Homeostasis: Maintaining Internal Balance

1. Homeostasis is the self-regulating process that maintains a stable internal environment for optimal body function, despite changes in external or internal conditions.
2. It ensures that physiological variables such as temperature, pH, heart rate, and blood pressure remain within narrow limits.

Key Characteristics of Homeostasis

1. Self-regulating: The body constantly monitors and adjusts conditions.
2. Dynamic equilibrium: Conditions fluctuate within a narrow range rather than being completely static.
3. Essential for survival: Disruptions in homeostasis can lead to disease or dysfunction.

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

In negative feedback, the output of a process inhibits its own production, helping to maintain homeostasis.

How Negative Feedback Works

1. A stimulus causes a change in the internal environment.
2. Receptors detect the change and send signals to the control center (often the brain).
3. The control center processes the information and signals an effector to reverse the change.
4. The effector restores the condition to its normal state, stopping the response.

Positive Feedback

Positive feedback amplifies the output of a process, making it more pronounced.

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

1. Intrinsic regulation occurs within the heart itself without external influence.
2. The heart has its own built-in pacemaker system that generates electrical impulses.
3. The conduction system of the heart consists of specialized autorhythmic cells that generate and spread electrical impulses:

1. Sinoatrial (SA) Node – The Pacemaker

1. Located in the right atrium, it generates spontaneous electrical impulses (action potentials).
2. Sets the normal sinus rhythm of the heart (60–100 beats per minute at rest).
3. Impulses travel through the atria, causing them to contract.

2. Atrioventricular (AV) Node – The Delay Center

1. Located at the junction between the atria and ventricles.
2. Delays the electrical impulse slightly to allow the atria to fully contract before the ventricles are stimulated.

3. Bundle of His & Purkinje Fibers – The Ventricular Conduction System

1. The impulse travels from the AV node to the Bundle of His, then to the left and right bundle branches and Purkinje fibers.
2. This ensures coordinated contraction of the ventricles, pumping blood efficiently.

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:

1. The Autonomic Nervous System (ANS)
2. Hormonal influences (e.g., epinephrine/adrenaline)

Extrinsic control mechanisms include:

1. Autonomic Nervous System (ANS)
   1. Sympathetic Nervous System (SNS): Increases heart rate & force of contraction (e.g., during exercise).
   2. Parasympathetic Nervous System (PNS): Decreases heart rate (e.g., at rest).
2. Hormonal Regulation
   1. Epinephrine & Norepinephrine: Released from adrenal glands, increasing heart rate and contractility.
   2. Thyroid Hormones: Affect metabolic rate and cardiac output

Thermoregulation (Regulation of Body Temperature)

1. Thermoregulation is the body's ability to maintain a core temperature of 37 ± 1°C, ensuring optimal enzyme activity and cellular function.
2. The cardiovascular, muscular, nervous, and integumentary systems work together to regulate body temperature.

Mechanisms of Thermoregulation

1. Heat Loss Mechanisms (Cooling the Body in Hot Conditions)

When the body is overheating, heat dissipation mechanisms are activated:

1. Sweat Response: Sweat glands produce sweat, which evaporates and removes heat from the skin surface.
2. Vasodilation: Blood vessels near the skin widen, increasing blood flow to the skin’s surface and promoting heat loss.
3. Behavioral Adaptations: Seeking shade, reducing activity, or wearing loose clothing helps prevent overheating.

Heat Retention Mechanisms (Warming the Body in Cold Conditions)

When the body detects a drop in temperature, heat conservation mechanisms activate:

1. Vasoconstriction: Blood vessels in the skin constrict to reduce blood flow and minimize heat loss.
2. Shivering: Rapid, involuntary muscle contractions generate heat to maintain body temperature.
3. Non-Shivering Thermogenesis: Brown adipose tissue (BAT) produces heat by metabolizing stored fat, especially in infants and hibernating animals.
4. Piloerection ("Goosebumps"): Hair stands up, trapping an insulating layer of air (less effective in humans).

Factors Affecting Thermoregulation

1. Training Status: Athletes develop a more efficient sweating response and better blood flow to the skin.
2. Body Composition: Individuals with higher fat percentages retain heat more efficiently, while those with more muscle generate more heat.
3. Environmental Conditions: Humidity reduces sweating efficiency, while extreme cold increases the risk of hypothermia.
4. Sex Differences & Hormonal Phases: Estrogen can increase blood vessel dilation, while progesterone has a thermogenic effect, slightly increasing body temperature.

Regulation of Blood Glucose

1. The body tightly regulates blood glucose levels (normally 4.0–7.8 mmol/L) to provide energy while preventing hyperglycemia or hypoglycemia.
2. This regulation primarily involves the hormones insulin and glucagon, released by the pancreas.

Insulin

When blood glucose levels rise after eating, beta cells in the pancreas release insulin. Insulin facilitates:

1. Glucose uptake by muscle, liver, and fat cells for storage and energy use.
2. Conversion of glucose to glycogen (glycogenesis) for storage in the liver and muscles.
3. Inhibition of gluconeogenesis (new glucose production from non-carbohydrate sources).

Glucagon

During fasting, alpha cells of the pancreas release glucagon, which:

1. Stimulates glycogen breakdown (glycogenolysis) in the liver to release glucose into the blood.
2. Promotes gluconeogenesis, generating glucose from proteins and fats.
3. Inhibits glucose uptake by muscle and liver cells.

Impact of Exercise on Blood Glucose Regulation

Exercise naturally helps regulate blood sugar levels by:

1. Increasing glucose uptake by muscle cells without requiring insulin.
2. Suppressing insulin release to prevent excessive glucose uptake.
3. Enhancing glycogen breakdown to fuel active muscles.