Cardiovascular System: Anatomy & Physiology Study Guide (Chapters 18 & 19)

Cardiovascular System: Anatomy & Physiology

Overview

This study guide covers the essential concepts of the cardiovascular system, focusing on both anatomy and physiology. It is structured to help students understand the structure and function of the heart, blood vessels, and the regulation of blood flow and pressure. The guide is organized by major topics and subtopics, with expanded academic context and definitions for key terms.

Cardiac Muscle Anatomy and Physiology

Structure of Cardiac Muscle

• Cardiac Myocytes: Short, branched, striated cells with one or two centrally located nuclei. Connected by intercalated discs containing desmosomes (for mechanical strength) and gap junctions (for electrical connectivity).

• Comparison to Skeletal Muscle: Skeletal muscle fibers are long, cylindrical, multinucleated, and lack intercalated discs. Cardiac muscle is involuntary and contracts as a functional syncytium due to gap junctions.

• Key Structures: Sarcomeres (contractile units), sarcoplasmic reticulum (Ca2+ storage), T-tubules (conduct action potentials).

Types of Cardiomyocytes

• Contractile Cells: Make up the majority of the myocardium; responsible for generating force during contraction.

• Autorhythmic (Pacemaker) Cells: Located in the sinoatrial (SA) node, atrioventricular (AV) node, and conduction system; initiate and conduct action potentials.

Comparison Table: Cardiac vs. Skeletal Muscle

Feature	Cardiac Muscle	Skeletal Muscle
Cell Shape	Short, branched	Long, cylindrical
Nuclei	1-2, central	Multiple, peripheral
Intercalated Discs	Present	Absent
Gap Junctions	Yes	No
Control	Involuntary	Voluntary
Contraction	Syncytium	Individual fibers
ATP Supply	Aerobic (many mitochondria)	Aerobic & anaerobic

Electrical Activity of the Heart

Action Potentials in Cardiac Cells

• Contractile Cells: Resting membrane potential ≈ -90 mV. Action potential has four phases: rapid depolarization (Na+ influx), initial repolarization, plateau (Ca2+ influx balances K+ efflux), and repolarization (K+ efflux).

• Autorhythmic Cells: No true resting potential; exhibit pacemaker potential (slow depolarization due to Na+ influx), threshold, rapid depolarization (Ca2+ influx), and repolarization (K+ efflux).

Key Ion Channels: Na+, Ca2+, and K+ channels are responsible for the phases of the action potential.

Conduction System of the Heart

• Sequence: SA node → AV node → Bundle of His → Right and left bundle branches → Purkinje fibers.

• Gap Junctions: Allow rapid spread of electrical impulses, enabling the heart to contract as a coordinated unit (functional syncytium).

• AV Node: Delays impulse, ensuring atrial contraction precedes ventricular contraction.

Electrocardiogram (ECG/EKG)

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization (and atrial repolarization, which is masked).

• T Wave: Ventricular repolarization.

• Segments: PR interval (atrial to ventricular conduction), ST segment (ventricular contraction).

Arrhythmias (Disruptions of Heart Rhythm)

• Sinus Arrest: SA node fails; AV node paces heart (40-60 bpm), P waves absent.

• Heart Block: AV node fails to conduct impulses; more P waves than QRS complexes.

• Ventricular Fibrillation: Disorganized electrical activity; chaotic ECG, life-threatening.

• Other Terms: Tachycardia (fast HR), Bradycardia (slow HR), Atrial Fibrillation (irregular atrial activity).

Mechanical Events of the Cardiac Cycle

Phases of the Cardiac Cycle

• Systole: Contraction phase; blood ejected from ventricles.

• Diastole: Relaxation phase; ventricles fill with blood.

• Valve Function: AV valves (tricuspid, mitral) prevent backflow into atria; semilunar valves (aortic, pulmonary) prevent backflow into ventricles.

Cardiac Output (CO) and Related Variables

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

  • Formula: $CO = SV \times HR$

  • Normal adult: $CO = 70\,ml/beat \times 75\,beats/min = 5250\,ml/min = 5.25\,L/min$

• Stroke Volume (SV): Volume of blood ejected by a ventricle per beat.

  • Formula: $SV = EDV - ESV$

  • Example: $SV = 120\,ml - 50\,ml = 70\,ml$

• Ejection Fraction (EF): Percentage of EDV ejected per beat.

  • Formula: $EF = \frac{SV}{EDV} \times 100$

  • Example: $EF = \frac{70}{120} \times 100 \approx 60\%$

Factors Affecting Cardiac Output

• Preload: Degree of stretch of cardiac muscle cells before contraction (related to EDV).

• Afterload: Resistance the ventricles must overcome to eject blood (related to aortic pressure).

• Contractility: Force of contraction at a given muscle length; increased by positive inotropic agents (e.g., epinephrine), decreased by negative inotropic agents (e.g., acidosis).

• Heart Rate (HR): Controlled by autonomic nervous system; sympathetic stimulation increases HR, parasympathetic decreases HR.

Blood Vessels and Circulation

Types of Blood Vessels

• Arteries: Carry blood away from the heart; thick walls, high pressure.

• Arterioles: Small arteries; major site of resistance and regulation of blood flow.

• Capillaries: Thin-walled; site of exchange between blood and tissues.

• Venules and Veins: Return blood to the heart; thin walls, valves prevent backflow.

Blood Pressure and Resistance

• Blood Pressure (BP): Force per unit area exerted by blood on vessel walls (mmHg).

• Mean Arterial Pressure (MAP): Average pressure in arteries during one cardiac cycle.

  • Formula: $MAP = \text{Diastolic Pressure} + \frac{1}{3}(\text{Pulse Pressure})$

  • Pulse Pressure = Systolic - Diastolic Pressure

• Resistance (R): Opposition to flow; determined by vessel diameter, length, and blood viscosity.

  • Poiseuille’s Law: $R = \frac{8 \eta l}{\pi r^4}$ (where $\eta$ = viscosity, $l$ = length, $r$ = radius)

• Relationship: $F = \frac{\Delta P}{R}$ (Flow = Pressure gradient / Resistance)

Regulation of Blood Pressure

• Short-term: Baroreceptor reflex (senses changes in BP, adjusts HR and vessel diameter via autonomic nervous system).

• Long-term: Renal regulation of blood volume.

• Hypertension: Chronic high BP (>130/80 mmHg); risk factors include genetics, lifestyle.

• Hypotension: Chronic low BP (<90/60 mmHg); can lead to shock if severe.

Capillary Exchange and Bulk Flow

Mechanisms of Exchange

• Diffusion: Movement of small molecules (O2, CO2) down concentration gradients.

• Vesicular Transport: Endocytosis and exocytosis for larger molecules.

• Bulk Flow: Movement of fluid driven by hydrostatic and osmotic pressures.

Starling Forces and Net Filtration

• Hydrostatic Pressure (P): Pushes fluid out of capillaries.

• Osmotic Pressure (OP): Pulls fluid into capillaries (due to plasma proteins).

• Net Filtration Pressure (NFP): Determines direction of fluid movement.

  • Formula: $NFP = (P_c - P_i) - (\pi_c - \pi_i)$

  • Filtration occurs at arterial end; reabsorption at venous end.

• Edema: Excess fluid in interstitial space due to imbalance in Starling forces.

Regulation of Blood Flow

Intrinsic (Local) Control

• Autoregulation: Tissues regulate their own blood flow via local factors (e.g., O2, CO2, pH).

• Active Hyperemia: Increased blood flow in response to increased metabolic activity.

• Reactive Hyperemia: Increased blood flow following a period of reduced blood supply.

Extrinsic Control

• Neural: Sympathetic and parasympathetic nervous systems adjust vessel diameter and heart function.

• Hormonal: Epinephrine, norepinephrine, angiotensin II, and others affect blood pressure and flow.

Clinical Correlations

• Heart Failure: Inability of the heart to pump sufficient blood; reduced ejection fraction.

• Aneurysm: Localized dilation of a blood vessel wall; risk of rupture.

• Shock: Inadequate tissue perfusion; can be hypovolemic, cardiogenic, or distributive.

Summary Table: Key Cardiovascular Formulas

Variable	Formula	Units
Cardiac Output (CO)	$CO = SV \times HR$	ml/min or L/min
Stroke Volume (SV)	$SV = EDV - ESV$	ml/beat
Ejection Fraction (EF)	$EF = \frac{SV}{EDV} \times 100$	%
Mean Arterial Pressure (MAP)	$MAP = \text{Diastolic} + \frac{1}{3}(\text{Pulse Pressure})$	mmHg
Flow (F)	$F = \frac{\Delta P}{R}$	ml/min
Resistance (R)	$R = \frac{8 \eta l}{\pi r^4}$	mmHg·min/ml

Additional info: This guide synthesizes and expands upon the objectives and key points from the provided syllabus slides, offering definitions, formulas, and clinical context for major cardiovascular concepts.