates include:
  • Body temperature
  • Blood pH
  • Blood pressure
  • Concentrations of dissolved substances in body fluids
  • Concentration of blood glucose
  • Concentration of O2 & CO2
  • Concentration of metabolic wastes
• Tolerance limits are the upper and lower limits of a normal range of a factor.
• Homeostasis provides the body with a degree of independence from the environment.
• Mammals are successful because they can maintain constant levels of activity despite fluctuations in external and internal environmental conditions.
• This steady state is maintained by feedback systems, usually operated through negative feedback.

Feedback systems

• A feedback system is a circular situation in which the body responds to a stimulus, with the response altering the original stimulus.
• Negative feedback:
  • Involved in maintaining conditions within a narrow range.
  • Responses counteract the original stimulus and restore the system to its original state.
  • The response reverses the direction of the stimulus.
  • Example: Controlling blood glucose levels.
• Positive feedback:
  • A stimulus causes responses that increase the stimulus in the same direction.
  • Acts as an amplifier of response.
  • Does not contribute to homeostasis and can be damaging to the body.
  • Example: High fever/heat stroke, uterine contractions during childbirth.

Components of a feedback system

• Receptor (detector): Specialized cells in the brain or organs detect the stimulus.
  • A stimulus never stays exactly constant but fluctuates around the set value (set point).
• Modulator (processing centre): Processes information from the receptor and sends information to the effector.
• Effector: Organs, muscles, or glands carry out a response that reverses the stimulus.
• Communication in the feedback system can be by hormones or nerve impulses.

Thermoregulation

• The process of regulating body temperature.
• Important because:
  • High body temperature (hyperthermia) denatures enzymes, causing metabolic reactions to fail.
  • Low body temperature (hypothermia) inactivates enzymes, slowing down metabolic activities.
• Involves controlling the amount of heat lost and heat gained across the body surface.
• Heat can be transferred in and out of the body through conduction, convection, and radiation.
• Evaporation can only remove heat.
• Heat can also be gained through metabolism.
• During cell respiration, food is oxidized to release energy.
  • Some energy is used for cellular work, like active transport and cell division.
  • Most of the energy is released as heat energy.
• Thermoregulation is controlled by the hypothalamus in the brain.
  • Monitors the body's core temperature.
• The skin is the major homeostatic organ involved in thermoregulation, due to its large surface area.

Components of the thermoregulatory system

• Receptors:
  • Peripheral thermoreceptors in the skin (warm and cold receptors) detect changes in external temperature.
  • Central thermoreceptors in the hypothalamus detect changes in blood temperature.
• Modulator: Thermoregulatory centre in the hypothalamus.
• Effectors:
  • Skin-based effectors:
    • Sweat glands
    • Hair erector muscles
    • Skin arterioles/capillaries
  • Skeletal Muscles
  • Endocrine glands:
    • Adrenal gland
    • Thyroid gland

Responding to decreased temperature (increase heat gain & reduce heat loss)

• Effectors:
  • Skin-based effectors:
    • Sweat glands: Not stimulated. No sweat production. No evaporation of sweat.
    • Hair erector muscles: Contract, raising the hairs and trapping a layer of air, which acts as an insulating layer to reduce heat loss.
    • Skin arterioles/capillaries: Vasoconstriction. Less blood flows to the surface of the skin, reducing heat loss by radiation.
  • Skeletal muscles: Shiver (involuntary contraction) to produce heat.
  • Endocrine glands:
    • Adrenal gland: Stimulated to release adrenaline, which increases the body's metabolic rate in the liver to generate heat.
    • Thyroid gland: Stimulated to release thyroxine, which also increases metabolic rate.

Responding to increased temperature (reduce heat gain & increase heat loss)

• Effectors:
  • Skin-based effectors:
    • Sweat glands: Stimulated to produce and release sweat. Evaporation of sweat removes heat from the skin surface.
    • Hair erector muscles: Relax, lowering the hairs and minimizing trapped air layer, allowing heat loss.
    • Skin arterioles/capillaries: Vasodilation. More blood flows closer to the surface of the skin, increasing heat loss by radiation.
  • Skeletal muscles: Not stimulated, no shivering. No heat production.
  • Endocrine glands:
    • Adrenal gland: Not stimulated, no adrenaline secretion. No heat production.
    • Thyroid gland: Not stimulated, no thyroxine secretion. No heat production.

Regulation of blood sugar levels

• Important because:
  • High blood glucose level (hyperglycemia) increases blood osmotic pressure, causing water to move out of cells. This can stop chemical reactions.
  • Low blood glucose level (hypoglycemia) means cells do not have enough glucose for respiration. This can also disrupt metabolism.
• Organs involved:
  • Liver
  • Pancreas
  • Adrenal glands

Role of the liver

• Converts glucose into glycogen for storage or glycogen to glucose for release into the blood.

Role of the pancreas

• Contains clusters of hormone-secreting cells called the islets of Langerhans.
• These clusters contain α cells and β cells:
  • α cells secrete glucagon.
  • β cells secrete insulin.
• Effects of insulin:
  • Increases glucose uptake by cells.
  • Promotes glycogen formation from glucose in the liver.
  • Promotes protein synthesis.
  • Suppresses gluconeogenesis.
  • Inhibits lipolysis.
• Effects of glucagon:
  • Increases glycogen breakdown to glucose in the liver.
  • Promotes gluconeogenesis.
  • Increases lipolysis.

Role of the adrenal glands

• Adrenal glands consist of two parts:
  • Adrenal medulla (inner): Secretes adrenaline and noradrenaline.
  • Adrenal cortex (outer): Secretes glucocorticoids (cortisol).

Treatment of diabetes mellitus using gene therapy

• Involves taking a copy of the insulin gene and finding a way to get it into target cells so they can produce insulin.
• A vector (like a virus) is used to carry the insulin gene and deliver it into the cell.
• Once inside the cell, the gene is expressed to produce therapeutic insulin.

Regulation of body fluid concentrations

• Osmoregulation is very important because:
  • Too much water: Cells undergo cytolysis (bursting) as water enters the cells by osmosis.
  • Too little water: Chemical reactions stop, blood pressure drops, and toxic wastes accumulate.
• Like thermoregulation, osmoregulation is based on a balance between fluid gain and fluid loss.
• A constant composition of body fluids is achieved when fluid gain equals fluid loss.

The kidneys

• Important excretory and osmoregulatory homeostatic organs.
• The functional units of the kidneys are called nephrons.

Antidiuretic hormone (ADH)

• Also known as vasopressin.
• Released from the posterior pituitary gland.
• Causes the distal convoluted tubule and collecting duct of the nephron to become more permeable to water.

Aldosterone

• Plays an important role in osmoregulation.
• Released from the adrenal cortex.
• Increases the permeability of kidney tubules, leading to greater sodium reabsorption into the bloodstream.
• As a result, more water is reabsorbed.

Regulating water intake by the thirst mechanism

• The thirst mechanism is triggered by a decrease in blood volume and an increase in blood osmolarity (concentration of solutes in the blood)
• Specialized receptors in the hypothalamus sense these changes and trigger the feeling of thirst.

Regulation of gas concentrations

• The levels of respiratory gases (oxygen and carbon dioxide) must be regulated properly to ensure a continuous supply of oxygen for cell respiration and to remove carbon dioxide, a toxic waste product.
• Too much carbon dioxide will cause a drop in blood pH, which can damage cells and disrupt metabolism.
• Carbon dioxide is carried to the lungs by blood and later excreted in expired air, while oxygen from inspired air is delivered to the cells by blood.
• Both the respiratory and circulatory systems are involved in the regulation of gas concentrations.

Control of breathing

• Breathing rate is regulated by the respiratory centre located in the medulla oblongata.
• The respiratory centre contains two regions:
  • Inspiratory centre: Controls inhalation.
  • Expiratory centre: Controls exhalation.
• Carbon dioxide and hydrogen ions (pH) are the main chemical factors that affect breathing rate.
  • Increases in their concentrations are detected by chemoreceptors:
    • Central chemoreceptors (located in the medulla oblongata) detect changes in carbon dioxide concentration in the blood.
    • Peripheral chemoreceptors (located in the aorta and carotid arteries) detect changes in hydrogen ion concentration in the blood.

Voluntary control of breathing

• Voluntary control comes from connections between the cerebral cortex and spinal cord.
• It bypasses the respiratory centre in the medulla oblongata.
• It provides protection by preventing harmful/irritating gases and water from entering the lungs.