Ch 14a Cardiovascular Physiology: Structure, Function, and Hemodynamics

Cardiovascular System Overview

Functions of the Cardiovascular System

The cardiovascular system is essential for the transport of materials throughout the body and for maintaining homeostasis. It consists of the heart, blood vessels, and blood.

• Transport: Delivers gases (O2, CO2), nutrients, and waste products to and from cells.

• Communication: Circulates hormones and signaling molecules.

• Defense: Distributes immune cells and antibodies.

• Temperature Homeostasis: Regulates body temperature by redistributing heat.

The heart acts as a pump:

• Atria: Receive blood returning to the heart.

• Ventricles: Pump blood out of the heart.

• Septum: Divides the left and right halves of the heart.

Blood Vessels and Circulation

Blood vessels form a closed circuit for blood flow, consisting of arteries, veins, and capillaries.

• Arteries: Carry blood away from the heart.

• Veins: Return blood to the heart.

• Capillaries: Sites of exchange between blood and tissues.

• Pulmonary Circulation: Moves blood between heart and lungs for gas exchange.

• Systemic Circulation: Delivers blood to the rest of the body.

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

Blood consists of:

• Cells: Red blood cells, white blood cells, platelets.

• Plasma: Liquid component containing water, proteins, nutrients, and waste.

Hemodynamics: Pressure, Flow, and Resistance

Pressure Change in the Cardiovascular System

Blood flow is driven by pressure gradients created by the heart and affected by vessel diameter and blood volume.

• Muscle Contraction: Generates pressure transferred to blood.

• Ventricular Contraction: Main source of driving pressure.

• Vasodilation: Vessel diameter increases, blood pressure decreases.

• Vasoconstriction: Vessel diameter decreases, blood pressure increases.

• Volume Changes: Affect blood pressure throughout the system.

Pressure Gradient and Blood Flow

Blood flows from regions of higher pressure to lower pressure. The magnitude of flow depends on the pressure difference, not the absolute pressure.

• Hydrostatic Pressure: Pressure exerted by a fluid in a container, proportional to fluid column height.

• Energy Loss: As blood flows, pressure decreases due to friction.

• Flow Equation:

$\text{Flow} \propto \Delta P$

• No Pressure Gradient: No flow occurs if $\Delta P = 0$.

• Equal Pressure Gradient: Flow is the same if $\Delta P$ is equal, regardless of absolute pressures.

Resistance to Flow

Resistance opposes blood flow and is determined by vessel length, blood viscosity, and vessel radius.

• Inverse Relationship: $\text{Flow} \propto \frac{1}{R}$

• Poiseuille’s Law:

$R = \frac{8L\eta}{\pi r^4}$

• Length (L): Resistance increases with vessel length.

• Viscosity (η): Resistance increases with blood thickness.

• Radius (r): Resistance decreases dramatically as radius increases (to the fourth power).

Vasoconstriction and vasodilation are key mechanisms for regulating resistance and thus blood flow.

• Vasoconstriction: Decreases vessel radius, increases resistance, decreases flow.

• Vasodilation: Increases vessel radius, decreases resistance, increases flow.

Summary Equation:

$\text{Flow} = \frac{\Delta P}{R}$

Structure of the Heart

Heart Anatomy

The heart is a muscular organ composed mainly of myocardium and surrounded by the pericardium, a fluid-filled sac.

• Valves: Ensure one-way flow of blood.

• Atrioventricular (AV) Valves: Between atria and ventricles (tricuspid on right, bicuspid/mitral on left).

• Semilunar Valves: Between ventricles and arteries (aortic and pulmonary valves).

• Pericardium: Protective sac around the heart.

Cardiac Cycle: Contraction and Relaxation

The heart alternates between contraction (systole) and relaxation (diastole) to pump blood.

• Ventricular Contraction: AV valves close to prevent backflow; semilunar valves open to allow ejection.

• Ventricular Relaxation: Semilunar valves close to prevent arterial backflow; AV valves open for filling.

Cardiac Muscle Physiology

Cardiac Muscle Structure

Cardiac muscle cells are striated, branched, and connected by intercalated disks.

• Contractile Cells: Generate force; organized into sarcomeres.

• Autorhythmic Cells (Pacemakers): Initiate and regulate heartbeat; fewer contractile fibers, no organized sarcomeres.

• Intercalated Disks: Contain desmosomes (transfer force) and gap junctions (electrical connection).

• Single Nucleus: Most cardiac cells have one nucleus.

• Large T-tubules and Small SR: Facilitate rapid signaling and contraction.

• Mitochondria: Occupy about one-third of cell volume, supporting high energy demand.

Cardiac Muscle Contraction

Contraction is initiated by action potentials and is graded based on calcium availability.

• Action Potential: Starts in pacemaker cells.

• L-type Ca2+ Channels: Allow extracellular calcium entry (10% of total).

• Ryanodine Receptors: Release Ca2+ from sarcoplasmic reticulum.

• Calcium-Troponin Binding: Initiates crossbridge cycling (similar to skeletal muscle).

• Relaxation: Ca2+ removed via Ca2+ ATPase (to SR) and Na+/Ca2+ exchanger (to extracellular space).

• Force Generation: Proportional to number of active crossbridges and amount of Ca2+ bound to troponin.

• Sarcomere Length: Affects force of contraction.

Cardiac vs. Skeletal Muscle

• Cardiac Muscle: Smaller, single nucleus, branched, connected by intercalated disks, large T-tubules, small SR, many mitochondria.

• Skeletal Muscle: Larger, multinucleated, unbranched, no intercalated disks, smaller T-tubules, larger SR, fewer mitochondria.

Cardiac Action Potentials

Myocardial Contractile Cells

Action potentials in contractile cells have a unique plateau phase that prevents tetanus.

• Depolarization: Due to Na+ entry.

• Repolarization: Due to K+ exit.

• Plateau Phase: Due to Ca2+ entry, prolongs action potential.

• Prevents Tetanus: Ensures rhythmic contractions.

Myocardial Autorhythmic Cells

Pacemaker cells have an unstable membrane potential and initiate heartbeats.

• Pacemaker Potential: Gradual depolarization until threshold is reached.

• Depolarization: Due to Ca2+ channel opening.

Key Equations and Tables

Hemodynamic Equations

$\text{Flow} = \frac{\Delta P}{R}$ $R = \frac{8L\eta}{\pi r^4}$

Comparison Table: Cardiac vs. Skeletal Muscle

Feature	Cardiac Muscle	Skeletal Muscle
Cell Size	Smaller	Larger
Nucleus	Single	Multiple
Branching	Branched	Unbranched
Intercalated Disks	Present	Absent
T-tubules	Larger, branched	Smaller
Sarcoplasmic Reticulum	Smaller	Larger
Mitochondria	One-third cell volume	Less abundant

Summary Table: Blood Vessel Types

Vessel Type	Function	Pressure
Arteries	Carry blood away from heart	High
Arterioles	Regulate flow to capillaries	Moderate
Capillaries	Exchange of materials	Low
Venules	Collect blood from capillaries	Lower
Veins	Return blood to heart	Lowest

Additional info: Academic context and definitions have been expanded for clarity and completeness. Diagrams referenced in the original materials have been described in text and tables.