Study Notes: The Cardiovascular System – Cardiac Function

Chapter 13: The Cardiovascular System – Cardiac Function

Overview

This chapter explores the structure and function of the heart, the flow of blood through the cardiac cycle, the electrical and mechanical events that drive heart function, and the regulation of cardiac output. Understanding these concepts is essential for comprehending how the cardiovascular system maintains homeostasis and responds to physiological demands.

Structure and Function of the Heart

Layers of the Heart Wall

• Epicardium: The outermost layer; a thin serous membrane that provides protection and reduces friction.

• Myocardium: The thick, muscular middle layer responsible for the contractile force of the heart.

• Endocardium: The innermost layer; a smooth lining that minimizes resistance to blood flow within the heart chambers.

Blood Flow Through the Heart

• Blood enters the right atrium from the systemic circulation via the superior and inferior vena cava.

• It passes through the tricuspid valve into the right ventricle, then is pumped through the pulmonary valve into the pulmonary arteries to the lungs.

• Oxygenated blood returns via the pulmonary veins to the left atrium, passes through the bicuspid (mitral) valve into the left ventricle, and is pumped through the aortic valve into the aorta for systemic distribution.

• Parallel flow: Most systemic and pulmonary circuits are in parallel, allowing for efficient oxygenation and nutrient delivery. Exceptions include the hepatic portal system.

Valves and Blood Flow Regulation

Heart Valves

• Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left) valves prevent backflow from ventricles to atria.

• Semilunar valves: Pulmonary and aortic valves prevent backflow from arteries into ventricles.

• Valve location and function: AV valves are between atria and ventricles; semilunar valves are between ventricles and major arteries.

Electrical Activity of the Heart

Autorhythmic Cells and the Cardiac Conduction System

• Autorhythmic cells: Specialized cardiac cells that generate and conduct action potentials, initiating the heartbeat independently of neural input.

• Sinoatrial (SA) node: The primary pacemaker, located in the right atrium, initiates electrical impulses.

• Atrioventricular (AV) node: Delays the impulse, allowing atrial contraction before ventricular contraction.

• Bundle of His, bundle branches, Purkinje fibers: Conduct impulses rapidly through the ventricles.

Electrical Conduction Pathway

1. Impulse originates at the SA node.

2. Spreads through atria, causing atrial contraction.

3. Impulse delayed at AV node.

4. Travels through Bundle of His, bundle branches, and Purkinje fibers, causing ventricular contraction.

Action Potentials in Cardiac Cells

• Pacemaker cells: Generate spontaneous action potentials due to unstable resting membrane potential.

• Contractile cells: Exhibit a prolonged action potential with a plateau phase due to calcium influx, preventing tetanus.

Excitation-Contraction Coupling

• Action potential triggers calcium influx.

• Calcium-induced calcium release from the sarcoplasmic reticulum increases cytosolic Ca2+.

• Ca2+ binds to troponin, enabling actin-myosin crossbridge cycling and contraction.

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

• Systole: Ventricular contraction and ejection of blood.

• Diastole: Ventricular relaxation and filling.

Heart Sounds

• First heart sound (S1): Closure of AV valves at the start of systole.

• Second heart sound (S2): Closure of semilunar valves at the start of diastole.

Cardiac Output and Its Regulation

Definitions and Formulas

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

• Formula: $CO = HR \times SV$

• Heart Rate (HR): Beats per minute.

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

• Formula: $SV = EDV - ESV$ where EDV = End-Diastolic Volume, ESV = End-Systolic Volume

• Ejection Fraction (EF): Percentage of EDV ejected per beat. Formula: $EF = \frac{SV}{EDV} \times 100\%$

Regulation of Heart Rate and Stroke Volume

• Intrinsic regulation: Frank-Starling law – increased venous return stretches the heart, increasing force of contraction and SV.

• Extrinsic regulation: Autonomic nervous system (sympathetic increases HR and contractility; parasympathetic decreases HR).

• Endocrine factors: Hormones such as epinephrine increase HR and contractility.

Preload and Afterload

• Preload: The degree of stretch of cardiac muscle fibers at the end of diastole (related to EDV).

• Afterload: The resistance the ventricles must overcome to eject blood (related to arterial pressure).

Starling's Law of the Heart

• States that the force of ventricular contraction increases as the heart is filled with more blood (increased EDV leads to increased SV).

Blood Pressure and Its Regulation

Systolic and Diastolic Pressure

• Systolic pressure: Maximum arterial pressure during ventricular systole.

• Diastolic pressure: Minimum arterial pressure during ventricular diastole.

• Measurement: Typically measured using a sphygmomanometer (blood pressure cuff).

The Lymphatic System

Function and Integration

• Lymphatic system: Returns excess interstitial fluid to the bloodstream, absorbs dietary fats, and provides immune defense.

• Complements the cardiovascular system by maintaining fluid balance and the immune system by transporting lymphocytes and filtering pathogens.

Summary Table: Key Cardiac Parameters

Parameter	Definition	Formula
Cardiac Output (CO)	Blood volume pumped per minute	$CO = HR \times SV$
Stroke Volume (SV)	Blood volume ejected per beat	$SV = EDV - ESV$
Ejection Fraction (EF)	Fraction of EDV ejected per beat	$EF = \frac{SV}{EDV} \times 100\%$

Example: Calculating Cardiac Output

• If HR = 70 bpm and SV = 70 mL/beat, then $CO = 70 \times 70 = 4900$ mL/min (or 4.9 L/min).

Additional info:

• Intrinsic and extrinsic controls of the heart ensure adaptability to changing physiological demands.

• Understanding preload and afterload is essential for interpreting cardiac function in health and disease.