8A Human Structure and Function: The Heart

Cardiovascular System and Heart Anatomy

  • The heart functions as a double pump driving two circuits:

    • Pulmonary circulation: Delivers blood to the lungs.

    • Systemic circulation: Delivers blood to the body.

  • Location: Situated in the chest wall, slightly left and posterior to the sternum within the pericardial cavity.

  • Orientation: The Base (superior end) is where major blood vessels attach; the Apex (inferior end) points downward.

  • Chambers:

    • Atria (Left and Right): Receiving chambers with thin muscular walls.

    • Ventricles (Left and Right): Pumping chambers with thicker muscular walls (the left ventricle is significantly thicker).

  • Vessels: Arteries carry blood away from the heart; veins carry blood to the heart.

Circulation Path and Heart Valves

  • Right Side: Receives deoxygenated blood from the Superior vena cava (upper body) and Inferior vena cava (lower body) into the Right Atrium. Blood flows through the Tricuspid valve into the Right Ventricle and is pumped through the Pulmonary valve into the Pulmonary artery (Left and Right) toward the lungs.

  • Left Side: Receives oxygenated blood from Pulmonary veins into the Left Atrium. Blood flows through the Bicuspid (mitral) valve into the Left Ventricle and is pumped through the Aortic valve into the Aorta for systemic circulation.

  • Valvular Function:

    • AV valves (Tricuspid/Bicuspid) prevent backflow into atria during ventricular contraction.

    • Semilunar valves (Pulmonary/Aortic) prevent backflow into ventricles during relaxation.

    • Chordae tendineae and papillary muscles help stabilize AV valves.

Electrical Conduction and ECG

  • The conducting system uses autorhythmic nodal cells to coordinate contraction.

  • Sinoatrial (SA) node: Located in the right atrial wall; acts as the primary pacemaker with a fast spontaneous activity rate.

  • Atrioventricular (AV) node: Located at the junction of atria and ventricles; possesses a slower spontaneous rate.

  • Electrocardiogram (ECG) Components:

    • P wave: Represents atrial contraction.

    • QRS complex: Represents atrial relaxation and ventricular contraction.

    • T wave: Represents ventricular relaxation.

Cardiac Cycle and Heart Sounds

  • The cycle consists of alternating phases of contraction (Systole) and relaxation (Diastole).

  • Systole: Chambers contract and eject blood.

  • Diastole: Chambers relax and fill with blood.

  • Heart Sounds:

    • "Lub" (S1): Produced by the closure of the Atrioventricular (AV) valves.

    • "Dub" (S2): Produced by the closure of the Semilunar valves.

Cardiac Output and Regulatory Factors

  • Cardiac Output (COCO): The total volume of blood pumped per minute (L.min−1L.min^{-1}).

  • Formula: CO=HR×SVCO = HR \times SV

  • Components:

    • Heart Rate (HRHR): Measured in bpmbpm or b.min−1b.min^{-1}.

    • Stroke Volume (SVSV): Measured in mL.beat−1mL.beat^{-1} (SV=EDV−ESVSV = EDV - ESV).

  • Example: If HR=75 bpmHR = 75\,bpm and SV=80 mL.beat−1SV = 80\,mL.beat^{-1}, then CO=75×80=6000 mL.min−1=6 L.min−1CO = 75 \times 80 = 6000\,mL.min^{-1} = 6\,L.min^{-1}.

  • Factors affecting HR:

    • Sympathetic Nervous System (SNS) and Adrenaline: Increase HRHR via the SA node.

    • Parasympathetic Nervous System (PSNS): Decreases HRHR.

    • Atrial reflex: Increased venous return stretches atrial cells, increasing SNS activity and HRHR.

  • Factors affecting SV:

    • Increase SV: Sympathetic stimulation, Adrenaline, and increased Venous return (Preload/stretching ventricular cells).

    • Decrease SV: Anti-hypertensive drugs and increased blood pressure (Afterload), which reduces ventricular ejection.