Anatomy

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Blood Vessels and Circulation

Objectives • Distinguish among the types of blood vessels. • Describe fluid and dissolved material transport of the cardiovascular system. • Describe the factors that influence blood pressure and blood pressure regulation. • Discuss the movement of fluids between capillaries and interstitial spaces. • Describe how blood flow and pressure in tissues is regulated. • Identify the principle blood vessels of each circuit and the areas they serve. • Describe fetal circulation and the changes at birth and during aging.

Structure of vessel walls • Walls of arteries and veins contain three distinct layers

• Tunic intima • Tunica media • Tunica externa (adventitia)

Differences between arteries and veins • Compared to veins, arteries

• Have thicker walls • Have more smooth muscle and elastic fibers • Are more resilient

Arteries • Undergo changes in diameter

• Vasoconstriction – decreases the size of the lumen • Vasodilation – increases the size of the lumen

• Classified as either elastic (conducting) or muscular (distribution) • Small arteries (internal diameter of 30 um or less) are called arterioles

Capillaries • An endothelial tube inside a basal lamina • These vessels

• Form networks • Surround muscle fibers • Radiate through connective tissue • Weave throughout active tissues

• Capillaries have two basic structures • Continuous • Fenestrated

• Flattened fenestrated capillaries = sinusoids

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Capillary beds • An interconnected network of vessels consisting of

• Collateral arteries feeding an arteriole • Metarterioles • Arteriovenous anastomoses • Capillaries • Venules

Veins • Collect blood from all tissues and organs and return it to the heart • Are classified according to size

• Venules • Medium-sized veins • Large veins

Venous valves • Venules and medium-sized veins contain valves

• Prevent backflow of blood

Distribution of blood • Total blood volume is unevenly distributed

• Venoconstriction maintains blood volume • Veins are capacitance vessels

• Capacitance = relationship between blood volume and pressure

Circulatory pressure • Circulatory pressure is divided into three components

• Blood pressure (BP) • Capillary hydrostatic pressure (CHP) • Venous pressure

Resistance (R) • Resistance of the cardiovascular system opposes the movement of blood • For blood to flow, the pressure gradient must overcome total peripheral resistance

• Peripheral resistance (PR) is the resistance of the arterial system

Overview of cardiovascular pressures • Factors involved in cardiovascular pressures include

• Vessel diameter • Cross-sectional area of vessels • Blood pressure • Blood viscosity

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Arterial blood pressure • Arterial blood pressure

• Maintains blood flow through capillary beds • Rises during ventricular systole and falls during ventricular diastole

• Pulse is a rhythmic pressure oscillation that accompanies each heartbeat • Pulse pressure = difference between systolic and diastolic pressures • Example of Pulse Pressure for BP of 120/90: PP = 120 – 90; PP = 30

• Mean arterial pressure (MAP) = diastolic pressure + (pulse pressure ÷ 3) Example of MAP for BP of 120/90 MAP = 90 + (30 ÷ 3) MAP = 90 ÷ 10 MAP = 100 mm Hg

Capillary exchange • Flow of water and solutes from capillaries to interstitial space

Processes that move fluids across capillary walls • Diffusion • Filtration • Hydrostatic pressure (CHP) • Reabsorption

Venous pressure and venous return • Assisted by two processes

• Muscular compression • The respiratory pump

Cardiovascular regulation • Autoregulation • Neural mechanisms • Endocrine mechanisms

Autoregulation of blood flow within tissues • Local vasodilators accelerate blood flow in response to:

• Decreased tissue O2 levels or increased CO2 levels • Generation of lactic acid • Release of nitric acid • Rising K+ or H+ concentrations in interstitial fluid • Local inflammation • Elevated temperature

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Neural mechanisms • Adjust CO and PR to maintain vital organ blood flow • Medullary centers of regulatory activity include

• Cardiac centers • Vasomotor centers control

• Vasoconstriction via adrenergic release of NE • Vasodilation via direct or indirect release of NO

Reflex control of cardiovascular function • Baroreceptors reflexes monitor stretch

• Atrial baroreceptors monitor blood pressure • Chemoreceptor reflexes monitor CO2, O2, or pH levels

Hormones and cardiovascular regulation • Antidiuretic hormone – released in response to decreased blood volume • Angiotensin II – released in response to a fall in blood pressure • Erythropoietin – released if BP falls or O2 levels are abnormally low • Natriuretic peptides – released in response to excessive right atrial stretch

Exercise and the cardiovascular system • Light exercise results in

• Extensive vasodilation • Increased venous return • A rise in cardiac output

• Heavy exercise results in • Increased blood flow to skeletal muscles • Restriction of blood flow to nonessential organs

Cardiovascular response to hemorrhaging: short term • Carotid and aortic reflexes increase CO and peripheral vasoconstriction • Sympathetic nervous system elevates blood pressure • E and NE increase cardiac output and ADH enhances vasoconstriction

Cardiovascular response to hemorrhaging: long term • Decline in capillary blood pressure recalls fluids from interstitial spaces • Aldosterone and ADH promote fluid retention • Increased thirst promotes water absorption across the digestive tract • Erythropoietin ultimately increases blood volume and improves O2 delivery

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Identify the Below Landmarks on the Heart

Chambers Right atrium Right ventricle Left atrium Left ventricle

Valves Right AV (tricuspid) valve Left AV (bicuspid or mitral) valve Chordae tendineae Papillary muscles Pulmonary semilunar valve Aortic semilunar valve Fibrous skeleton

Vessels Superior vena cava Inferior vena cava Pulmonary trunk Pulmonary arteries Pulmonary veins Right coronary artery Left coronary artery Anterior interventricular (anterior

descending) coronary artery

Great cardiac vein Coronary sinus, and it’s opening to

right atrium Other Structures

Foramen ovale Fossa ovalis Ductus arteriosus Ligamentum arteriosum Parietal pericardium Visceral pericardium (epicardium) Myocardium Endocardium Interatrial septum Interventricular septum Sinoatrial (SA) node Internodal pathways Atrioventricular (AV) node AV bundle Bundle branches Purkinje fibers

Identify the Below Systemic Arteries

Ascending aorta Thoracic aorta Brachiocephalic trunk Subclavian Axillary Brachial Deep brachial Radial Ulnar Common Carotid Internal Carotid External Carotid Facial Superficial Temporal Occipital Vertebral Basilar

Cerebral arterial circle (circle of Willis) Anterior communicating Anterior cerebral Posterior communicating Posterior cerebral

Celiac trunk Common hepatic Superior mesenteric Renal Inferior mesenteric Common iliac Internal iliac External iliac Femoral Popliteal Anterior tibial Posterior tibial

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Identify the Below Systemic Veins

Superior vena cava Brachiocephalic trunk Subclavian Axillary Cephalic Brachial Basilic Radial Ulnar Median cubital Internal jugular External jugular Temporal Facial Vertebral Inferior vena cava Hepatic Renal Gonadal Common iliac Internal iliac External iliac Femoral Poplilteal Posterior tibial Great saphenous Hepatic portal circulation

Superior mesenteric Inferior mesenteric Splenic Hepatic portal vein

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Special circulation • The brain

• Four arteries which anastomose insuring constant blood flow • Cerebral arterial circle (Circle of Willis)

• The heart • Coronary arteries arising from the ascending aorta

• The lungs • Pulmonary circuit, regulated by local responses to O2 levels

• Opposite other tissues (declines in O2 cause vasodilation) • Hepatic portal system

• Contains substance absorbed by the stomach and intestines • Delivers these compounds to the liver for

• Storage • Metabolic conversion • Detoxification and Excretion

• Fetal Circulation • Fetal blood flow to the placenta is supplied via paired umbilical arteries • A single umbilical vein drains from the placenta to the ductus venosus

• Collects blood from umbilical vein and liver • Empties into the inferior vena cava • No need for pulmonary function in the fetus • Two shunts bypass the pulmonary circuit

• Foramen ovale • Ductus arteriosus

• Cardiovascular changes at birth • Lungs and pulmonary vessels expand • Ductus arteriosus constricts and becomes ligamentum arteriosum • A valvular flap closes the foramen ovale forming fossa ovalis • Ductus venosus closes and forms ligamentum arteriosum

Age-related changes in blood may include • Decreased hematocrit • Constriction or blockage of peripheral veins by a thrombus • Pooling of blood in the veins of the legs Vessels are less elastic, prone to Ca2+ deposits and thrombi formation