Cardiovascular Physiology

Blood Vessels and Circulation

The cardiovascular system consists of a network of blood vessels that transport blood throughout the body, supporting tissue perfusion and homeostasis.

• Types of Blood Vessels:

  • Arteries: Carry blood away from the heart; thick and elastic to withstand high pressure.

  • Veins: Return blood to the heart; thinner walls, often contain valves to prevent backflow.

  • Capillaries: Smallest vessels; site of exchange between blood and tissues.

• Portal Systems: Two capillary beds in series (e.g., hepatic portal system).

• Blood Composition: Blood consists of cells (RBCs, WBCs, platelets) and plasma.

• Pulmonary vs. Systemic Circulation:

  • Pulmonary: Right ventricle pumps blood to lungs via pulmonary arteries; returns via pulmonary veins.

  • Systemic: Left ventricle pumps blood to body via aorta; returns via vena cava.

Blood Flow Dynamics

Blood flow is driven by pressure gradients and is influenced by vessel diameter, resistance, and blood viscosity.

• Pressure Gradient (ΔP): Difference in pressure between two regions drives flow.

• Flow Equation:

  • Where is resistance, which is inversely proportional to flow.

• Poiseuille's Law:

  • Resistance increases with viscosity () and length (), decreases with radius () to the fourth power.

• Vasodilation: Increased diameter, decreased resistance, increased flow.

• Vasoconstriction: Decreased diameter, increased resistance, decreased flow.

• Blood Viscosity: Increased viscosity increases resistance.

• Flow Rate: Amount of fluid passing a point per unit time.

• Velocity of Flow: , where is flow rate and is cross-sectional area.

Heart Structure and Function

The heart is a muscular organ divided into four chambers, responsible for pumping blood through pulmonary and systemic circuits.

• Chambers: Right and left atria (receive blood), right and left ventricles (pump blood out).

• Valves:

  • Atrioventricular (AV) Valves: Tricuspid (right), Mitral/Bicuspid (left).

  • Semilunar Valves: Pulmonary (right), Aortic (left).

• Septum: Divides heart into left and right halves.

• Myocardium: Cardiac muscle tissue; contains contractile and autorhythmic cells.

Electrical Activity and Cardiac Cycle

The heart's rhythmic contractions are coordinated by electrical signals originating from pacemaker cells and conducted through specialized pathways.

• Autorhythmic Cells: Generate spontaneous action potentials; about 1% of myocardial cells.

• Contractile Cells: Striated muscle cells organized into sarcomeres; responsible for force generation.

• Intercalated Discs: Contain desmosomes and gap junctions for mechanical and electrical coupling.

• Action Potential Phases (Contractile Cells):

  1. Phase 4: Resting membrane potential.

  2. Phase 0: Depolarization (Na+ influx).

  3. Phase 1: Initial repolarization (K+ efflux).

  4. Phase 2: Plateau (Ca2+ influx balances K+ efflux).

  5. Phase 3: Rapid repolarization (K+ efflux).

• Action Potential Phases (Pacemaker Cells):

  1. Unstable resting potential (pacemaker potential).

  2. Depolarization via Ca2+ channels.

  3. Repolarization via K+ channels.

• Calcium-Induced Calcium Release: Ca2+ influx triggers further Ca2+ release from sarcoplasmic reticulum via ryanodine receptors.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart, providing information about heart rhythm and conduction.

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization.

• T Wave: Ventricular repolarization.

• Segments: P-R and T-P intervals.

Cardiac Cycle and Phases

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole).

• Five Phases:

  1. Atrial contraction

  2. Isovolumetric ventricular contraction

  3. Ventricular ejection

  4. Isovolumetric ventricular relaxation

  5. Ventricular filling

• One-way Flow: Maintained by heart valves.

• Portal Systems: Liver and kidneys have specialized portal circulation.

Regulation of Blood Flow and Pressure

Blood flow and pressure are regulated by vessel diameter, blood viscosity, and neural/hormonal mechanisms.

• Vasodilation: Increases diameter, decreases resistance, increases flow.

• Vasoconstriction: Decreases diameter, increases resistance, decreases flow.

• Mean Arterial Pressure (MAP):

  • Where is cardiac output and is peripheral resistance.

• Volume Changes: Affect blood pressure and flow.

• Contractions: Heart creates pressure without changing blood volume.

Cellular Elements of Blood

Blood contains several cellular components, each with specialized functions.

• Red Blood Cells (RBCs): Also called erythrocytes; transport oxygen.

• White Blood Cells (WBCs): Also called leukocytes; immune defense.

• Platelets: Split off from megakaryocytes; involved in clotting.

Differences Between Skeletal Muscle and Cardiac Muscle

Cardiac muscle cells differ from skeletal muscle cells in structure and function.

• Cardiac Muscle:

  • Branched, interconnected via intercalated discs.

  • Autorhythmic cells generate spontaneous action potentials.

  • Longer action potentials due to Ca2+ influx.

  • Contraction can be graded; strength varies with Ca2+ availability.

• Skeletal Muscle:

  • Long, multinucleated fibers.

  • Action potentials triggered by motor neurons.

  • Shorter action potentials; contraction is all-or-none.

Additional info:

• Cardiac muscle contraction strength can vary (graded) due to variable Ca2+ influx.

• Autorhythmic cells lack organized sarcomeres and are responsible for initiating heartbeats.

• Desmosomes and gap junctions in intercalated discs allow for mechanical strength and electrical continuity.

• ECG segments (P-R, T-P) represent intervals between depolarization and repolarization events.