5.1 Heart Anatomy, Blood Components & Vascular System

5.1 Heart Anatomy, Blood Components & Vascular System

CIDESCO Exam Tip: The cardiovascular system is a core topic in CIDESCO sanitation, physiology, and electrotherapy theory. Candidates are routinely evaluated on heart valve mechanics, cardiac conduction sequence, blood cell functional classification, blood pressure dynamics, and microvascular gas exchange at the capillary bed.

The cardiovascular system (blood vascular system) is a closed tubular network designed to pump and distribute blood throughout the human body. Driven by the rhythmic muscular contractions of the heart, blood transports oxygen, essential nutrients, hormones, and metabolic enzymes to peripheral tissues while picking up carbon dioxide and metabolic nitrogenous wastes for excretion. For aesthetic therapists and beauty professionals, a thorough knowledge of circulatory anatomy is essential when evaluating skin perfusion, administering body massage, operating electrotherapy equipment (such as faradic or galvanic machines), and identifying vascular contraindications.

Gross Anatomy of the Heart & Structural Layering

The human heart is a hollow, muscular, cone-shaped organ roughly the size of a closed fist, located in the thoracic cavity within the mediastinum (between the lungs), leaning slightly to the left of the midline. The heart wall is constructed from three distinct structural layers enveloped within a protective fibroserous sac:

1. Pericardium: The outer protective sac consisting of an external tough, fibrous layer (fibrous pericardium) and an internal two-layered serous membrane (serous pericardium). The pericardial cavity between the parietal and visceral serous layers contains pericardial fluid to reduce friction during cardiac contractions.
2. Epicardium: The visceral layer of the serous pericardium forming the smooth, thin outermost boundary of the heart wall.
3. Myocardium: The middle and thickest layer, composed of specialized, involuntary, striated cardiac muscle fibers interconnected by intercalated discs. The myocardium performs the actual mechanical pumping action, with the left ventricular myocardium being three times thicker than the right due to systemic pumping resistance.
4. Endocardium: The smooth, thin innermost layer of simple squamous endothelium lining the heart chambers and covering the heart valves, ensuring frictionless blood flow.

                    ┌────────────────────────────────────────┐
                    │          SUPERIOR VENA CAVA            │
                    └──────────────────┬─────────────────────┘
                                       │
                                       ▼
                            ┌─────────────────────┐
                            │    RIGHT ATRIUM     │
                            └──────────┬──────────┘
                                       │ (Tricuspid Valve)
                                       ▼
                            ┌─────────────────────┐
                            │   RIGHT VENTRICLE   │
                            └──────────┬──────────┘
                                       │ (Pulmonary Semilunar Valve)
                                       ▼
                            ┌─────────────────────┐
                            │  PULMONARY ARTERY   │
                            └──────────┬──────────┘
                                       │
                                       ▼
                            ┌─────────────────────┐
                            │  LUNGS (Gas Exch.)  │
                            └──────────┬──────────┘
                                       │
                                       ▼
                            ┌─────────────────────┐
                            │   PULMONARY VEINS   │
                            └──────────┬──────────┘
                                       │
                                       ▼
                            ┌─────────────────────┐
                            │     LEFT ATRIUM     │
                            └──────────┬──────────┘
                                       │ (Bicuspid / Mitral Valve)
                                       ▼
                            ┌─────────────────────┐
                            │   LEFT VENTRICLE    │
                            └──────────┬──────────┘
                                       │ (Aortic Semilunar Valve)
                                       ▼
                            ┌─────────────────────┐
                            │    AORTA (Systemic) │
                            └─────────────────────┘

Heart Chambers & Unidirectional Valve Mechanics

The internal cardiac cavity is divided longitudinally by a muscular septum (interatrial and interventricular septa) into right and left pumps, each containing an upper receiving chamber (atrium) and a lower pumping chamber (ventricle):

• Right Atrium: Receives deoxygenated venous blood returning from the systemic circulation via the superior vena cava (head, neck, upper limbs), inferior vena cava (lower body), and coronary sinus (heart wall).
• Right Ventricle: Pumps deoxygenated blood into the pulmonary trunk toward the lungs for re-oxygenation.
• Left Atrium: Receives oxygen-rich blood returning from the pulmonary circulation via four pulmonary veins.
• Left Ventricle: Pumps high-pressure oxygenated blood into the aorta to supply systemic body tissues.

To prevent backflow and enforce strict unidirectional circulation, four cardiac valves open and close passively in response to pressure differentials:

The Intrinsic Cardiac Conduction System

Unlike skeletal muscle, cardiac muscle possesses autorhythmicity—the innate ability to generate spontaneous electrical action potentials without external nervous system stimulation. The intrinsic conduction network consists of specialized non-contractile cardiac cells:

1. Sinoatrial (SA) Node: Located in the posterior wall of the right atrium near the opening of the superior vena cava. Known as the primary pacemaker, the SA node spontaneously depolarizes at an intrinsic rate of 60-100 impulses per minute, setting the baseline sinus rhythm.
2. Atrioventricular (AV) Node: Situated in the lower interatrial septum. It receives the wave of depolarization from the SA node and delays impulse transmission by approximately 0.1 seconds, allowing the atria to finish contracting and empty their blood completely into the ventricles before ventricular contraction begins.
3. Bundle of His (AV Bundle): Located in the upper interventricular septum; the sole electrical connection bridging the atria and ventricles.
4. Right & Left Bundle Branches: Course down the interventricular septum toward the apex of the heart.
5. Purkinje Fibers: Large-diameter fibers that sweep upward through the outer ventricular myocardium, triggering rapid, coordinated ventricular contraction beginning at the apex and squeezing blood upward toward the arterial trunks.

[SA Node] ──► (Atrial Wall Depolarization) ──► [AV Node] (0.1s delay)
                                                   │
                                                   ▼
                                            [Bundle of His]
                                                   │
                                                   ▼
                                       [Bundle Branches (Left/Right)]
                                                   │
                                                   ▼
                                            [Purkinje Fibers] ──► (Ventricular Contraction)

The Cardiac Cycle, Hemodynamics & Blood Pressure

The cardiac cycle includes all physiological events occurring during one complete heart beat, lasting approximately 0.8 seconds at a resting rate of 75 bpm. Each cycle consists of two primary phases:

• Systole: The contraction phase during which heart chambers actively expel blood under pressure.
• Diastole: The relaxation phase during which chambers refill with blood.

Blood pressure (BP) is the lateral force exerted by circulating blood against the internal walls of systemic arteries. It is measured in millimeters of mercury (mmHg) and expressed as two values:

Blood Pressure = Systolic Pressure / Diastolic Pressure (e.g., 120/80 mmHg)

• Systolic Blood Pressure (~120 mmHg): The peak arterial pressure achieved during left ventricular contraction (ventricular systole).
• Diastolic Blood Pressure (~80 mmHg): The baseline arterial pressure remaining during left ventricular relaxation (ventricular diastole), maintained by the elastic recoil of major arteries.
• Pulse Pressure: The numerical difference between systolic and diastolic pressure (120 - 80 = 40 mmHg).

Formed Elements & Plasma Composition of Blood

Blood is a specialized liquid connective tissue comprising approximately 7-8% of total adult body weight (average volume 5 liters). Whole blood separates into two primary fractions upon centrifugation:

1. Liquid Plasma (~55% Volume)

Plasma is a straw-colored, aqueous fluid comprising 90-92% water and 8-10% solutes. Plasma proteins synthesized predominantly by the liver include:

• Albumin (~60%): Maintains plasma colloidal oncotic pressure (osmotic balance) to prevent fluid transudation into tissue spaces.
• Globulins (~36%): Includes alpha/beta globulins (transporting lipids, iron, and fat-soluble vitamins) and gamma globulins (immunoglobulins/antibodies) essential for adaptive immune protection.
• Fibrinogen (~4%): Soluble plasma precursor converted to insoluble fibrin threads during blood coagulation.

2. Formed Elements (~45% Volume)

Vascular Hierarchy & Microvascular Exchange

The vascular system comprises five structural vessel types forming a continuous circuit:

1. Arteries: Thick-walled vessels carrying oxygenated blood away from the heart under high pressure. Their walls feature three structural tunics: an inner simple squamous endothelial tunica intima, a thick muscular tunica media containing smooth muscle and elastic tissue, and an outer fibrous tunica adventitia (externa).
2. Arterioles: Small muscular branches of arteries that function as primary resistance vessels. Arteriolosclerosis or sympathetic stimulation alters arteriolar caliber, regulating peripheral blood pressure and directing blood flow.
3. Capillaries: Microscopic, thin-walled vessels composed of a single endothelial layer resting on a basement membrane. Capillaries form vast capillary beds where gas exchange (O₂ ↔ CO₂), nutrient delivery, and waste extraction occur between blood and interstitial fluid. Precapillary sphincters (rings of smooth muscle) control capillary bed perfusion.
4. Venules: Small vessels formed by the union of several capillaries that collect deoxygenated blood.
5. Veins: Thin-walled vessels returning deoxygenated blood toward the heart under low pressure. Veins possess a thin tunica media, a wide lumen, and internal semilunar valves to prevent retrograde flow. Venous return is augmented by the skeletal muscle pump and respiratory intra-thoracic pressure shifts.

Pulmonary vs. Systemic Circulation

The cardiovascular system functions through two interconnected circulatory loops:

• Pulmonary Circulation: Deoxygenated blood exits the right ventricle via the pulmonary trunk, branches into right and left pulmonary arteries, enters lung capillaries for alveolar gas exchange (CO₂ release, O₂ absorption), and returns as oxygenated blood via four pulmonary veins to the left atrium.
• Systemic Circulation: Oxygenated blood exits the left ventricle through the ascending aorta, branching into systemic arteries to supply all body tissues, organs, skin, and extremities. Deoxygenated blood returns through systemic veins into the superior and inferior vena cava to re-enter the right atrium.