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Chapter 16 The Cardiovascular System: Blood Vessels and Circulation Blood Vessels • arteries • carry blood away from ventricles of heart • arterioles • receive blood from arteries • carry blood to capillaries • capillaries • sites of exchange of substances between blood and body cells • venules • receive blood from capillaries • veins • carry blood toward atria of heart 15-30 Blood Vessels Three layers (tunica): external, middle, inner Arteries: thicker Arterioles: Elastic: large smooth muscle helps regulate blood pressure Sympathetic activity to smooth muscle vasoconstriction (narrowing) Decreased sympathetic activity or NONE causes relaxation (dilation) Capillaries lack muscle fibers 15-33 Metarteriole • connects arteriole directly to venule 15-34 Walls of Artery and Vein 15-32 Blood Vessel Structure: Arteries, Veins Blood Vessel Structure: Arteries, Veins Capillaries • smallest diameter blood vessels • extensions of inner lining of arterioles • walls are endothelium only • semipermeable • sinusoids – leaky capillaries 15-35 Capillary Network 15-36 Regulation of Capillary: Blood Flow Precapillary sphincters • may close a capillary • respond to needs of the cells • low oxygen and nutrients cause sphincter to relax Autoregulation: ability of a tissue to adjust blood flow into the area according to demands 15-37 Exchange in the Capillaries • water and other substances leave capillaries because of net outward pressure at the capillaries’s arteriolar ends • water enters capillaries’ at the venular ends because of a net inward pressure • substances move in and out along the length of the capillaries according to their respective concentration gradients 15-38 Capillary Details Capillary Details Capillary Exchange Colloid osmotic pressure (pulls into capillary) Plasma proteins create this “pulling” pressure Causes reabsorption of fluid from outside to inside Excess fluid returned via lymphatic system Capillary Exchange Blood Vessel Structure: Veins Venules Larger lumen, thinner walls Valves prevent backflow Very thin, no valves Blood enters veins at very low pressure Inadequate to overcome gravity and return blood to heart Venous Valves 15-40 Venous Return: Two Mechanisms Skeletal muscle contractions 1. Especially in lower limbs squeeze veins - emptying them Because of valves, flow is heart Systemic venules and veins serve as blood reservoirs hold ~ 64% total blood volume Venous Return: Two Mechanisms Venous Return: Two Mechanisms Respiratory pump 2. Inhalation decreases thoracic pressure & increases abdominal pressure blood to heart Exhalation allows refilling of abdominal veins Blood Flow Through Vessels BP highest in aorta: 110/70 mm Hg Pulse in large arteries BP declines as flows through more vessels Arterioles: major drop in BP due to smooth muscle contraction vasoconstriction Capillary beds ~ 35-16 mm Hg 16 mm Hg at venules 0 at right atrium Blood Flow Through Vessels Factors that regulate blood flow and BP 1. Blood volume and ventricular contraction cardiac output Under control of cardiovascular (CV) center (medulla) 2. Vascular resistance: lumen diameter vessel length Smaller lumen (with vasoconstriction) greater resistance Greater vessel length (with weight gain) greater resistance blood viscosity Higher viscosity (as with high hematocrit) greater resistance Cardiovascular Center Located in medulla Helps regulate Heart rate Stroke volume Blood pressure Blood flow to specific tissues Mechanisms By neural mechanisms By hormonal mechanisms Input to Cardiovascular Center (Medulla) Input from different parts of brain Cerebral cortex: thoughts, decisions Limbic system: emotions Hypothalamus: changes in temperature or blood volume blood flow adjusted accordingly Input from sensory receptors and nerves Proprioceptors, baroreceptors, chemoreceptors Input to Cardiovascular Center (Medulla) Proprioceptors: Baroreceptors in aorta and carotid: if BP Cause heart rate as exercise begins cardiac output (CO) BP sympathetic stimulation CO BP parasympathetic CO BP Chemoreceptors in aorta and carotid bodies If low O2, high CO2, or high H+ (acidity) resistance by vasoconstriction BP Input to Cardiovascular Center (Medulla) Output to Cardiovascular Effectors ANS nerves to heart Sympathetic HR and force of contraction cardiac output (CO) BP Parasympathetic HR CO BP Vasomotor (sympathetic nerves) To arterioles contract smooth muscle vasomotor tone vascular resistance BP To veins contract smooth muscle move blood to heart BP Hormone Regulation of Blood Flow + BP Renin-angiotensin aldosterone (RAA) system Epinephrine + norepinephrine CO BP ADH = vasopressin Angiotensin II vasoconstriction BP aldosterone retain Na++ water BP vasoconstriction BP Thirst + water retention in kidney BP ANP from cells in atria Vasodilation, loss of Na+ water in urine BP Hormone Regulation of Blood Flow + BP Arterial Blood Pressure Blood Pressure – force the blood exerts against the inner walls of the blood vessels • rises when ventricles contract • falls when ventricles relax • systolic pressure – maximum pressure • diastolic pressure – minimum pressure 15-42 Checking Circulation: Pulse Pulse in arteries = heart rate (HR) Press artery against bone or muscle. Radial artery (thumb side of wrist) Carotid artery (neck) Brachial artery (arm) Tachycardia: rapid resting HR (>100 bpm) Bradycardia= slow resting HR (<50 bpm) Pulse 15-43 Central Venous Pressure • pressure in the right atrium • weakly beating heart increases central venous pressure • increase in central venous pressure causes blood to back up into peripheral vein 15-49 Blood Pressure Device used: sphygmomanometer Inflate cuff to raise pressure > systolic BP First sound indicates systolic BP Lower pressure further until sound become faint Briefly stop blood flow there Lower pressure in cuff until flow just starts Usually on brachial artery Diastolic BP Normal BP values <120 mm Hg for systolic and < 80 mm Hg for diastolic Circulatory Routes Two main routes: systemic + pulmonary Systemic circulation Oxygenated blood travels from heart throughout body, deoxygenating as it goes All systemic arteries branch from aorta All systemic veins empty into superior vena cava, inferior vena cava, or the coronary sinus Circulatory Routes Circulatory Routes: Aorta Circulatory Routes: Aorta Circulatory Routes: Aorta Circulatory Routes: Aorta Circulatory Routes: Pelvis, Lower Limb Circulatory Routes: Principle Veins Circulatory Routes: Principle Veins of the Hands and Neck Circulatory Routes: Principle Veins of the Right Upper Limb Circulatory Routes: Principle Veins of the Pelvis and Lower Limbs Pulmonary Circulation Carries blood from right side of heart to lungs to get O2 and eliminate CO2 Right ventricle (RV) pulmonary trunk R + L pulmonary arteries both lungs Carry “blue blood” low O2 in and high in CO2 Pulmonary capillaries: gas exchange R and L pulmonary veins L atrium Carry “red blood” (high in O2 in and low in CO2) Pulmonary Circuit 15-50 Blood Flow Through Alveoli • cells of alveolar wall are tightly joined together • the high osmotic pressure of the interstitial fluid draws water out of them 15-51 Cerebral Arterial Circle • Circle of Willis • formed by anterior and posterior cerebral arteries, which join the internal carotid arteries 15-58 Hepatic Portal Circulation Portal vein: transports blood from one organ’s capillary bed to another GI organs Splenic and superior mesenteric veins Hepatic portal vein (“blue blood”) Sinusoids (“leaky capillaries” in liver) Mixes “blue blood” with “red blood” Hepatic vein inferior vena cava (IVC) Hepatic Portal Circulation Hepatic Portal Vein 15-68 Hepatic Portal Circulation Fetal Circulation Specialized for exchange of materials with maternal blood/bypass lungs Exchange in placenta umbilical vein ductus venosus (bypasses liver) inferior vena cava R atrium (mixes with deoxygenated blood from lower body) foramen ovale L atrium Or R Ventricle pulmonary trunk ductus arteriosus aorta internal iliac arteries umbilical arteries placenta Fetal Circulation Changes at Birth Umbilical arteries medial umbilical ligaments Umbilical vein ligamentum teres Ductus venosus ligamentum venosum Placenta expelled after Foramen ovalis closes fossa ovale Ductus arteriosus ligamentum arteriosum Aging Stiffening of aorta Loss of cardiac muscle strength Reduced CO & increased systolic pressure Higher risk for Coronary artery disease (CAD) Congestive heart failure (CHF) Atherosclerosis