The Cardiovascular System

Overview and Functions

• Cardiovascular system is a closed system consisting of the heart and blood vessels.

• The heart pumps blood, while blood vessels circulate blood throughout the body.

• Primary functions:

  • Transport oxygen, nutrients, cell wastes, and hormones to and from cells.

Anatomy of the Heart

Location, Size, and Orientation

• Approximately the size of a human fist; weighs less than a pound.

• Located in the thoracic cavity, between the lungs, in the inferior mediastinum.

• Apex: Points toward the left hip and rests on the diaphragm.

• Base: Where great blood vessels emerge; points toward the right shoulder.

Coverings of the Heart

• Pericardium: Double-walled sac surrounding the heart.

  • Fibrous pericardium: Outer, tough, and loose connective tissue layer.

  • Serous pericardium: Deep to fibrous layer; two layers:

    • Parietal pericardium: Lines inner surface of fibrous pericardium.

    • Visceral pericardium (epicardium): Next to the heart.

  • Pericardial cavity: Space between layers, filled with serous fluid to reduce friction.

Heart Wall Structure

• Epicardium: Outermost layer (visceral pericardium).

• Myocardium: Middle layer, composed of cardiac muscle; responsible for contraction.

• Endocardium: Innermost layer; endothelium lining the heart chambers and blood vessels.

Chambers and Associated Great Vessels

Heart Chambers

• Four chambers:

  • Atria (right and left): Superior, receiving chambers; blood enters under low pressure from veins.

  • Ventricles (right and left): Inferior, discharging (pumping) chambers; thick-walled.

• Interatrial septum: Separates the two atria.

• Interventricular septum: Separates the two ventricles.

Double Pump Function

• Arteries: Carry blood away from the heart.

• Veins: Carry blood toward the heart.

• Pulmonary circuit (right side): Pumps blood to lungs for oxygenation.

• Systemic circuit (left side): Pumps oxygenated blood to the body.

Pulmonary and Systemic Circulation

• Pulmonary circulation:

  • Right side pumps oxygen-poor blood to lungs via pulmonary arteries.

  • Oxygen-rich blood returns to left side via pulmonary veins.

• Systemic circulation:

  • Left side pumps oxygen-rich blood to body via the aorta.

  • Oxygen-poor blood returns to right atrium via superior/inferior vena cava.

  • Left ventricle has thicker walls due to higher workload.

Heart Valves

Types and Functions

• Valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) valves: Between atria and ventricles.

  • Left AV: Bicuspid (mitral) valve.

  • Right AV: Tricuspid valve.

• Semilunar valves: Between ventricles and arteries.

  • Pulmonary semilunar valve.

  • Aortic semilunar valve.

Valve Operation

• AV valves: Anchored by chordae tendineae; open during relaxation, closed during contraction.

• Semilunar valves: Closed during relaxation, open during contraction.

• Valves respond to pressure changes within the heart.

Cardiac Circulation

Coronary Arteries and Veins

• Heart muscle (myocardium) is supplied by its own blood vessels.

• Coronary arteries: Branch from the aorta.

  • Left: Anterior interventricular and circumflex arteries.

  • Right: Posterior interventricular and marginal arteries.

• Cardiac veins: Drain deoxygenated blood from myocardium into the coronary sinus, which empties into the right atrium.

• Coronary veins fill mainly during heart relaxation (diastole).

Physiology of the Heart

Intrinsic Conduction System (Nodal System)

• Cardiac muscle contracts spontaneously and regularly, independent of nerve impulses.

• Intrinsic conduction system coordinates heart activity and sets basic rhythm (~75 bpm).

• Components:

  • Sinoatrial (SA) node: Pacemaker; initiates heartbeat (right atrium).

  • Atrioventricular (AV) node: Junction of atria and ventricles; delays impulse.

  • AV bundle (bundle of His) and bundle branches: In interventricular septum.

  • Purkinje fibers: Spread within ventricle walls.

• Impulse pathway: SA node → atria → AV node (delay) → AV bundle → bundle branches → Purkinje fibers → ventricles contract.

Cardiac Cycle and Heart Sounds

• Cardiac cycle: One complete heartbeat (systole + diastole); ~0.8 seconds.

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Heart sounds:

  • "Lub": Closing of AV valves (start of ventricular systole).

  • "Dup": Closing of semilunar valves (end of ventricular systole).

Phases of the Cardiac Cycle

• Atrial diastole (ventricular filling): Heart relaxed, AV valves open, blood flows into ventricles.

• Atrial systole: Atria contract, complete ventricular filling.

• Isovolumetric contraction: Ventricles contract, AV valves close, all valves briefly closed.

• Ventricular systole (ejection phase): Semilunar valves open, blood ejected.

• Isovolumetric relaxation: Ventricles relax, semilunar valves close, AV valves reopen when atrial pressure exceeds ventricular pressure.

Cardiac Output (CO), Stroke Volume (SV), and Heart Rate (HR)

• Cardiac output (CO): Volume of blood pumped by each ventricle per minute.

• Stroke volume (SV): Volume of blood pumped per beat (~70 mL).

• Heart rate (HR): Beats per minute (typically 75 bpm).

• Formula:

  • Example:

Regulation of Stroke Volume and Heart Rate

• Starling’s law: The more the cardiac muscle is stretched (preload), the stronger the contraction.

• Preload: Determined by venous return and ventricular filling time.

• Contractility: Ability of cardiac muscle to contract; influenced by sympathetic stimulation.

• Afterload: Pressure ventricles must overcome to eject blood; increases with age and atherosclerosis.

• Factors modifying heart rate:

  • Neural: Sympathetic (increases HR), Parasympathetic (decreases HR).

  • Hormonal: Epinephrine, thyroxine (increase HR); ion imbalances affect HR.

  • Physical: Age, gender, exercise, temperature.

Blood Vessels

Types and Functions

• Arteries and arterioles: Carry blood away from the heart.

• Capillaries: Sites of exchange between blood and tissues.

• Venules and veins: Return blood toward the heart.

Microscopic Anatomy of Blood Vessels

• Three layers (tunics) except in capillaries:

  • Tunica intima: Endothelium; reduces friction.

  • Tunica media: Smooth muscle and elastic tissue; controls vessel diameter.

  • Tunica externa: Fibrous connective tissue; supports and protects.

• Arteries: Thicker tunica media; withstand higher pressure.

• Veins: Thinner tunica media, larger lumen, valves to prevent backflow; aided by skeletal muscle contraction.

• Capillaries: Single layer of endothelium; allow exchange of substances.

Capillary Beds and Microcirculation

• Capillaries form networks called capillary beds.

• Blood flow through capillary beds is called microcirculation.

• Precapillary sphincters: Regulate blood flow into capillary beds.

• Vascular shunt: Direct connection between arteriole and venule, bypassing capillaries when sphincters are closed.

Gross Anatomy of Blood Vessels

Major Arteries of Systemic Circulation

• Aorta: Largest artery; leaves left ventricle.

  • Regions: Ascending aorta, aortic arch, thoracic aorta, abdominal aorta.

• Branches:

  • Ascending aorta: Right and left coronary arteries (heart).

  • Aortic arch: Brachiocephalic trunk (right common carotid, right subclavian), left common carotid, left subclavian.

  • Thoracic aorta: Intercostal, bronchial, esophageal, phrenic arteries.

  • Abdominal aorta: Celiac trunk (left gastric, splenic, common hepatic), superior/inferior mesenteric, renal, gonadal, lumbar, common iliac arteries.

Major Veins of Systemic Circulation

• Superior vena cava: Drains head and arms.

• Inferior vena cava: Drains lower body.

• Veins draining into superior vena cava: Radial, ulnar, brachial, axillary, cephalic, basilic, median cubital, subclavian, vertebral, internal/external jugular, brachiocephalic, azygos veins.

• Veins draining into inferior vena cava: Tibial, fibial, popliteal, femoral, iliac, great saphenous, gonadal, renal, hepatic portal, hepatic veins.

Special Circulations

• Cerebral arterial circle (Circle of Willis): Provides collateral blood flow to the brain.

• Hepatic portal circulation: Veins from digestive organs, spleen, and pancreas drain into hepatic portal vein, which carries blood to the liver for processing before entering systemic circulation.

Physiology of Circulation

Vital Signs and Arterial Pulse

• Vital signs: Arterial pulse, blood pressure, respiratory rate, body temperature.

• Arterial pulse: Pressure wave from heart contraction; normal 70–76 bpm at rest.

Blood Pressure

• Definition: Force exerted by blood against vessel walls; drives blood flow.

• Blood pressure gradient: Highest in arteries, decreases through capillaries, lowest in veins.

• Measurement: Systolic (peak contraction) over diastolic (relaxation); e.g., 120/80 mm Hg.

• Auscultatory method: Indirect measurement, usually at the brachial artery.

Factors Affecting Blood Pressure

• Blood pressure equation: (CO = cardiac output, PR = peripheral resistance)

• Neural: Sympathetic stimulation increases BP (vasoconstriction); parasympathetic has little effect.

• Renal: Kidneys regulate blood volume; release renin to increase BP if low, excrete water to decrease BP if high.

• Temperature: Heat causes vasodilation (decreases BP); cold causes vasoconstriction (increases BP).

• Chemicals: Epinephrine increases HR and BP; other substances may increase or decrease BP.

• Diet: Low salt, saturated fat, and cholesterol help prevent hypertension.

Blood Pressure Variations and Disorders

• Normal systolic: 110–140 mm Hg; diastolic: 70–80 mm Hg.

• Hypotension: Systolic below 100 mm Hg; may indicate illness or shock.

• Hypertension: Sustained BP ≥ 140/90 mm Hg; risk for atherosclerosis.

Capillary Exchange and Fluid Movements

• Exchange mechanisms: Diffusion (direct, through clefts or pores), vesicular transport.

• Oxygen/nutrients: Move from blood to tissues; CO2/wastes: Move from tissues to blood.

• Fluid movement: Determined by balance of blood pressure (forces out) and osmotic pressure (draws in).

• At arterial end: BP > osmotic pressure → fluid exits capillary. At venous end: BP < osmotic pressure → fluid re-enters capillary.

Developmental Aspects of the Cardiovascular System

Fetal Circulation

• Heart develops as a simple tube; pumps blood by week 4 of pregnancy.

• Becomes a four-chambered, double-pump organ by week 7.

• Umbilical cord: One vein (oxygen/nutrient-rich blood to fetus), two arteries (waste/CO2 to placenta).

• Shunts:

  • Ductus venosus: Bypasses liver; blood enters inferior vena cava.

  • Foramen ovale: Shunts blood from right to left atrium (bypasses lungs).

  • Ductus arteriosus: Connects aorta and pulmonary trunk (bypasses lungs).

• At birth, shunts close (foramen ovale → fossa ovalis; ductus arteriosus → ligamentum arteriosum).

Age-Related Changes

• Weakening of venous valves → pooling of blood in extremities.

• Varicose veins, progressive arteriosclerosis, hypertension (loss of vessel elasticity), and coronary artery disease (fatty, calcified deposits).

Summary Table: Major Arteries and Veins

Additional info:

• Table entries inferred for clarity and completeness.

• For more detailed vessel lists, refer to anatomical atlases or textbooks.