27. Cardiovascular system. Systemic and pulmonary circulation. The heart as an organ – functional morphology of the endocardium, myocardium, and pericardium. Nerve supply. Myocardial blood supply

1. Systemic and Pulmonary Circulation

• pulmonary circulationL

• Deoxygenated blood enters the right atrium from the superior and inferior vena cava.

• Pumped into the right ventricle, then through the pulmonary arteries to the lungs.

• Low arterial pressure to allow effective gas exchange.

• If pressure is too high → risk of hypoxia (↓ O₂) and hypercapnia (↑ CO₂).

Systemic Circulation

• Oxygenated blood returns from the lungs via pulmonary veins into the left atrium.

• Pumped into the left ventricle, then out through the aorta to supply body tissues.

• High arterial pressure due to:

  • Thick smooth muscle layer in systemic arteries.

  • Vessels can constrict/dilate via smooth muscle receptors.

2. The Heart as an Organ – Functional Morphology

heart as an organ: The heart has four chambers:

• 2 atria (right and left)

• 2 ventricles (right and left)

• Interventricular septum separates the right and left sides.

• Valves ensure unidirectional blood flow:

  • Atrioventricular (AV) valves:

    • Tricuspid valve – between right atrium and right ventricle

    • Bicuspid (mitral) valve – between left atrium and left ventricle

  • Semilunar valves:

    • Pulmonary valve – between right ventricle and pulmonary artery

    • Aortic valve – between left ventricle and aorta

    • Endocardium

• Inner lining of the heart chambers.

• Made of endothelial cells.

• Smooth surface to reduce friction and support valve function.

Myocardium

• Thick muscular layer composed of cardiac muscle fibers.

• Responsible for the contractile function of the heart.

• Thicker in the left ventricle due to higher pressure demands.

Pericardium

• Double-walled sac surrounding the heart.

  • Fibrous pericardium – outer tough layer for protection and anchoring.

  • Serous pericardium – inner layer:

    • Parietal layer

    • Visceral layer (epicardium)

• Contains pericardial fluid to reduce friction during heartbeats.

3. Nerve Supply of the Heart

Sympathetic Nervous System

• Controlled by adrenaline binding to β1-adrenergic receptors.

• Innervates both atria and ventricles.

• Effects:

  • Positive chronotropic (↑ heart rate)

  • Dromotropic (↑ conduction speed)

  • Batmotropic (↑ excitability)

  • Inotropic (↑ contraction strength)

  • Tonotropic (↑ tone)

Parasympathetic Nervous System

• Controlled by acetylcholine binding to M2 muscarinic receptors.

• Innervates only the atria.

• Vagus nerve divisions:

  • Right vagus → innervates the Sinoatrial Node (SAN)

  • Left vagus → innervates the Atrioventricular Node (AVN)

4. Myocardial Blood Supply

• -Left coronary artery s
  Supplies left atrium and left ventricle.

• Right Coronary Artery (RCA):

  • Supplies right atrium, right ventricle, SAN, and AVN.