25 Patho

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Structure and Function of the Cardiovascular System

Functions of the Circulatory System

The circulatory system is essential for maintaining homeostasis by transporting substances throughout the body. Its main function is transport, but it also plays roles in defense and temperature regulation.

Delivers oxygen and nutrients to tissues, supporting cellular metabolism and energy production.
Carries waste products from cellular metabolism to the kidneys and other excretory organs for removal from the body.
Circulates electrolytes and hormones, ensuring proper cellular function and communication between organs.
Transports immune substances that contribute to the body's defense mechanisms against pathogens.
Helps to regulate body temperature by redistributing heat.

Parts and Components of the Circulatory System

Pulmonary Circulation

Pulmonary circulation moves blood through the lungs, facilitating gas exchange as part of the respiratory system.

Components: Right heart, pulmonary artery, pulmonary capillaries, and pulmonary veins.

Systemic Circulation

Systemic circulation supplies oxygenated blood to all other tissues of the body.

Components: Left heart, aorta and its branches, capillaries supplying the brain and peripheral tissues, systemic venous system, and vena cava.

Types of Circulation

Central circulation: Blood in the heart and pulmonary circulation.
Peripheral circulation: Blood outside the central circulation, supplying the rest of the body.

Characteristics of the Pulmonary and Systemic Circulations

Both circulations have similar structural components but serve different functions.

Arteries and arterioles: Function as a distribution system to move blood to the tissues.
Capillaries: Serve as an exchange system where transfer of gases, nutrients, and wastes occurs.
Venules and veins: Serve as collection and storage vessels that return blood to the heart.

Circulatory System Pressure

The circulatory system is a closed system with two pumps in series: the right and left sides of the heart. Blood pressure varies between the arterial and venous systems.

Arterial pressure: Higher pressure (90–100 mm Hg), propels blood to all other tissues (systemic circulation).
Venous pressure: Lower pressure (12 mm Hg), propels blood through the lungs (pulmonary circulation).

Hemodynamics

Hemodynamics refers to the physical principles governing pressure, flow, and resistance as they relate to the cardiovascular system.

Laminar blood flow: Smooth, orderly pattern of fluid movement in a vessel, reducing friction and preventing clotting factors from contacting the vessel wall.
Turbulent flow: High velocity, constrictions, or bends in vessels can cause disordered flow, as seen in atherosclerosis, aneurysms, or stenoses (narrowing).

Functional Anatomy of the Heart

The heart is a muscular organ with specialized structures to ensure efficient blood flow and prevent backflow.

Pericardium: Fibrous covering that holds the heart in position and provides physical protection and a barrier to infection.
Myocardium: Muscular portion forming the walls of the atria and ventricles.
Endocardium: Thin, three-layered membrane lining the heart chambers.
Heart valves: Atrioventricular and semilunar valves are pressure valves that ensure one-way flow, preventing backflow of blood. There are no valves at the atrial sites where blood enters the heart; if the atria are distended, excess blood is pushed back into the veins, leading to jugular vein distention in right-sided heart failure or pulmonary congestion in left-sided failure.
Cardiac skeleton: Provides structural support and electrical isolation for impulse conduction.

The Cardiac Cycle

The cardiac cycle describes the rhythmic pumping action of the heart, divided into two main phases:

Systole: Period during which the ventricles contract and eject blood.
Diastole: Period during which the ventricles relax and fill with blood.

Factors Determining the Workload of the Heart

Preload: The volume of blood in the ventricles just before contraction (end-diastolic volume), analogous to filling a balloon with water.
Afterload: The force or resistance the heart must overcome to eject blood into the arteries.

Cardiac Output

Cardiac output is the amount of blood the heart pumps each minute, determined by stroke volume and heart rate.

Formula:

Stroke volume (SV): The amount of blood ejected with each heartbeat.
Heart rate (HR): The number of heartbeats per minute.
Venous return and contractility: Influence stroke volume and thus cardiac output.

Frank-Starling Mechanism and Cardiac Reserve

Frank-Starling Law: The greater the volume of blood in the heart before contraction, the greater the volume ejected (increased contractility from end-diastolic volume stretch).
Cardiac reserve: The maximum percentage increase in cardiac output above the resting level, reflecting the heart's ability to increase output during increased demand (e.g., exercise).

Composition of the Arterial System

Arteries: Thick-walled vessels with elastic fibers, stretch during systole and recoil during diastole.
Arterioles: Serve as resistance vessels, acting as control valves for blood flow into capillaries.

Peripheral Resistance and Regulation of Blood Flow

Blood pressure is determined by cardiac output and peripheral vascular resistance (PVR).

Formula:

CO (Cardiac Output): Variable, depending on heart function and venous return.
PVR (Peripheral Vascular Resistance): Regulated by the baroreceptor reflex to maintain pressure and perfusion.
Factors affecting hemodynamics: Volume, pressure, and resistance.

Control of Blood Flow

Blood flow is regulated by changes in vessel diameter and local tissue needs.

Lack of oxygen or accumulation of metabolites signals local tissues to increase blood flow during increased metabolic activity.
Endothelial control: Endothelial cells release substances that cause vessels to relax (vasodilation) or constrict (vasoconstriction).
Hyperemia: Increased blood flow during metabolic demand (e.g., exercise, digestion).
Long-term regulation: Collateral circulation develops through arteriogenesis (enlargement of existing vessels) or formation of anastomotic channels (natural or surgical connections between vessels).

Control of Vascular Function

Vascular function is regulated by circulating chemical substances, including hormones and local mediators.

Epinephrine: Stress hormone and neurotransmitter; causes vasoconstriction and raises blood pressure.
Norepinephrine: Can constrict most blood vessels, increasing blood pressure; some tissues (e.g., skeletal muscle) may respond differently.
Angiotensin II: Strong vasoconstrictor, plays a key role in regulating blood pressure and fluid balance.
Serotonin: Usually causes vasoconstriction in blood vessels, but effects can vary by location.
Histamine: Released during allergic reactions and inflammation; causes vasodilation and increases vessel permeability.
Bradykinin: Powerful vasodilator; causes vessels to relax and become more permeable.
Prostaglandins: Can act as vasodilators or vasoconstrictors depending on the type; many forms (e.g., prostacyclin) cause vasodilation.

Summary Table: Major Vasoactive Substances

Substance	Main Effect
Epinephrine	Vasoconstriction, increases BP
Norepinephrine	Vasoconstriction, increases BP
Angiotensin II	Vasoconstriction, increases BP
Serotonin	Vasoconstriction (location-dependent)
Histamine	Vasodilation, increases permeability
Bradykinin	Vasodilation, increases permeability
Prostaglandins	Vasodilation or vasoconstriction (type-dependent)

Lymphatic System

The lymphatic system is a network of vessels that helps maintain fluid balance and immune defense.

Lymphatic vessels often travel alongside arteries and veins.
Lymph is derived from interstitial fluid and contains plasma proteins and other osmotically active particles.
Lymph nodes filter the fluid, removing foreign particles such as bacteria.

Forces Determining the Movement of Capillary Fluid

Fluid movement between capillaries and interstitial spaces is governed by several pressures:

Capillary fluid pressure: Drives fluid out of capillaries into interstitial spaces.
Plasma colloidal osmotic pressure: Created by plasma proteins, restrains fluid loss from capillaries by pulling water back in.
Interstitial colloidal osmotic pressure: Pressure exerted by proteins in the interstitial fluid, can draw fluid out of capillaries.

Autonomic Control Centers for Cardiac Function and Blood Pressure

Cardiac function and blood pressure are regulated by centers in the medulla oblongata of the brainstem.

Vasomotor center: Controls sympathetic-mediated acceleration of heart rate and blood vessel tone.
Cardioinhibitory center: Controls parasympathetic-mediated slowing of heart rate.

Baroreceptors and Chemoreceptors

These specialized sensory neurons provide continuous feedback to the cardiovascular control centers.

Baroreceptors: Stretch-sensitive receptors that monitor blood pressure.
Chemoreceptors: Monitor blood oxygenation, carbon dioxide levels, and pH to help maintain cardiovascular and respiratory homeostasis.