BLOOD VESSELS

transport blood throughout the body.

The vascular system comprises the following components:

• The arterial system, which includes arteries and arterioles that carry blood away from the heart.
• The capillary system, which comprises complex networks of capillaries termed ‘beds’ that facilitate molecular exchange between the tissues and circulation.
• The venous system, which includes postcapillary venules, venules and veins that return blood to the heart.
  • These vessels join to form a closed system through which the heart pumps blood.

Capillary Details:

• Capillary beds merging into precapillary venules; the precapillary venules facilitate exchange like the capillaries do, but also function as a major transit point for white blood cells (a key component of the immune response) as they move into and out of the circulation.
• Precapillary venules merge into a venule; venules continue to permit molecular change and white blood cell movement.
• The venules connect to veins; veins are the largest of the vessels returning blood to the heart.

Vessel Cross-Sections & Physiologic Functions

• Blood vessel walls are generally organized into three layers:
  • Tunica intima, the innermost layer, which contains a layer of epithelium surrounded by connective tissue
  • Tunica media, the middle layer, which contains variable amounts of smooth muscle and elastic connective tissue
  • Tunica adventitia, the outermost layer, which comprises mainly collagen; elastic lamina supports the tunica adventitia, anchoring blood vessels to nearby organs and providing stability.
• The thickness and relative compositions of these layers vary between vessels.

Three types of arteries:

• Elastic arteries, which are the largest.
• Muscular arteries, which branch from elastic arteries.
• Arterioles, which are the smallest and branch from muscular arteries.

Cross section of an elastic artery:

• From deep to superficial, draw:
  • A thin tunica intima, a thick tunica media, and a thin tunica adventitia.
  • The tunica media in elastic arteries contains a notably large amount of elastic connective tissue, which enables them to expand and recoil as the heart contracts and ejects blood into circulation.
  • The aorta is an example of an elastic artery.

Cross section of a muscular artery as follows

• From deep to superficial
  • A thin tunica intima, a thick tunica media and a thin tunica adventitia.
  • Show that the tunica media in muscular arteries contains abundant smooth muscle, which allows them to regulate blood flow by vasoconstriction (a contraction of smooth muscle that narrows the lumen) and vasodilation (a relaxation of smooth muscle that widens the lumen).

Cross section of an arteriole as follows:

• A thin tunica intima, a thick tunica media, and a thin tunica adventitia.
  • Tunica media includes a varying amount of smooth muscle.
  • Smooth muscle in the tunica media of arterioles also functions in vasoconstriction and vasodilation, allowing arterioles to regulate the flow of blood into capillary beds.

Components of a capillary bed:

• 4 types of vessels broadly referred to as capillaries:
  • Metarterioles, or precapillaries, which are not true capillaries but pass through capillary beds
  • Continuous capillaries, which are the most abundant
  • Fenestrated capillaries, which are found in the kidneys, small intestine and endocrine glands
  • Sinusoidal capillaries, which are found in the spleen, liver and bone marrow

Features of true capillaries

• Comprise endothelial cells.
• Continuous capillary have continuous walls of endothelial cells connected by tight junctions.
• Lacks smooth muscle, and a basement membrane surrounds it.
• This thin endothelial wall facilitates molecular exchange between the lumen of the continuous capillary and the surrounding tissue.

Features of fenestrated capillaries

• Continuous wall of endothelial cells with a number of pores called fenestrations.
• Continuous basement membrane surrounds the wall of endothelial cells.
• Fenestrations produce greater permeability than that of continuous capillaries.
• Fenestrated capillaries can be found in specialized tissues that require more rapid and extensive molecular exchange.

Features of sinusoidal capillaries

• Large gaps between endothelial cells.
• Discontinuous basement membrane surrounds these endothelial cells.
• The large gaps between endothelial cells and incomplete basement membrane permit even greater permeability than that of fenestrated capillaries.
• Sinusoidal capillaries facilitate the passage of red and white blood cells and are found in specialized organs within which such movement regularly occurs.

Vessels of the venous system

• Postcapillary venules, which form when capillaries merge
• Venules, which form when postcapillary venules merge
• Veins, such as the venae cava, which are the largest of these vessels and form when venules merge

• The venous system returns blood to the heart.
• In general, the vessels of the venous system have thinner walls and larger lumens than the arterial system, with less defined elastic and muscular features.

Cross section of a postcapillary venule

• Walls are porous and that the tunica media is very thin.
  • The porous walls of postcapillary venules facilitate the movement of white blood cells into and out of circulation. – White blood cells function in the immune response, which will be discussed elsewhere.

Cross section of a venule

• Walls are thin, but thicker than postcapillary venule.
  • Porous walls for passage of white blood cells.

Cross section of a vein

• Deep to superfiical: thin tunica media and a thick tunica adventitia.
  • Tunica adventitia is the largest layer in veins and contains a small amount of smooth muscle, unlike arterial walls where smooth muscle is limited to the tunica media.
  • Tunica intima and the tunica media are thinner than in the elastic and muscular arterial walls.
  • The venae cava are the largest veins in the body and return blood to the right atrium of the heart.
  • Many veins, particularly in the limbs and extremities, contain valves formed by the tunica intima, which prevent backflow of blood.

Clinical Correlations:

• Faulty valves cause a back-flow of blood, which manifests as varicose veins and can result in stasis dermatitis.

Overview of Heart Anatomy:

• Inferior chambers are the right and left ventricles, which are separated by the interventricular septum.
• Superior chambers are the right and left atria, which are separated by the interatrial septum.

• Great vessels that enter and exit the heart:
  • The paired pulmonary veins enter the left atrium.
  • The aorta arises from the left ventricle.
  • The pulmonary trunk arises from the right ventricle and splits to form the pulmonary arteries.
  • The superior vena cava and inferior vena cava both drain into the right atrium.

• Four valves ensure unidirectional blood flow through the chambers of the heart:
  • Right and left atrioventricular (AV) valves are between the atria and ventricles.
  • Semi-lunar valves are at the bases of the pulmonary trunk and aorta.

Path of blood flow through lungs, heart, and body:

1. Within the lungs, blood picks up oxygen.
2. Travels through pulmonary veins to left atrium.
3. From the left atrium, oxygen-rich blood passes through the left atrioventricular valve, then enters the left ventricle.
4. The left ventricle pumps this blood through the aortic semilunar valve and into the aorta, which is the major systemic artery.
5. From the aorta, oxygen-rich blood is distributed via systemic arteries to the body tissues.
6. Oxygen moves out of the bloodstream and into the body tissues, and,
   Metabolic waste, including carbon dioxide, move into the blood.
7. Systemic veins carry the oxygen-poor blood away from body tissues,
   and drain into the superior and inferior vena cavae.
8. The superior and inferior vena cavae drain directly into the right atrium of the heart.
9. From the right atrium, blood passes through the right atrioventricular valve, and into the right ventricle.
10. The right ventricle contracts and pumps the blood past the pulmonary semi-lunar valve, and into the pulmonary trunk, which splits to form the left and right pulmonary arteries.
11. The pulmonary arteries carry the oxygen-poor blood to the lungs, where carbon dioxide is released and the blood is re-oxygenated; from here, the re-oxygenated blood enters the pulmonary veins, and the cycle continues.

The cardiac veins carry deoxygenated blood from the myocardium to the right atrium.

• Cardiac veins are classified by their blood-flow return to the heart:
  • The coronary sinus (and its four major tributaries) drains into the right atrium.
  • The anterior cardiac veins drain directly into the right atrium.
  • The small cardiac veins drain directly into the nearest chamber.

Pathway of venous return to the coronary sinus:

The coronary sinus is a wide, short vessel that drains directly into the right atrium. It runs in the posterior coronary sulcus.

• Four major tributaries:
  • Great cardiac vein
  • Left posterior ventricular vein
  • Middle cardiac vein
  • Small cardiac vein

Anterior interventricular vein

• Arises near the apex of the heart and travels superiorly (within the anterior interventricular sulcus).
• Once it reaches the coronary sulcus, the anterior interventricular vein becomes the great cardiac vein.
  • Travels posteriorly to drain directly into the left end of the coronary sinus (be aware that some texts do not name the anterior interventricular vein separately; instead, they refer to this entire vessel as the great cardiac vein).

Right marginal vein

• Travels superiorly along the right side of the heart.
• At the coronary sulcus, the right marginal vein becomes the small cardiac vein.
  • Wraps posteriorly to drain directly into the right end of the coronary sulcus (in some individuals, the right marginal vein drains directly into the right atrium).

Middle cardiac vein

• Arises near the apex and runs in the posterior interventricular sulcus (hence, it is also called the posterior interventricular vein).

Left posterior ventricular vein

• Arises between the great and middle cardiac veins.

Anterior cardiac veins

• Arise on the superior surface of the right ventricle, and drain directly into the right atrium.

Smallest cardiac veins

aka, venae cordis minimae, aka, thebesian veins

• Valveless vessels that drain directly into the cardiac chambers, particularly the right atrium.

Vein – Artery Associations:

• The anterior interventricular vein travels with the anterior interventricular artery.
• The middle cardiac vein travels with the posterior interventricular artery.
• The small cardiac vein travels with the right coronary artery.
• The right marginal vein travels with the right marginal artery.

In Summary:

The great cardiac and left posterior ventricular veins drain regions of the myocardium supplied by the left coronary artery;
The middle and small cardiac veins drain regions of the myocardium supplied by the right coronary artery.

Coronary arteries supply the myocardium with oxygenated blood.

Right coronary artery:

• Arises directly from the right side of the aorta (specifically, from the right aortic sinus) and travels posteriorly within the coronary sulcus.
• Branches:
  • Sinoatrial nodal branch (aka, sinoatrial nodal branch).
  • Right marginal branch.
  • Posterior interventricular branch (aka, the PDA, posterior descending artery), which travels within the posterior interventricular groove (aka, sulcus).

Left coronary artery:

• Arises directly from the left side of the aorta and travels posteriorly within the coronary sulcus (the left coronary artery is sometimes referred to as the left main stem vessel).
• Branches:
  • Circumflex artery, which gives off the left marginal artery.
  • Anterior interventricular branch (aka, the LAD, left anterior descending artery), which travels within the anterior interventricular groove (aka, sulcus).

Areas served by the right coronary artery:

• The right atrium
• Most of the right ventricle
• The sinoatrial node (in 60% of individuals)
• The atrioventricular node (in 80% of individuals)

Areas served by the left coronary artery:

• Left atrium
• Most of the left ventricle
• Most of the interventricular septum
• The sinoatrial node (in 40% of individuals)

Three common patterns of arterial supply:

• Right-dominant pattern, in which the posterior interventricular artery branches from the right coronary artery; this is the most common pattern.
• Left-dominant pattern, in which the posterior interventricular artery branches from the left coronary artery; this is less common.
• Codominance, in which both the right and left coronary arteries contribute to the posterior interventricular artery.
• Occasionally, individuals will have only one coronary artery, or will have accessory coronary arteries.

Additional Information:

• In general, the coronary arteries are functional end vessels, which means that there is little to no redundancy in the blood supply of the myocardium.

• Two common exceptions to this rule:
  • Anastomoses between the anterior and posterior interventricular arteries
  • Anastomoses between the right and left coronary arteries

• Anastomoses provide alternative pathways, called collateral channels, through which oxygenated blood can reach the myocardium.

Clinical correlations:

• Occlusion (aka, blockage) of a coronary artery can cause ischemia and myocardial infarction (heart attack).
  • Treatments for coronary artery occlusion include:
    Coronary angioplasty, in which the clogged artery is widened at the site of the obstruction.
    Coronary artery bypass graft surgery, in which vascular segments from elsewhere in the body (eg, the great saphenous vein, internal thoracic arteries, or radial arteries) are grafted to the heart to circumvent the obstructed coronary arteries.

Endocardium

• Innermost layer of heart wall
• Endocarditis (inflammation of the endocardium) can destroy the valves and disrupt blood flow through the heart.

Myocardium

• Cardiac muscle fibers (cells) are anchored to the dense regular connective tissue of the fibrous skeleton. In addition to anchoring the cardiac muscle fibers, the fibrous skeleton maintains the structural and physiological integrity of the heart.
• The myocardial layer is injured in myocardial infarction (aka, heart attack), which occurs when obstructed coronary artery blood flow causes cardiac muscle cell death.

Epicardium

• Most superficial layer of heart wall
• Often filled with fat
• Two sublayers; outermost layer is visceral layer of pericardium.

Pericardium:

Fibrous layer

• Most superficial layer
• Tough layer of dense connective tissue
• Because it is inelastic, it prevents overfilling of the heart.
• Arises from the diaphragm
• Covers the heart and the roots of the great vessels, with which it is continuous

Serous layer

• Parietal layer:
  Lines the fibrous pericardium
• Pericardial cavity
  Between parietal and visceral layers; contains thin layer of fluid to reduce friction and allow movement of heart
• Visceral layer
  Forms most superficial layer of the epicardium

Pericarditis (inflammation of the pericardium) causes the pericardium to rub against the heart, which causes friction. As a result, patients experience pain and can even suffer from impaired heart function, which, sometimes necessitates medical or surgical intervention.

Base of heart

Located posteriorly, widest part of heart

Apex of heart

Located inferiorly, narrowest part of heart, points towards left side of body

4 Chambers of the heart:

Left and Right Atria, superiorly

• Auricles are ear-like extensions of the atria; expand to accommodate blood flow
• Superior and inferior vena cavae return blood to right atrium
• Pulmonary veins return blood to left atrium

Left and Right Ventricles, inferiorly

• Anterior and posterior interventricular sulci separate right from left
• Aorta carries blood away from left ventricle
• Pulmonary trunk carries blood away from right ventricle

Coronary sulcus (aka, atrioventricular groove)

Groove between the atria and ventricles where coronary vessels travel.

Key Features of the Internal Heart:

• The right side of the heart receives deoxygenated blood from the body and sends it to the lungs.
• The left side of the heart receives oxygenated blood from the lungs and sends it to the body.

Septi (singular = septum)

• Structurally and functionally divide the heart into right and left sides; each side operates as a muscular pump.
  • Interventricular septum divides right and left ventricles, inferiorly.
  • Interatrial septum divides right and left atria, superiorly.

Chambers

• Atria
  • Superior chambers
• Ventricles
  • Inferior chambers

Valves

• Ensure unidirectional blood flow through the heart.

Right atrioventricular valve

• Three cusps (aka, leaflets):
  • Anterior
  • Posterior
  • Septal
• Because it has three cusps, this valve is called the “tricuspid valve.”

Left atrioventricular valve

• Two cusps:
  • Anterior
  • Posterior
• Because it has two cusps, it is called the bicuspid valve (aka, the mitral valve, because it is mitre-shaped).

Semilunar valves:

• Aortic and pulmonary semilunar valves ensure that blood travels from through the aorta and pulmonary trunk unidirectionally.

Papillary muscles

• Anchor AV valves.
• Special extensions of the trabeculae carneae in the ventricles (papillary refers to their nipple-like shape).
• The moderator band (aka, septomarginal trabecula) spans from the interventricular septum to the base of the anterior papillary muscle; it prevents the ventricle from overfilling, and contains a portion of the cardiac conduction system (which is addressed in a separate tutorial).

Right ventricle has three papillary muscles, each named for its location:

• Anterior
• Posterior
• Septal (which is sometimes absent)

Left ventricle typically has two papillary muscles

• Anterior
• Posterior

Chordae tendineae

• Short cords that attach flaps of valves to papillary muscles to prevent prolapse (aka, eversion) of the valves, and ensure unidirectional blood flow through the chambers.

Features of the Ventricles:

• Walls of the left ventricle are more muscular and thicker than the walls of the right.
  • The left ventricle must produce more muscular force to pump blood to the body; in contrast, the right ventricle produces less force, as it sends blood to the nearby lungs.
• Trabeculae carnae
  Irregular ridges of muscle on internal ventricular surface

Features of the Atria:

• Pectinate muscles line the anterior wall of the right atrium. These muscles also exist in the left atrium but are less abundant.
• Fossa ovalis is a shallow depression in the wall of the interatrial septum.
  • The fossa ovalis is clinically significant from the moment of birth, when it seals off an opening in the interatrial septum called the foramen ovale.
  • In utero, the foramen ovale shunts blood directly from the right to left atrium, which allows the blood to bypass the nonfunctional lungs (the fetus receives oxygen directly from the maternal blood via the placenta).
    Immediately after birth, the interatrial septum fuses, which closes the foramen ovale; the fossa ovalis represents this fusion.
  • In some cases, septal fusion is incomplete (aka, patent foramen ovale), which can impede blood flow and, consequently, blood oxygenation.

Great vessels

• Arteries send blood away from the heart
• Veins return blood to the heart

Aorta

• Arises from the left ventricle and arches posteriorly; carries oxygenated blood away from the left ventricle.

Pulmonary trunk

• Arises from the right ventricle and splits to form the right and left pulmonary arteries; pulmonary arteries carry deoxygenated blood away from the right ventricle, to the lungs.

Pulmonary veins

• Drain into the left atrium; return blood to the heart from the lungs. (“pulmonary” refers to the lungs”).

Inferior and Superior vena cavae

• Return deoxygenated blood from the body to the right atrium

Opening of coronary sinus

• Returns deoxygenated blood from the myocardium to the heart to right atrium.