2.3 The Cardiovascular & Circulatory System

2.3 The Cardiovascular & Circulatory System

The cardiovascular system is the body’s primary transport network, distributing oxygen, nutrients, hormones, and immune cells to tissues while carrying away carbon dioxide and metabolic wastes. Composed of the heart, blood vessels, and blood, this system is a major focal point on the ATI TEAS 7 exam.

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Anatomy of the Heart and Wall Structure

The heart is a muscular organ located within the mediastinum of the thoracic cavity. It is enclosed in a protective double-walled sac called the pericardium.

Layers of the Heart Wall

1. Epicardium (Visceral Pericardium): The outer serous membrane covering the heart surface.
2. Myocardium: The thick middle layer composed of cardiac muscle tissue. Cardiomyocytes are branching cells connected by intercalated discs containing gap junctions (which allow rapid electrical conduction) and desmosomes (which resist mechanical stress).
3. Endocardium: The smooth inner endothelial lining continuous with the inner tunic of blood vessels.

Heart Chambers and Septa

The heart contains four internal chambers:

• Right Atrium: Receives deoxygenated blood from the superior vena cava (head and upper body), inferior vena cava (lower body), and coronary sinus (heart wall).
• Right Ventricle: Receives blood from the right atrium and pumps it under low pressure into the pulmonary trunk toward the lungs.
• Left Atrium: Receives oxygenated blood from the lungs via four pulmonary veins.
• Left Ventricle: Has a significantly thicker myocardial wall than the right ventricle because it must generate high pressure to pump blood through the entire systemic body circuit.
• Interventricular & Interatrial Septa: Muscular partitions separating the left and right sides of the heart, preventing the mixing of oxygenated and deoxygenated blood.

Heart Valves

Four non-return valves ensure unidirectional blood flow through the heart:

• Atrioventricular (AV) Valves: Prevent backflow into the atria during ventricular contraction (systole).
  • Tricuspid Valve: Positioned between the right atrium and right ventricle (3 cusps).
  • Bicuspid (Mitral) Valve: Positioned between the left atrium and left ventricle (2 cusps).
  • AV valves are anchored to papillary muscles via fibrous cords called chordae tendineae.
• Semilunar (SL) Valves: Prevent backflow into the ventricles during ventricular relaxation (diastole).
  • Pulmonary Semilunar Valve: Located at the exit of the right ventricle into the pulmonary trunk.
  • Aortic Semilunar Valve: Located at the exit of the left ventricle into the aorta.

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Path of Blood Flow Through the Circuits

Blood travels continuously through two separate circulatory circuits in a closed double loop.

Systemic Capillaries (Deoxygenated) -> Venae Cavae -> Right Atrium -> Tricuspid Valve -> Right Ventricle -> Pulmonary Valve -> Pulmonary Trunk & Arteries -> Lungs (Gas Exchange) -> Pulmonary Veins -> Left Atrium -> Bicuspid/Mitral Valve -> Left Ventricle -> Aortic Valve -> Aorta -> Systemic Arteries -> Systemic Tissues

1. Deoxygenated blood returns from systemic tissues via the superior and inferior venae cavae into the right atrium.
2. Blood passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk, which divides into right and left pulmonary arteries.
4. In the pulmonary capillaries of the lungs, blood releases $CO_2$ and absorbs $O_2$.
5. Oxygenated blood returns via four pulmonary veins into the left atrium.
6. Blood passes through the bicuspid (mitral) valve into the left ventricle.
7. The left ventricle contracts, forcing blood through the aortic semilunar valve into the aorta.
8. Oxygen-rich blood is distributed through systemic arteries to capillary beds nationwide before returning to the right atrium via systemic veins.

TEAS 7 Concept Warning: Arteries ALWAYS carry blood away from the heart, and veins ALWAYS carry blood toward the heart. In the pulmonary circuit, the pulmonary arteries carry deoxygenated blood, while pulmonary veins carry oxygenated blood.

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Intrinsic Cardiac Conduction System

The heart contracts rhythmically and autonomously due to specialized autorhythmic cardiac cells that generate action potentials without nervous system stimulation.

[SA Node] -> [AV Node] -> [Bundle of His] -> [Bundle Branches] -> [Purkinje Fibers]

1. Sinoatrial (SA) Node: Located in the superior wall of the right atrium. Known as the natural pacemaker of the heart, it initiates electrical impulses at an intrinsic rate of 70–80 beats per minute. Impulses spread across both atria, causing atrial contraction.
2. Atrioventricular (AV) Node: Located in the lower interatrial septum. It receives the impulse and delays it by approximately 0.1 seconds, allowing the atria to contract fully and complete ventricular filling before the ventricles contract.
3. Atrioventricular (AV) Bundle (Bundle of His): Located in the superior interventricular septum; connects the electrical connection between atria and ventricles.
4. Right and Left Bundle Branches: Course down the interventricular septum toward the heart apex.
5. Purkinje Fibers: Subendocardial branches that distribute the electrical impulse upward through the ventricular myocardium, causing synchronized ventricular contraction from the apex toward the base.

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Blood Vessel Structure and Classification

Blood vessels form a closed system of tubes categorized into three main types:

Layers of Blood Vessel Walls (Tunics)

1. Tunica Intima: Innermost endothelial lining; provides a smooth surface to prevent friction and clot formation.
2. Tunica Media: Middle layer composed of smooth muscle and elastic fibers. Controlled by the autonomic nervous system to mediate vasoconstriction (narrowing) and vasodilation (widening), controlling blood pressure.
3. Tunica Externa (Adventitia): Outer protective layer of collagen fibers.

Key Vessel Features:

• Arterioles: Smallest arteries; serve as the primary site of vascular resistance and blood pressure regulation.
• Capillaries: Consist of only a single layer of endothelial cells, allowing rapid exchange of gases, nutrients, and wastes between blood and interstitial fluid.
• Veins: Act as blood reservoirs (~65% of total blood volume). Because venous pressure is very low, blood return to the heart depends on one-way venous valves, the skeletal muscle pump, and breathing movements (respiratory pump).

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Blood Composition and Formed Elements

Adults possess approximately 5 liters of blood with a tightly regulated pH between 7.35 and 7.45. Blood is a specialized fluid connective tissue consisting of liquid plasma (55%) and formed elements (45%).

1. Blood Plasma (55%)

Plasma is 90% water and 10% dissolved solutes:

• Plasma Proteins: Manufactured mainly by the liver. Includes albumin (maintains blood osmotic pressure), globulins (antibodies and transport proteins), and fibrinogen (essential for blood clotting).
• Solutes: Electrolytes ($Na^+$, $K^+$, $Cl^-$), nutrients (glucose, amino acids), metabolic wastes (urea), and hormones.

2. Formed Elements (45%)

• Erythrocytes (Red Blood Cells - RBCs): Biconcave, flexible cells that lack nuclei and organelles when mature. Packed with hemoglobin, an iron-bearing protein that binds and transports oxygen. RBC production (erythropoiesis) occurs in red bone marrow, stimulated by the renal hormone erythropoietin (EPO) in response to hypoxia.
• Leukocytes (White Blood Cells - WBCs): Nucleated cells involved in immune defense.
  • Granulocytes: Neutrophils (most abundant; phagocytize bacteria), Eosinophils (fight parasitic infections/allergies), Basophils (release histamine during inflammation).
  • Agranulocytes: Lymphocytes (B cells produce antibodies; T cells attack infected cells) and Monocytes (become tissue macrophages).
• Thrombocytes (Platelets): Cell fragments derived from giant megakaryocytes. Essential for hemostasis, forming platelet plugs and releasing clotting factors to form fibrin blood clots.