Chapter 18Anatomy of the

Cardiovascular System

Location of the heart

– Lies in the mediastinum, behind the body of the sternum between the points of attachment of ribs 2 through 6; approximately two thirds of its mass is to the left of the midline of the body and one third to the right

– Posteriorly the heart rests on the bodies of thoracic vertebrae 5 through 8

– Apex lies on the diaphragm, pointing to the left

– Base lies just below the second rib

– Boundaries of the heart are clinically important as an aid in diagnosing heart disorders

Coverings of the heart

– Structure of the heart coverings• Pericardium

– Fibrous pericardium—tough, loose-fitting inextensible sac

– Serous pericardium—parietal layer lies inside fibrous pericardium, and visceral layer (epicardium) adheres to outside of the heart; pericardial space with pericardial fluid separates the two layers

– Function of the heart coverings—provides protection against friction

Structure of the heart

– Wall of the heart—made up of three distinct layers:

• Epicardium—outer layer of heart wall• Myocardium—thick, contractile middle layer of heart

wall; compresses the heart cavities, and the blood within them,with great force

• Endocardium—delicate inner layer of endothelial tissue

Structure of the heart (cont.)

– Chambers of the heart—divided into four cavities with the right and left chambers separated by the septum:

• Atria– Two superior chambers, known as “receiving chambers,” because they

receive blood from veins– Atria alternately contract and relax to receive blood and then push it into

ventricles– Myocardial wall of each atrium is not very thick, because little pressure is

needed to move blood such a small distance– Auricle—earlike flap protruding from each atrium

• Ventricles– Two lower chambers, known as “pumping chambers,” because they push

blood into the large network of vessels– Ventricular myocardium is thicker than myocardium of the atria, because

great force must be generated to pump blood a large distance; myocardium of left ventricle is thicker than the right, because it must push blood much further

Structure of the heart (cont.)

– Valves of the heart—mechanical devices that permit the flow of blood in one direction only

• Atrioventricular (AV) valves—prevent blood from flowing back into the atria from the ventricles when the ventricles contract

– Tricuspid valve (right AV valve)—guards the right atrioventricular orifice; free edges of three flaps of endocardium are attached to papillary muscles by chordae tendineae

– Bicuspid, or mitral, valve (left AV valve)—similar in structure to tricuspid valve except only two flaps present

Structure of the heart (cont.)

• Valves of the heart (cont.)• Semilunar (SL) valves—half moon–shaped flaps

growing out from the lining of the pulmonary artery and aorta; prevent blood from flowing back into ventricles from aorta and pulmonary artery

– Pulmonary semilunar valve—at entrance of pulmonary artery

– Aortic semilunar valve—at entrance of aorta

Structure of the heart (cont.)

– Blood supply of heart tissue • Coronary arteries—myocardial cells receive blood from

right and left coronary arteries– First branches to come off aorta– Ventricles receive blood from branches of both right and

left coronary arteries– Each ventricle receives blood only from a small branch of

corresponding coronary artery– Most abundant blood supply goes to myocardium of left

ventricle– The right coronary artery is dominant in approximately 50%

of all hearts and the left in about 20%; in approximately 30%, neither coronary artery is dominant

– Few anastomoses exist between the larger branches of the coronary arteries

Structure of the heart (cont.)

– Blood supply of heart tissue (cont.)• Veins of the coronary circulation

– As a rule, veins follow a course that closely parallels that of coronary arteries

– After going through cardiac veins, blood enters coronary sinus to drain into right atrium

– Several veins drain directly into right atrium

Structure of the heart (cont.)

– Conduction system of the heart—comprising the sinoatrial (SA) node, atrioventricular (AV) node, AV bundle, and Purkinje fibers; made up of modified cardiac muscle (Figure 18-11)

• Sinoatrial node (SA node or pacemaker)— hundreds of cells in right atrial wall near opening of superior vena cava

• Atrioventricular node (AV node)—small mass of special cardiac muscle in right atrium along lower part of interatrial septum

• Atrioventricular bundle (AV bundle or bundle of His) and Purkinje fibers

– AV bundle originates in AV node, extends by two branches down the two sides of the interventricular septum, and continues as Purkinje fibers

– Purkinje fibers extend out to papillary muscles and lateral walls of ventricles

Structure of the heart (cont.)

– Nerve supply of the heart• Cardiac plexuses—located near arch of aorta,

made up of the combination of sympathetic and parasympathetic fibers

• Fibers from cardiac plexus accompany right and left coronary arteries to enter the heart

• Most fibers end in the SA node, but some end in the AV node and in the atrial myocardium

• Sympathetic nerves—accelerator nerves• Vagus fibers—inhibitory, or depressor, nerves

Blood Vessels

• Types of blood vessels– Arteries

• Carry blood away from heart—all arteries except pulmonary artery carry oxygenated blood

• Elastic arteries—largest in body– Examples: aorta and its major branches– Able to stretch without injury– Accommodate surge of blood when heart contracts

and able to recoil when ventricles relax

Blood Vessels– Arteries (cont.)

• Muscular (distributing) arteries

– Smaller in diameter than elastic arteries

– Muscular layer is thick

– Examples: brachial, gastric, superior mesenteric

• Arterioles (resistance vessels)

– Smallest arteries

– Important in regulating blood flow to end-organs

• Metarterioles

– Short connecting vessel between true arteriole and 20 to 100 capillaries

– Encircled by precapillary sphincters

– Distal end called thoroughfare channel, which is free of precapillary sphincters

Blood Vessels

• Types of blood vessels (cont.)– Capillaries—primary exchange vessels

• Microscopic vessels• Carry blood from arterioles to venules—together,

arterioles, capillaries and venules constitute the microcirculation

• Not evenly distributed—highest numbers in tissues with high metabolic rate; may be absent in some “avascular” tissues such as cartilage

Blood Vessels

•Types of capillaries – True capillaries—receive blood flowing from metarteriole with input

regulated by precapillary sphincters– Continuous capillaries

» Continuous lining of endothelial cells» Openings called intercellular clefts exist between adjacent

endothelial cells– Fenestrated capillaries

» Have both intercellular clefts and “holes” or fenestrations through plasma membrane to facilitate exchange functions

– Sinusoids» Large lumen and tortuous course» Absent or incomplete basement membrane» Very porous—permit migration of cells into or out of vessel lumen

Blood Vessels

• Types of blood vessels (cont.)– Veins

• Carry blood toward the heart• Act as collectors and as reservoir vessels;

called capacitance vessels

Blood Vessels

• Structure of blood vessels – Layers

• Tunica adventitia—found in arteries and veins• Tunica media—found in arteries and veins• Tunica intima—found in all blood vessels; only

layer present in capillaries

Blood Vessels

• Structure of blood vessels (cont.)– “Building blocks” commonly present

• Lining endothelial cells– Only lining found in capillary– Line entire vascular tree– Provide a smooth luminal surface—protects against

intravascular coagulation– Intercellular clefts, cytoplasmic pores, and

fenestrations allow exchange to occur between blood and tissue fluid

– Capable of secreting a number of substances– Capable of reproduction

Blood Vessels

– “Building blocks” commonly present (cont.)• Collagen fibers

– Exhibit woven appearance– Formed from protein molecules that aggregate into fibers – Visible with light microscope– Have only a limited ability to stretch (2% to 3%) under

physiological conditions– Function to strengthen and keep lumen of vessel open

Blood Vessels

– “Building blocks” commonly present (cont.)• Elastic fibers

– Composed of insoluble protein called elastin– Form highly elastic networks– Fibers can stretch over 100% under physiological

conditions– Play important role in creating passive tension to help

regulate blood pressure throughout cardiac cycle

• Smooth muscle fibers– Present in all segments of vascular system except

capillaries– Most numerous in elastic and muscular arteries– Exert active tension in vessels when contracting

Major Blood Vessels

• Circulatory routes – Systemic circulation—blood flows from the left

ventricle of the heart through blood vessels to all parts of the body (except gas exchange tissues of lungs) and back to right atrium

– Pulmonary circulation—venous blood moves from right atrium to right ventricle to pulmonary artery to lung arterioles and capillaries where gases are exchanged; oxygenated blood returns to left atrium via pulmonary veins; from left atrium, blood enters left ventricle

Major Blood Vessels

• Systemic circulation– Systemic arteries

• Main arteries give off branches, which continue to rebranch, forming arterioles and then capillaries

• End-arteries—arteries that eventually diverge into capillaries

• Arterial anastomosis—arteries that open into other branches of the same or other arteries; incidence of arterial anastomoses increases as distance from the heart increases

• Arteriovenous anastomoses or shunts occur when blood flows from an artery directly into a vein

Major Blood Vessels• Systemic circulation (cont.)

– Systemic veins

• Veins are the ultimate extensions of capillaries; unite into vessels of increasing size to form venules and then veins

• Large veins of the cranial cavity are called dural sinuses

• Veins anastomose as do arteries

• Venous blood from the head, neck, upper extremities, and thoracic cavity (except lungs) drains into superior vena cava

• Venous blood from thoracic organs drains directly into superior vena cava or azygos vein

• Hepatic portal circulation

– Veins from the spleen, stomach, pancreas, gallbladder, and intestines send their blood to the liver via the hepatic portal vein

– In the liver, venous blood mingles with arterial blood in the capillaries and is eventually drained from liver by hepatic veins that join the inferior vena cava

• Venous blood from lower extremities and abdomen drains into inferior vena cava

Major Blood Vessels

• Fetal circulation– Basic plan of fetal circulation—additional vessels needed to allow fetal

blood to secure oxygen and nutrients from maternal blood at the placenta (Figure 18-31)

• Two umbilical arteries—extensions of internal iliac arteries; carry fetal blood to placenta

• Placenta—attached to uterine wall, where exchange of oxygen and other substances between the separated maternal and fetal blood occurs (Figure 18-30)

• Umbilical vein—returns oxygenated blood from placenta to fetus; enters body through umbilicus and goes to undersurface of liver, where it gives off two or three branches and then continues as ductus venosus

• Ductus venosus—continuation of umbilical vein, drains into inferior vena cava

• Foramen ovale—opening in septum between right and left atria• Ductus arteriosus—small vessel connecting pulmonary artery with

descending thoracic aorta

Major Blood Vessels

• Fetal circulation (cont.)– Changes in circulation at birth

(compare Figures 18-31 and 18-32)• When umbilical cord is cut, the two umbilical arteries, the

placenta and the umbilical vein no longer function

• Umbilical vein within the baby’s body becomes the round ligament of the liver

• Ductus venosus becomes the ligamentum venosum of the liver

• Foramen ovale—functionally closed shortly after a newborn’s first breath and pulmonary circulation is established; structural closure takes approximately 9 months

• Ductus arteriosus—contracts with establishment of respiration, becomes ligamentum arteriosum

Cycle of Life: Cardiovascular Anatomy

• Birth—change from placenta-dependent system• Heart and blood vessels maintain basic structure

and function from childhood through adulthood– Exercise thickens myocardium and increases supply of

blood vessels in skeletal muscle tissue

• Adulthood through later adulthood—degenerative changes– Atherosclerosis—blockage or weakening of critical

arteries

– Heart valves and myocardial tissue degenerate—reduces pumping efficiency