Functional Organization of Cardiovascular System

Introduction

The cardiovascular system ensures continuous delivery of oxygen and nutrients by maintaining blood flow throughout the body. This chapter explains the heart as the central pumping organ, the differences between systemic and pulmonary circulations, and the importance of central blood volume in regulating venous return, cardiac filling, and cardiac output.

• The cardiovascular system transports oxygen, nutrients, hormones, and other essential substances to body tissues.
• It removes carbon dioxide and metabolic waste products, thereby maintaining cellular function.
• The system plays a vital role in homeostasis by regulating fluid balance, body temperature, and tissue perfusion.
• Cardiovascular function is adjusted by altering cardiac output and changing the diameter of blood vessels.
• These mechanisms enable the body to respond effectively to exercise, environmental temperature changes, alterations in posture, emotional stress, and other physiological demands.
• Efficient circulation is necessary to ensure adequate oxygen delivery to all organs.
• Cardiovascular diseases are major causes of morbidity and mortality worldwide.
• Conditions such as hypertension, diabetes mellitus, and ischemic heart disease significantly affect cardiovascular health.
• Understanding cardiovascular physiology is essential for diagnosing and managing cardiovascular disorders.

Components of Cvs

The cardiovascular system consists of the heart and an extensive network of blood vessels that transport blood throughout the body.

Central Pump: The Heart

• Its primary functions are the delivery of oxygen and nutrients and the removal of carbon dioxide and metabolic waste products.
• The heart serves as the central pump that generates the pressure required for blood circulation.
• It functions as a dual pump composed of right and left sides that operate in series.
• The right side pumps blood through the pulmonary circulation for gas exchange in the lungs.
• The left side pumps oxygenated blood through the systemic circulation to supply body tissues.
• Under resting conditions, cardiac output is approximately 5 liters per minute in a healthy adult.
• Cardiac output increases substantially during exercise, emotional stress, and other physiological demands.
• The heart contains four chambers: two atria and two ventricles.
• The atria receive blood returning to the heart, whereas the ventricles generate the force required for blood ejection.
• The right atrium and right ventricle form the right heart, while the left atrium and left ventricle form the left heart.

Right Side of the Heart

• The right atrium receives deoxygenated blood from the systemic circulation through the superior and inferior venae cavae and the coronary sinus.
• Blood flows from the right atrium into the right ventricle through the tricuspid valve.
• The tricuspid valve prevents backward flow of blood into the right atrium during ventricular contraction.
• The right ventricle pumps blood into the pulmonary trunk and pulmonary arteries.
• Blood then travels to the lungs, where oxygen uptake and carbon dioxide elimination occur.
• The pulmonary valve is located between the right ventricle and pulmonary trunk.
• This valve prevents regurgitation of blood into the right ventricle during ventricular relaxation.
• Proper coordination of cardiac chambers and valves ensures efficient forward blood flow and maintains adequate tissue perfusion throughout the body.

Left side of the Heart

• The left atrium receives oxygenated blood from the lungs through the pulmonary veins.
• Blood passes from the left atrium to the left ventricle through the mitral valve.
• The mitral valve ensures one-way flow and prevents regurgitation into the left atrium during ventricular contraction.
• The left ventricle pumps oxygenated blood into the systemic circulation through the aorta.
• The aortic valve prevents backflow of blood from the aorta into the left ventricle during ventricular relaxation.
• The left ventricle generates the highest pressure within the heart to maintain systemic blood flow.

Circulatory System: The Blood Vessels

• The blood vessels form a closed vascular network that transports blood between the heart and body tissues.
• Oxygenated blood is pumped from the left ventricle into the systemic circulation, which supplies tissues with oxygen and nutrients.
• Deoxygenated blood returns to the right side of the heart through the systemic veins.
• The right ventricle pumps this blood into the pulmonary circulation for gas exchange in the lungs.
• Systemic arteries are longer, more extensively branched, and provide greater resistance to blood flow than pulmonary arteries.
• Consequently, the left ventricle must generate much higher pressures than the right ventricle.
• Peak systolic pressure in the left ventricle is approximately 120 millimeters of mercury, whereas the right ventricle generates about 25 millimeters of mercury.
• Because of this increased workload, the left ventricular wall is significantly thicker and more muscular than the right ventricular wall.
• The coordinated function of the heart and blood vessels ensures efficient tissue perfusion and maintenance of cardiovascular homeostasis.

Functional Histology of Blood Vessels

• Blood vessels are composed of three concentric layers: tunica intima, tunica media, and tunica externa, each contributing to vascular structure and function.
• The tunica intima forms the innermost layer and consists of a simple squamous endothelial lining supported by a thin subendothelial connective tissue layer.
• The vascular endothelium provides a smooth surface for blood flow and plays an important role in regulating vascular permeability, coagulation, and vascular tone.
• The tunica media is primarily composed of circularly arranged smooth muscle cells with varying amounts of elastic fibers.
• Contraction of smooth muscle in the tunica media narrows the vessel lumen, producing vasoconstriction, whereas relaxation widens the lumen, causing vasodilation.
• The tunica externa forms the outermost layer and contains collagen fibers, elastic fibers, fibroblasts, and supporting connective tissue.
• Four principal structural components of the vessel wall are endothelial cells, smooth muscle tissue, elastic tissue, and collagen fibers.
• The relative proportion of these components differs among arteries, arterioles, veins, and capillaries according to their functional requirements.
• Variations in wall composition determine vessel elasticity, strength, compliance, and resistance to blood flow.
• The ratio of wall thickness to lumen diameter is an important factor influencing hemodynamics, blood pressure regulation, and vascular function.

Endothelial Cells

• Endothelial cells form the innermost lining of all blood vessels and collectively constitute the vascular endothelium.
• These cells are arranged as a single continuous layer resting on a basement membrane and are connected by tight junctions and other intercellular junctions.
• The endothelial lining provides a smooth, non-thrombogenic surface that facilitates efficient blood flow.
• In capillaries, the vessel wall consists primarily of endothelial cells and their basement membrane, creating a thin barrier for exchange between blood and tissues.
• The permeability of capillaries varies among organs according to the structural characteristics of endothelial junctions.
• Capillaries in the brain possess highly restrictive junctions that contribute to the blood–brain barrier and limit the passage of many substances.
• In contrast, liver capillaries are highly permeable, allowing extensive exchange of water, solutes, and plasma proteins.
• Endothelial cells synthesize and release several biologically active substances that regulate vascular tone, blood coagulation, inflammation, and vessel growth.

Elastic Tissue

• Elastic tissue provides the ability of blood vessels to stretch and recoil during changes in blood pressure.
• Large elastic arteries, particularly the aorta and its major branches, contain abundant elastic fibers and exhibit high compliance.
• Elastic fibers are composed of elastin and microfibrils arranged in an extensible network.
• Elastin is a resilient protein rich in amino acids such as glycine, alanine, valine, and proline.
• Microfibrils consist mainly of glycoproteins that provide structural support to elastic fibers.
• Elastic tissue is abundant in large arteries, present in veins, less prominent in arterioles, and largely absent in capillaries.

Smooth Muscle

• Smooth muscle is present in the walls of arteries, arterioles, veins, and larger venules, but is absent in capillaries.
• Small arteries and arterioles contain a relatively large amount of smooth muscle and function as major resistance vessels.
• Contraction or relaxation of vascular smooth muscle regulates vessel diameter, blood flow, and arterial pressure.

Fibrous Tissue

• Fibrous tissue, composed mainly of type I and type III collagen fibers, provides mechanical strength to vessel walls.
• Collagen limits excessive distension, maintains structural integrity, and helps vessels withstand hemodynamic stress.

Physiological Classification of Blood Vessels

Blood vessels can be classified functionally into elastic vessels, resistance vessels, exchange vessels, and capacitance vessels according to their primary physiological roles.

Windkessel Vessels

• Elastic vessels include the aorta and large elastic arteries.
• These vessels contain abundant elastic fibers in their walls, allowing them to expand and recoil with each cardiac cycle.
• During ventricular systole, they stretch and temporarily store part of the stroke volume and the associated pressure energy.
• During diastole, elastic recoil releases the stored energy and helps maintain forward blood flow.
• This elastic reservoir function smooths fluctuations in arterial pressure and converts intermittent ventricular ejection into a more continuous flow.
• The high compliance of large arteries reduces excessive increases in systolic pressure.

Resistance Vessels

• Resistance vessels include small arteries and arterioles.
• Their walls contain a prominent smooth muscle layer and relatively less elastic tissue.
• These vessels provide the greatest resistance to blood flow within the systemic circulation.
• Arterioles are the principal determinants of peripheral vascular resistance.
• Changes in arteriolar diameter markedly influence tissue perfusion and arterial blood pressure.
• A substantial pressure drop occurs as blood passes through these vessels.

Exchange Vessels

• Exchange vessels are represented mainly by capillaries.
• Capillary walls consist of a single layer of endothelial cells resting on a basement membrane.
• Their thin walls facilitate efficient diffusion of oxygen, carbon dioxide, nutrients, water, electrolytes, and metabolic waste products.
• The combined cross-sectional area of capillaries is extremely large, which slows blood flow and enhances exchange efficiency.
• Specialized capillaries in different organs exhibit varying permeability according to functional requirements.

Capacitance Vessels

• Capacitance vessels include venules and veins.
• These vessels possess high compliance and can accommodate large volumes of blood with only small increases in pressure.
• Approximately 60–70% of the total blood volume is normally contained within the venous system.
• Veins serve as a blood reservoir and help regulate venous return, cardiac filling, and circulatory stability.
• Sympathetic stimulation can reduce venous capacitance and mobilize stored blood into the circulation.

Divisions of Vascular System

• The vascular system is organized into three major components: the arterial system, capillary networks, and the venous system.
• Together, these components ensure the transport of blood, nutrients, gases, hormones, and metabolic waste products throughout the body.

Arterial System

• The arterial system includes the aorta, large arteries, medium and small arteries, arterioles, and metarterioles.
• Its primary function is to deliver oxygenated blood from the left side of the heart to systemic tissues.
• The pulmonary arteries are an exception because they carry deoxygenated blood from the heart to the lungs.
• Blood flows through arteries under relatively high pressure and velocity.

Arteries

• Arterial walls are thick and contain a prominent tunica media composed of smooth muscle and elastic tissue.
• Arterial smooth muscle receives extensive sympathetic innervation.
• Increased sympathetic activity causes vasoconstriction, whereas reduced sympathetic activity promotes vasodilation.
• Changes in arterial diameter influence blood flow and contribute to the regulation of arterial pressure.
• Large and medium arteries are common sites of atherosclerosis, a condition characterized by lipid-rich plaque formation within the arterial wall.
• Progressive atherosclerosis can reduce tissue perfusion and may lead to serious conditions such as myocardial infarction, ischemic stroke, and peripheral arterial disease.

Arterioles

• Arterioles arise from small arteries and represent the principal resistance vessels of the circulation.
• They possess a relatively thick smooth muscle layer and a small lumen.
• Contraction or relaxation of arteriolar smooth muscle markedly alters vascular resistance.
• Arterioles regulate the distribution of blood flow to different organs according to metabolic demand.
• Because they determine most peripheral vascular resistance, they play a central role in blood pressure regulation.
• Pathological thickening and narrowing of arteriolar walls are referred to as arteriolosclerosis.

Metarterioles

• Metarterioles are short vascular channels that connect arterioles to capillary networks.
• Their walls contain discontinuous smooth muscle cells rather than a continuous muscular layer.
• Smooth muscle rings at capillary entrances, known as precapillary sphincters, regulate blood flow into individual capillary beds according to local tissue requirements.

System of Capillary Network

• The capillary network arises from arterioles and metarterioles and forms an extensive vascular bed within tissues.
• Capillaries connect the arterial side of the circulation to venules, creating a pathway for exchange between blood and cells.
• They are the principal exchange vessels of the cardiovascular system.
• Capillary walls consist of a single layer of endothelial cells resting on a basement membrane.
• The absence of additional vascular layers makes the capillary wall extremely thin and highly suitable for diffusion.
• Oxygen, carbon dioxide, nutrients, water, electrolytes, hormones, and metabolic waste products move across the capillary wall.
• Blood flow through capillaries is referred to as microcirculation.
• Capillaries do not contain a continuous smooth muscle layer within their walls.
• Therefore, they cannot actively regulate their diameter to the same extent as arteries and arterioles.
• Blood flow into capillary beds is primarily controlled by precapillary sphincters and the tone of upstream arterioles.
• Some capillaries are associated with pericytes, which are contractile cells located outside the endothelial layer.
• Pericytes contribute to capillary stability, repair, and limited regulation of capillary blood flow.
• The capillary network is highly branched, resulting in a very large total cross-sectional area.
• Because of this large cross-sectional area, the velocity of blood flow decreases markedly within capillaries.
• Slow blood flow provides sufficient time for efficient exchange of substances between blood and surrounding tissues.
• Structural variations in capillaries, such as continuous, fenestrated, and sinusoidal types, reflect the specialized functional requirements of different organs.

Table 84.1: The major differences between systemic and the pulmonary circulations.

Feature	Systemic Circulation	Pulmonary Circulation
Primary role	Delivers blood to body tissues	Exchanges gases in the lungs
Pressure level	High-pressure circuit (mean ≈ 100 mm Hg)	Low-pressure circuit (mean ≈ 15 mm Hg)
Arteries and arterioles	Thicker walls with relatively smaller lumens	Thinner walls with larger lumens
Capillaries	Generally longer and narrower	Generally shorter and wider
Vascular resistance	High peripheral resistance	Low vascular resistance
Sympathetic vasoconstrictor tone	Well developed at rest	Minimal under normal conditions
Capillary flow pattern	Largely continuous	More influenced by the cardiac cycle

Venous System

• The venous system begins at the capillary bed and consists of collecting venules, venules, small veins, and large veins.
• Its primary functions are to collect blood from tissues, store blood, and return it to the atria of the heart.
• The movement of blood from peripheral veins to the heart is known as venous return.
• Collecting venules receive blood directly from capillaries and merge to form larger venules.
• Venules subsequently unite to form veins, which progressively increase in size as they approach the heart.

Venules

• Venules are thin-walled vessels that operate under low intravascular pressure.
• Their walls contain a small amount of smooth muscle and connective tissue.
• Many venules receive sympathetic nerve supply, allowing limited changes in venous tone.
• Alterations in venular diameter contribute to regulation of venous capacitance and blood distribution.· 

Veins

• Veins have thinner walls and a larger lumen than arteries of comparable size.
• They contain less smooth muscle and elastic tissue but possess greater compliance.
• Approximately 60–70% of the total blood volume is normally present within the systemic venous circulation.
• Because of their high capacity, veins function as the major blood reservoir of the body.
• Many veins of the limbs, particularly those of the lower limbs, contain venous valves.
• These valves are formed by folds of the tunica intima and are directed toward the heart.
• Venous valves prevent backward flow of blood and promote efficient venous return.
• Contraction of surrounding skeletal muscles compresses veins and propels blood toward the heart.
• This mechanism, known as the skeletal muscle pump, is especially important during standing and walking.
• Respiratory movements also assist venous return by creating pressure changes within the thoracic cavity.

Systemic vs Pulmonary Circulation

• The circulatory system is divided into two interconnected circuits: systemic circulation and pulmonary circulation.
• These circuits function together to deliver oxygen, nutrients, and hormones to tissues and to remove carbon dioxide and metabolic waste products.

Systemic Circulation

• Systemic circulation carries blood between the heart and the tissues of the body.
• It begins when the left ventricle ejects oxygenated blood into the aorta.
• Oxygenated blood is distributed through arteries, arterioles, capillaries, venules, and veins before returning to the right atrium.
• This circulation contains approximately 80–85% of the total blood volume and therefore represents the major blood reservoir of the body.
• Systemic arteries operate under relatively high pressure, with normal systolic arterial pressure typically ranging from about 100 to 140 millimeters of mercury.
• Blood pressure decreases progressively as blood flows from large arteries to capillaries and veins.
• Arteries and arterioles possess relatively thick walls and well-developed smooth muscle layers.
• Extensive sympathetic innervation allows these vessels to regulate vascular tone and blood distribution.
• A resting degree of vasoconstrictor tone is normally present in systemic resistance vessels.
• Systemic arterioles provide the greatest resistance to blood flow and play a major role in regulating arterial pressure.
• Systemic capillaries are numerous and form extensive exchange networks within tissues.
• Their structure supports efficient transfer of gases, nutrients, water, and waste products between blood and cells.

Pulmonary Circulation

• Pulmonary circulation transports blood between the heart and the lungs.
• It begins when the right ventricle pumps deoxygenated blood into the pulmonary trunk and pulmonary arteries.
• Blood passes through pulmonary capillaries surrounding the alveoli, where gas exchange occurs.
• Oxygenated blood then returns to the left atrium through the pulmonary veins.
• Pulmonary circulation contains approximately 10–12% of the total blood volume.
• Its primary function is to facilitate oxygen uptake and carbon dioxide elimination.
• Pulmonary arteries and arterioles have thinner walls and larger lumens than their systemic counterparts.
• Pulmonary vascular resistance is normally much lower than systemic vascular resistance.
• Pulmonary vessels receive sympathetic innervation, but resting vasoconstrictor tone is minimal under normal conditions.
• The low-resistance pulmonary circuit allows the entire cardiac output of the right ventricle to pass through the lungs efficiently.
• Under normal physiological conditions, the outputs of the right and left ventricles are essentially equal.
• The low-pressure pulmonary circulation accommodates this blood flow while maintaining effective gas exchange and minimizing the workload of the right ventricle.

Volume Distribution in Vascular Compartments

• An average healthy adult weighing about 70 kilograms contains approximately 5–6 liters of blood.
• Blood is distributed unevenly among different vascular compartments according to their physiological functions.
• Approximately 80–85% of the total blood volume is present in the systemic circulation.
• About 10–12% is contained within the pulmonary circulation.
• The heart and great intrathoracic vessels contain roughly 5–10% of the total blood volume.
• Within the pulmonary circulation, blood is distributed relatively evenly among pulmonary arteries, capillaries, and veins.
• In contrast, the systemic venous system contains the largest proportion of blood.
• Nearly 60% of the total blood volume, or about three-fourths of systemic blood volume, is present in systemic veins and venules.
• Because of their high compliance, veins function as the principal blood reservoir of the body.
• Although capillaries contain only about 5% of the total blood volume, they are essential for tissue oxygenation, nutrient delivery, and waste removal.

Central vs Peripheral Blood Volume

Central Blood Volume

• Central blood volume refers to blood present within the thoracic compartment.
• It includes blood in the heart, pulmonary circulation, thoracic aorta, superior vena cava, and intrathoracic segment of the inferior vena cava.
• Central blood volume constitutes approximately 20–25% of the total blood volume.
• It plays a crucial role in determining venous return, cardiac filling, stroke volume, and cardiac output.
• Changes in central blood volume directly influence cardiovascular performance.
• Central venous pressure is often used as an indicator of central blood volume and right atrial filling.

Peripheral Blood Volume

• Peripheral blood volume comprises blood located outside the thoracic cavity.
• Most of this volume is present in the veins of the limbs, abdomen, and other extrathoracic regions.
• Peripheral blood volume accounts for approximately 75–80% of the total blood volume.
• The abdominal and limb veins contribute most of the peripheral blood reservoir.
• Redistribution of blood from the peripheral veins to the central circulation increases venous return and helps maintain cardiac output during physiological demands.