The Cardiovascular System

Overview

• The cardiovascular system is a closed system composed of the heart and blood vessels.
• Primary function: deliver oxygen and nutrients, and remove carbon dioxide and other wastes.

The Heart: Location and Size

• Located in the thorax, between the lungs; apex directed toward the left hip.
• Roughly the size of a fist.

Heart Coverings and Wall

• Pericardium: double serous membrane with visceral (epicardium) and parietal layers; serous fluid between layers.
• Heart wall consists of three layers:
  • Epicardium: outer layer; part of the pericardium.
  • Myocardium: middle, mostly cardiac muscle.
  • Endocardium: inner, endothelium-lined.

External Heart Anatomy (major landmarks)

• Four chambers: right atrium, left atrium (receiving); right ventricle, left ventricle (discharging).
• Major vessels: aorta, pulmonary trunk, superior and inferior vena cavae, pulmonary veins.
• Coronary arteries and veins supply the myocardium; blood drains via the coronary sinus into the right atrium.

The Heart: Valves

• Four valves ensure one-way blood flow:
  • Atrioventricular (AV) valves: Tricuspid (right) and Bicuspid/Mitral (left).
  • Semilunar valves: Pulmonary (right ventricle to pulmonary trunk) and Aortic (left ventricle to aorta).
• Valves are held by chordae tendineae (heart strings) and prevent backflow.

The Heart: Conduction System

• Intrinsic conduction system sets pace of the heart:
  • Sinoatrial (SA) node: pacemaker.
  • Atrioventricular (AV) node.
  • Atrioventricular bundle (Bundle of His).
  • Bundle branches and Purkinje fibers.
• Contraction is intrinsic; autorhythmic cells coordinate timing.

Heart Contractions and the Cardiac Cycle

• Contractions begin at the SA node and propagate through the conduction system.
• Cardiac cycle includes:
  • Atrial contraction (atrial systole) and ventricular filling.
  • Isovolumetric contraction (AV valves close, no blood change in ventricles).
  • Ventricular systole with ejection (semilunar valves open).
  • Isovolumetric relaxation (all valves closed; ventricles relax).
• Systole = contraction; Diastole = relaxation.

Cardiac Output and Regulation

• Cardiac output (CO) = amount of blood pumped by each ventricle per minute:
  CO = HR \cdot SV
  where HR = heart rate, SV = stroke volume.
• Stroke volume: volume pumped by a ventricle with each beat.
• Regulation:
  • Autonomic nervous system: parasympathetic slows HR; sympathetic increases HR and contractility.
  • Hormones: epinephrine, thyroxine.
  • Other influences: exercise, venous return, blood volume.
• Starling’s law: the more the cardiac muscle is stretched during filling, the stronger the contraction.

Blood Vessels: Structure and Function

• Vessels form a circuit: arteries → arterioles → capillaries → venules → veins.
• Three tunics (layers):
  • Tunica interna (intima): endothelium.
  • Tunica media: smooth muscle; controlled by sympathetic nervous system.
  • Tunica externa (adventitia): fibrous connective tissue.
• Arteries have thick walls; veins have larger lumens; capillaries are one cell thick to enable exchange.
• Movement of blood: arteries primarily pump blood; veins rely on skeletal muscle pumps to return blood to the heart.

Capillary Bed and Exchange

• Capillary beds contain true capillaries (exchange vessels) and a vascular shunt (direct arteriole-to-venule connection).
• True capillaries permit diffusion of oxygen, nutrients, carbon dioxide, and wastes between blood and tissues.
• Diffusion occurs via direct diffusion, intercellular clefts, fenestrations, and vesicular transport.

Systemic Circulation: Major Vessels (Overview)

• Arteries (e.g., aorta and branches) carry oxygen-rich blood away from the heart.
• Veins (e.g., superior/inferior vena cava) return deoxygenated blood to the heart.
• Capillary networks connect arterial and venous systems in tissues.

Hepatic Portal Circulation

• The hepatic portal vein drains blood from digestive organs to the liver for processing.
• Key vessels: splenic vein, superior and inferior mesenteric veins, left gastric vein.
• Blood then flows through the liver to be filtered before entering general circulation.

Brain Circulation: Circle of Willis

• Circle of Willis provides collateral blood flow to the brain.
• Major contributors: internal carotid arteries and basilar artery.
• Key arteries include the middle cerebral artery, anterior/posterior cerebral arteries, and anterior/posterior communicating arteries.

Fetal Circulation

• Fetal circulation uses placental gas exchange rather than lungs.
• Key shunts: foramen ovale (right to left atrial flow), ductus arteriosus (pulmonary artery to aorta), ductus venosus (umbilical vein to inferior vena cava).
• Oxygenation is high in blood entering the fetus via the umbilical vein and umbilical arteries return blood to the placenta.

Pulse and Blood Pressure

• Pulse: pressure wave of blood, palpable at various points (temporal, carotid, radial, etc.).
• Blood pressure: measured in large arteries; systolic pressure is peak during ventricular contraction; diastolic is when ventricles relax.
• Pressure decreases with distance from the heart.

Measuring Blood Pressure

• Measured with cuff and stethoscope; typical reading example: 120/70\ \text{mmHg} (systolic/diastolic).
• Sounds (Korotkoff) are heard as cuff pressure crosses systolic and diastolic levels.

Factors Affecting Blood Pressure

• Neural factors: autonomic adjustments (sympathetic activation raises BP).
• Renal factors: regulation by altering blood volume (renin system).
• Temperature: heat causes vasodilation; cold causes vasoconstriction.
• Chemicals and diet can raise or lower BP.
• Exercise, posture changes, and hormonal influences alter BP.

Regulation of Blood Pressure and Circulation

• Blood pressure is influenced by:
  • Blood volume and venous return
  • Cardiac output (CO) and peripheral resistance
  • Hormones and neural inputs
• Short-term regulation mainly via autonomic nervous system; long-term regulation via renal control of blood volume.

Developmental Aspects

• The heart begins as a simple tubular heart in the embryo and functions by the fourth week.
• By the end of week seven, the heart is a four-chambered organ.
• After week seven, few structural changes occur.