20/2-Homeostasis and Feedback Loops

Homeostasis

• Homeostasis is crucial for understanding how different body systems are regulated.
• It refers to maintaining the body in a functional condition, both internally and in response to the external environment.
• Warm-blooded creatures rely on homeostasis to regulate their internal environment.

Regulators vs. Conformers

• Humans are regulators, maintaining a stable internal temperature regardless of external conditions.
• Other organisms are conformers, where their internal temperature is influenced by environmental factors.

Homeostasis as a Balancing Act

• The body is constantly balancing internal and external environmental factors to maintain a set point.
• The set point isn't a fixed degree but a range within which the body functions optimally (e.g., around 37.5°C for humans).

Schematic of Homeostasis

• Stimulus: A change in a variable that disrupts balance.
• Receptor: Detects the change (sensory receptor).
• Control Center: Receives information from the receptor and determines the appropriate response.
• Effector: Implements the response to counteract the change and restore balance.

Feedback Loops

• Feedback loops are essential for maintaining homeostasis.
• They can be positive or negative, with negative feedback loops being more common for maintaining balance.

Negative Feedback Loop Example: Blood Glucose

• Stimulus: Elevated blood glucose levels.
• Sensor: Pancreatic beta cells.
• Effector: Insulin release.
• Response: Tissues take up glucose, lowering blood glucose levels back to the normal range.

Negative Feedback Loop Example: Temperature Regulation

• When body temperature increases, a signal triggers physiological responses to reduce the temperature.
• Mechanisms include radiation, respiration, convection, and evaporation.
• Evaporation: Perspiration releases water droplets, removing warmth.
• Convection: Heat transfer through air or liquid contact (e.g., a cool breeze or swimming in cool water).
• Conduction: Heat transfer to a solid surface.

Body Response to Temperature Changes

• Control Center: Hypothalamus in the brain.
• Response to Increased Temperature:
  • Blood vessels enlarge to release heat.
  • Sweat glands activate for evaporative cooling.
• Response to Decreased Temperature:
  • Warming mechanisms are activated.
• Core temperature is maintained within a small variation around the set point.
• Core and skin temperature receptors constantly signal the hypothalamus to evoke compensatory mechanisms.

Energy Investment in Homeotherms

• Homeotherms (endotherms) like birds and mammals require a higher energy investment per gram of tissue to maintain internal temperature.
• They generally have a narrower range of set points for optimal activity and health.
• $Energy <br />\newline use <br />\newline increases <br />\newline with <br />\newline temperature$ (illustrated by a graph).

Endocrine vs. Nervous System

• The endocrine system (hormonal system) and the nervous system are the two main regulatory systems in the body.
• The endocrine system is anatomically discontinuous, relying on hormones to signal tissues without direct connection.

Negative Feedback Loop: Conditions in the Body

• Change from the set point is detected.
• Corrective mechanisms are activated.
• Condition returns to the set point.
• Signals turn off corrective mechanisms.

Circulatory System

• The circulatory system in humans is a closed mechanism, bringing blood back to the heart and lungs for reoxygenation.
• Main components: heart, arteries, arterioles, veins, venules, capillaries, cells, and plasma.
• Functions: Transport of oxygen, nutrients, hormones, waste, antibodies, and fluids.
• Blood consists of plasma and cellular elements.

Blood Composition

• Plasma: High water content, electrolytes, nutrients, waste products (metabolites), respiratory gases, and hormones.
• Cells: Involved in responses to disease and maintaining homeostasis; includes platelets and red blood cells.
• Key components for hormone transport: nutrients, metabolites, respiratory gases, and hormones.

Cellular Physiology

• Plasma: Fluid component of blood.
• Red Blood Cells: Facilitate oxygen transport.
• Interstitial Fluid: Fluid between cells.
• Intracellular Fluid: Fluid within cells.
• Extracellular Fluid: Everything outside the cells.

Comparison of Nervous and Endocrine Systems

• Nervous System:
  • Evolved before hormonal systems.
  • Neurons conduct electrical signals.
  • Signals are generally fast.
• Endocrine System:
  • Hormones are circulated as chemicals.
  • Signals are generally slower.
• Nerves are closely connected, whereas endocrine cells release hormones to broadcast throughout the body.
• Target cells respond if they have receptors for a particular hormone.

Hormone Action

• Hormones require receptors on target cells to elicit a response.
• The cardiovascular system transports hormones.
• Secreting cells release hormones into the bloodstream, which then bind to target cells to produce an effect.
• Many cells can respond to multiple hormones if they have the corresponding receptors.

Blood Glucose Regulation (Revisited)

• High Glucose Levels: Pancreas releases insulin, signaling cells to take up glucose, and the liver converts glucose to glycogen.
• Low Glucose Levels: Pancreas stops insulin production and releases glucagon, causing the liver to release glucose.

Countercurrent Thermal Flow

• Arteries and veins exchange heat to maintain temperature.
• Blood moving away from the body core cools, transferring heat to veins returning to the core, warming the blood before it reaches internal organs.
• Examples: Arms, legs, bird legs (kookaburras on a BBQ), and testes in mammals.
• This helps reduce energy expenditure and maintain optimal temperature.

Positive Feedback Mechanisms

Characteristics

• Less common than negative feedback loops.
• Amplify a response rather than return to homeostasis.

Childbirth (Parturition)

• Uterine contractions stimulate oxytocin production, which further stimulates contractions until the baby is born.
• Smooth muscle assists the body in releasing the baby.
• Prostaglandins also contribute to uterine contractions.
• Once baby is born, negative feedback loop takes place for body to return to normal state.

Cascade Effect of Hormone Release

• A hormone binds to a receptor, initiating a cascade of hormone production within the cell.
• This results in a large amount of a substance required for a response.

Blood Clotting

• A break or tear in a blood vessel initiates a clotting cascade.
• Platelets are produced to cover the exposed tissue until it is healed.
• The response continues until a protective covering is formed.

Other Examples of Positive Feedback

• Menstrual cycle (estrogen and luteinizing hormone).
• Milk production in lactating mothers.