Cardiovascular Physiology: Structure, Function, and Regulation

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Cardiovascular Physiology

Functions of the Cardiovascular System

The cardiovascular system is essential for maintaining homeostasis by transporting materials throughout the body, facilitating cell-to-cell communication, supporting immune responses, and regulating body temperature.

Transport of Materials: Includes oxygen, nutrients, hormones, metabolic wastes, and immune cells.
Cell-to-Cell Communication: Hormones and signaling molecules are distributed via the blood.
Immune Responses: Immune cells circulate to detect and respond to pathogens.
Thermoregulation: Blood flow adjusts to regulate heat distribution.

Substance Moved	From	To
Oxygen	Lungs	All cells
Nutrients and water	Intestinal tract	All cells
Wastes	Some cells	Liver for processing
Immune cells, antibodies, clotting proteins	Present in blood continuously	Available to any cell that needs them
Hormones	Endocrine cells	Target cells
Stored nutrients	Liver and adipose tissue	All cells
Metabolic wastes	All cells	Kidneys
Heat	All cells	Skin
Carbon dioxide	All cells	Lungs

Transport in the Cardiovascular System

Organization of the Cardiovascular System

The cardiovascular system consists of the heart and blood vessels, which are organized into two main circuits: systemic and pulmonary. Blood flows in a closed loop, starting and ending at the aorta.

Heart: Pumps blood through vessels.
Arteries: Carry blood away from the heart.
Veins: Return blood to the heart.
Capillaries: Sites of exchange between blood and tissues.

Human cardiovascular system showing arteries and veins Systemic and pulmonary circuits diagram

Blood Flow, Pressure, and Resistance

Blood flow is driven by pressure gradients and opposed by resistance. The relationship among these factors is described by several key equations and principles.

Pressure Gradient (ΔP): Drives blood flow from high to low pressure.
Resistance (R): Opposes flow; determined by vessel length, viscosity, and radius.
Flow (Q): Proportional to ΔP and inversely proportional to R.
Velocity of Flow (v): Depends on flow rate and vessel cross-sectional area.

Mean systemic blood pressure across vessel types Pressure in static and flowing fluids Fluid flow through a tube depends on the pressure gradient Flow is proportional to pressure difference Flow is inversely proportional to resistance Poiseuille's Law equation for resistance Resistance is proportional to length and viscosity divided by radius to the fourth power Resistance is inversely proportional to radius to the fourth power As the radius of a tube decreases, resistance increases Velocity of flow depends on cross-sectional area Velocity equation: v = Q/A

Key Equations:

Internal and External Anatomy of the Heart

The heart is a hollow, muscular organ located in the thoracic cavity. It has three layers: epicardium, myocardium, and endocardium. The heart's structure ensures efficient blood flow and separation of oxygenated and deoxygenated blood.

Epicardium: Outer layer.
Myocardium: Muscular middle layer responsible for contraction.
Endocardium: Inner lining.

Heart position in thorax Superior view of heart in transverse plane Heart in thoracic cavity Structure of the heart Ventricular contraction and AV valve position Ventricular relaxation and semilunar valve position

Major Blood Vessels and Blood Flow Pathways

Blood flows through a series of vessels, with the heart acting as a central pump. The organization of the heart and major vessels ensures proper oxygenation and distribution of blood.

Structure	Receives Blood From	Sends Blood To
Right atrium	Venae cavae	Right ventricle
Right ventricle	Right atrium	Lungs
Left atrium	Pulmonary vein	Left ventricle
Left ventricle	Left atrium	Body except for lungs
Venae cavae	Systemic veins	Right atrium
Pulmonary trunk (artery)	Right ventricle	Lungs
Pulmonary vein	Veins of the lungs	Left atrium
Aorta	Left ventricle	Systemic arteries

Heart and major blood vessels table Systemic and pulmonary circuits diagram

Myocardial Cells: Autorhythmic vs. Contractile

The heart contains two types of myocardial cells: autorhythmic (pacemaker) cells and contractile cells. Their arrangement and function are critical for coordinated heart contractions.

Autorhythmic Cells: Generate action potentials spontaneously; lack a stable resting membrane potential.
Contractile Cells: Responsible for forceful contractions; have prolonged action potentials to prevent summation.
Intercalated Disks: Specialized junctions for rapid electrical signal transmission.

Intercalated disks in cardiac muscle Electrical conduction in heart muscle

Electrical Conduction and Action Potentials

Electrical signals originate in the SA node and propagate through the heart, triggering coordinated contractions. The conduction system includes the SA node, AV node, bundle of His, and Purkinje fibers.

SA Node: Primary pacemaker.
AV Node: Delays signal to allow atrial contraction.
Bundle of His and Purkinje Fibers: Rapidly distribute the signal to ventricles.

Conduction system of the heart Electrical events of the cardiac cycle

Electrocardiogram (ECG) and Cardiac Cycle

An ECG records the summed electrical activity of the heart. The waves and segments correspond to specific mechanical events in the cardiac cycle.

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization.
T wave: Ventricular repolarization.

ECG trace ECG vs. myocardial action potential Einthoven's triangle for ECG Vector of current flow in heart ECG deflection and current flow direction ECG waves and segments Electrical events of the cardiac cycle

Pressure Changes and Flow During the Cardiac Cycle

Pressure changes in the heart drive blood flow through the chambers and into the vessels. The cardiac cycle includes systole (contraction) and diastole (relaxation).

Systole: Ventricular contraction ejects blood.
Diastole: Chambers relax and fill with blood.
Wiggers Diagram: Illustrates the relationship between electrical, pressure, and volume changes.

Heart cycle: contraction and relaxation Wiggers diagram

Heart Rate, Stroke Volume, and Cardiac Output

Cardiac output is the volume of blood pumped by the heart per minute, determined by heart rate and stroke volume.

Stroke Volume (SV): Difference between end-diastolic volume (EDV) and end-systolic volume (ESV).
Cardiac Output (CO): Product of heart rate (HR) and stroke volume.

Stroke volume equation

Normal stroke volume calculation

Cardiac output equation

Cardiac output calculation

Autonomic Regulation of Heart Rate

The autonomic nervous system (ANS) regulates heart rate and contractility. The parasympathetic division decreases heart rate, while the sympathetic division increases both heart rate and contractility.

Parasympathetic (PNS): Negative chronotropic effect; affects only heart rate.
Sympathetic (SNS): Positive chronotropic and inotropic effects; affects heart rate and contractility.

Autonomic control of heart rate Concept map of autonomic control

Factors Influencing Stroke Volume

Stroke volume is influenced by venous return, myocardial stretch (preload), arterial resistance (afterload), contractility, and mechanisms such as the skeletal muscle pump and respiratory pump.

Venous Return: Amount of blood returning to the heart.
Preload: Degree of stretch before contraction.
Afterload: Resistance the heart must overcome to eject blood.
Contractility: Force of contraction at a given preload.
Skeletal Muscle Pump: Muscle contractions help return blood to the heart.
Respiratory Pump: Breathing movements facilitate venous return.
Inotropic Agents: Substances that affect contractility.

Frank-Starling Law: The force of contraction is directly proportional to the initial length of the cardiac muscle fibers (sarcomeres).

Additional info: The Frank-Starling mechanism ensures that the heart pumps out all the blood returned to it, maintaining balance between venous return and cardiac output.

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