Blood Vessels

n    Blood is carried in a closed system of vessels that begins and ends at the heart

n    The three major types of vessels are arteries, capillaries, and veins

n    Arteries carry blood away from the heart, veins carry blood toward the heart

n    Capillaries contact tissue cells and directly serve cellular needs

n    Arterioles

n    Venules

Vessel Anatomy

n    Arteries and veins are composed of three tunics

n    Tunica externa

–    Collagen fibers that protect and reinforce vessels

n    Tunica media

–    External elastic lamina

–    Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system

–    Controls vasoconstriction/vasodilation of vessels

n    Tunica interna (intima)

–    Internal elastic lamina

–    Endothelial layer that lines the lumen of all vessels

n    Lumen

–    Central blood-containing space surrounded by tunics

Generalized Structure of Blood Vessels

Elastic (Conducting) Arteries

n   Thick-walled arteries near the heart; the aorta and its major branches

–  Large lumen allow low-resistance conduction of blood

–  Contain elastin in all three tunics

–  Withstand and smooth out large blood pressure fluctuations

–  Serve as pressure reservoirs

Muscular (Distributing) Arteries and Arterioles

n   Muscular arteries – distal to elastic arteries; deliver blood to body organs

–  Have thick tunica media with more smooth muscle

–  Active in vasoconstriction

n   Arterioles – smallest arteries; lead to capillary beds

–  Control flow into capillary beds via vasodilation and constriction

CAPILLARIES

n   Smallest blood vessels

–  Walls consisting of a thin tunica interna, one cell thick

–  Allow only a single RBC to pass at a time

n   There are three structural types of capillaries: continuous, fenestrated, and sinusoids

Continuous Capillaries

n   Abundant in the skin and muscles

–  Endothelial cells provide an uninterrupted lining

–  Adjacent cells are connected with tight junctions

–  Intercellular clefts allow the passage of fluids

Continuous Capillaries

n   In the brain:

–  Have tight junctions completely around the endothelium

–  Constitute the blood-brain barrier

Fenestrated Capillaries

n    Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

n    Characterized by:

–   An endothelium riddled with pores (fenestrations)

–   Greater permeability than other capillaries

Sinusoids

n    Highly modified, leaky, fenestrated capillaries with large lumens

n    Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

n    Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

n    Blood flows sluggishly, allowing for modification in various ways

Capillary Beds

n    A microcirculation of interwoven networks of capillaries, consisting of:

–   Metarteriole

•   Thoroughfare channel connecting an arteriole directly with a postcapillary venule

–   True capillary

•   10 to 100 per capillary bed

•   Capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

•   Precapillary sphincter

–   Cuff of smooth muscle that surrounds each true capillary

–   Regulates blood flow into the capillary

Capillary Beds

Capillary Beds

VEINS

n    Venules are formed when capillary beds unite

–    Allow fluids and WBCs to pass from the bloodstream to tissues

n    Veins are:

–    Formed when venules converge

–    Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks

–    Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

n     Veins have much lower blood pressure and thinner walls than arteries

n     To return blood to the heart, veins have special adaptations

–    Large-diameter lumens, which offer little resistance to flow

–    Valves (resembling semilunar heart valves), which prevent backflow of blood

–    Venous return

–    Varicose veins

Blood Flow

n   Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period:

–  Is measured in ml per min.

–  Is equivalent to cardiac output (CO), considering the entire vascular system

–  Is relatively constant when at rest

–  Varies widely through individual organs

Blood Pressure (BP)

n   Force per unit area exerted on the wall of a blood vessel by its contained blood

–  Expressed in millimeters of mercury (mm Hg)

–  Measured in reference to systemic arterial BP in large arteries near the heart

n   The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

Resistance

n   Opposition to flow

–  Measure of the amount of friction blood encounters

–  Generally encountered in the systemic circulation

–  Referred to as peripheral resistance (PR)

n   The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameter

Resistance Factors

n    Resistance factors that remain relatively constant are:

–  Blood viscosity – thickness of the blood

–  Blood vessel length – the longer the vessel, the greater the resistance encountered

n    Changes in vessel diameter are frequent and significantly alter peripheral resistance

Blood Pressure

n    Systemic pressure:

–    Is highest in the aorta

–    Declines throughout the length of the pathway

–    Is 0 mm Hg in the right atrium

n    The steepest change in blood pressure occurs in the arterioles

n    Systolic pressure

–    Pressure exerted on arterial walls during ventricular contraction

n    Diastolic pressure

–    Lowest level of arterial pressure during a ventricular cycle

n    Pulse pressure

–    The difference between systolic and diastolic pressure

n    Mean arterial pressure (MAP)

–    Pressure that propels the blood to the tissues

–    MAP = diastolic pressure + 1/3 pulse pressure

Systemic Blood Pressure

Venous Return

n    Venous blood flow must overcome gravity

n    Venous BP alone is too low to promote adequate blood return and is aided by the:

–    Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins

–    Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart      

n    Valves prevent backflow during venous return

BP Regulation

n   Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter

–  Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles

n   Cardiovascular center – vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter

BP Regulation

n   Neural mechanisms

–  Baroreceptor reflexes (pressure-sensitive)

•   Carotid sinus reflex

•   Aortic reflex

–  Chemoreceptor reflexes (O₂, CO₂, and H⁺ sensitive)

BP Regulation

n   Hormonal mechanisms

–  ADH

•   Causes intense vasoconstriction in cases of extremely low BP

–  Renin-angiotensin-aldosterone system

•   Kidney release of renin generates angiotensin II, which causes vasoconstriction

–  E and NE

•   Increase blood pressure         

–  Atrial natriuretic peptide (ANP)

•   Causes blood volume and pressure to decline

Capillary Exchange of Respiratory Gases and Nutrients

n    Oxygen, carbon dioxide, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid along concentration gradients

–   Oxygen and nutrients pass from the blood to tissues

–   Carbon dioxide and metabolic wastes pass from tissues to the blood

–   Water-soluble solutes pass through clefts and fenestrations

–   Lipid-soluble molecules diffuse directly through endothelial membranes

Capillary Exchange

n   Capillary hydrostatic pressure (CHP)

n   Capillary colloid osmotic pressure (CCOP)

n   Net filtration pressure (NFP)

SHOCK

n    Circulatory shock – any condition in which blood vessels are inadequately filled and blood cannot circulate normally

n    Results in inadequate blood flow to meet tissue needs

n    Types

–   Hypovolemic shock – results from large-scale blood loss

–   Cardiogenic shock – the heart cannot sustain adequate circulation

–   Vascular shock – poor circulation resulting from extreme vasodilation

SHOCK

n   Homeostatic responses

–  Activate RAA system

–  Secretion of ADH

–  Sympathetic activation