Cardiovascular Anatomy and Physiology

Cardiovascular Anatomy and Physiology

The Human Heart

  • The heart is located in the anterior mediastinum, above the diaphragm, and in front of the vertebral column, esophagus, and posterior mediastinum.
  • Position: The heart sits approximately 1-2 cm left of the midline of the body.
  • Asymmetry: The left lung is slightly smaller than the right lung due to the heart's protrusion into the left chest cavity.
  • Apex Location: The apex of the heart is at the fifth intercostal space at the left midclavicular line.
  • Major Vessels Position: The superior and inferior vena cava, pulmonary artery, pulmonary vein, and aorta are situated above the heart.
  • Size and Weight:
    • Description: The human heart is pear-shaped and about the size of an adult fist.
    • Measurements (Normal Adults):
    • Length: Approximately 12 cm
    • Width: 8-9 cm
    • Thickness: 6 cm
    • Weight: 250-350 grams (depending on age and gender)
    • Notable Trend: Weight and size of the heart increase with age, especially in men.

The Pericardium

  • Definition: The pericardium is a double-layered sac that contains the heart.
  • Layers:
    • Fibrous Pericardium:
    • Structure: Outermost layer made of dense fibrous connective tissue.
    • Functions: Protects the heart, anchors it to surrounding structures, limits movement, prevents overfilling with increased blood volume.
    • Serous Pericardium:
    • Structure: Thinner and more fragile than the fibrous layer, consists of two layers of flat cells.
    • Functions: Lubricates the heart, decreasing friction and absorbing shock during heart activity.
  • Pericardial Cavity: Space between parietal and visceral layers containing pericardial fluid.

The Epicardium

  • Definition: The innermost layer of the pericardium, also known as visceral pericardium.
  • Structure: Cone-shaped sac that envelops the heart, made of thin connective tissue covered by fat.
  • Functions: Covers the outer surface of myocardium, forms a protective cover, produces pericardial fluid, contains vessels supplying blood to the myocardium.

The Myocardium

  • Definition: The middle muscular layer of the heart.
  • Structure: Surrounded by epicardium externally and lined by endocardium internally.
  • Composition: Composed of cardiac muscle, described as striated when viewed microscopically, involuntary muscle requiring no central nervous system input to contract.
  • Functions: Stimulates the heart to pump blood from the ventricles to the body and relaxes atria to collect blood.

The Endocardium

  • Definition: The innermost layer of the heart lining the myocardium.
  • Structure: Thin, smooth membrane made of a layer of endothelial cells, continuous with the inner lining of large blood vessels.
  • Functions: Conduit for blood flow, contains Purkinje fibers participating in myocardial contraction by rapidly conducting electrical signals.

Heart Chambers and Blood Flow

Right Atrium

  • Function: Receives deoxygenated blood from the body via the superior and inferior vena cava.
  • Structure: Larger than the left atrium, capable of expanding to accommodate larger blood volumes at low pressure (0-3 mmHg).
  • Valves: Separated from the right ventricle by the tricuspid valve.

Right Ventricle

  • Function: Pumps blood to the lungs for oxygenation.
  • Structure: Thick muscular walls, contracts to pump blood from atrium through the tricuspid valve, and opens pulmonary valve to enter pulmonary artery.
  • Pressure Maintenance: High pressure for effective blood expulsion despite inability to expand significantly.

Left Atrium

  • Function: Receives oxygenated blood from the lungs via four pulmonary veins.
  • Structure: Capable of expanding under lower pressure (6-10 mmHg), but not as much as the right atrium.
  • Valves: Separated from the left ventricle by the mitral (bicuspid) valve.

Left Ventricle

  • Function: Pumps oxygenated blood to the entire body.
  • Structure: Thick muscular walls capable of generating high pressure; pumps a larger blood volume than right ventricle.
  • Valves: Mitral valve prevents backflow to the atrium, opens aortic valve to release blood into the aorta.

Cardiac Valves

  • Atrioventricular (AV) Valves:
    • Tricuspid Valve: Between right atrium and right ventricle.
    • Mitral or Bicuspid Valve: Between left atrium and left ventricle.
    • Function: Allow blood flow in one direction; structures made of endocardium with fibrous chords (chordae tendineae) to prevent bulging.
  • Semilunar Valves:
    • Pulmonary Valve: Between right ventricle and pulmonary artery.
    • Aortic Valve: Between left ventricle and aorta.
    • Structure: Three crescent-shaped flaps ensuring unidirectional blood flow; no chordae tendineae.

Control Mechanisms for Cardiac Output

  • Definition: Cardiac output is determined by heart rate (HR) and stroke volume (SV). Formula:
    CO = HR imes SV
  • Changes in HR or SV directly affect cardiac output.
  • Stroke Volume: Controlled by intrinsic factors (heart's ability to respond to input) and extrinsic factors (contractility, influenced by hormones, autonomic nervous system, etc.).

Cardiac Blood Flow

  • Sequence: Blood flows from the body to the right atrium (via superior/inferior vena cava), to right ventricle, then to lungs (via pulmonary artery), back to left atrium (via pulmonary veins), and then pumped to the body via left ventricle (through aorta).

Heart Sounds

  • S1: Closing of AV valves, signals the beginning of systole.
  • S2: Closing of semilunar valves, marks end of systole.
  • S3: Can be heard in children, young adults, or heart failure patients; indicates rapid ventricular filling.
  • S4: Heard just before S1, associated with atrial contraction; found in patients with weak ventricles.

Important Terms

  • Ischemia: Lack of oxygen delivery to an organ due to obstruction of blood vessels.
  • Myocardial Ischemia: Specific to the left ventricle not receiving adequate oxygenated blood; often due to atherosclerosis.
  • Atherosclerosis: Fatty plaque in coronary arteries obstructing blood flow; can lead to myocardial injury or infarction (heart attack).
  • Peripheral Resistance: Total resistance in arterioles, affects blood pressure.
  • Baroreceptors/Chemoreceptors: Senses changes in blood pressure and cardiovascular response mechanisms.

Vascular Terminology

  • Anterior or Ventral: Near the front of the body.
  • Posterior or Dorsal: Near the back of the body.
  • Superior: Toward the head; synonymous with "cephalic".
  • Inferior: Toward the feet; synonymous with "caudal".
  • Medial: Near the middle of the body.
  • Lateral: To the side of the body.
  • Distal: Away from the center of the body.
  • Proximal: Nearest to the center of the body.

Blood Circulation Pathways

Systemic Circulation

  • Description: System of blood vessels carries blood from the heart to the body's tissues and returns deoxygenated blood back to the heart.

Arterial Trees

  • Arteries: Blood vessels carrying oxygen-rich blood, branch from larger to smaller sizes: Large (1.0-4.0 mm), Small (0.5-1.0 mm), Arterioles (0.01-0.50 mm).
  • Major Arteries:
    • Coronary Arteries: Supply blood to the heart muscle.
    • Aorta: Main artery branching to supply head, upper body, abdominal organs, and lower body.

Venous Circulation

  • Venules and Veins: Small blood vessels that rejoin to form veins, returning oxygen-poor blood to the heart (0.5-5.0 mm).
  • Major systemic veins return deoxygenated blood from various body parts to the right atrium.

Capillaries

  • Structure: Smallest blood vessels composed entirely of endothelial cells; diameter: 0.006-0.01 mm.
  • Function: Sites of exchange for oxygen, carbon dioxide, water, electrolytes, proteins, and metabolic waste between blood and tissues.

Blood Pressure Parameters

  • Mean Arterial Pressure (MAP):

    • Calculated as:
      MAP = rac{(2 imes ext{diastolic}) + ext{systolic}}{3}
    • Normal MAP range: 70-110 mmHg; needed MAP for organ perfusion: 60 mmHg.

  • Pulse Pressure: Difference between systolic and diastolic pressures; regulated by stroke volume and arterial elasticity.

Measuring Blood Pressure

  • Method: Auscultatory method using a sphygmomanometer; normal systolic: 120 mmHg, normal diastolic: 80 mmHg.

Electrical Conduction System

  • Components:
    • Sinoatrial (SA) Node: Pacemaker of the heart, generates spontaneous impulses.
    • Atrioventricular (AV) Node: Slows impulse for proper atrial contraction before ventricular contraction.
    • Bundle of His, Bundle Branches, Purkinje Fibers: Conduct impulses rapidly throughout the ventricles for effective contraction.

Heart Rate Regulation

  • Influences: Effects from sympathetic (norepinephrine increases HR) and parasympathetic (acetylcholine decreases HR) nervous systems.
  • Normal heart rate: 60-100 beats per minute; limit is approximately 250 bpm, where faster rates may decrease cardiac output due to insufficient ventricular filling.