Blood vessels require oxygen and nutrients, so larger ones have blood vessels in their walls.
The walls of arteries and veins have three layers. The inner layer is composed largely of endothelium, with a basement membrane that has elastic fibers; the middle layer is smooth muscle tissue; the outer layer is connective tissue (largely collagen fibers). Arteries (on left) have a thicker wall than veins because they have a larger middle layer than veins. Capillary walls (center) are one-cell-thick endothelium. Veins (on right) are larger in diameter than arteries, so that collectively veins have a larger holding capacity than arteries.
The inner layer of an artery wall is a simple squamous epithelium called endothelium with a connective tissue basement membrane with elastic fibers. The outer layer is fibrous connective tissue near the middle layer, but it becomes loose connective tissue at its periphery.
Capillaries have one-cell-thick walls composed only of endothelium with a basement membrane. Capillaries form vast networks with a total surface area of 6,000 square meters in humans.
A capillary bed forms a maze of capillary vessels that lies between an arteriole and a venule. When sphincter muscles are relaxed, the capillary bed is open, and blood flows through the capillaries. When sphincter muscles are contracted, blood flows through a shunt that carries blood directly from an arteriole to a venule. As blood passes through a capillary in the tissues, it gives up its oxygen (O2). Therefore, blood goes from being O2-rich in the arteriole (red color) to being O2-poor in the vein (blue color).
The superior vena cava and the pulmonary trunk are attached to the right side of the heart. The aorta and pulmonary veins are attached to the left side of the heart. The right ventricle forms most of the ventral surface of the heart, and the left ventricle forms most of the dorsal surface.
The coronary arteries and cardiac veins pervade cardiac muscle. The coronary arteries bring oxygen and nutrients to cardiac cells, which derive no benefit from blood coursing through the heart.
If the SA node fails to work properly, the heart still beats due to impulses generated by the AV node, but the beat is slower (40 to 60 beats per minute). To correct this condition, it is possible to implant an artificial pacemaker, which automatically gives an electrical stimulus to the heart.
The SA node sends out a stimulus, which cause the atria to contract. When this stimulus reaches the AV node, it signals the ventricles to contract. Impulses pass down the two branches of the atrioventricular bundle to the Purkinje fibers, and thereafter the ventricles contract.
Although the blood pressure in the brachial artery is typically about 120/80, blood pressure actually varies throughout the body. Blood pressure is highest in the aorta and lowest in the venaecavae.
Thromboembolism is a clot that has moved and is now stationary in a new blood vessel where it can cause damage.
• The cardiovascular system is dividedfor descriptive purposes into two mainparts :-• The circulatory system :
• The lymphatic system :1.lymph nodes2.lymph vessels.
• The heart is a hollow muscular organ• It is about 10 cm long and is aboutthe size of the owner’s fist• It weighs about 225 g in women and isheavier in men(about 310 g).
• The heart lies in the thoracic cavity inthe mediastinum (the space betweenthe lungs)• It lies obliquely , a little more to theleft than the right• It presents a base above and an apexbelow• The apex is about 9 cm to the left ofthe midline at the level of the 5thintercostal space.
• The two systems communicate withone another and are intimatelyassociated• The heart pumps blood into twoanatomically separate systems ofblood vessels :-• The Pulmonary circulation• The Systemic circulation.
• Pulmonary Circulation : The right sideof the heart pumps blood to the lungswhere gas exchange occurs• Systemic Circulation : The left side ofthe heart pumps blood into thesystemic circulation, which suppliesthe rest of the body.
• The heart pumps blood into vesselsthat vary in structure,size andfunction• The cardiovascular system has threetypes of blood vessels:• Arteries (and arterioles) – carry bloodaway from the heart• Capillaries – where nutrient and gasexchange occur• Veins (and venules) – carry bloodtoward the heart.
• Tunica adventitia or outer layer offibrous tissue• Tunica media or middle layer of smoothmuscle and elastic tissue• Tunica intima or inner lining of squamousepithelium called endothelium.
• Arteries and arterioles take bloodaway from the heart• The largest artery is the aorta• The middle layer of an artery wallconsists of smooth muscle that canconstrict to regulate blood flow andblood pressure• Arterioles can constrict ordilate, changing blood pressure.
• Capillaries have walls only one cellthick to allow exchange of gases andnutrients with tissue fluid• Capillary beds are present in allregions of the body but not allcapillary beds are open at the sametime• Contraction of a sphincter musclecloses off a bed and blood can flowthrough an arteriovenous shunt thatbypasses the capillary bed.
• Venules drain blood fromcapillaries, then join to form veinsthat take blood to the heart• Veins have much less smooth muscleand connective tissue than arteries• Veins often have valves that preventthe backward flow of blood whenclosed• Veins carry about 70% of the body’sblood and act as a reservoir duringhemorrhage.
• The heart muscle forms themyocardium, with tightly interconnectcells of cardiac muscle tissue• The pericardium is the outermembranous sac with lubricating fluid• The heart has four chambers: twoupper, thin-walled atria, and twolower, thick-walled ventricles.
• The septum is a wall dividing the rightand left sides• Atrioventricular valves occur betweenthe atria and ventricles – thetricuspid valve on the right and thebicuspid valve on the left; both valvesare re enforced by chordae tendinaeattached to muscular projectionswithin the ventricles.
• Inferiorly - The apex rests on thecentral tendon of the diaphragm• Superiorly - The great bloodvessels, superior vena cava,pulmonaryartery and pulmonary veins• Posteriorly - Theoesophagus, trachea,left and rightbronchus,descending aorta,inferiorvena cave and thoracic vertebrae
• Laterally - The lungs - the left lungoverlaps the left side of the heart• Anteriorly –The sternum , ribs andintercostal muscles.
• Two coronary arteries :-• Right coronary artery• Left coronary artery.• Right coronary artery.• Marginal branch.• Posterior interventricular branch.• Right atrial branch.• Infundibular branch.• Terminal branch.
• Left coronary artery.• Anterior interventricularbranch.• Circumflex branch.• Diaphragmatic branch.• Left atrial branch.• Terminal branch.
• Blood follows this sequence throughthe heart:• superior and inferior vena cava →right atrium → tricuspid valve → rightventricle → pulmonary semilunar valve→ pulmonary trunk and arteries to thelungs → pulmonary veins leaving thelungs → left atrium → bicuspid valve→ left ventricle → aortic semilunarvalve → aorta → to the body.
• The pumping of the heart sends outblood under pressure to the arteries• Blood pressure is greatest in theaorta; the wall of the left ventricle isthicker than that of the rightventricle and pumps blood to theentire body• Blood pressure then decreases as thecross-sectional area of arteries andthen arterioles increases.
• Each heartbeat is called a cardiaccycle• When the heart beats, the two atriacontract together, then the twoventricles contract; then the wholeheart relaxes• Systole is the contraction of heartchambers; diastole is their relaxation• The heart sounds, lub-dup, are due tothe closing of the atrioventricularvalves, followed by the closing of thesemilunar valves.
• The SA (sinoatrial) node, orpacemaker, initiates the heartbeatand causes the atria to contract onaverage every 0.85 seconds• The AV (atrioventricular) node conveysthe stimulus and initiates contractionof the ventricles• The signal for the ventricles tocontract travels from the AV nodethrough the atrioventricular bundle tothe smaller Purkinje fibers.
• The beating of the heart isnecessary to homeostasisbecause it creates pressure thatpropels blood in arteries and thearterioles• Arterioles lead to the capillarieswhere nutrient and gas exchangewith tissue fluid takes place.
• Blood pressure due to the pumping ofthe heart accounts for the flow ofblood in the arteries• Systolic pressure occurs when theheart expels the blood• Diastolic pressure occurs when theheart ventricles are relaxing• Both pressures decrease with distancefrom the left ventricle because bloodenters more and more arterioles andarteries.
• Blood moves slowly in capillariesbecause there are morecapillaries than arterioles• This allows time for substancesto be exchanged between theblood and tissues.
• Venous blood flow is dependentupon:1) skeletal muscle contraction,2) presence of valves in veins, and3) respiratory movements.• Compression of veins causes blood tomove forward past a valve that thenprevents it from returningbackward.
• Cardiovascular disease (CVD) is theleading cause of death in Westerncountries• Modern research efforts haveimproved diagnosis, treatment, andprevention• Major cardiovascular disorders includeatherosclerosis, stroke, heartattack, aneurysm, and hypertension.
• Atherosclerosis is due to a build-up offatty material (plaque), mainlycholesterol, under the inner lining ofarteries.• The plaque can cause a thrombus(blood clot) to form• The thrombus can dislodge as anembolus and lead to thromboembolism.
• A cerebrovascular accident, orstroke, results when an emboluslodges in a cerebral blood vessel or acerebral blood vessel bursts• A portion of the brain dies due tolack of oxygen• A myocardial infarction, or heartattack, occurs when a portion ofheart muscle dies due to lack ofoxygen.
• About 70% of Indians suffer fromhypertension (high blood pressure)• Hypertension is present when systolicpressure is 140 or greater or diastolicpressure is 100 or greater; diastolicpressure is emphasized when medicaltreatment is considered.