Chapter 20: The Cardiovascular System Blood Vessels PDF

Chapter 20 The Cardiovascular System: Blood Vessels and Circulation Blood Vessels and Hemodynamics Hemodynamics are the forces involved in the circulation of blood throughout the body. Blood vessels transport blood throughout the body. They can be classified into five categories. Ar...

Chapter 20 The Cardiovascular System: Blood Vessels and Circulation Blood Vessels and Hemodynamics Hemodynamics are the forces involved in the circulation of blood throughout the body. Blood vessels transport blood throughout the body. They can be classified into five categories. Arteries Arterioles Capillaries Venules Veins Blood Vessels Arteries carry blood away from the heart. Arterioles are small blood vessels formed from splitting arteries. Capillaries are tiny vessels where gas and substance exchanges between tissues and the blood occur. Veins carry blood towards the heart. Venules are small blood vessels that merge to form veins. Angiogenesis is the formation of new blood vessels. Blood Vessels (cont.) An anastomosis is a union of two or more blood vessels in the same body region. They allow for alternative routes for blood flow to reach a tissue or organ. This alternative blood flow is called collateral circulation.  An artery that does not have anastomoses is called an end artery. Anastomoses may occur between two arteries, two veins, or an artery and a vein. Blood Vessel Structure The wall of a blood vessel has three tunics, or layers. The tunica interna (intima) is the innermost layer. The tunica externa (adventitia) is the outermost layer. The tunica media lies between the tunica interna and the tunica externa. Blood flows through the lumen, or interior opening of the blood vessel. Blood Vessel Structure (cont.) The tunica interna is the layer of the blood vessel directly in contact with blood. It is composed of three layers.  The endothelium is an epithelial layer responsible for maintaining efficient blood flow, assisting with capillary permeability, and influencing the smooth muscle tissue surrounding the blood vessel.  The internal elastic lamina is a thin layer of connective tissue composed of elastic fibers supporting the endothelium.  The basement membrane anchors the endothelium to the internal elastic lamina. Blood Vessel Structure (cont.) The tunica media contains smooth muscle fibers and elastic fibers. It is responsible for controlling blood flow by altering the width of the lumen. The external elastic lamina is the outermost part of the tunica media. The tunica externa consists of collagen and elastic fibers. It contains nerves and blood vessels for the vessel wall.  Those blood vessels are called vasa vasorum. Arteries There are three types of arteries. Elastic (or conducting) arteries are responsible for propelling blood onward towards the rest of the body.  Their vessel walls are very thin, and have a tunica media mostly composed of elastic fibers called the elastic lamellae. Muscular (or distributing) arteries are responsible for maintaining proper blood pressure and flow.  Their vessel walls are thick, and have a tunica media made primarily from smooth muscle. Resistance arteries are the smallest arteries, Arterioles Arterioles are branches of arteries responsible for regulating blood flow into the capillaries. The metarteriole links arterioles and venules, and capillaries with both blood vessels. Arterioles have individual muscle fibers called precapillary sphincters that can close off blood flow to capillaries. The tunica externa of arterioles have many nerve cells that can control blood flow into the arteries. Capillaries The flow of blood from the arterioles into the capillaries and venules is called the microcirculation of the body. The capillary bed is composed of 50-100 capillaries originating from a single metarteriole. There are three types of capillaries in the body. Continuous capillaries Fenestrated capillaries Sinusoids Capillaries (cont.) Continuous capillaries have a mostly continuous endothelium. Intercellular clefts, gaps between endothelial cells, form occasional interruptions. Continuous capillaries are found in the CNS, lungs, muscles, and the skin. Fenestrated capillaries have many pores in the endothelium. They are found in the kidneys, villi of the small intestine, and most endocrine glands. Capillaries (cont.) Sinusoids are much wider and more winding than continuous or fenestrated capillaries. It has an incomplete (or lacks a) basement membrane. The intercellular clefts in sinusoids are much larger.  They allow proteins and even blood cells to pass from a tissue into the bloodstream. Sinusoids may also contain specialized cells, depending on the functions of the tissue it is supplying. Blood Circulation In blood circulation, most blood follows conventional blood flow properties. Blood flows from arteries into arterioles, capillaries, venules, and finally veins. In some regions, blood flows from one capillary system to another via veins called portal veins. This type of circulation is called a portal system. They function to transport products of a body region directly to another body region. Venules Most venules have far thinner walls than their arteriole counterparts. Postcapillary venules drain blood from the capillaries, and begin the process of returning blood to the heart. They are also very porous, and form part of the microcirculation. Muscular venules have thicker walls, and often serve as blood reservoirs in the body. Veins Veins have much thinner walls than arteries. They have very thin tunica interna and media layers. They lack internal and external elastic laminae. Most veins have unidirectional valves to prevent blood backflow. The blood pressure in veins is far less than that of arteries. A vascular sinus is a vein without smooth muscle fibers to alter its diameter. Blood Circulation About two-thirds of the blood volume resides systemic veins and venules. They function as blood reservoirs, and provide blood when needed. During increased muscular activity, the brain send signals to veins to constrict. The vasoconstriction results in greater blood flow to the muscles. During hemorrhage, vasoconstriction compensates for the blood loss. Capillary Exchange The primary purpose of blood flow throughout the body is that of capillary exchange, the movement of substances between blood and interstitial fluid. Substance exchange functions through three mechanisms.  Diffusion, or movement down the concentration gradient.  Transcytosis, or transport through endothelial cells.  Bulk flow, or movement due to pressure differences. Blood Flow Blood flow is the volume of blood that flows through any tissue over a period of time. Blood pressure determines the direction of blood flow, as blood flows from high to low pressure regions. Vascular resistance is the opposition to blood flow due to friction between blood and the vessel walls. Venous return refers to blood flowing back to the heart through the systemic veins.  The skeletal muscle pump and respiratory pump both assist in venous return. Control of Blood Flow The cardiovascular (CV) center is also responsible for regulating blood flow and blood pressure. There are two negative feedback loops involved in regulating blood pressure that occur as reflexes. Baroreceptors are pressure-sensitive sensory receptors in major arteries. When blood pressure changes, the CV center alters heart rate and blood vessel diameters to return blood pressure to normal. This is called the baroreceptor reflex. Control of Blood Flow (cont.) Chemoreceptors are sensory receptors located close to baroreceptors within major arteries. They detect changes in the concentrations of oxygen, carbon dioxide, and protons within the blood. In response, the CV center stimulates changes in blood pressure. Changes in the rate of breathing may also be induced to return the blood composition to normal. Hormone Regulation Hormones play a key role in the regulation of blood pressure. Cardiac output is increased by epinephrine and norepinephrine. Vasoconstriction increases blood pressure, and may be caused by several hormones.  Angiotensin II  Vasopressin (or antidiuretic hormone)  Aldosterone Hormone Regulation (cont.) Vasodilation decreases blood pressure, and may be caused by several hormones. Atrial natriuretic peptide (ANP) Epinephrine Nitric oxide An increase in blood volume increases blood pressure, and can be caused by aldosterone and vasopressin. A decrease in blood volume decreases blood pressure, and can be caused by ANP. Autoregulation Autoregulation is the ability of tissues to adjust their blood flows to fit their requirements. Blood flow to regions of the body changes according to the activity being performed. There are two main causes of autoregulatory change. Physical changes, such as temperature or muscle stretching. Chemicals released by tissues that affect blood vessel diameter. Shock and Homeostasis Shock is defined as the failure of the cardiovascular system to deliver oxygen and nutrients to the body. There are four types of shock. Hypovolemic shock is due to decreased blood volume. Cardiogenic shock is due to low heart function. Vascular shock is due to inappropriate vasodilation.  Anaphylactic, neurogenic, and septic shock Obstructive shock is due to blood flow Shock and Homeostasis (cont.) The body employs hormones to counteract the effects of shock. In addition, baroreceptors and chemoreceptors initiate responses from the autonomic nervous system to combat shock. Local autoregulatory processes also reduce the effects of shock. If blood volume decreases more than 10-20%, the body may not be able to resist the effects of shock. Circulation and Vessels The systemic circulation delivers oxygen and nutrients to the body, and has three major parts. Arteries and arterioles that carry blood from the left ventricle to the rest of the body. Systemic capillaries. Venules and veins that carry blood from the rest of the body to the right atrium. The pulmonary circulation serves to oxygenate the blood. Systemic Circulation Systemic circulation has several subdivisions. Coronary circulation supplies the heart. Cerebral circulation supplies the brain. Bronchial circulation supplies the lungs.  The bronchial and pulmonary circulations have distinctly different roles. Hepatic portal circulation conveys blood from the GI tract to the liver. Notable Arteries Aorta (ascending/descending/thoracic/abdominal) Brachiocephalic trunk L/R carotid (common, internal, and external) L/R vertebral Cerebral arterial circle (circle of Willis) L/R renal Inferior suprarenal L/R suprarenal (superior and middle) Notable Arteries (cont.) Celiac trunk Common hepatic, left gastric, splenic Superior and inferior mesenteric L/R subclavian, axillary, brachial, radial, and ulnar L/R iliac (common, internal, and external) L/R femoral L/R popliteal L/R tibial (anterior and posterior) L/R peroneal Notable Veins Superior and inferior vena cava L/R brachiocephalic and subclavian Dural venous sinuses L/R jugular (external and internal) L/R hepatic, renal, and suprarenal Hepatic portal vein Mesenteric (superior and inferior), splenic, L/R gastric Notable Veins (cont.) L/R radial, ulnar, brachial, cephalic, basilic, and axillary L/R saphenous (great and small) L/R iliac (common, internal, and external) L/R femoral L/R popliteal L/R tibial (anterior and posterior) L/R peroneal Hypertension Hypertension is defined as persistently high blood pressure. Over 50 million individuals in the US suffer from hypertension. Systolic blood pressure is the blood pressure level corresponding to the force of blood on arterial walls after ventricular contraction. Diastolic blood pressure is the blood pressure level corresponding to the force of blood on arterial walls after ventricular relaxation. Hypertension (cont.) Category Systolic (mmHg) Diastolic (mmHg) Normal Less than 120 Less than 80 Prehypertension 120-139 80-89 Stage 1 Hypertension 140-159 90-99 Stage 2 Hypertension 160-179 100-109 Hypertensive Crisis 180 and above 110 and above Hypertension (cont.) Hypertension falls into two categories. Primary hypertension can not be attributed to any identifiable cause.  90-95% of all hypertension cases are primary hypertension. Secondary hypertension can be attributed to a specific cause.  5-10% of all hypertension cases are secondary hypertension. Hypertension can damage blood vessels and the heart over time, and is a major risk factor for heart disease and stroke. Nomenclature Hypoxia is the decrease of [O2] in the blood. Acidosis is a decrease of pH in the blood. Alkalosis is an increase of pH in the blood. Hypercapnia is the increase of [CO2] in the blood. Tachycardia is a rapid resting heart rate (over 100 beats/minute). Bradycardia is a slow resting heart rate (under 50 beats/minute). Nomenclature (cont.) An aneurysm is the expansion of a weakened blood vessel, forming a bulging sac. Aortagraphy is the X-ray examination of the aorta and its main branches. Deep vein thrombosis is the formation of a blood clot in a deep vein of the lower limbs. Femoral angiography is the X-ray examination of the femoral artery and its branches. Hypotension is low blood pressure. Nomenclature (cont.) An occlusion is the closure or blockage of the lumen of a structure, such as a blood vessel. Postural hypotension is low blood pressure caused by assuming a standing position. Phlebitis is inflammation of a vein. Thrombectomy is the surgical removal of a blood clot. Thrombophlebitis is the inflammation of a vein caused by the formation of blood clots.