CVS

• Circulatory system

  • Pumps and directs blood cells and substances carried in blood to all tissues of the body
  • Includes both the blood and lymphatic vascular systems
  • Total length of vessels in an adult is estimated at between 100,000 and 150,000 km
• Blood vascular system

  Also called cardiovascular system
• Components of the blood vascular system

  • Heart
  • Arteries
  • Capillaries
  • Veins
• Heart

  Propels blood through the system
• Arteries

  A series of vessels efferent from the heart that become smaller as they branch into the various organs, carry blood to the tissues
• Capillaries

  • The smallest vessels, are the sites of O2, CO2, nutrient, and waste product exchange between blood and tissues
  • Together with the smallest arterial and venous branches carrying blood to and from them, capillaries in almost every organ form a complex network of thin, anastomosing tubules called the microvasculature or microvascular bed
• Veins

  Result from the convergence of venules into a system of larger channels which continue enlarging as they approach the heart, toward which they carry the blood to be pumped again
• Major divisions of the circulatory system

  • Pulmonary circulation
  • Systemic circulation
• Lymphatic vascular system

  • Begins with the lymphatic capillaries, which are thin-walled, closed-ended tubules carrying lymph which merge to form vessels of steadily increasing size
  • The largest lymph vessels connect with the blood vascular system and empty into the large veins near the heart, returning fluid from tissue spaces all over the body to the blood
• Endothelium

  • The internal surface of all components of the blood and lymphatic systems is lined by a simple squamous epithelium
  • Cardiovascular endothelial cells have crucial physiologic and medical importance
  • They must maintain a selectively permeable, antithrombogenic (inhibitory to clot formation) barrier
  • They also determine when and where white blood cells leave the circulation for the interstitial space of tissues and secrete a variety of paracrine factors for vessel dilation, constriction, and growth of adjacent cells
• Heart wall

  Consists of three major layers: the internal endocardium, the middle myocardium, and the external epicardium
• Endocardium

  Consists of the lining endothelium, its supporting layer of fibroelastic connective tissue with scattered fibers of smooth muscle, and a deeper layer of connective tissue (continuous with that of the myocardium and often called the subendocardial layer) surrounding variable numbers of modified cardiac muscle fibers which comprise the heart's impulse conducting system
• Myocardium

  • Consists mainly of typically contractile cardiac muscle fibers arranged spirally around each heart chamber
  • The myocardium is much thicker in the walls of the ventricles, particularly the left, than in the atrial walls
• Epicardium

  • A simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves
  • The epicardium corresponds to the visceral layer of the pericardium, the membrane surrounding the heart
• Cardiac skeleton

  • Prominent dense irregular connective tissue separating the musculature of the atria from that of the ventricles, forming part of the interventricular and interatrial septa, and extending into the valve cusps and the chordae tendineae to which they are attached
  • Functions include surrounding, anchoring, and supporting all heart valves, providing firm points of insertion for cardiac muscle in the atria and ventricles, and helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles
• Cardiac conducting system

  • Modified cardiac muscle cells in the subendocardial layer and adjacent myocardium that generate and conduct waves of depolarization to stimulate rhythmic contractions in adjacent myocardial fibers
  • Consists of the sinoatrial (SA) node (pacemaker) in the right atrial wall, the atrioventricular (AV) node in the floor of the right atrium, the AV bundle (of His) which gives rise to right and left bundle branches, and the Purkinje fibers which extend into the myocardium of the ventricles
• Innervation of the heart

  • Both parasympathetic and sympathetic neural components are present, affecting heart rate and rhythm
  • Stimulation of the parasympathetic division (vagus nerve) slows the heartbeat, whereas stimulation of the sympathetic nerve accelerates activity of the pacemaker
• Tissues of the Vascular Wall

  • Walls of all blood vessels except capillaries contain smooth muscle and connective tissue in addition to the endothelial lining
  • Amount and arrangement of these tissues influenced by mechanical factors like blood pressure and metabolic factors reflecting local tissue needs
• Endothelium

  Specialized epithelium that acts as a semipermeable barrier between blood and interstitial tissue fluid
• Functions of endothelial cells

  • Present a nonthrombogenic surface
  • Regulate local vascular tone and blood flow by secreting various factors
  • Have roles in inflammation and local immune responses
  • Secrete various growth factors
• Vasculogenesis

  Formation of the vascular system from embryonic mesenchyme
• Angiogenesis

  Sprouting and outgrowth of capillaries from small existing vessels
• The normal vascular endothelium is antithrombogenic, allowing no adhesion of blood cells or platelets and preventing blood clot formation
• When endothelial cells are damaged, collagen is exposed and induces aggregation of blood platelets, initiating a cascade that produces fibrin and forms a thrombus
• Solid masses called emboli may detach from large thrombi and be carried by blood to obstruct distant vessels
• Tissue plasminogen activator (tPA) is used to quickly dissolve clots in conditions like myocardial infarct, stroke, or pulmonary embolism
• Smooth muscle fibers

  • Occur in walls of all vessels larger than capillaries, arranged helically in layers
  • Allow vasoconstriction and vasodilation to regulate blood pressure
• Connective tissue components in vascular walls

  • Collagen fibers provide structure
  • Elastic fibers provide resiliency
  • Ground substance components like proteoglycans affect permeability
• Tunica intima

  Innermost layer, consists of endothelium and subendothelial connective tissue, sometimes contains smooth muscle
• Tunica media

  Middle layer, consists mainly of concentric layers of smooth muscle cells with elastic fibers and lamellae
• Tunica adventitia

  Outer layer, connective tissue of collagen and elastic fibers
• Large vessels have vasa vasorum (arterioles, capillaries, venules) in the adventitia and outer media to provide nutrients, as the wall is too thick to be nourished solely by diffusion from the lumen
• Large veins commonly have more vasa vasorum than arteries because they carry deoxygenated blood
• The adventitia of larger vessels contains a network of unmyelinated autonomic nerve fibers (vasomotor nerves) that release the vasoconstrictor norepinephrine
• Elastic arteries

  • Aorta, pulmonary artery, and largest branches
  • Prominent thick tunica media with many elastic lamellae alternating with smooth muscle layers
  • Well-developed tunica intima with smooth muscle cells
  • Internal elastic lamina between intima and media
  • Thin adventitia
• The numerous elastic laminae of elastic arteries contribute to making blood flow more uniform by distending the wall during systole and rebounding passively during diastole to maintain arterial pressure
• Atherosclerosis is a disease of elastic and large muscular arteries, initiated by damaged or dysfunctional endothelial cells oxidizing low-density lipoproteins in the intima, leading to accumulation of lipid-filled macrophages and development of fibro-fatty plaques
• Atheromas

  Fibro-fatty plaques consisting of a gruel-like mix of smooth muscle cells, collagen fibers, and lymphocytes with necrotic regions of lipid, debris, and foam cells
• Predisposing factors for atheromas

  • Dyslipidemia (> 3:1 ratios of LDL to HDL)
  • Hyperglycemia of diabetes
  • Hypertension
  • Presence of toxins introduced by smoking
• In elastic arteries, atheromas

  Produce localized destruction within the wall, weakening it and causing arterial bulges or aneurysms which can rupture