Cardiovascular System: Heart Function, Blood Pressure, and Related Concepts

Cardiovascular System: The Heart and Blood Pressure

Heart Function and Cardiac Cycle

The heart is a muscular organ responsible for pumping blood throughout the body. Its function is regulated by electrical and mechanical events that ensure efficient circulation.

• Cardiac Cycle: The sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.

• Resting State of the Heart: The heart at rest is in diastole, allowing chambers to fill with blood.

• Ventricular Systole: Contraction of the ventricles pumps blood into the pulmonary artery and aorta.

• Electrical Conduction: The sinoatrial (SA) node initiates the heartbeat, followed by the atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers.

• Stroke Volume (SV): The amount of blood ejected by a ventricle in one contraction.

• End-Diastolic Volume (EDV): The volume of blood in a ventricle at the end of filling (diastole).

• End-Systolic Volume (ESV): The volume of blood remaining in a ventricle after contraction (systole).

• Ejection Fraction (EF): The percentage of EDV ejected with each heartbeat. Calculated as $EF = \frac{SV}{EDV} \times 100\%$

Example: If EDV = 120 mL and ESV = 50 mL, then SV = 70 mL. EF = (70/120) × 100% ≈ 58%.

Blood Pressure and Its Regulation

Blood pressure (BP) is the force exerted by circulating blood on the walls of blood vessels. It is a vital sign and is regulated by cardiac output, blood volume, and vascular resistance.

• Systolic Pressure: The maximum pressure during ventricular contraction.

• Diastolic Pressure: The minimum pressure during ventricular relaxation.

• Mean Arterial Pressure (MAP): The average pressure in the arteries during one cardiac cycle. Calculated as $MAP = \frac{1}{3} (Systolic\ Pressure - Diastolic\ Pressure) + Diastolic\ Pressure$

• Pulse Pressure: The difference between systolic and diastolic pressure.

• Factors Affecting BP: Cardiac output (CO), total peripheral resistance (TPR), blood volume, and vessel elasticity.

• Cardiac Output (CO): The volume of blood pumped by the heart per minute. $CO = SV \times HR$ (where HR is heart rate).

Example: If SV = 70 mL and HR = 75 bpm, then CO = 70 × 75 = 5250 mL/min or 5.25 L/min.

Frank-Starling Law of the Heart

The Frank-Starling law states that the strength of the heart's contraction is directly proportional to its initial length (preload). This means that increased venous return stretches the ventricular walls, resulting in a more forceful contraction and increased stroke volume.

• Preload: The degree of stretch of cardiac muscle fibers at the end of diastole.

• Afterload: The resistance the ventricles must overcome to eject blood.

• Contractility: The intrinsic ability of cardiac muscle to contract at a given fiber length.

Example: During exercise, increased venous return increases preload, enhancing cardiac output via the Frank-Starling mechanism.

Common Cardiovascular Conditions

Several conditions can affect heart function and blood pressure regulation.

• Hypertension: Chronically elevated blood pressure, increasing the risk of heart disease and stroke.

• Heart Failure: The inability of the heart to pump sufficient blood to meet the body's needs.

• Stenosis: Narrowing of heart valves, impeding blood flow and increasing cardiac workload.

• Symptoms of Heart Disease: May include chest pain, shortness of breath, fatigue, and edema.

Table: Key Cardiac Volumes and Formulas

Term	Definition	Formula
Stroke Volume (SV)	Blood ejected per beat	$SV = EDV - ESV$
Cardiac Output (CO)	Blood ejected per minute	$CO = SV \times HR$
Ejection Fraction (EF)	Percent of EDV ejected	$EF = \frac{SV}{EDV} \times 100\%$
Mean Arterial Pressure (MAP)	Average arterial pressure	$MAP = \frac{1}{3} (Systolic - Diastolic) + Diastolic$

Additional info:

• Some content was inferred based on standard cardiovascular physiology due to fragmented and unclear notes.

• Key terms and formulas were expanded for clarity and completeness.