Anatomy & Physiology: Vessels

• Chapter 20 covers vessels through lecture outline
• Includes anatomy & physiology, with an integrative approach for the fourth edition

Vessel Types

• Arteries convey blood from the heart to capillaries
• Microscopic porous blood vessels exchanging substances between blood and tissues are capillaries
• Veins transport blood from capillaries to the heart
• Lumen is the space inside of a vessel
• Artery and vein walls have 3 layers: tunica intima (mainly epithelium), tunica media (smooth muscle), tunica externa (areolar CT)
• Capillaries only have tunica intima

General Structure of Vessels

• Companion vessels (arteries and veins serving same body region) are situated next to each other
• Arteries have a thicker tunica media and narrower lumen than veins
• Arteries have more elastic and collagen fibers than veins, so are more resilient and resistant to changes in blood pressure
• Veins have a thicker tunica externa and larger lumen than arteries
• Veins have valves

Arteries

• Arteries branch as they get farther from the heart, becoming smaller and less elastic, and decreasing the amount of smooth muscle in tunica media
• Elastic arteries allow for most stretch and recoil to accommodate big blood pressure changes, like the aorta, pulmonary trunk, common carotid, common iliac arteries
• Muscular arteries serve specific body regions; less elastic but still muscular to allow vasoconstriction and dilation, like most named arteries like brachial and coronary
• Arterioles have smooth muscle that is usually somewhat constricted (vasomotor tone)
• Vasomotor tone is regulated in the brainstem and controls systemic blood pressure and blood flow

Clinical View: Atherosclerosis

• Atheroma (a thickening of tunica intima) will narrow the arterial lumen in elastic and muscular arteries
• Cause not fully understood but common theory is the response-to-injury hypothesis: infection, trauma, hypertension injuries endothelium results in inflammation reaction, atheroma formation
• Risk factors: increased LDL cholesterol in blood, males > females, smoking, and high blood pressure

Capillaries

• Small vessels connecting arterioles to venules that function as groups (capillary beds) but not independently
• Average length is 1 mm and diameter is 8 to 10 micrometers, so small that erythrocytes travel single file!
• Walls consist of an endothelial layer on a basement membrane
• Thin walls and small diameters are optimal for exchange between blood and tissue fluid
• Three types of capillaries: continuous, fenestrated, and sinusoid

Types of Capillaries

• Capillary wall structure depends on location and needs of adjacent tissues
• Continuous capillaries are in most places (muscle, skin, CNS): endothelial cells separated by intercellular clefts (small gaps), so large particles (cells, proteins) cannot pass, but small molecules (e.g., glucose) can pass through
• Fenestrated capillaries are where fluid transport is maximal (intestines, kidneys): endothelial cells have fenestrations, allowing movement of smaller plasma proteins
• Sinusoids transport of large substances like formed elements or large proteins and provide large openings; Found in bone marrow, spleen, and some endocrine glands

Capillary Beds

• A metarteriole (branch of an arteriole) feeds a capillary bed and a venule drains it
• True capillaries branch from metarterioles and make up bulk of capillary bed
• Precapillary sphincters are smooth muscle rings at true capillary origins
• Relaxation of precapillary sphincters permits blood flow, and contraction causes blood to bypass capillary bed
• At any time, only ~25% of body's capillary beds are open
• Capillary perfusion is the amount of blood entering capillaries per unit time per gram of tissue (mL/min/g)

Veins

• Like arteries, veins have large, medium and small-sized, and venules
• Largest veins travel with elastic arteries and small and medium-sized veins travel with muscular arteries
• Most veins of these sizes have numerous valves to prevent blood from pooling in the limbs and ensure flow toward heart
• Similar to how semilunar heart valves work.
• Venules are companion vessels with arterioles; merge to form veins

Systemic Veins as Blood Reservoirs

• At rest, 70% of blood is in systemic circulation, and most of that (55% of total blood) is in systemic veins
• Vasoconstriction of systemic veins can move blood into circulation, when more blood needed during exertion
• Blood can be shifted back into reservoirs via vasodilation, when less blood is needed during rest

Pathways of Blood Vessels

• Simple pathway: one end artery branches into smaller arteries and then arterioles to deliver blood to organ or region via a capillary bed, then blood returns via venules, veins, and back to the heart
• Anastomosis (arterial or venous): greater than (>)1 arteries supply or >1 veins drain the same region
• Arteriovenous anastomosis (shunt): transports blood from artery directly to veins, allowing areas to be bypassed if hypothermic, like in fingers, toes, palms, and ears
• Portal system (two capillary beds in sequence), hepatic portal system

Capillary Exchange Intro: Velocity of Blood Flow

• Cross-sectional area of a vessel is its lumen diameter
• Total cross-sectional area is sum of diameters of all vessels of a certain type (artery, capillary, or vein)
• Collectively, the total cross-sectional areas of capillaries is largest because 60,000 miles of capillaries exists!
• The larger the total cross-sectional area of a vessel type, the slower the blood flow velocity
• Blood flow is slow in capillaries, allowing for exchange between blood and tissue fluid

Capillary Exchange

• Capillaries exchange substances (gases, nutrients, wastes, hormones) between blood and surrounding tissues by:
• Diffusion: substances move down their concentration gradient (oxygen, hormones, nutrients move from blood to interstitial fluid, while carbon dioxide and wastes diffuse from tissue to blood)
• Vesicular transport (certain hormones and fatty acids)
• Bulk flow: large amounts of fluids and dissolved substances move down a pressure gradient
• Movement direction depends on net pressure of opposing forces (hydrostatic pressure versus colloid osmotic pressure)
• Filtration: fluid moves out of blood on arterial end of capillary
• Reabsorption: fluid moves back into blood on venous end of capillary

Bulk Flow

• Hydrostatic pressure (HP): force exerted by a fluid
• Blood hydrostatic pressure (HPb) is the force exerted by blood on vessel wall; promotes filtration out of capillary
• HPif is close to zero in most tissues; "if" subscript means interstitial fluid.
• Colloid osmotic pressure (COP): the "pull" on water (water follows solutes!) due to the presence of proteins (colloid) primarily
• Blood colloid osmotic pressure (COP) (aka oncotic pressure) draws fluid into blood due to blood proteins (e.g., albumins); promotes reabsorption into capillary
• COPif is very low (0 to 5 mm Hg) since few proteins present in interstitial fluid
• Net filtration pressure (NFP) is the difference between net hydrostatic pressure and net colloid osmotic pressure: NFP = (HPb - HPif) - (COPb - COPif)

Bulk Flow at Capillaries

• Since IF pressures are negligible, and blood hydrostatic pressure drops through the capillary bed but blood COP remains constant, NFP is higher at the arterial end (more filtration) than at the venous end (more reabsorption)
• If NFP is positive, we have filtration; if negative, we have absorption

Role of the Lymphatic System

• About 85% of blood is reabsorbed by capillaries before draining into the venule
• The lymphatic system picks up excess fluid not reabsorbed at the venous capillary end, filters it and returns it to venous circulation

Local Blood Flow

• Not all capillaries are filled simultaneously—in addition to total blood flow (creates systemic blood pressure gradient), local blood flow is dependent upon:
• Degree of tissue vascularity (metabolic demands
• Increased metabolism or growth of vascular tissue (muscle/fat tissue/tumors/etc) causes the creation of new vessels (angiogenesis)
• Myogenic response: Smooth muscle in blood vessel wall keeps local flow relatively constant
• If systemic blood pressure rises, arteriole walls stretch and will constrict in response, and if BP drops, arteriole walls will stretch less and will relax in response

Local Short-Term Regulation

• Local regulatory factors (vasodilators and vasoconstrictors) alter blood flow in response to:
• Changing metabolic needs (autoregulation): Lack of oxygen/ nutrients/increased lactate (as with exercise) causes vasodilation
• Reactive hyperemia: Blood flow increases after it is temporarily disrupted with additional blood required to resupply oxygen, nutrients, and eliminate wastes
• Damage: Chemicals released during inflammation (due to trauma/allergy/infection) cause arterioles to dilate and chemicals released during tissue damage can cause vasoconstriction to limit blood loss through damaged vessel

Clinical View: Detecting a Pulse Point

• Pulse is the throbbing of an arterial wall to the rhythm of heartbeat; More forceful pulse associated with higher pressure, while absence indicates flow to body part lacking
• Pulse points are where an artery may be palpated for pulse

Blood Pressure Intro

• Blood pressure is the force of blood against inside walls of a vessel (hydrostatic pressure!)
• A blood pressure gradient is the difference in BP between one end of a vessel and the other
• The systemic blood pressure gradient is the difference in pressure between arteries and where blood is returned to the heart (vena cava) and drives blood moving through the system, from higher to lower pressure areas
• Blood pressure varies throughout the body depending on vessel type, size, and location (distance from heart)

Blood Pressure in Different Vessel Types

• Blood flow in arteries pulses with cardiac cycle (per heartbeat)
• Systolic pressure is when a ventricle contracts (systole) and arterial pressure peaks - the upper number of the blood pressure ratio
• Diastolic pressure is when ventricles relax (diastole) and arterial pressure is lowest - lower number of blood pressure ratio
• Pulse pressure adds pressure in arteries added by heart contraction
• Meaning, the difference between systolic and diastolic blood pressure and tends to decline w/age/disease
• Mean arterial pressure (MAP): A weighted average arterial blood pressure across an entire cardiac cycle that provides a measure of perfusion
• CAP approximates the systemic blood pressure gradient (the difference between MAP and BP when blood reaches the heart at vena cava)
• Capillary blood pressure has no difference between systolic and diastolic pressure

Venous Blood Pressure

• Venous return to the heart depends on:
• Pressure gradient: small in veins; BP is 20 mm Hg in venules, almost 0 in vena cava

• Skeletal muscle pump: Return from limbs is assisted as muscle contracts and veins are squeezed
• Respiratory pump assists return in the thorax
• Inspiration: diaphragm contracts, so abdominal pressure increases->blood in abdominal veins driven toward thoracic cavity