Cardiovascular Physiology: Structure, Function, and Cardiac Adaptations

The Cardiovascular System

Overview

The cardiovascular system is responsible for transporting blood, nutrients, gases, and wastes throughout the body. It consists of the heart, blood vessels, and blood, working together to maintain homeostasis and support cellular function.

• Heart: A muscular organ that pumps blood through the circulatory system.

• Blood vessels: Include arteries, veins, and capillaries that transport blood to and from the heart and tissues.

• Blood: The fluid medium carrying oxygen, nutrients, hormones, and waste products.

Example: The heart pumps oxygenated blood to the body via the systemic circuit and deoxygenated blood to the lungs via the pulmonary circuit.

Location and Basic Structure of the Heart

Position and Size

The heart is located slightly to the left of the midline in the thoracic cavity, posterior to the sternum, and rests on the diaphragm. It is about the size of a fist and typically weighs between 250 and 350 grams.

• Cardiomegaly: An enlarged heart, which can be visualized on a chest X-ray, may indicate underlying pathology.

Chambers of the Heart

The heart has four chambers:

• Atria (singular: atrium): Superior right and left chambers that receive blood from veins.

• Ventricles: Inferior right and left chambers that pump blood into arteries.

Blood flows from the atria to the ventricles, and then the ventricles pump blood into the arteries.

Cardiac Valves

Valves ensure unidirectional blood flow through the heart:

• Tricuspid valve: Between right atrium and right ventricle.

• Pulmonary valve: Between right ventricle and pulmonary artery.

• Bicuspid (mitral) valve: Between left atrium and left ventricle.

• Aortic valve: Between left ventricle and aorta.

Function: Prevent backflow of blood during the cardiac cycle.

Cardiac Muscle Adaptations

Hypertrophy vs Hyperplasia

• Hypertrophy: Increase in cell size (common in cardiac muscle).

• Hyperplasia: Increase in cell number (rare in adult cardiac muscle).

Cardiomegaly can result from hypertrophy or, less commonly, hyperplasia.

Cardiac Cycle and Blood Flow

Systole and Diastole

• Systole: The contraction phase of the heart, during which blood is pumped out of the chambers.

• Diastole: The relaxation phase, during which the chambers fill with blood.

Pulmonary vs Systemic Circuits

• Pulmonary circuit: Carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.

• Systemic circuit: Carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.

Cardiac Compensatory Mechanisms

Preload and Afterload

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

• Afterload: The pressure the heart must overcome to eject blood, primarily determined by aortic pressure.

Example: Increased preload (more blood in the ventricle before contraction) stretches the heart muscle more, potentially increasing stroke volume.

Stroke Volume and Cardiac Output

• Stroke Volume (SV): The amount of blood pumped by one ventricle in a single heartbeat.

• Cardiac Output (CO): The amount of blood pumped by the heart in one minute.

Formulas:

• Stroke Volume: $SV = EDV - ESV$

• Cardiac Output: $CO = SV \times HR$

Where:

• EDV: End-diastolic volume (volume in ventricle after filling)

• ESV: End-systolic volume (volume in ventricle after contraction)

• HR: Heart rate (beats per minute)

Cardiac Hypertrophy: Volume vs Pressure Overload

Eccentric Hypertrophy (Volume Overload)

Occurs when the heart is exposed to increased volume (preload), such as in aortic regurgitation. Sarcomeres are added in series, leading to longer cardiac muscle cells and a dilated ventricle with normal wall thickness.

• Initial effect: Increased force of contraction (Frank-Starling mechanism).

• Long-term effect: Cell damage, decreased contractile force, increased ESV, decreased SV (systolic failure).

Example: A patient with aortic regurgitation develops an enlarged left ventricle and symptoms of heart failure.

Concentric Hypertrophy (Pressure Overload)

Occurs when the heart faces increased pressure (afterload), such as in chronic hypertension. Sarcomeres are added in parallel, resulting in thicker cardiac muscle cells and a thickened ventricular wall with reduced chamber volume.

• Effect: Increased force to overcome high pressure, but decreased filling and SV (diastolic failure).

Example: A patient with long-standing hypertension develops a thick, stiff left ventricle and symptoms of heart failure.

Comparison Table: Eccentric vs Concentric Hypertrophy

Feature	Eccentric Hypertrophy (Volume Overload)	Concentric Hypertrophy (Pressure Overload)
Stimulus	Increased Preload (Volume)	Increased Afterload (Pressure)
Sarcomere Pattern	In Series (Longer Cells)	In Parallel (Thicker Cells)
Geometry	Dilated, normal wall thickness	Thick walls, small cavity
Failure Type	Systolic Failure (HFrEF)	Diastolic Failure (HFpEF)
Effect on SV & CO	SV decreases due to increased ESV	SV decreases due to decreased EDV

Key Terminology

• Systole: Contraction phase of the heart.

• Diastole: Relaxation phase of the heart.

• Preload: Stretch of the ventricle before contraction (related to EDV).

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

• Hypertrophy: Increase in cell size.

• Hyperplasia: Increase in cell number.

• Stroke Volume (SV): Blood pumped per beat.

• Cardiac Output (CO): Blood pumped per minute.

Sample Questions for Review

1. What is the function of the atrioventricular valves in the heart?

   • Answer: To prevent backflow of blood into the atria when the ventricles contract.

2. Afterload is directly related to which of the following factors?

   • Answer: Aortic pressure or resistance to ejection.

3. Sarcomeres are added in series primarily when the ventricle experiences:

   • Answer: Increased volume during filling.

Summary Table: Core Concepts

Concept	Volume Overload	Pressure Overload
Stimulus	Preload (Volume)	Afterload (Pressure)
Sarcomere Pattern	In Series (Longer Cells)	In Parallel (Thicker Cells)
Geometry	Dilated, normal wall thickness	Thick walls, small cavity
Failure Type	Systolic Failure (HFrEF)	Diastolic Failure (HFpEF)
Effect on SV & CO	SV decreases due to increased ESV	SV decreases due to decreased EDV

Additional info: HFrEF = Heart Failure with reduced Ejection Fraction; HFpEF = Heart Failure with preserved Ejection Fraction.