1. Chapter 18: Anatomy of the Cardiovascular System

3. HEART Location of the heart (Figure 18-2)
Lies in the mediastinum, behind the body of the sternum between the points of attachment of ribs two through six
Approximately two thirds of its mass is to the left of the midline of the body, and one third is to the right
Posteriorly the heart rests on the bodies of vertebrae T5-T8
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

5. HEART (cont.) Size and shape of the heart (Figures 18-1 and 18-2)
At birth, is transverse and appears large in proportion to the diameter of the chest cavity
Between puberty and 25 years of age the heart attains its adult shape and weight
In an adult, the shape of the heart tends to resemble that of the chest

7. HEART (cont.) Coverings of the heart Structure of the heart coverings
Pericardium (Figure 18-4)
Fibrous pericardium: tough, loose-fitting inextensible sac
Serous pericardium:
parietal layer lies inside the fibrous pericardium
visceral layer (epicardium) adheres to the outside of the heart
pericardial space with pericardial fluid separates the two layers
Heart coverings protect against friction

10. HEART (cont.) Structure of the heart
Wall of the heart: composed of three distinct layers (Figure 18-5)
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

12. HEART: STRUCTURE
Chambers: heart is divided into four cavities with the right and left chambers separated by the septum (Figures 18-6 and 18-7)
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

13. HEART: STRUCTURE (cont.)
Ventricles
Two lower chambers known as pumping chambers because they push blood into the large network of vessels
Ventricular myocardium is thicker than the myocardium of the atria because great force must be generated to pump the blood a large distance; myocardium of left ventricle is thicker than the right because it must push blood much further

15. HEART: STRUCTURE (cont.)
Valves of the heart: mechanical devices that permit the flow of blood in one direction only (Figure 18-8)
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 has only two flaps

17. HEART: STRUCTURE (cont.)
Semilunar valves: half-moon–shaped flaps growing out from the lining of the pulmonary artery and aorta; prevent blood from flowing back into the ventricles from the aorta and pulmonary artery
Pulmonary valve: valve at entrance of the pulmonary artery
Aortic valve: valve at entrance of the aorta

19. HEART: STRUCTURE (cont.)
Skeleton of the heart
Set of connected rings that serve:
as a semirigid support for the heart valves
as the attachment of cardiac muscle of the myocardium
as an electrical barrier between the myocardium of the atria and that of the ventricles
Flow of blood through heart (Figure 18-7)

21. HEART: STRUCTURE (cont.)
Surface projection (Figure 18-9)

22. HEART: STRUCTURE (cont.)
Flow of blood through heart (Figure 18-7)

23. HEART: STRUCTURE (cont.)
Systemic circulation
Blood flows from the left side of the heart through the body tissues and back to the right side of the heart
Pulmonary circulation
Blood flows from the right side of the heart to the lungs and back to the left side of the heart

24. HEART: STRUCTURE (cont.)
Blood supply of heart tissue (Figures and 18-11)
Coronary arteries: myocardial cells receive blood from the right and left coronary arteries
First branches to come from the aorta
Ventricles receive blood from branches of both right and left coronary arteries
Each atria receives blood only from a small branch of the corresponding coronary artery
Most abundant blood supply goes to the myocardium of the left ventricle
Right coronary artery is dominant in approximately 50% of all hearts and the left in about 20%; neither coronary artery is dominant in approximately 30%
Few anastomoses exist between the larger branches of the coronary arteries

26. HEART: STRUCTURE (cont.)
Veins of the coronary circulation
After going through cardiac veins, blood enters the coronary sinus to drain into the right atrium
Several veins drain directly into the right atrium
As a rule, veins follow a course that closely parallels that of coronary arteries

27. HEART: STRUCTURE (cont.)
Nerve supply of the heart
Myocardium is autorhythmic, generates its own AP
System of myocardial fibers specialized for rapid electrical conduction is known as the conduction system
The body still needs to adjust the heart rate and regulates the heart via afferent nerves
Both sympathetic and parasympathetic innervations
Some fibers innervate the SA node, others the AV node
Cardiac plexuses: located near the arch of the aorta; composed of sympathetic and parasympathetic fibers
Sympathetic nerves: accelerator nerves
Parasympathetic Vagus fibers: inhibitory, or depressor, nerves

28. BLOOD VESSELS Types of blood vessels Arteries
Carry blood away from heart
all arteries except pulmonary artery carry oxygenated blood
Elastic arteries are largest in body (e.g., aorta and its major branches)
Able to stretch without injury
Accommodate surge of blood when heart contracts and able to recoil when ventricles relax

29. BLOOD VESSELS (cont.) Types of blood vessels (cont.) 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

30. BLOOD VESSELS (cont.) 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 (Figure 18-14)
Not evenly distributed; highest numbers in tissues with high metabolic rate; may be absent in some “avascular” tissues, such as cartilage

32. BLOOD VESSELS (cont.) Types of blood vessels (cont.)
Capillaries (cont.)
Types of capillaries (Figure 18-15)
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

34. BLOOD VESSELS (cont.) Types of blood vessels (cont.) Veins
Carry blood toward the heart
Act as collectors and reservoir vessels; called capacitance vessels
One way valves prevent backflow

35. BLOOD VESSELS (cont.) Structure of blood vessels (Figure 18-13) Layers
Tunica externa: found in arteries and veins (tunica adventitia)
Prevents tearing of the of the vessels during body movements
Attachment point to nearby structures
Tunica media: found in arteries and veins
Smooth muscle layer that permits change in diameter
Thicker in arteries than veins
Tunica intima: found in all blood vessels; only layer present in capillaries

37. BLOOD VESSELS (cont.) “Building blocks” commonly present
Lining endothelial cells
Only lining found in capillary
Line entire vascular tree
Provide a smooth luminal surface; protect 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
Collagen fibers
Exhibit woven appearance
Have only a limited ability to stretch (2% to 3%) under physiological conditions
Strengthen and keep lumen of vessel open

38. BLOOD VESSELS (cont.) Elastic fibers Smooth muscle fibers
Composed of insoluble protein called elastin
Form highly elastic networks
Fibers can stretch more than 100% under physiological conditions
Play important role in creating passive tension to help regulate blood pressure throughout the 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

39. MAJOR BLOOD VESSELS Circulatory routes (Figure 18-16)
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 the 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 by pulmonary veins; from left atrium, blood enters the left ventricle

41. MAJOR BLOOD VESSELS (cont.)
Systemic circulation
Systemic arteries (Tables 18-2 to 18-6; Figures to 18-22)
Main arteries give off branches, which continue to rebranch, forming arterioles and then capillaries
End arteries: arteries that eventually diverge into capillaries
Arterial anastomoses: 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

48. MAJOR BLOOD VESSELS (cont.)
Systemic circulation (cont.)
Systemic veins (Figures to 18-29)
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 the same as arteries
Hepatic portal circulation (Figures and 18-28)
Veins from the spleen, stomach, pancreas, gallbladder, and intestines send blood to the liver by the hepatic portal vein
In the liver the venous blood mingles with arterial blood in the capillaries and is eventually drained from the liver by hepatic veins that join the inferior vena cava
Venous blood from the lower extremities and abdomen drains into the inferior vena cava

56. MAJOR BLOOD VESSELS (cont.)
Fetal circulation
The 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 the internal iliac arteries that carry fetal blood to the placenta
Placenta: where exchange of oxygen and other substances between the separate maternal and fetal blood occurs; attached to uterine wall (Figure 18-30)
Umbilical vein: returns oxygenated blood from the placenta to the fetus; enters body through the umbilicus and goes to the undersurface of the liver, where it gives off two or three branches and then continues as the ductus venosus
Ductus venosus: continuation of the umbilical vein; drains into inferior vena cava; allows blood to bypass liver
Foramen ovale: opening in septum between the right and left atria; allows blood to bypass lungs
Ductus arteriosus: small vessel connecting the pulmonary artery with the descending thoracic aorta; blood bypasses lungs

58. MAJOR BLOOD VESSELS (cont.)
Fetal circulation (cont.)
Changes in circulation at birth (Figures and 18-32)
When umbilical cord is cut, the two umbilical arteries, placenta, and 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

60. 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 the 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, reducing pumping efficiency