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: White blood cells 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 right and left sides, each with an atrium (upper chamber) and a ventricle (lower chamber). It is responsible for pumping blood through two main circuits:

• 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 deoxygenated blood to the right atrium.

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

• Pulmonary Veins: Carry 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.

Arteries vs. Veins

• Arteries: Carry blood away from the heart (not always oxygenated; e.g., pulmonary arteries carry deoxygenated blood).

• Veins: Return blood to the heart (not always deoxygenated; e.g., pulmonary veins carry oxygenated blood).

Note: Blood is always red due to hemoglobin; the blue appearance of veins is due to light refraction through tissue.

Example: Horseshoe crabs have blue blood due to hemocyanin (copper-based oxygen carrier), unlike humans who use iron-based hemoglobin.

Pressure Differences in Circuits

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

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

• Systolic Pressure: Pressure during heart contraction.

• Diastolic Pressure: Pressure during heart relaxation.

Heart Valves and Unidirectional Flow

Types of Heart Valves

• Atrioventricular (AV) Valves: Separate atria from ventricles.

  • Tricuspid Valve: Right AV valve (right atrium to right ventricle).

  • Mitral (Bicuspid) Valve: Left AV valve (left atrium to left ventricle).

• Semilunar Valves: Separate ventricles from major arteries.

  • Pulmonary Semilunar Valve: Right ventricle to pulmonary artery.

  • Aortic Semilunar Valve: Left ventricle to aorta.

Valves ensure unidirectional flow by preventing backflow of blood. The closing of these valves produces the characteristic "lub-dub" heart sounds.

Valve Function and Pressure-Volume Relationships

• When ventricles contract (systole), pressure increases, AV valves close, and semilunar valves open to eject blood.

• When ventricles relax (diastole), pressure decreases, semilunar valves close, and AV valves open to allow filling.

• Valves act as one-way doors, opening and closing in response to pressure gradients.

Example: The syringe model demonstrates how decreasing volume increases pressure to drive flow, and valves prevent backflow during relaxation.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events during one heartbeat, including changes in pressure, volume, and valve status.

1. Late Diastole: Both atria and ventricles are relaxed; blood flows passively from veins into atria and 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), but semilunar valves remain closed; volume does not change.

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

5. Isovolumic Ventricular Relaxation: Ventricles relax, semilunar valves close (second heart sound), AV valves remain closed; volume does not change.

6. Early Diastole: AV valves open, and the cycle repeats.

Key Volumes and Stroke Volume

• 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 and Volume Changes

• During ventricular contraction, pressure rises sharply, opening semilunar valves and ejecting blood.

• When ventricular pressure falls below arterial pressure, semilunar valves close, producing the dicrotic notch in the aortic pressure curve.

• Ventricular relaxation allows pressure to drop, AV valves open, and filling begins again.

Cardiac Muscle Physiology

Structure of Cardiac Myocytes

• Uninucleated: Each cell contains a single nucleus.

• Striated: Contains organized sarcomeres, similar to skeletal muscle.

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

• Highly Aerobic: Cardiac muscle relies on aerobic metabolism and can utilize fatty acids as an energy source.

Electrical Properties and Action Potentials

• Autorhythmic Cells: Specialized pacemaker cells generate spontaneous action potentials, setting the heart rate (typically ~60 beats/min at rest).

• Action Potential Phases in Cardiac Myocytes:

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

  2. Depolarization: Rapid influx of sodium through voltage-gated sodium channels.

  3. Initial Repolarization: Fast-opening and closing potassium channels briefly open.

  4. Plateau Phase: Calcium channels open, allowing Ca2+ influx, maintaining depolarization.

  5. Repolarization: Slow-opening potassium channels open, calcium channels close, returning the cell to resting potential.

• No Undershoot: Cardiac muscle action potentials do not exhibit the hyperpolarization seen in neurons.

Significance of the Plateau Phase

• The plateau phase prolongs the action potential, ensuring that the refractory period lasts as long as the contraction.

• This prevents tetanus (sustained contraction), which would be fatal for heart function.

• Allows the heart to contract and relax rhythmically without summation of contractions.

Comparison: Skeletal muscle can undergo summation and tetanus due to shorter action potentials; cardiac muscle cannot, due to the extended refractory period.

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 contraction duration is intermediate between skeletal and smooth muscle (~200 ms).

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

• Heart sounds are produced by the closing of valves: "lub" (AV valves), "dub" (semilunar valves).