Lecture 6 ExPhys

Anatomy of the Heart and Vascular Systems

Required Drawings and Diagrams: Knowledge of the following is essential for understanding cardiac function: * Heart anatomy, specifically identifying all chambers and major vessels. * The path of blood flow through the heart.
Major Anatomical Structures of the Heart:
* SVC (Superior Vena Cava): Returns deoxygenated blood from the upper body to the right atrium.
* IVC (Inferior Vena Cava): Returns deoxygenated blood from the lower body to the right atrium.
* Right Atrium: The receiving chamber for deoxygenated blood.
* Tricuspid Valve: The atrioventricular valve between the right atrium and right ventricle. *
Right Ventricle: Pumps deoxygenated blood to the lungs.
* Pulmonary Valve: The semilunar valve leading to the pulmonary artery.
* Pulmonary Artery (Right and Left): Carries deoxygenated blood to the lungs. *
Pulmonary Veins (Right and Left): Carry oxygenated blood from the lungs back to the left atrium.
* Left Atrium: The receiving chamber for oxygenated blood.
* Mitral Valve: The atrioventricular valve (also called the bicuspid valve) between the left atrium and left ventricle.
* Left Ventricle: The thickest chamber; pumps oxygenated blood to the entire body. *
Aortic Valve: The semilunar valve leading to the aorta.
* Aorta: The main artery transporting oxygenated blood to the systemic circulation.
Key Anatomical Terms:
* Pericardium: A tough, membranous sac that surrounds and protects the entire heart. *
Myocardium: The muscular tissue of the heart responsible for contraction.
* Myocardial Cells: Specialized cardiac muscle cells that make up the myocardium. *
Intercalated Disks: Specialized cell junctions within the myocardium where individual muscle cells connect. These disks anchor cells together and allow the heart to contract as a single functional unit through electrical coupling.

Characteristics of Cardiac Muscle (The Myocardium)

Fiber Composition: The myocardium contains only one fiber type. This fiber is highly unique but shares characteristics with skeletal Type I muscle fibers: * It is highly oxidative. * It possesses high capillary density. * It contains a high number of mitochondria.
Nature of Contraction: Contraction is strictly involuntary.
Blood Supply: The myocardium has its own dedicated blood supply known as the coronary blood vessels.
Functional Movement during Exercise: The heart does not contract all four chambers at once. It utilizes a torsion movement, similar to twisting a wet towel, to efficiently pump blood.

The Cardiac Conduction System and Control of Heart Rate

Unique Electrical Properties: Myocardial cells have the ability to spontaneously depolarize and directionally conduct electrical signals throughout the heart.
Intrinsic Control:
* Definition: The heart's unique ability to generate its own rhythmical electrical signal without external stimulation (spontaneous rhythmicity).
* Intrinsic Heart Rate: Without neural or hormonal influence, the heart rate averages approximately $100\,bpm$ .
* The Cardiac Conduction System Components:
1. Sinoatrial (SA) node: Located in the upper right atrium; known as the heart's pacemaker.
2. Atrioventricular (AV) node: Conducts the impulse from the atria to the ventricles; includes a small delay to allow atria to finish contracting.
3. AV bundle (Bundle of His): Found in the interventricular septum.
4. Bundle branches: Left and right branches leading to the ventricles.
5. Purkinje fibers: Terminal branches that spread the impulse through the ventricular myocardium.
Extrinsic Control:
* Autonomic Nervous System (ANS):
* Parasympathetic Nervous System (PNS): Connects via the Vagus Nerve (Cranial Nerve X). It carries impulses to the SA and AV nodes. When stimulated, it releases acetylcholine, which decreases heart rate. At rest, the PNS dominates, a state known as "vagal tone." * Sympathetic Nervous System (SNS): Increases the rate of depolarization of the SA node, increasing heart rate. It also increases the force of ventricular contraction. Maximal SNS stimulation can increase HR to $250\,bpm$ .
* Relationship to Heart Rate: * PNS predominates when HR is less than $100\,bpm$ . * SNS predominates when HR is greater than $100\,bpm$ (during physical or emotional stress). * At the start of exercise or at low intensities, HR initially increases due to the withdrawal of vagal tone. Further increases are driven by sympathetic activation. *
Endocrine System: The adrenal medulla releases catecholamines (Norepinephrine and Epinephrine), which increase both heart rate and contractility.

The Cardiac Cycle and Electrical Activity

Definitions:
* Cardiac Cycle: Includes all mechanical and electrical events during one single heartbeat.
* Diastole: The relaxation phase where chambers fill with blood.
* Systole: The contraction phase where ventricles expel blood.
* Pressure: The driving force that moves blood from one chamber to the next. *
Exercise Impact: As exercise intensity increases, the time available for both filling (diastole) and contraction (systole) is shortened.
Phases of the Cardiac Cycle:
1. Atrial Diastole: Atria fill passively. AV valves open once atrial pressure exceeds ventricular pressure, and blood pours into the ventricles.
2. Atrial Systole: The SA node activates an action potential, causing the atria to contract and push remaining blood into the ventricles.
3. Ventricular Diastole: The semilunar (SV) valves close. Pressure builds within the ventricles while volume remains constant; pressure must eventually exceed that of the lungs or body to eject blood.
End-Diastolic Volume (EDV) is the total blood volume in the ventricle at the end of this phase.
4. Ventricular Systole: The electrical impulse reaches the AV node, travels down the Bundle of His, and causes contraction. Once ventricular pressure exceeds pulmonary/aorta pressure, SL valves open and blood is ejected. The amount of blood remaining is the End-Systolic Volume (ESV).
Electrocardiogram (ECG) Waves:
* P-wave: Represents Atrial Depolarization (SA node to AV node).
* QRS Complex: Represents Ventricular Depolarization (AV bundle to Purkinje fibers). Note: Atrial repolarization occurs here but is hidden by the QRS complex.
* T-wave: Represents Ventricular Repolarization.
Heart Sounds:
* 'Lubb' (First Sound): Caused by the closure of the atrioventricular (AV) valves during ventricular systole.
* 'Dupp' (Second Sound): Caused by the closure of the semilunar (SL) valves during ventricular diastole.

Hemodynamics and Cardiac Output

Key Equations:
* Cardiac Output ( $Q$ ): The total volume of blood pumped by the ventricle per minute. * $Q = HR \times SV$ * Resting value: ~ $5\,L$ . * Exercise value: $25-30+\,L$ . *
Stroke Volume (SV): The volume of blood pumped per beat. * $SV = EDV - ESV$ * Normal resting SV: $60-80\,ml$ .
* Ejection Fraction (EF): The fraction of blood pumped out of the LV relative to the amount present before contraction. It is an index of the heart's pumping ability. * $EF = \frac{SV}{EDV} \times 100$ * Standard resting example: $\frac{60\,ml}{100\,ml} = 60\%$ .
Components of Stroke Volume:
* Preload: The force that stretches the ventricles, related to the volume of blood filling the heart (EDV).
* Afterload: The resistance or force the heart must overcome to eject blood (associated with ESV).
* Contractility: The inherent ability of the heart muscle to contract.

The Vascular System Structure and Function

Blood Vessel Types and Characteristics:
* Arteries: Carry blood away from the heart at high pressure. They have thick walls with large amounts of muscle and elastic fibers and narrow lumens.
* Arterioles: The smallest arteries leading to capillaries. Known as "resistance vessels," they offer the greatest control of circulation via the SNS.
* Capillaries: Site of material exchange with tissues. Walls are extremely thin (single cell thick) and the lumen is narrow (one cell wide). They lack muscle and valves.
* Venules: Smallest veins connecting capillaries to larger veins.
* Veins: Carry blood back to the heart at low pressure. They have thin walls, wide lumens, and less muscle/elastic tissue. They contain valves to prevent backflow and act as a blood reservoir.
Vessel Wall Layers:
* Tunica Intima: Innermost layer (Endothelium and loose connective tissue).
* Tunica Media: Middle layer (Smooth muscle and elastic fibers).
* Tunica Externa: Outermost layer (Collagen fibers).

Blood Pressure and Flow Dynamics

Blood Pressure Definitions:
* Systolic Blood Pressure (SBP): Highest pressure in the artery during ventricular systole.
* Diastolic Blood Pressure (DBP): Lowest pressure in the artery during ventricular diastole.
* Mean Arterial Pressure (MAP): The average pressure exerted by blood as it travels through arteries. * $MAP = \frac{2}{3} DBP + \frac{1}{3} SBP$ * (Note: Diastole lasts twice as long as systole).
Hemodynamic Principles: * The cardiovascular system is a closed-loop system. Blood flows due to a pressure gradient (High Pressure to Low Pressure). * At rest, MAP in the Aorta is approximately $100\,mmHg$ and is approximately $0\,mmHg$ in the Right Atrium. *
Resistance: Dictated by vessel length, vessel radius, and blood viscosity. * $Resistance = \text{viscosity} \times \frac{\text{length}}{\text{radius}^4}$ * $Blood\,Flow = \frac{\Delta Pressure}{Resistance}$ * Small changes in vessel radius (vasoconstriction and vasodilation) are the most effective way to regulate blood flow.

Regulation and Distribution of Blood Flow

Distribution Shifts: * Blood is diverted to tissues with the highest metabolic need.
* At Rest: Liver and kidneys receive ~ $50\%$ of cardiac output; muscles receive $15-20\%$ . *
During Exercise: Muscles can receive $80\%$ or more; supply to liver and kidneys decreases.
* Specific Needs: Digestive system increases flow after meals; skin increases flow in heat to facilitate sweating.
Methods of Control: *
Intrinsic (Local) Control: Local tissues dilate or constrict arterioles. Vasodilation is triggered by: increases in $CO_2$ , $H+$ (lactic acid), Nitric Oxide ( $NO$ ), Acetylcholine ( $ACh$ ), or decreased $O_2$ and blood pressure.
* Extrinsic Neural Control: Regulated by the SNS. Increased SNS activity causes vasoconstriction. At rest, "vasomotor tone" maintains slight vasoconstriction to support blood pressure.
* Muscle-Specific Regulation: During exercise, functional sympatholysis occurs. Endothelium-derived factors like Nitric Oxide prevent SNS-induced vasoconstriction in working muscles, allowing for necessary vasodilation.
* Venous Return: Aided by the Muscle Pump, where rhythmic skeletal muscle contractions compress veins and push blood back toward the heart.
Baroreceptors: Pressure sensors in the aortic arch and carotid arteries that sense vessel stretch.
* If BP Increases: Signals decrease SNS and increase vagal tone (PNS), leading to vasodilation and decreased heart rate.
* If BP Decreases: Signals cause vagal withdrawal and increased SNS, leading to vasoconstriction and increased heart rate.

Blood Composition and Clinical Pathologies

Blood Functions: Transportation of gases and waste, temperature regulation, and acid-base ( $pH$ ) balance.
Key Components:
* Erythrocytes: Red blood cells (RBCs).
* Hemoglobin: Protein on RBCs that binds to oxygen.
* Viscosity: Thickness of the blood; increasing RBC count requires an increase in plasma to maintain balance.
Heart Rate Conditions:
* Normal HR: $60-100\,bpm$ (Ideal: $60-80\,bpm$ ).
* Bradycardia: Resting HR below $60\,bpm$ . Common in athletes but classified as an arrhythmia.
* Tachycardia: Resting HR exceeding $100\,bpm$ . Can be caused by stress, meds, or heart conditions.
Cardiovascular Conditions:
* Angina Pectoris: Severe chest pain from inadequate heart blood supply.
* Arrhythmia: Irregular heart rhythm.
* Hypertension: Consistently high resting blood pressure.
* Ischemia: Inadequate blood supply to any organ, especially the heart.
* Syncope: Temporary loss of consciousness (fainting) due to reduced brain blood flow.
* Claudication: Leg pain/cramping during exercise, relieved by rest; associated with peripheral artery disease.
* Atherosclerosis: Plaque buildup in artery walls, reducing flow.
* Hyperlipidemia/Dyslipidemia: Imbalance or elevation of blood fats/cholesterol.
* Peripheral Artery Disease (PAD): Narrowed arteries reducing flow to limbs (legs/feet).
* Coronary Artery Disease (CAD): Atherosclerosis in coronary arteries; most common heart disease.
* Myocardial Infarction: A heart attack caused by sudden blockage of blood flow resulting in tissue death.
* Stroke: Interrupted blood flow to the brain.
* Heart Failure: A clinical condition where the heart cannot pump enough blood. Includes hypertrophy (initially) and later thinning/wasting of muscle walls. Often characterized by fluid buildup in lungs.
Cholesterol Types:
* HDL: "Good" cholesterol; carries cholesterol back to the liver for waste processing.
* LDL: "Bad" cholesterol; carries cholesterol to cells but can build up in artery walls as plaque.
* Triglycerides: Lipids that store unused calories for energy.
* Total Cholesterol: Sum of HDL, LDL, and $20\%$ of triglycerides.

Questions & Discussion

Muddiest Points?: This section is reserved for clarifying difficult concepts like functional sympatholysis or pressure gradients.
Knowledge Check Questions:
* Define systole and diastole: Systole is contraction/ejection; Diastole is relaxation/filling.
* Which nerve controls the PSNS activation on the heart?: The Vagus Nerve (Cranial Nerve X).
* What is the intrinsic control of the heart in bpm?: Approximately $100\,bpm$ .
* How does heart rate increase during exercise?: First via vagal tone withdrawal (up to $100\,bpm$ ), then through sympathetic nervous system activation.
* T-wave represents which phase of the cardiac cycle?: Ventricular repolarization (occurs during late ventricular systole/early diastole).
* During which phase of the cardiac cycle do we get end-diastolic volume?: At the end of ventricular diastole (after atrial systole).