Homeostasis: Maintaining Internal Balance
Homeostasis
Homeostasis is the maintenance of a stable internal environment within an organism, ensuring optimal conditions for cellular processes.
- Homeostasis is the self-regulating process that maintains a stable internal environment for optimal body function, despite changes in external or internal conditions.
- It ensures that physiological variables such as temperature, pH, heart rate, and blood pressure remain within narrow limits.
Key Characteristics of Homeostasis
- Self-regulating: The body constantly monitors and adjusts conditions.
- Dynamic equilibrium: Conditions fluctuate within a narrow range rather than being completely static.
- Essential for survival: Disruptions in homeostasis can lead to disease or dysfunction.
Students often think homeostasis means keeping the body completely unchanging, but it actually involves constant small adjustments to maintain stability.
Feedback Mechanisms
Regulation of Hormone Secretion
Hormone secretion is typically regulated by feedback mechanisms, ensuring that hormone levels are kept within a certain range.
Negative Feedback
Negative feedback
Negative feedback is a homeostatic mechanism in which a change in a physiological condition triggers a response that counteracts the initial change, maintaining balance.
In negative feedback, the output of a process inhibits its own production, helping to maintain homeostasis.
How Negative Feedback Works
- A stimulus causes a change in the internal environment.
- Receptors detect the change and send signals to the control center (often the brain).
- The control center processes the information and signals an effector to reverse the change.
- The effector restores the condition to its normal state, stopping the response.
Effector
A specialized cell or structure that detects changes in the internal or external environment and sends signals to the control center.
Effector
A structure (e.g., muscle or gland) that responds to signals from the control center to restore homeostasis.
| Homeostatic Process | Stimulus (Change) | Response (Negative Feedback) |
|---|---|---|
| Body Temperature Regulation | Increase in body temperature | Sweating and vasodilation (heat loss) |
| Blood Glucose Control | High blood glucose | Insulin release → Lowers glucose |
| Blood Pressure Regulation | High blood pressure | Heart rate decreases, blood vessels dilate |
| Oxygen Levels in Blood | Low oxygen levels | Increased breathing rate to absorb more oxygen |
- When thyroid hormone (T3 and T4) levels in the blood are low, the hypothalamus releases TRH (thyrotropin-releasing hormone), which stimulates the pituitary gland to release TSH (thyroid-stimulating hormone).
- This stimulates the thyroid gland to produce more thyroid hormones.
- Once thyroid hormone levels are restored to normal, negative feedback inhibits the release of TRH and TSH.
Positive Feedback
Positive feedback
Positive feedback is a physiological mechanism in which a change in a condition leads to an amplified response, moving further away from homeostasis.
Positive feedback amplifies the output of a process, making it more pronounced.
| Process | Stimulus (Change) | Response (Positive Feedback) |
|---|---|---|
| Childbirth (Labor Contractions) | Pressure on cervix | Oxytocin release → Stronger contractions |
| Blood Clotting | Vessel injury | Platelets release chemicals → More clotting |
| Lactation (Milk Ejection Reflex) | Baby suckles | Oxytocin release → More milk production |
Oxytocin and childbirth:
- During labor, the stretching of the cervix stimulates the release of oxytocin from the pituitary gland.
- Oxytocin causes uterine contractions, which push the baby toward the cervix, causing further stretching and even more oxytocin release.
- This process continues until the baby is delivered.
Students often assume all feedback is negative feedback.
Remember:
- Negative feedback = Stabilizes the body (homeostasis).
- Positive feedback = Intensifies a process (not homeostasis).
Regulation of the Heart: Intrinsic and Extrinsic Control
The regulation of heart function depends on both intrinsic and extrinsic mechanisms, ensuring that oxygenated blood reaches tissues efficiently.
Intrinsic Regulation
- Intrinsic regulation occurs within the heart itself without external influence.
- The heart has its own built-in pacemaker system that generates electrical impulses.
- The conduction system of the heart consists of specialized autorhythmic cells that generate and spread electrical impulses:
1. Sinoatrial (SA) Node – The Pacemaker
Sinoatrial (SA) Node
The heart’s natural pacemaker, responsible for initiating electrical impulses that regulate heartbeat.
- Located in the right atrium, it generates spontaneous electrical impulses (action potentials).
- Sets the normal sinus rhythm of the heart (60–100 beats per minute at rest).
- Impulses travel through the atria, causing them to contract.
2. Atrioventricular (AV) Node – The Delay Center
Atrioventricular (AV) Node
A structure in the heart that delays electrical impulses to allow the atria to contract before the ventricles.
- Located at the junction between the atria and ventricles.
- Delays the electrical impulse slightly to allow the atria to fully contract before the ventricles are stimulated.
- Think of the SA node as a drummer setting the rhythm for the entire band (heart).
- The AV node ensures the band plays in sync, preventing chaotic beats.
3. Bundle of His & Purkinje Fibers – The Ventricular Conduction System
Bundle of His
A part of the heart’s conduction system that transmits impulses from the AV node to the ventricles.
Purkinje Fibers
Specialized fibers in the heart that distribute electrical impulses to the ventricles, ensuring coordinated contraction.
- The impulse travels from the AV node to the Bundle of His, then to the left and right bundle branches and Purkinje fibers.
- This ensures coordinated contraction of the ventricles, pumping blood efficiently.
Even when removed from the body, a heart can continue beating for a short period due to its intrinsic conduction system (as long as it has an oxygen supply).
Extrinsic Regulation
While the heart can beat on its own, external factors regulate its rate and force based on the body's needs. This is controlled by:
- The Autonomic Nervous System (ANS)
- Hormonal influences (e.g., epinephrine/adrenaline)
Extrinsic control mechanisms include:
- Autonomic Nervous System (ANS)
- Sympathetic Nervous System (SNS): Increases heart rate & force of contraction (e.g., during exercise).
- Parasympathetic Nervous System (PNS): Decreases heart rate (e.g., at rest).
- Hormonal Regulation
