The Cardiovascular System: Anatomy

• The cardiovascular system consists of the heart and vascular network, ensuring tissue perfusion.

Functional Anatomy

• The heart, a four-chambered muscular organ, resides in the mid-mediastinum behind the sternum.
• Surface grooves, or called sulci, mark the boundaries of the heart chambers.
• The heart is enclosed in the pericardium:
  • Fibrous pericardium: an inelastic sac surrounds the heart.
  • Serous pericardium: the inner lining of the fibrous pericardium.
    • Parietal layer: inner lining of the fibrous pericardium.
    • Visceral layer (epicardium): covers the heart's outer surface and great vessels.
• Pericardial fluid lubricates the serous pericardium layers.
• Pericarditis occurs when the pericardium becomes inflamed.
• A pericardial effusion, caused by excess fluid, can lead to cardiac tamponade, impairing heart function.
• The heart wall has three layers: the outer epicardium, the middle myocardium with striated muscle fibers, and the inner endocardium.
• Four atrioventricular (AV) rings provide support and electrically isolate the atria from the ventricles.
• Each ring is composed of dense connective tissue called annulus fibrosus cordis, which encircles the bases of the pulmonary trunk, aorta and heart valves.
• The atria are thin-walled chambers, separated by an intra-atrial septum that contains the fossa ovalis cordis.
• The ventricles form the bulk of the heart muscle, with the left ventricle having greater mass than the right.
• The interventricular septum separates the right and left ventricles.
• Atrioventricular (AV) valves (tricuspid on the right, mitral on the left) prevent backflow into the atria during ventricular contraction.
• Chordae tendineae cordis anchor the AV valves to papillary muscles in the endocardium.
• Semilunar valves prevent backflow into the ventricles during diastole.
• The coronary circulation is the heart's circulatory system, meeting high metabolic demands with extensive branching.
• Coronary arteries fill during diastole and partial obstruction can cause ischemia, while complete obstruction causes myocardial infarction (MI).
• Acute Coronary Syndrome (ACS) includes unstable angina, Non-ST segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI).
• The coronary sinus collects venous blood and empties into the right atrium; thebesian veins also shunt blood into the heart chambers.

Heart Muscle Properties

• Myocardial tissue's excitability allows response to electrical, chemical, or mechanical stimuli.
• Inherent rhythmicity (automaticity) allows spontaneous impulse initiation, especially in the sinoatrial (SA) and atrioventricular (AV) nodes.
• Conductivity enables electrical impulse spread, facilitating coordinated contraction.
• Contractility is the myocardium's primary function, with a refractory period preventing sustained contractions.

Vascular System and Circulation

• The vascular system includes systemic and pulmonary circulations.
• Systemic circulation starts with the aorta (left ventricle) and ends in the right atrium.
• Pulmonary circulation starts with the pulmonary artery (right ventricle) and ends in the left atrium.
• Venous blood from the head and upper extremities flow to the right atrium via the superior vena cava (SVC).
• Venous blood from the abdomen and lower body flows to the right atrium via the inferior vena cava (IVC).
• The right ventricle pumps venous blood into the pulmonary arteries for gas exchange in the lungs.
• Arterial blood returns to the left atrium through the pulmonary veins for gas exchange in the body.
• The systemic circulation consists of arterial, capillary, and venous systems and regulates blood flow and distribution.
• Large arteries transmit pressure, arterioles control flow resistance, and capillaries enable nutrient and waste exchange.
• Arteriovenous anastomoses shunt blood away from capillary beds.
• Smooth muscle rings, called precapillary sphincters, control local blood flow.
• The venous system acts as a reservoir, with capacitance vessels altering blood volume distribution.
• Venous return to the heart is aided by sympathetic venous tone, skeletal muscle pumping, cardiac suction, and the thoracic pump.

Vascular Resistance

• Systemic vascular resistance (SVR) opposes blood flow in systemic circulation.
• Pulmonary vascular resistance (PVR) opposes blood flow in pulmonary circulation.
• Resistance changes regulate blood flow in organs, with arterial and venous pressures maintained within a range.

Blood Pressure Determinants

• Blood pressure is vital in the regulation of blood flow
• Mean arterial pressure (MAP) = (cardiac output (CO) × systemic vascular resistance (SVR)) + central venous pressure (CVP). The cardiovascular system maintains sufficient pressure to propel blood. Relative blood volume and vascular capacity affects MAP. Vasoconstriction increases blood pressure, while vasodilation decreases it. Septic shock happens if MAP falls below a certain threshold.

Cardiovascular System Control

• The system maintains adequate tissue perfusion through coordinated heart and vascular functions.
• Blood flow changes regulating blood volume and vascular capacity are important for homeostasis.
• Control mechanisms are intrinsic (local) and extrinsic (neural) functions, operating independently or centrally.

Peripheral Vasculature Regulation

• Vascular muscle tone is maintained for effective regulation.
• Local control involves precapillary sphincters adjusting flow based on tissue needs.
• Central control involves the nervous system and hormones affecting arterioles and veins.
• Myogenic and metabolic factors regulate local tissue blood flow, with metabolites affecting smooth muscle tone.
• Central control is mainly through the sympathetic nervous system, causing vasoconstriction or vasodilation.

Cardiac Output Regulation

• Cardiac output (CO) (the amount of blood pumped by the heart per minute)
• cardiac output (CO) = heart rate (HR) × stroke volume (SV).
• Stroke volume (SV) is the ejected blood volume per contraction and SV is primarily effected by preload, afterload, and contractility. SV = end-diastolic volume (EDV) - end-systolic volume (ESV)
• Ejection fraction (EF) is the proportion of EDV ejected per stroke. EF = (SV / EDV)x100

Factors Affecting Stroke Volume

• Preload: force stretching ventricles at the end of diastole.
• Afterload: force the left ventricle must overcome and is impacted by peripheral vascular resistance and physical blood composition.
• Higher stroke volume for a given preload indicates positive inotropism, while the opposite indicates negative inotropism.

Changes to Heart Rate

• Factors increasing heart rate are called positive chronotropic factors.
• Factors decreasing heart rate are called negative chronotropic factors.

Cardiovascular Control Mechanisms Central Control

• Central control integrates brainstem and peripheral receptors and maintains blood flow unless conditions are abnormal.
• These inputs are then integrated to have adequate blood flow and pressure under normal conditions.
• Central control uses baroreceptors and chemoreceptors to maintain control. Combining baro and chemoreceptors creates a negative feedback response.