ch.19 Cardiovascular system blood vessels

• Blood vessels are the delivery system of dynamic structures that begin and end at the heart
• Arteries carry blood away from the heart; they are oxygenated except for pulmonary circulation and umbilical vessels of the fetus
• Capillaries have direct contact with tissue cells and directly serve cellular needs
• Veins carry blood toward the heart; they are deoxygenated except for pulmonary circulation and umbilical vessels of the fetus
• All vessels consist of a lumen, central blood-containing space, surrounded by a wall
• Walls of all vessels, except capillaries, have three layers or tunics:

  • Tunica intima
  • Tunica media
  • Tunica externa
• Tunica intima is the innermost layer that is in "intimate" contact with blood
• Tunica media is the middle layer composed mostly of smooth muscle and sheets of elastin
• Tunica externa is the outermost layer of the wall, also called tunica adventitia, composed mostly of loose collagen fibers that protect and reinforce the wall and anchor it to surrounding structures
• Capillary beds consist of two types of vessels:

  • Vascular shunt: channel that connects arteriole directly with venule (metarteriole-thoroughfare channel)
  • True capillaries: actual vessels involved in exchange
• Veins carry blood toward the heart; formation begins when capillary beds unite in postcapillary venules and merge into larger and larger veins
• Veins have a large lumen and thin walls, making them good storage vessels called capacitance vessels or blood reservoirs because they contain up to 65% of the blood supply
• Venous valves prevent backflow of blood and are most abundant in veins of limbs
• Blood pressure varies directly with cardiac output (CO), peripheral resistance (PR), and blood volume
• Blood pressure is directly proportional to CO and PR
• Changes in one variable are quickly compensated for by changes in other variables
• Cardiac output (CO) is stroke volume (SV) multiplied by heart rate (HR)
• MAP (mean arterial pressure) is directly proportional to SV, HR, and PR
• Anything that increases SV, HR, or PR will also increase MAP
• Short-term regulation of blood pressure can be achieved through neural controls, hormonal controls, and renal controls
• Factors affecting blood pressure can be affected by short-term regulation through neural controls, hormonal controls, and long-term regulation through renal controls
• Short-term regulation through neural controls involves altering blood vessel diameter to alter resistance
• Neural controls operate via reflex arcs involving the cardiovascular center of the medulla, baroreceptors, chemoreceptors, and higher brain centers
• Baroreceptor reflexes help maintain blood pressure homeostasis by initiating reflex vasoconstriction when MAP is low
• Chemoreceptor reflexes detect changes in CO2, pH, or O2 and increase blood pressure by signaling the cardioacceleratory center and vasomotor center
• Higher brain centers such as the medulla, hypothalamus, and cerebral cortex can modify arterial pressure via relays to the medulla
• Short-term regulation through hormonal controls involves hormones such as epinephrine, norepinephrine, angiotensin II, ADH, and atrial natriuretic peptide
• Long-term regulation of blood pressure is achieved through renal mechanisms that alter blood volume via the kidneys
• Direct renal mechanisms alter blood volume independently of hormones by increasing or decreasing urine production
• Indirect renal mechanisms involve the renin-angiotensin-aldosterone mechanism
• The goal of blood pressure regulation is to keep blood pressure high enough for adequate tissue perfusion but not too high to damage blood vessels
• Factors that increase MAP include the activity of the muscular pump and respiratory pump, fluid loss, crisis stressors, baroreceptors, blood volume, blood pH, O2, CO2, chemoreceptors, venous return, and activation of vasomotor and cardioacceleratory centers in the brain stem
• Transient elevations in blood pressure can occur during changes in posture, physical exertion, emotional upset, fever, and other factors like age, sex, weight, race, mood, and posture
• Circulatory shock can result from inadequate blood flow to meet tissue needs and can be classified as hypovolemic shock, vascular shock, or cardiogenic shock
• Blood flow through body tissues is controlled by extrinsic factors (sympathetic nervous system and hormones) and intrinsic factors (autoregulation)
• Autoregulation involves local control of blood flow to meet specific tissue requirements
• During exercise, skeletal muscles can receive over 70% of blood due to redistribution of blood flow
• Intrinsic controls involve skeletal muscle arterioles dilating to increase blood flow to muscles while extrinsic controls decrease blood flow to other organs
• Autoregulation involves local conditions that regulate blood flow to specific areas through metabolic and myogenic controls
• Two types of intrinsic mechanisms regulate blood flow to specific areas:

  • Metabolic controls
  • Myogenic controls