The Cardiovascular System

Overview and Importance

The cardiovascular system is essential for transporting gases, nutrients, hormones, and waste products throughout the body. It consists of the heart (the pump), blood vessels (arteries, veins, capillaries), and blood. This system ensures that all cells receive oxygen and nutrients while removing metabolic wastes efficiently.

• Transport of Gases: Oxygen is delivered from the lungs to tissues; carbon dioxide is transported from tissues to the lungs for exhalation.

• Nutrient Distribution: Nutrients absorbed from the digestive system are distributed to cells.

• Waste Removal: Metabolic wastes are transported to the liver and kidneys for detoxification and excretion.

• Hormone Delivery: Endocrine glands release hormones into the bloodstream for regulation of physiological processes.

• Temperature Regulation: Blood flow can be adjusted to regulate heat loss or retention.

• Immune Function: Leukocytes and immune mediators are distributed to defend against pathogens.

Heart Anatomy and Circuits

Heart Structure and Chambers

The heart is a muscular organ divided into four chambers: two atria (upper chambers) and two ventricles (lower chambers). It is functionally divided into right and left sides, each serving a distinct circuit.

• Right Side: Receives deoxygenated blood from the body and pumps it to the lungs (pulmonary circuit).

• Left Side: Receives oxygenated blood from the lungs and pumps it to the rest of the body (systemic circuit).

• Major Vessels:

  • Vena Cava: Superior and inferior veins returning blood to the right atrium.

  • Pulmonary Arteries: Carry deoxygenated blood from the right ventricle to the lungs.

  • Pulmonary Veins: Return oxygenated blood from the lungs to the left atrium.

  • Aorta: Distributes oxygenated blood from the left ventricle to the systemic circuit.

• Coronary Circulation: The heart muscle (myocardium) is supplied by coronary arteries and veins, as the blood within the chambers does not nourish the heart tissue directly.

Valves of the Heart

Valves ensure unidirectional blood flow through the heart and prevent backflow.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Mitral (Bicuspid) Valve: Between left atrium and left ventricle.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary artery.

  • Aortic Valve: Between left ventricle and aorta.

Key Principle: Valves open and close in response to pressure changes, ensuring blood flows in one direction only.

Blood Flow Pathway

Systemic and Pulmonary Circuits

• Systemic Circuit: Left ventricle → aorta → body tissues → vena cava → right atrium.

• Pulmonary Circuit: Right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium.

Note: Arteries always carry blood away from the heart; veins return blood to the heart. Oxygenation status depends on the circuit (e.g., pulmonary veins carry oxygenated blood).

Blood Color and Oxygenation

• Human Blood: Always red due to hemoglobin; never blue. The appearance of blue veins is due to light refraction through tissue.

• Other Animals: Some, like horseshoe crabs, have blue blood due to copper-based hemocyanin.

Pressure and Flow in the Circulatory System

Pressure Gradients

The heart generates pressure to drive blood through vessels. Blood flows from regions of higher pressure to lower pressure.

• Pulmonary Circuit Pressure: Lower (approx. 25/8 mmHg) due to short distance between heart and lungs.

• Systemic Circuit Pressure: Higher (approx. 120/80 mmHg) to propel blood throughout the body.

• Systolic Pressure: Pressure during ventricular contraction.

• Diastolic Pressure: Pressure during ventricular relaxation.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.

1. Late Diastole: Both chambers relaxed; blood flows passively from atria to ventricles through open AV valves.

2. Atrial Systole: Atria contract, pushing additional blood into ventricles.

3. Isovolumic Ventricular Contraction: Ventricles begin to contract; AV valves close (first heart sound, "lub"). No blood ejected yet.

4. Ventricular Systole (Ejection): Ventricular pressure exceeds arterial pressure; semilunar valves open, blood is ejected into aorta and pulmonary artery.

5. Isovolumic Ventricular Relaxation: Ventricles relax; semilunar valves close (second heart sound, "dub").

6. Return to Diastole: AV valves open, cycle repeats.

Heart Sounds

• "Lub": Closure of AV valves at the start of ventricular contraction.

• "Dub": Closure of semilunar valves at the start of ventricular relaxation.

Volume and Pressure Changes

During the cardiac cycle, ventricular volume and pressure change in a coordinated manner:

• End-Diastolic Volume (EDV): Maximum volume in ventricle at end of filling (approx. 135 mL).

• End-Systolic Volume (ESV): Volume remaining after contraction (approx. 65 mL).

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

Formula:

For example,

Pressure-Volume Relationships

• Ventricular contraction increases pressure, opening semilunar valves and ejecting blood.

• Relaxation decreases pressure, allowing AV valves to open and ventricles to fill.

• The dicrotic notch on the aortic pressure curve marks closure of the aortic valve.

Cardiac Muscle Physiology

Structure of Cardiac Myocytes

• Uninucleated, Striated Cells: Cardiac muscle cells are smaller than skeletal muscle fibers but have striations due to organized sarcomeres.

• Intercalated Discs: Specialized connections between cells containing gap junctions and desmosomes, allowing electrical and mechanical coupling.

• Gap Junctions: Permit rapid spread of action potentials, enabling the heart to contract as a functional syncytium.

Autorhythmicity

• Pacemaker Cells: Specialized cells generate spontaneous action potentials, setting the heart rate (typically ~60 beats per minute at rest).

Cardiac Action Potential

The cardiac muscle action potential is distinct from that of skeletal muscle and neurons, featuring a prolonged plateau phase.

• Resting Membrane Potential: Approximately -90 mV, close to the equilibrium potential for potassium.

• Phases:

  1. Depolarization: Rapid influx of Na+ through voltage-gated sodium channels.

  2. Initial Repolarization: Brief opening of fast K+ channels.

  3. Plateau: Influx of Ca2+ through voltage-gated calcium channels balances K+ efflux, prolonging depolarization.

  4. Repolarization: Closure of Ca2+ channels and opening of slow K+ channels returns membrane to resting potential.

• No Undershoot: Cardiac muscle cells do not hyperpolarize as neurons do.

Functional Significance of the Plateau

• Prevents Tetany: The long refractory period (plateau phase) ensures that cardiac muscle cannot be tetanized (sustained contraction), which would be fatal.

• Ensures Rhythmic Contraction: Each contraction is followed by relaxation, allowing the heart to refill and maintain effective pumping.

Comparison of Muscle Types (Described in Text)

• Skeletal Muscle: Fast contraction, can be tetanized, voluntary control.

• Cardiac Muscle: Intermediate contraction speed (~200 ms), cannot be tetanized, involuntary, autorhythmic.

• Smooth Muscle: Slowest contraction, involuntary, found in walls of hollow organs.

Summary Table: Key Features of the Cardiac Cycle

Key Equations

• Stroke Volume:

• Cardiac Output:

Where is cardiac output, is stroke volume, and is heart rate.

Additional Info

• Cardiac muscle cells are highly aerobic and can utilize fatty acids as an energy source.

• The left ventricle has a thicker wall than the right, reflecting its role in generating higher pressures for systemic circulation.

• Heart valves are responsible for the characteristic "lub-dub" sounds heard during auscultation.