Activity of the Heart

Heart Rate Regulation

• Definition: The number of heartbeats per minute, influenced by various factors.
• Regulatory Mechanisms:
  • Autonomic Nervous System: Sympathetic (increases heart rate) and parasympathetic (decreases heart rate) branches.
  • Hormonal Influence: Adrenaline and noradrenaline increase heart rate; thyroid hormones can also elevate it.
  • Baroreceptors: Detect changes in blood pressure, signaling adjustments in heart rate.
  • Chemoreceptors: Monitor blood pH and gas concentration, affecting heart rate accordingly.

Blood Flow Dynamics

• Circulatory Pathways:
  • Systemic Circulation: Oxygenated blood from the heart to the body; deoxygenated blood returns.
  • Pulmonary Circulation: Deoxygenated blood from the heart to the lungs; oxygenated blood returns.
• Cardiac Output (CO): Volume of blood the heart pumps per minute (CO = Heart Rate x Stroke Volume).
• Resistance and Compliance:
  • Vascular Resistance: Impacts blood flow; higher resistance reduces flow.
  • Compliance: Ability of blood vessels to expand; affects blood pressure and flow.

Physiological Response to Exercise

• Increased Heart Rate: During exercise, heart rate rises to meet oxygen demands.
• Improved Stroke Volume: Enhanced efficiency of the heart leads to increased stroke volume.
• Blood Redistribution: Blood flow is redirected to muscles and skin, away from non-essential organs.
• VO2 Max: Maximum rate of oxygen consumption during intense exercise; a measure of cardiovascular fitness.

Electrical Conduction System

• Components:
  • Sinoatrial (SA) Node: Primary pacemaker, initiating electrical impulses.
  • Atrioventricular (AV) Node: Delays impulse to allow atrial contraction before ventricles contract.
  • Bundle of His and Purkinje Fibers: Conduct impulses through ventricles for coordinated contraction.
• Action Potentials: Triggered by ion exchange (Na+, K+, Ca2+) across cell membranes, leading to heart muscle contraction.

Cardiac Cycle

• Phases:
  • Diastole: Heart muscle relaxes; chambers fill with blood.
  • Systole: Heart muscle contracts; blood is ejected into arteries.
• Key Events:
  • Atrial Systole: Atria contract to fill ventricles.
  • Ventricular Systole: Ventricles contract to pump blood to lungs and body.
• Heart Sounds:
  • S1 (lub): Closure of AV valves during ventricular systole.
  • S2 (dub): Closure of semilunar valves at the end of ventricular systole.

Heart Rate Regulation

• Heart rate is defined as the number of heartbeats occurring in one minute, influenced by multiple physiological factors.
• The autonomic nervous system regulates heart rate through its two branches: the sympathetic nervous system, which increases heart rate, and the parasympathetic nervous system, which reduces it.
• Hormones such as adrenaline and noradrenaline elevate heart rate, while thyroid hormones also have a stimulating effect on heart function.
• Baroreceptors are specialized sensors that detect blood pressure fluctuations, prompting the body to adjust the heart rate accordingly.
• Chemoreceptors monitor changes in blood pH and gas levels, influencing heart rate as a response to these chemical changes.

Blood Flow Dynamics

• Systemic circulation refers to the pathway where oxygenated blood is distributed from the heart to the rest of the body, returning deoxygenated blood back to the heart.
• Pulmonary circulation involves the transportation of deoxygenated blood from the heart to the lungs, where it becomes oxygenated before returning to the heart.
• Cardiac output (CO) represents the volume of blood that the heart pumps in one minute, calculated as the product of heart rate and stroke volume (CO = Heart Rate x Stroke Volume).
• Vascular resistance plays a critical role in regulating blood flow, with higher resistance leading to decreased blood flow.
• Compliance describes the blood vessels' ability to expand in response to pressure, impacting overall blood pressure and flow.

Physiological Response to Exercise

• Heart rate significantly increases during exercise to meet the heightened oxygen demands of the body.
• Stroke volume improves as the heart becomes more efficient, allowing for greater blood pumped per beat.
• During physical activity, blood flow is strategically redirected to major muscle groups and the skin, while reducing circulation to less critical organs.
• VO2 Max is the maximum rate of oxygen consumption achieved during intense exercise, serving as a crucial indicator of cardiovascular fitness levels.

Electrical Conduction System

• The sinoatrial (SA) node functions as the heart's primary pacemaker, responsible for initiating the electrical impulses that trigger heartbeats.
• The atrioventricular (AV) node delays the electrical impulse briefly to ensure proper timing, allowing atrial contraction before ventricular contraction.
• The Bundle of His and Purkinje fibers are vital for conducting electrical impulses throughout the ventricles, ensuring synchronized contraction of the heart muscle.
• Action potentials are generated by the exchange of ions (sodium, potassium, and calcium) across cell membranes, leading to the contraction of heart muscle tissue.

Cardiac Cycle

• The cardiac cycle consists of two main phases: diastole, during which the heart muscle relaxes and chambers fill with blood, and systole, when the muscle contracts and ejects blood into the arteries.
• Atrial systole involves the contraction of the atria to facilitate the filling of the ventricles with blood.
• Ventricular systole is the phase where the ventricles contract to pump blood towards the lungs and the rest of the body.
• Heart sounds are produced by the closing of heart valves: S1 (lub), occurs with the closure of the AV valves during ventricular systole; S2 (dub), corresponds to the closure of the semilunar valves at the end of ventricular systole.

Explore the mechanisms behind heart rate regulation and the dynamics of blood flow in the circulatory system. Understand how the autonomic nervous system, hormonal influences, and various receptors contribute to these physiological processes. This quiz will test your knowledge on systemic and pulmonary circulation, cardiac output, and vascular resistance.