Cardiovascular System — General Characteristics, Heart Structure, Blood Vessels, and Regulation

15.1 General Characteristics of the Cardiovascular System

• Cardiovascular system consists of the heart and blood vessels.
• Heart: hollow, cone-shaped, muscular pump; generates force to transport respiratory gases, nutrients, and wastes through the body.
• Blood vessels transport blood:
  • Arteries transport blood away from the heart.
  • Veins transport blood toward the heart.
  • Capillaries transport blood between arteries and veins; sites of nutrient, gas, and waste exchange.
• Two closed circuits (pathways):
  • Pulmonary circuit: carries oxygen-poor blood from the heart to the lungs, releases CO2, picks up O2, returns to the heart.
  • Systemic circuit: transports oxygen-rich blood and nutrients to body cells, removes wastes, returns to the heart.
• Organization of blood flow between heart sides:
  • Right side pumps to the pulmonary circuit; blood returns to the right side from the lungs.
  • Left side pumps to the systemic circuit; blood returns to the right side.

15.2 Structure of the Heart

• Size and location:
  • Size of a fist; averages about 14 cm long, 9 cm wide.
  • Located in the thoracic cavity, within the mediastinum, behind the sternum, above the diaphragm, near the lungs.
  • Base lies beneath the 2nd rib; apex at the 5th intercostal space.
  • Hollow, cone-shaped organ; a double pump with left and right halves.
• Coverings of the heart (pericardium):
  • Pericardium (pericardial sac) covers the heart and proximal ends of large vessels.
  • Fibrous pericardium: tough outer layer; surrounds a double-layered serous membrane.
  • Parietal pericardium: outer layer of serous membrane, deep to fibrous pericardium.
  • Visceral pericardium: inner layer of serous membrane; attached to the surface of the heart; also called the epicardium.
  • Pericardial cavity: space between visceral and parietal layers of serous pericardium.
• Wall anatomy:
  • Epicardium (visceral pericardium): outer, thin layer; reduces friction.
  • Myocardium: middle, thickest layer; cardiac muscle tissue.
  • Endocardium: inner, thin layer; lines all heart chambers.
• Heart chambers and valves:
  • Four chambers: Right atrium, Right ventricle, Left atrium, Left ventricle.
  • Auricles: flap-like projections from atria; allow atrial expansion.
  • Interatrial septum: separates left and right atria.
  • Interventricular septum: separates left and right ventricles.
  • Valves:
  • Tricuspid valve: between right atrium and right ventricle.
  • Pulmonary semilunar valve: between right ventricle and pulmonary trunk.
  • Mitral (bicuspid) valve: between left atrium and left ventricle.
  • Aortic semilunar valve: between left ventricle and aorta.
  • Chordae tendineae: fibrous strings attaching valve cusps to papillary muscles; prevent cusps from bulging too far.
  • Pulmonary veins: return oxygen-rich blood from the lungs to the left atrium.
• Skeleton of the heart:
  • Rings of dense connective tissue surround origins of great vessels and valve openings.
  • Provide attachments for heart valves and muscle fibers; prevent excess dilation during contraction.
  • The rings plus fibrous masses in the interventricular septum make up the skeleton of the heart.
• Blood supply to the heart:
  • Coronary arteries supply heart tissues; first two branches of the aorta.
  • Coronary veins drain myocardium; coronary sinus drains into the right atrium.

15.3 Heart Actions

• Definitions:
  • Systole: contraction of a heart chamber.
  • Diastole: relaxation of a heart chamber.
• Cardiac cycle: coordinated sequence of events during a heartbeat.
  • Phase 1: atria contract (atrial systole) while ventricles relax (ventricular diastole).
  • Phase 2: ventricles contract (ventricular systole) while atria relax (atrial diastole).
  • Phase 3: brief period when both chambers relax.

15.4 Blood Vessels

• Blood vessels form a closed circuit: heart → body or lungs → heart.
• Vessel types and pathways:
  • Arteries: carry blood away from the heart.
  • Arterioles: receive blood from arteries; lead to capillaries.
  • Capillaries: exchange sites between blood and tissue fluid.
  • Venules: carry blood from capillaries to veins.
  • Veins: carry blood back to the heart.
• Vessel structure (Table 15.3 overview):
  • Artery: thick, strong walls with three tunics; endothelium; smooth muscle; connective tissue; transports blood under high pressure.
  • Arteriole: thinner than arteries but with three tunics; regulate flow into capillaries via vasoconstriction/vasodilation.
  • Capillary: single layer of simple squamous endothelium; highly permeable; site of exchange; flow regulated by precapillary sphincters.
  • Venule: thinner walls; connect capillaries to veins.
  • Vein: thinner walls; less developed middle layer; valves in many veins; serve as blood reservoirs; return blood under low pressure.
• Capillary exchange and flow regulation:
  • Precapillary sphincters regulate blood flow into capillaries.
  • Higher tissue metabolic rate → denser capillary networks (e.g., muscle, nerve tissue).
  • Patterns vary; some capillaries directly connect arterioles to venules; others form networks.
  • Blood flow redistribution occurs with activity (e.g., during exercise vs post-meal).
• Capillary exchange mechanisms (Exchanges in the Capillaries):
  • Diffusion: primary mechanism; lipid-soluble substances diffuse through membranes; water-soluble substances diffuse through channels and slits.
  • Filtration: hydrostatic pressure drives molecules through membranes; pressure derives from ventricular contraction.
  • Osmosis: osmotic pressure from plasma proteins draws water into capillaries; colloid osmotic pressure is due to plasma proteins.
  • Transcytosis: large substances (e.g., antibodies, lipoproteins) move via endocytosis and exocytosis.
• Capillary exchange example (Figure 15.31) numbers:
  • Arteriolar end: hydrostatic pressure ~ 35 mm Hg; osmotic pressure ~ 24 mm Hg; net outward pressure ~ 11 mm Hg.
  • Venular end: hydrostatic pressure ~ 16 mm Hg; osmotic pressure ~ 24 mm Hg; net inward pressure ~ 8 mm Hg.

15.5 Blood Pressure

• Definition: blood pressure (BP) is the force the blood exerts against the inner walls of blood vessels; circulates blood.
• Common reference: BP usually refers to systemic arteries; exists throughout the vascular system; blood moves from high to low pressure.
• Arterial blood pressure basics:
  • Systolic pressure (SP): maximum pressure during ventricular contraction.
  • Diastolic pressure (DP): minimum pressure just before next contraction.
  • Pulse pressure (PP): difference between SP and DP; PP = SP - DP
  • Mean arterial pressure (MAP): average pressure in the arterial system; represents average driving pressure for blood to tissues.
• Measurement: BP is measured in mm Hg with a sphygmomanometer.
• Relationships to cardiac output and resistance:
  • Cardiac output (CO) is the volume discharged from a ventricle per minute; BP varies with CO and peripheral resistance (PR).
  • Equations:
  • Cardiac output: CO = HR imes SV
  • Stroke volume: SV = EDV - ESV
  • Blood pressure: BP = CO imes PR
• Other factors influencing BP:
  • Blood viscosity (related to hematocrit and plasma proteins).
  • Vascular elasticity and peripheral resistance.
  • Baroreceptor reflexes regulate BP via autonomic pathways (cardioaccelerator and cardioinhibitor centers).

15.6 Paths of Circulation

• Pulmonary circuit (small loop):
  • Right ventricle → pulmonary trunk → right and left pulmonary arteries → alveolar capillaries in lungs → pulmonary venules and veins → left atrium.
  • Blood in pulmonary arteries/arterioles is low in O2 and high in CO2; gas exchange occurs in alveolar capillaries.
• Systemic circuit (large loop):
  • Left atrium → left ventricle → aorta → arteries/arterioles → systemic capillaries → systemic venules/veins → right atrium.
• Coronary circulation is part of systemic circuit feeding the heart muscle itself.

15.7 Arterial System

• Aorta and branches:
  • Aorta: largest-diameter artery; sections include ascending aorta, aortic arch, descending aorta (thoracic and abdominal portions).
  • Aortic semilunar valve at root; aortic sinuses near valve cusps; left and right coronary arteries begin at the aortic sinuses.
  • Major arch branches: brachiocephalic trunk; left common carotid artery; left subclavian artery.
• Circle of Willis: cerebral arterial circle supplying the brain; formed by internal carotid arteries and basilar artery.
• Major branches overview (selected):
  • Arch gives rise to brachiocephalic trunk (→ right subclavian and right common carotid); left common carotid; left subclavian.
  • Descending aorta gives thoracic branches (bronchial, pericardial, esophageal, mediastinal, posterior intercostal).
  • Abdominal aorta gives celiac trunk, superior mesenteric, suprarenal, renal, gonadal, inferior mesenteric, lumbar, middle sacral, common iliac.
• Tables/figures summarize branches and regions supplied (Table 15.4).

15.8 Venous System

• General characteristics:
  • Systemic venous circulation returns blood to the heart after exchanges.
  • Veins originate from merging capillaries → venules → small veins → larger veins; complex and irregular networks.
  • Major systemic veins converge into the superior and inferior venae cavae → right atrium.
• Veins of the head/neck and upper limb:
  • Brain and head/neck venous drainage via cerebral venous sinuses → internal jugular veins.
  • Upper limb drainage through deep (radial/ulnar → brachial → axillary) and superficial (cephalic, basilic, with median cubital at the elbow) systems.
  • Azygos system drains thoracic wall and empties into superior vena cava.
• Abdominal viscera and portal systems:
  • Hepatic portal system drains abdominal viscera and sends blood to the liver via the hepatic portal vein; liver processes nutrients before blood enters systemic circulation.
  • Renal portal system (and others) drain to renal veins and inferior vena cava.
• Veins of the lower limb and pelvis:
  • Deep veins: foot → tibial veins → popliteal → femoral → external iliac.
  • Superficial veins: great and small saphenous; trunkal drainage via alternative routes.
  • Major venous network from lower limb returns to the heart via the inferior vena cava.
• Central venous pressure:
  • Pressure in the right atrium; influenced by blood volume and venous return; affects peripheral venous pressures and can lead to edema if elevated.

15.9 Cardiac Conduction System

• Specialized group of cardiac muscle cells that initiate and distribute cardiac action potentials:
  • SA (sinoatrial) node: natural pacemaker; initiates rhythmic contractions.
  • Internodal atrial pathways: conduct impulses from SA node to atria.
  • Atrioventricular (AV) node: conducts impulses to AV bundle; delays impulse to allow atrial contraction completion before ventricles contract.
  • AV bundle (Bundle of His): conducts impulses rapidly between SA node and bundle branches.
  • Left and right bundle branches: conduct impulses to Purkinje fibers.
  • Purkinje fibers: large fibers that distribute impulses to ventricular myocardium; drive contraction in a twisting, apex-first pattern.
• Pathways are illustrated in Figures 15.16–15.17 and described in text.

15.10 Electrocardiogram (ECG/EKG) and Heart Sounds

• ECG: recording of electrical changes in the myocardium during the cardiac cycle; assesses conduction.
  • P wave: atrial depolarization just before atrial contraction.
  • QRS complex: ventricular depolarization just before ventricular contraction (three waves: Q, R, S).
  • T wave: ventricular repolarization just before ventricular relaxation.
  • Atrial repolarization is masked within the QRS complex.
• Heart sounds:
  • S1 (lubb): first sound; occurs during ventricular systole; closure of AV valves.
  • S2 (dupp): second sound; occurs during ventricular diastole; closure of semilunar valves (pulmonary and aortic).
  • Murmur: abnormal sound due to incomplete valve closure or other valve abnormalities.
• Cardiac impulse path and ECG correlation are shown in Figures 15.16–15.21 and related captions.

15.11 Control of the Cardiac Cycle and Heart Rate

• Autonomic regulation via the cardiac center in the medulla oblongata.
• SA node normally controls heart rate; sympathetic and parasympathetic inputs modify rate and force.
• Parasympathetic innervation (vagus nerves) lowers SA node rate from ~100 bpm to ~60–80 bpm; also affects AV node.
• Sympathetic innervation (accelerator nerves) increases heart rate and contractility via SA, AV nodes, atrial and ventricular myocardium.
• Baroreceptor reflexes:
  • Involve a cardioinhibitor reflex center and a cardioaccelerator reflex center in the medulla.
  • Aortic arch and carotid sinus baroreceptors detect blood pressure changes; impulses modulate heart rate and vessel tone via autonomic pathways.
• Examples and dynamics:
  • Baroreceptors respond to increased pressure by increasing parasympathetic activity and decreasing sympathetic activity to lower BP.
  • Stretch receptors in venae cavae contribute to venous return regulation.
  • Other factors influencing heart rate: hypothalamic and cerebrum input, body temperature, ion concentrations.

15.12 Arrhythmias and Cardiac Disorders (Clinical Applications)

• Arrhythmias: altered heart rhythms; several types include:
  • Fibrillation: uncoordinated, chaotic contraction; atrial fibrillation often non-lethal; ventricular fibrillation often fatal.
  • Tachycardia: HR > 100 bpm at rest.
  • Bradycardia: HR < 60 bpm at rest.
  • Flutter: rapid, regular contractions of a chamber (~250–350 bpm).
  • Premature beat: impulse occurs early in the cycle from ectopic regions.
  • Ectopic pacemaker: SA node damage may lead AV node to take over; slower pace (~40–60 bpm).
  • Artificial pacemaker: device to regulate conduction system.
• Coronary artery disease (CAD): cholesterol plaque deposition in coronary arteries; can reduce oxygen supply to myocardium; treatment options include PTCA and coronary bypass surgery.
• Hypertension: chronic high BP; risk factor for atherosclerosis, heart disease, stroke, and other conditions; lifestyle modifications and medications are used for management.

Notable numerical values and formulas referenced in the material

• Heart size/location details:
  • Average heart length: ~14 cm; width: ~9 cm.
• Cardiac muscle cell features:
  • One central nucleus per cardiac cell; cells form branching networks; intercalated discs with desmosomes and gap junctions; functional syncytia (atrial and ventricular).
• Cardiac output and stroke volume:
  • SV ≈ 70 mL/beat (typical value given for discussion).
  • CO = HR × SV.
  • SV = EDV − ESV.
• Blood pressure concepts:
  • SP = systolic pressure (max pressure during contraction).
  • DP = diastolic pressure (min pressure during relaxation).
  • PP = SP − DP.
  • BP = CO × PR (relationship given in the text).
• Capillary exchange end values (arteriolar end vs venular end):
  • Arteriolar end: hydrostatic pressure ≈ 35 mm Hg; osmotic pressure ≈ 24 mm Hg; net outward pressure ≈ 11 mm Hg.
  • Venular end: hydrostatic pressure ≈ 16 mm Hg; osmotic pressure ≈ 24 mm Hg; net inward pressure ≈ 8 mm Hg.
• Pulmonary vs systemic circuit flow patterns described in 15.6 paths of circulation.
• Capillary flow regulation:
  • Precapillary sphincters regulate capillary entry.
  • Blood flow distribution shifts with activity (e.g., exercise vs post-meal).
• Areas of the heart supplied by specific arteries (summary):
  • Right coronary artery branches: posterior interventricular artery; right marginal branch.
  • Left coronary artery branches: circumflex branch; anterior interventricular (left anterior descending) artery.
• Life-span considerations (summary): aging brings cholesterol deposition, arterial stiffening, possible changes in heart mass and compliance, and BP changes.

Connections and implications

• Foundational links:
  • Structure-function relationship: heart chambers and valves create unidirectional, coordinated blood flow; valve mechanics relate to heart sounds and murmurs.
  • Electrical conduction underpins the timing of contractions (ECG correlates with systole/diastole in different chambers).
  • Capillary exchange mechanisms explain how nutrients, gases, and wastes pass between blood and tissues, tying into systemic physiology.
• Real-world relevance:
  • Understanding the baroreceptor reflex is essential for predicting responses to posture changes, dehydration, or hemorrhage.
  • Knowledge of arterial and venous dynamics informs treatments for hypertension and venous insufficiency.
  • CAD pathophysiology links cholesterol metabolism to occlusion risk and the rationale for interventions like angioplasty or bypass surgery.

Key formulas to remember

• Cardiac output: CO = HR \times SV
• Stroke volume: SV = EDV - ESV
• Blood pressure: BP = CO \times PR
• (Frank-Starling concept) Preload relates to EDV and influences the next contraction strength; increased preload generally increases SV in healthy myocardium.
• (Note) MAP and other detailed BP formulas are discussed conceptually as measures of average perfusion pressure, but explicit MAP formula is not provided in the text.

Ethical, philosophical, and practical implications

• Hypertension and atherosclerosis present major public health challenges (silent progression, lifestyle vs medical management).
• Advances in stem cell therapy and artificial hearts raise questions about accessibility, long-term outcomes, and ethical considerations of organ replacement and tissue engineering.
• Understanding disease mechanisms (e.g., CAD, arrhythmias) informs prevention strategies and clinical decision-making to improve patient quality of life.