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Anatomy of the Heart

  • Superior vena cava

  • Right atrium

  • Tricuspid valve

  • Right pulmonary veins

  • Right ventricle

  • Papillary muscles

  • Aorta

  • Right and left pulmonary arteries

  • Pulmonary semilunar valve

  • Left pulmonary veins

  • Left atrium

  • Bicuspid (mitral) valve

  • Chordae tendineae

  • Left ventricle

  • Interventricular septum

  • Inferior vena cava

Heart Structure and Function

Septum

  • Function: Separates the left and right side of the heart.

Valves

Function

  • Control forward motion of blood flow through the heart and prevent backflow of blood within the heart chambers.

Types of Valves

  • Atrioventricular (AV) Valves:

    • Bicuspid valve: Separates the left atrium and left ventricle.

    • Tricuspid valve: Separates the right atrium and right ventricle.

  • Semi-Lunar (SL) Valves:

    • Pulmonary Valve: Exits the right ventricle into the pulmonary artery.

    • Aortic Valve: Exits the left ventricle into the aorta.

Blood Flow Pathway

  • Superior vena cava: Carries deoxygenated blood from the body to the heart.

  • Inferior vena cava: Also carries deoxygenated blood from the lower part of the body.

  • Pulmonary Artery: Deoxygenated blood from the right ventricle to the lungs.

  • Pulmonary Veins: Oxygenated blood from the lungs to the left atrium.

  • Aorta: Carries oxygenated blood from the left ventricle to the entire body.

Coronary Circulation

  • Coronary Arteries: Left and right branches from the aorta that encircle and supply the heart muscle with oxygen and glucose.

  • Coronary Veins: Drain deoxygenated blood directly back into the right atrium via the coronary sinus.

The Journey of Blood Through the Heart

  1. Blood enters the heart. Deoxygenated blood flows into the right atrium from the body via the vena cava.

    • The superior vena cava drains blood from the upper portion of the body.

    • The inferior vena cava drains blood from the lower portion of the body.

  2. Blood moves to the right ventricle, traveling through the tricuspid valve.

  3. Blood exits the heart via the pulmonary artery, becoming oxygenated in the lungs.

  4. Fresh blood leaves the lungs via the pulmonary vein to the left atrium.

  5. Blood enters the left ventricle through the mitral valve.

  6. Blood leaves the heart via the aorta to reach the rest of the body.

  7. The cycle repeats, returning to the vena cava.

Cardiac Conduction System

Components

  • SA Node: Responsible for initiating the heartbeat, located in the right atrium.

  • AV Node: Transmits cardiac impulse initiated by the SA node.

  • Bundle of His: Transmits impulses from the AV node to the ventricles.

  • Purkinje Fibres: Help the ventricles to contract and maintain a consistent heart rhythm.

Cardiac Cycle

  • Defined as the electrical and mechanical events that take place during one complete heartbeat.

  • At rest, one complete heartbeat occurs every 0.8 seconds, resulting in approximately 72 heartbeats per minute.

  • Diastolic phase (0.5 sec): Heart relaxes and fills with blood.

  • Systolic phase (0.3 sec): Heart contracts, forcing blood out of the heart.

Diastolic Phase

  • Atria fill with blood and atrioventricular valves are closed.

  • Rising atrial pressure forces open the AV valves and ventricles begin to fill.

  • Semi-lunar valves leading to the aorta and pulmonary arteries are closed.

Systolic Phase

  • The SA node initiates an impulse causing atrial contraction.

  • Atrioventricular valves close after blood passes.

  • The AV node sends a contraction wave through the ventricles, forcing open the semi-lunar valves to the lungs and arteries.

  • Blood is ejected into the circulatory systems.

Dual Action Pump

  • The heart is a dual-action pump, consisting of two separate pumps for two different locations:

  • Pulmonary Circuit: Low oxygen, high CO₂ blood moves from the heart to the lungs.

  • Systemic Circuit: Oxygenated blood goes from the heart to the body tissues.

Heart Rate, Stroke Volume, and Cardiac Output

Definitions

  • Heart Rate: The number of times the heart ventricles beat in one minute, with an average resting HR around 70 bpm. Bradycardia is defined as a heart rate lower than 60 bpm.

  • Maximal HR: Estimated using the formula: 220 - ext{age}.

  • Stroke Volume (SV): The volume of blood ejected from the heart ventricles per beat.

  • Cardiac Output (Q): The amount of blood pumped out of the heart per minute, calculated by the formula: Q = ext{Heart Rate} imes ext{Stroke Volume} .

Stroke Volume and Cardiac Output

  • Untrained performer resting HR: 60-75 bpm, maximal HR: 220 - ext{age}.

  • Trained athlete resting HR: 40-60 bpm, maximal HR: 220 - ext{age}.

    • Typical Stroke Volumes:

  • Untrained: 60-80 ml/beat, 51/min; Trained: 80-100 ml/beat, 30-40/ min.

  • Cardiac Output increases during exercise, up to 25-40 liters per minute, depending on fitness levels.

Cardiac Response to Exercise

Heart Rate Changes

  • Anticipatory Rise: HR can increase before exercise due to adrenaline.

  • HR directly increases with exercise intensity and decreases as intensity decreases.

  • Plateaus during sub-maximal work and decreases rapidly post-exercise due to reduced oxygen demand.

Stroke Volume Changes

  • SV initially increases with exercise intensity, reaches a plateau at 40-60% of maximal effort. After this point, if cardiac output needs to increase further, it relies on increased heart rate rather than stroke volume.

Venous Return and Starling's Law

Definitions

  • Venous Return: Blood returning from the capillaries through veins back to the right atrium.

  • End-systolic volume (ESV): Volume in a ventricle at the end of contraction.

  • End-diastolic volume (EDV): Volume in a ventricle at the end of filling.

Starling's Law of the Heart

  • Stroke volume is dependent on venous return; an increase in VR leads to an increase in SV and vice versa.

  • Mechanisms affecting VR include muscle pumps, respiratory pumps, and gravity.

Blood Pooling

  • Blood pooling occurs when there is insufficient pressure to return blood to the heart, often resulting in a feeling of heavy legs.

  • Active cool downs and maintaining elevated heart rates help combat blood pooling post-exercise.

Distribution of Cardiac Output During Rest and Exercise

At Rest

  • Only 15-20% of resting cardiac output is supplied to muscles; the remainder supplies body organs.

During Exercise

  • Increased cardiac output, predominantly going to working muscles, while reducing supply to organs; blood supply to the brain remains constant.

    • Vascular Shunt Mechanism: Redistributes cardiac output as exercise intensity increases.

    • Controlled by the Vasomotor Control Centre in the medulla oblongata, influenced by chemoreceptors and baroreceptors.

Vascular Shunt Mechanism

Mechanism of Blood Flow Redistribution

  • During exercise:

    • Skeletal muscle arterioles vasodilate, increasing blood flow to active muscles.

    • Arterioles in organs vasoconstrict, decreasing their blood flow.

Vascular Shunt Mechanism Summary

  • At rest, blood flows primarily to organs; during exercise, it shifts to working muscles.

  • The effectiveness of blood distribution is critical for maximizing performance and maintaining physiological function during physical activity.