Adult Circulation!!!

Transition from Fetal to Adult Circulation

• Quick Review topic: THE TRANSITION FROM FETAL TO ADULT CIRCULATION

• Fetal circulation diagram (a): Major structures and changes to become adult circulation (b).

• Key fetal-to-adult changes:

  • DUCTUS ARTERIOSUS becomes Ligamentum arteriosum.

  • Foramen ovale becomes Fossa ovalis.

  • DUCTUS VENOSUS becomes Ligamentum venosum; Hepatic portal vein remains in the liver area.

  • UMBILICAL ARTERIES become Medial umbilical ligaments.

  • UMBILICAL VEIN becomes Ligamentum teres.

  • UMBILICAL CORD connects placenta to fetus; at birth circulation re-routes from placental blood flow to pulmonary circulation.

• Circulation at birth (b): Transition of blood flow and oxygenation.

  • After birth, blood becomes mixed oxygenated and deoxygenated as the placental source is removed from the circuit and the lungs begin oxygenating blood.

• Visual cues in the diagram indicate:

  • Oxygenated blood flows to the left heart (Left ventricle) before being pumped to the systemic circulation.

  • Deoxygenated blood returns via the right heart to the lungs for oxygenation.

• Significance: The neonatal transition establishes the separation of systemic and pulmonary blood flow and establishes shunt closures (foramen ovale, ductus arteriosus).

Required Reading and References

• OEGAN’S Chapter 10

• Cardiopulmonary A&P Chapter 5

Knowledge Check: Classifications of Congenital Heart Disease

• Question: What are the 2 classifications for congenital heart disease?

• Note: This is a knowledge check question presented in the transcript; the answer is not provided in the slides.

Anatomy of the Heart and General Cardiovascular System

• The heart is a hollow organ with four chambers, muscular, roughly the size of a fist.

• Location and orientation:

  • Sits behind the sternum.

  • 2/3 of the heart lies to the left side of the chest.

  • The apex sits at the fifth intercostal space.

• Surface grooves (sulci) mark the boundaries of the heart chambers.

Cardiac Anatomy Landmarks (Auscultation Areas)

• Aortic area: 2nd Right Intercostal Space (ICS)

• Pulmonic area: 2nd Left ICS

• Tricuspid area: Left sternal border, 3rd–5th ICS

• Mitral (apical) area: Apex, usually 5th ICS at the midclavicular line

Pericardium and Pericardial Space

• Pericardium is a double-walled sac surrounding the heart:

  • Outer wall: Fibrous pericardium

  • Inner walls: Serous pericardium

  • Parietal layer lines the pericardial sac

  • Visceral layer lines the heart (epicardium)

• Pericardial fluid: thin serous fluid between parietal and visceral layers acting as a lubricant

• Pericardial cavity, parietal pericardium, visceral pericardium (epicardium), fibrous pericardium

Heart Wall Layers

• Epicardium (visceral pericardium): outer layer

• Myocardium: middle, bulk of heart muscle

• Endocardium: inner lining of the heart chambers

• Pericardial sac surrounds the heart and contains the pericardial fluid

Key Disorders of the Pericardium

• Pericarditis

• Pericardial effusion

• Large pericardial effusion can cause a serious drop in blood flow, shock, and death (cardiogenic compromise)

Anatomy Review Question

• What is the name of the sac that the heart sits in? Answer: Pericardium

Heart Layers (Recap)

• The heart wall consists of three layers:

  • Epicardium (outer)

  • Myocardium (bulk of heart muscle)

  • Endocardium (inner lining)

Coronary Circulation

• Rationale: The heart has a high metabolic rate and requires substantial coronary blood flow.

• Key features:

  • High metabolic rate; requires more blood flow per gram than most other organs.

  • Extensive network of branches in the coronary circulation.

  • About 70% of coronary blood flow occurs during diastole.

• Major arteries:

  • Left Coronary Artery (LCA): supplies left atrium, left ventricle, interventricular septum, and part of the right atrium.

  • Right Coronary Artery (RCA): supplies right ventricle, rest of the right atrium, and the sinus node.

• Venous drainage:

  • Coronary veins drain venous blood into the coronary sinus, which empties into the right atrium.

  • Thebesian veins drain into all heart chambers.

  • This venous drainage system creates a normal anatomic shunt of approximately 2–3% of cardiac output.

Anatomy – Coronary Veins and Nodes (Posterior and Anterior views)

• Posterior view highlights the coronary sinus region and the sinus node area.

• Anterior view shows the great cardiac vein, circumflex branch, and right coronary artery distribution.

Myocardial Ischemia and Infarction

• Myocardial Ischemia: partial obstruction of a coronary artery leading to decreased oxygen supply to tissue, aka angina pectoris.

  • Stable angina: chest pain triggered by activity or exertion.

  • Can become unstable when the vessel is almost completely obstructed.

• Myocardial Infarction (MI): complete obstruction of a coronary artery causing death of heart tissue; this is an emergency.

Cardiac Chambers and Valves

• Four muscular chambers:

  • Atria: right and left (upper chambers)

  • Ventricles: right and left (lower chambers)

• Valves:

  • Atrioventricular (AV) valves:

  • Tricuspid valve: between right atrium and right ventricle; has three flaps.

  • Mitral (bicuspid) valve: between left atrium and left ventricle; has two flaps.

  • Semilunar valves:

  • Pulmonary (Pulmonic) valve: between right ventricle and pulmonary artery.

  • Aortic valve: between left ventricle and aorta.

Valve Disorders (Overview)

• Know the two main valve disorders:

  • Regurgitation: backflow of blood due to an incompetent or damaged valve.

  • Stenosis: pathologic narrowing or constriction of a valve.

Cardiac Cycle – Lob-Dub Concept

• The Cardiac Cycle consists of four stages:

  • Systole: ventricular contraction; semilunar valves open; AV valves close; blood ejected from ventricles into the pulmonary artery and aorta.

  • Diastole: ventricular filling; AV valves open; semilunar valves closed; blood is pushed from atria into ventricles.

  • LUB-DUB is the characteristic heart sound pattern associated with AV valve closure (S1) and semilunar valve closure (S2).

• The cycle includes phases: Passive filling, Atrial contraction, AV valves close, Semilunar valves open; ventricular ejection; Ventricular diastole; Ventricular systole (repetition of the four-stage cycle).

Normal Heart Sounds (S1–S4)

• S1: closure of AV valves during ventricular contraction (normal)

• S2: closure of semilunar valves during ventricular relaxation (normal)

• S3: abnormal; rapid rush of blood from atria to ventricles (ventricular filling heard after S2)

• S4: abnormal; atrial contraction into a stiff ventricle (late diastole)

Properties of Heart Muscle

• The four key properties that enable pumping:

  • Excitability: cells’ ability to respond to electrical, chemical, or mechanical stimulation.

  • Conductivity: ability to spread electrical impulses quickly through myocardial tissue.

  • Contractility: myocardial contraction in response to electrical impulses.

  • Automaticity (inherent rhythmicity): ability to initiate spontaneous electrical impulse.

Microanatomy and Refractory Period

• Cardiac cells are short, fat, branched, and interconnected; each fiber contains myofibrils organized into sarcomeres (~2 μm).

• Cardiac muscle is striated; typically one nucleus per cell.

• Intercalated discs: special junctions that aid in electrical conduction between cells.

• Refractory period: time during which myocardium cannot be stimulated again; approximately 250 milliseconds.

• Notes:

  • 1 μm = 1×10^-6 meters.

Key Properties Review Question

• What are the 4 key properties of heart muscle? (Answer: Excitability, Conductivity, Contractility, Automaticity)

The Vascular System – Overview

• Systemic circulation

  • Begins at the aorta

  • Ends at the right atrium

• Pulmonary circulation

  • Begins at the pulmonary artery

  • Ends at the left atrium

The Vascular System – Components and Functions

• Systemic circulation components:

  • Arterial: conductance vessels; large, elastic, low-resistance arteries.

  • Resistance vessels: arterioles; small vessels that control blood flow to capillaries; major role in blood pressure.

• Capillary and Venous systems:

  • Capillary system: constant exchange of nutrients and wastes between blood and cells/tissues.

  • Venous system: capacitance vessels; reservoir for the circulatory system; contains approximately 60–75% of total blood volume; conducts blood back to the heart.

Quick Review: Blood Volume in Venous System

• How much blood is contained in the venous system? (Answer is implied to be substantial; the slide prompts recall, exact percentage not stated here.)

Vascular Resistance – Quick Review

• Systemic Vascular Resistance (SVR): sum of all opposing forces to blood flow through systemic circulation.

• Pulmonary Vascular Resistance (PVR): sum of all opposing forces to blood flow through the pulmonary circulation.

Determinants of Blood Pressure

• Cardiovascular Function:

  • Force of left ventricular contraction.

  • Systemic vascular resistance.

  • Blood volume.

• Perfusion:

  • First priority is to keep perfusion to tissues and organs normal.

• MAP (Mean Arterial Pressure):

  • Normal range is approximately $70 ext{–} 105 ext{ mmHg}.$ (Below 60 mmHg implies decreased perfusion to kidneys and brain; organ failure can occur within minutes.)

Mean Arterial Pressure (MAP) and Calculation

• MAP: the most important perfusion pressure for organs.

• MAP equation (as given in slides):

  • $MAP = rac{2 imes DBP + SBP}{3}$

• Practical example: If a BP is 120/80, MAP ≈ 93 mmHg (calculation typically performed as shown in class; see MAP formula).

Autonomic and Neural Regulation of the Cardiovascular System

• Central control:

  • Sympathetic nervous system: release of norepinephrine increases heart rate and contractility; influences vascular tone.

• Precapillary local controls (autoregulation):

  • Myogenic mechanisms maintain constant flow to the capillary bed despite changes in pressure.

  • Metabolic controls: changes in CO2, pH, O2, lactate, etc., cause vasoconstriction or dilation depending on tissue needs; different organs have different sensitivity; brain is among the most sensitive.

• Neural (central) control summary:

  • Medullary ischemic reflex center: decrease in O2 delivery to the medulla increases BP.

  • Vasomotor center (medulla oblongata): governs the amount of sympathetic impulses to vascular system; maintains vasomotor tone (a baseline level of constriction).

Cardiovascular controls : Peripheral receptors

• Baroreceptors: located in the aortic arch, carotid sinuses, walls of the atria, and thoracic & pulmonary veins; respond to changes in vascular volume; output proportional to vessel stretch; may adapt to chronic changes (new normal).

• Chemoreceptors:

  • Peripheral: located in the aortic arch and carotid sinuses; central: located in the medulla oblongata.

  • Primarily responsive to changes in O2, CO2, and pH; influence respiratory and cardiovascular responses.

Cardiac Output (CO)

• Definition: amount of blood ejected from the left ventricle in one minute.

• QT (cardiac output) equation: $QT=HR\cdot SV$

Stroke Volume (SV) and Determinants

• Stroke Volume: amount of blood ejected with each beat.

• SV equation: $SV = EDV - ESV$

  • End Diastolic Volume (EDV): amount of blood in the ventricle before contraction.

  • End Systolic Volume (ESV): amount of blood in the ventricle after contraction.

• Determinants of SV include:

  • Preload: ventricular stretch (related to EDV).

  • Contractility: amount of force the myocardium produces during contraction.

  • Afterload: sum of all external factors opposing ventricular ejection.

Ejection Fraction (EF)

• EF equation: $EF=\frac{SV}{EDV}$

• Normal EF: typically, 65%-70%

• Reduced EF: < 50%

• Severely reduced EF: < 30% which severely limits exercise tolerance.

Cardiac Output and Stroke Volume Recap

• Cardiac Output (QT) depends on heart rate and stroke volume: QT = HR × SV.

• SV is determined by the balance of preload, contractility, and afterload.

• EF provides a measure of the proportion of blood ejected per beat relative to EDV.

Quick Formula Recap

• Cardiac Output: $QT = HR imes SV$

• Stroke Volume: $SV = EDV - ESV$

• Ejection Fraction: $EF = rac{SV}{EDV}$

• Mean Arterial Pressure: $MAP = rac{2 imes DBP + SBP}{3}$

Foundational Concepts and Real-World Relevance

• The transition from fetal to adult circulation is critical for ensuring oxygen delivery after birth and establishing normal systemic-pulmonary separation.

• Coronary circulation highlights the heart’s high metabolic demand and the timing of blood flow (diastole) as crucial for myocardial perfusion.

• Understanding the cardiac cycle, valve mechanics, and heart sounds (S1–S4) helps in diagnosing valvular and structural heart diseases.

• MAP and overall perfusion pressure are essential for evaluating organ perfusion and guiding clinical decisions in hypotension or shock.

• Autonomic and local regulatory mechanisms ensure tissue perfusion is maintained across various physiological states and pathologies.

Practical Implications (Ethical/Clinical)

• Prompt recognition of myocardial infarction can be life-saving; timely reperfusion strategies are critical.

• Pericardial diseases (pericarditis, effusion) can progress to tamponade; early detection and management prevent shock.

• Valve disorders (regurgitation, stenosis) affect hemodynamics and may require surgical or medical management.

• Accurate BP interpretation (SBP, DBP, and MAP) guides therapy and monitoring in critical care settings.

Quick Practice Questions

• What are the 4 key properties of heart muscle? Answer: Excitability, Conductivity, Contractility, Automaticity.

• What are the 2 main classifications of congenital heart disease? (Prompts recall from the knowledge check.)

• Identify the two semilunar valves. Answer: Aortic valve and Pulmonary (Pulmonic) valve.

• If BP is 120/80, what is the MAP? Answer: Use MAP formula; approx $MAP <br>ightarrow 93 ext{ mmHg}$ depending on rounding.

• Name the layers of the heart wall from outermost to innermost. Answer: Epicardium, Myocardium, Endocardium.