Explain the interal structure o of human heart

Human Circulatory System

• Humans have a closed circulatory system: Blood pumped by the heart always flows through a closed network of blood vessels.

• Human circulatory system consists of:

  • Muscular, four-chambered heart

  • A network of closed, branching blood vessels − veins, arteries and capillaries

  • Blood

Heart

• Location: Thoracic cavity in between the lungs; slightly tilted to the left

• Protected by a double-walled pericardium, enclosing the pericardial fluid

• Has 4 chambers: 2 upper chambers − right and left atria
  2 lower chambers − right and left ventricles

• Inter-atrial septum: Separates the right and the left atria

• Inter-ventricular septum: Separates the right and the left ventricles

• Atrio-ventricular septum: Separates the atria and the ventricles of the same sides

• Septa have openings through which the two chambers on the same sides are connected.

• Tricuspid valve: Present between the right atria and the right ventricle

• Bicuspid (mitral) valve: Present between the left atria and the left ventricle

• Semilunar valves: Guard the openings of the right and the left ventricles into the pulmonary artery and the aorta respectively.

• Special cardiac musculature called nodal tissue is distributed throughout the heart.

• Sinoatrial node (SAN): Present at the upper right corner of the right atrium

• Atrio-ventricular node (AVN): Present at the lower left corner of the right atrium

• AV bundle (a bundle of nodal fibres) continues from the AVN and passes through the atrio-ventricular septa to reach the inter-ventricular septum.

• There, it divides immediately into right and left bundles. From these branches, minute fibres arise throughout the ventricular musculature. These fibres are called purkinje fibres.

• Right and left bundles + Purkinje fibres = Bundle of His

• Significance of nodal musculature: Auto-excitable; generates and maintains action potential to sustain the rhythmic contraction activity of the heart

• Pacemaker of the heart − Sino-atrial node (SAN)

• Heart beats 70−75 times/min

Double Circulation

• In human beings, oxygenated blood is received by the left atria while deoxygenated blood is received by the right atria, which then pass it on to their respective ventricles.

• This prevents the oxygenated and deoxygenated blood from mixing. This unique pathway is called double circulation.

Blood Circulation Pathway

Cardiac Cycle

• Cardiac cycle is the sequence of events which occur from the beginning of one heart beat to the beginning of the next heart beat.

• In the beginning, all the 4 chambers of the heart are in a state of joint diastole (relaxation).

• Tricuspid and bicuspid valves open and blood from the veins and the vena cava flow into the atria, and then into the ventricles because of the opening of the valves.

• SAN generates an action potential, and both atria undergo contraction (Atrial systole).

• The flow of blood into the ventricles increases by 30%.

• The action potential is conducted towards the ventricles through the AVN and the AV bundles, from where the bundle of His transmits this action potential over the entire cardiac musculature.

• The ventricles contract (ventricular systole) and the atria relax (atrial diastole) as a result of the conduction of action potential.

• Ventricular pressure increases. Hence, bicuspid and tricuspid valves close, to prevent the backflow of blood into the atria. Further increase in pressure in the ventricles leads to the opening of the semilunar valves.

• Blood from the ventricles flow into the pulmonary artery and the aorta, and subsequently into the circulatory pathways.

• Consequently, the ventricles relax (ventricular diastole), ventricular pressure falls, and the semilunar valves close to prevent the backflow of blood into the ventricles.

• Ventricular pressure further falls. As a result, the bicuspid and tricuspid valves open. This is because pressure is exerted on the atria by the blood entering them through the veins.

• Once again, joint diastole is experienced and the entire cycle is repeated.

Cardiac output

• • Heart beats: Average 72 times/minute (heart rate)

  • Duration of cardiac cycle is 0.8 seconds.

  • Stoke volume: Amount of blood pumped by the heart in one cardiac cycle
    Stoke volume = 70 mL

  • Cardiac output = Stoke volume × Heart rate
    = 70 mL × 72 times / min
    ~ 5000 mL

Heart Sounds 

• • Lub: First heart sound, associated with the closure of the tricuspid and bicuspid valves

  • Dub: Second heart sound, associated with the closure of the semilunar valves

  • These heart sounds are of diagnostic significance.
    
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