Chapter 21: Cardiovascular System: Blood Vessels & Circulation

Intro

• The circulatory system is composed of blood vessels that carry the blood to the tissues of the body, allowing for the exchange of substances, such as O2, nutrients, CO2, and waste products, necessary to maintain homeostasis 

• The heart is the pump that provides the major force causing blood to circulate, and the blood vessels are the pipes that carry blood to the body tissues and back to the heart 

• the circulatory system distributes the many substances that are necessary for and produced by the various metabolic activities throughout the body. In addition to providing the routes for blood movement, the blood vessels participate in regulating blood pressure and determining the degree of blood flow to the body’s most active tissues. Blood pressure must be high enough to ensure sufficient blood flow to meet the tissues’ metabolic needs. Regulation of both the blood vessels and the heart ensure that homeostatic blood pressure is maintained 

Vessels of the Circulatory System

The blood vessels are part of the cardiovascular system and comprise the portion commonly referred to as the circulatory system

• pulmonary vessels transport blood from right ventricle → lungs → left atrium

• systemic vessels transport blood from left ventricle → all parts of the body → right atrium

• the heart provides the major force that causes blood to move through these vessels

Functions of the Circulatory System

1. carries blood to body tissues & back to heart

2. exchanges nutrients, waste products & gases (O2 & CO2)

Nutrients and O2 diffuse from blood vessels to cells in all areas of the body. Waste products and CO2 diffuse from the cells, where they are produced, to blood vessel

3. transports hormones, components of immune system, molecules required for coagulation, enzymes, nutrients, gases, waste products, etc

4. helps regulate blood pressure (MAP) → tissue perfusion

the circulatory system and the heart work together to maintain blood pressure within a normal range of values 

5. directs blood flow to tissues by controlling degree or volume of flow

the circulatory system regulates the degree of blood flow, and, therefore, the volume of blood delivered to tissues to maintain homeostasis 

Structures Features of Blood Vessels

3 types of blood vessels form continuous passageway from heart to tissues & back

heart → arteries → capillaries → veins → heart

Tissue Layers:

tunica intima (tunica interna) → most internal layer of a blood vessel wall, consists of 4 layers

• endothelium

• basement membrane

• lamina propia (CT layer)

• internal elastic membrane

  • fenestrated layer of elastic fibers (separates the tunica intima from the next layer, openings)

Structures Features of Blood Vessels 2

tunica media (middle layer): smooth muscle cells arranged circularly around blood vessel (The amount of blood flowing through a blood vessel can be regulates by contraction or relaxation of the smooth muscle in the tunica media)

• contains variable amounts of elastic & collagen fibers depending on vessel size

• vasoconstriction: smooth muscles contract & dec. blood flow to the vessel (decrease in blood vessel diameter)

• vasodilation: smooth muscles relax & inc. blood flow (increase in blood vessel diameter)

tunica externa: CT that varies from dense regular near vessel to loose that merges w/ surrounding CT

Histology of a Blood Vessel

The layers, or tunics, of the blood vessel wall are the tunica intima, media, and externa. Vasa vasorum are blood vessels that supply blood to the wall of the blood vessel 

Comparison of Artery & Vein

The typical structure of a medium-sized artery (A) and a vein (V). Note that the artery has a thicker wall than the vein. The predoexternalayer in the wall of the artery is the tunica media, with its circular layers of smooth muscle. The predominant layer in the wall of the vein is the tunica externa, and the tunica media is thinner than in the artery 

Types of Arteries

carry blood away from heart & get smaller with distance

• elastic arteries

• muscular arteries

• arterioles

as arteries get smaller they transition from:

• high proportion of elastic fibers → fewer elastic fibers

• smaller proportion of smooth muscle → more smooth muscle

The ventricles of the heart pump blood into large, elastic arteries that branch repeatedly to form many progressively smaller arteries. As they become smaller, the artery walls undergo a gradual transition from having a large amount of elastic tissue and a smaller amount of smooth muscle to having less elastic tissue and more smooth muscle. From these muscular arteries, blood flows into the arterioles, the smallest of the arteries 

Elastic Arteries (conducting arteries)

includes most of named arteries

• largest diameter (10 mm)

• high pressure that fluctuates between systolic & diastolic

• more elastic tissue than smooth muscle

• relatively thick tunica intima

  • The elastic fibers of the internal and external elastic membranes merge and are not recognizable as distinct layers 

• thin tunica externa

• Often called conducting arteries 

• Because these vessels are the first to receive blood from the heart, blood pressure is relatively high in elastic arteries 

• also, due to the pumping action of the heart, blood pressure in the elastic arteries fluctuates between higher and lower diastolic values 

• when stretched, the walls of elastic arteries recoil, preventing drastic decreases in blood pressure 

• elastic arteries have a greater amount of elastic tissue and a smaller amount of smooth muscle in their walls, compared with other arteries 

• the elastic fibers are responsible for the elastic characteristics of the blood vessel wall, but collagenous connective tissue determines the degree to which the arterial wall can stretch 

• the tunica media consists of a meshwork of elastic fibers with interspersed, circular smooth muscle fibers and some collagen fibers 

Muscular Arteries (40 um-300 um)

medium muscular arteries:

• thick walls (25-40 layers of smooth muscle)

  • allows vessels to partially regulate blood supply to different regions of body

• called distributing arteries

smaller muscular arteries

• adapted for vasodilation & vasoconstriction

  • fewer smooth muscle layers

• include medium-sized and small arteries 

• the use of muscular in the name of these vessels refers to their thick tunica media 

• the walls of some muscular arteries are relatively thick, compared with their diameter, mainly because the tunica media contains 25-40 layers of smooth muscle 

• the tunica intimate of the muscular arteries has a well-developed internal elastic membrane 

• the tunica externa is composed of a relatively thick layer of collagenous connective tissue that blends with the surrounding connective tissue 

• muscular arteries are frequently called disturbing arteries because the smooth muscle fibers allow them to partially regulate blood flow to different body regions by either constricting or dilating 

• smaller muscular arteries range from 40 um to 300 um in diameter 

• those that are 40 um in diameter have approximately 3 or 4 layers of smooth muscle in their tunica media, whereas arteries that are 300 um across have essentially the same structure as the larger muscular arteries 

• the small muscular arteries are adapted for vasodilation and vasoconstriction 

Arterioles (9 um-40 um)

• transport blood from small arteries to capillaries

• smallest arteries where 3 tunics can still be differentiated

• capable of vasoconstriction & vasodilation

• They range in diameter from approximately 40 um, which is less than half the thickness of a sheet of printed paper, to as small as 9 um 

• The tunica intimate has no observable internal elastic membrane, and the tunica media consists of one or two layers of circular smooth muscle fibers 

Capillaries (7 um-9 um)

• wall consists of:

  • endothelium (simple squamous epithelium)

    • In the vessels associated with the heart, this endothelial lining is continuous with the endocardium of the heart 

  • basement membrane

    • The capillary wall consists primarily of a single layer of endothelial cells that resists on a basement membrane 

  • delicate layer of loose CT

• scattered pericapillary cells are fibroblasts, macrophages or undifferentiated smooth muscle cells

• vessels for exchanges between blood & interstitial spaces

• Blood flows from arterioles into capillaries, the most common type of blood vessel 

• Capillary walls are the thinnest of all blood vessels 

• most of the exchange that occurs between the blood and interstitial spaces occurs across the thin walls of capillaries 

• precapillary cells -> closely associated with the endothelial cells 

• these scattered cells lie between the basement membrane and the endothelial cells and are fibroblasts, macrophages, or undifferentiated smooth muscle fibers 

• most capillaries range from 7 um to 9 um in diameter, and they branch without changing in diameter

• capillaries are variable in length, but in general they are approximately 1 mm long 

• RBCs flow through most capillaries single file and are frequently folded as they pass through the smaller-diameter capillaries 

Types of Capillaries

Variation in size and permeability, or the degree to which materials enter or leave the blood 

Substances cross capillary walls by diffusion either (1) through or between endothelial cells or (2) through fenestrae

• continuous- many locations, including muscle & nervous tissue

  • no gaps between endothelial cells

    • Approximately 7-9 um in diameter, and their walls exhibit no gaps between the endothelial cells. They are located in muscle, nervous tissue, and many other locations. No pores 

  • no fenestrae

  • less permeable to large molecules than other capillaries

• fenestrated- intestinal villi, ciliary process of eye, choroid plexus, glomeruli of kidney

  • endothelial cells have numerous fenestrae where cytoplasm is absent & PM is thin & porous

  • highly permeable

• sinusoidal- endocrine glands, liver

  • large diameter w/ large fenestrae

  • less basement membrane

  • Sinusoidal capillaries, also called sinusoids, are larger in diameter than either continuous or fenestrated capillaries, and their basement membrane is less prominent or completely absent 

  • Their fenestrate are larger than those in fenestrated capillaries, and gaps can exist between endothelial cells 

  • the sinusoidal capillaries occur in places where large molecules, or sometimes, whole cells move across their wall (ex: in the liver or endocrine glands)

  • Lipid-soluble substances, such as O2 and CO2, and small, water-soluble molecules readily diffuse through the endothelial cells 

  • larger water-soluble substances must pass through the fenestra or gaps between the endothelial cells 

  • in addition, transport by pinocytosis occurs, but little is known about its role in the capillaries 

  • the walls of the capillaries are effective permeability barriers because RBCs and large, water-soluble molecules, such as proteins, cannot readily pass through them, with the exception of some specialized capillaries 

Structure of Capillary Walls

(A) continuous capillaries have no gaps between endothelial cells and no fenestrate. (B) fenestrated capillaries have fenestrate 7-100 nm in diameter, covered by thin, porous diaphragms, which are not present in some capillaries. (C) sinusoidal capillaries have larger fenestrate without diaphragms and can have gaps between endothelial cells 

Capillary Network (site of oxygen/nutrient exchange)

Capillaries do not exist individually in tissues but form branching networks. Arterioles supply blood to each capillary network 

• blood flows from arterioles → metarterioles → capillary networm

• flows through thoroughfare channel to a venule is consistent (allows to bypass capillary network)

• flow through arterial capillaries is intermittent

• precapillary sphincters of metarterioles regulates blood flow into capillary network (slow bloodflow down to allow exchange to happen)

• arteriole → metarteriole → arteriole capillaries → venous capillaries → venule

• Blood flows from arterioles to capillary networks through met arterioles, vessels with isolated smooth muscle fibers along their walls 

• Blood then flows from a met arteriole into a thoroughfare channel, a vessel within the capillary netwvenulest extends in a relatively direct fashion from a metarteriole to a venue 

• blood flow through thoroughfare channels in relatively continuous 

• several capillaries branch from the thoroughfare channels, forming the capillary network 

• blood flow is regulated in the capillary branches by pre capillary spaincters, smooth muscle fibers located at the origin of the branches 

• blood flows through the capillary network into the venules 

the ends of capillaries closest to the arterioles are arterial capillaries, and the ends closest to venules are venous capillaries 

Figure 21.4 

• a capillary network stems from an arteriole. Blood flows from the arteriole, through met arterioles, through the capillary network, to venules. Smooth muscle fibers, called pre capillary sphincters, regulate blood flow through the capillaries. Blood flow decreases when the pre capillary sphincters constrict and increases when they dilate. Exchange between the blood and other tissues occurs primarily at capillary networks 

• capillary networks are more numerous and extensive in highly metabolic tissues, such as in the lungs, liver, kidneys, skeletal muscle, and cardiac muscle 

• capillaries in the skin function in thermoregulation, and heat loss results from the flow of a large volume of blood through them 

• capillary networks in the serous of skin have many more thoroughfare channels than capillary networks in cardiac or skeletal muscle 

• the major function of the capillaries in these muscle tissues is nutrient and waste product exchange 

Capillary Network

• tissues w/ high metabolism have more capillary networks:

  • lungs, liver, kidneys, skeletal muscle, cardiac muscle

  • thermoregulation function in skin (maintain homeostasis)

  • have more thoroughfare channels

• nutrient & waste product exchange is major function in muscles

Arteriovenous Anastomes

• specialized vascular connections that allow blood to flow directly from arterioles → small veins (skip capillaries, quicker movement)

• glomus: arteriovenous anastomosis that consists of arterioles w/ abundant smooth muscle in walls

  • The vessels are branched and coiled and are surrounded by CT sheets. Glom era are present in large numbers in the sole of the foot, the palm of the hand, the terminal phalanges, and the nail bets 

• help regulate body temp by adjusting blood flow through them

  • As body temperature increases, glom era dilate and more blood flows through them, increasing the rate of heat loss from the body 

• abundant in sole of foot, palm of hand, terminal phalanges, nail beds

• pathologic arteriovenous anastomoses

• form from injury or tumors & are considered abnormal

• very large & can lead to heart failure (↑ venous return)

  • Abnormal vascular connections allow for direct flow of blood from arteries to veins

Veins

From capillaries, blood flows into veins. walls of veins are thinner. Vein walls also contain less elastic tissue and fewer smooth muscle fibers. As the blood returns to the heart, it flows through veins with thicker walls and greater diameters 

• carry blood toward heart

• vessels get larger as they approach heart

• classifications:

  • venules (smallest)

  • small veins

  • medium or large veins

Venules (diameter = up to 50 um)

• drain capillary network

• endothelial cells & basement membrane w/ few smooth muscle cells

• as diameter of venules inc. proportion of smooth muscle inc.

• pass blood to small veins

• Smallest veins. Their structure is very similar to that of capillaries in that they are tubes composed of endothelium resting on a delicate basement membrane 

• a few isolated smooth muscle fibers exist outside the endothelial cells, especially in the larger venules 

• as the vessels increase to 0.2-0.3 mm in diameter, the smooth muscle fibers form a continuous layer; the vessels are then called small veins

• in addition to a larger diameter compared to venules, small veins also have a tunica externa composed of collagenous CT 

• the venules collect blood from the capillaries and transport it to small veins, which in turn transport it to medium veins 

• nutrient exchange occurs across the venue walls, but, as the walls of the small veins increase in thickness, the degree of nutrient exchange decreases 

Small & Medium Veins

• small veins (0.2-0.3 mm)

  • smooth muscle cells form continuous layer

  • tunica adventitia made of collagenous CT

  • pass blood to medium veins

• medium veins (1 mm-1 cm)

  • thin tunica intima

  • circular smooth muscle in tunica media

  • tunica externa is pred.

  • pass blood to large veins

Large Veins

• thin tunica intima of endothelial cells, thin layer of CT & scattered elastic fibers

• tunica media has circularly arranged smooth muscle cells

• tunica externa is predominant layer

• The large veins transport blood from the medium veins to the heart 

• In the mediuexternaarge veins, the tunica intimate is thin and consists of endothelial cells, a relatively thin layer of collagenous CT, and a few scattered elastic fibers 

• the tunica media is also thin and is composed of a thin layer of circularly arranged smooth muscle fibers containing some collagen fibers and a few sparsely distributed elastic fibers 

• the tunica externa, which is composed of collagenous CT, is the predominant layer 

Portal Veins

In some areas of the body, a capillary network is directly connected to another capillary network by portal veins 

begin in a primary capillary network & extend to a secondary capillary network w/out a pumping mechanism

• This connection is unique in that there is no pumping mechanism like the heart between the two capillary networks 

humans have 3 portal veins systems:

1. hepatic portal veins- carry nutrient rich blood from GI & spleen capillaries to liver where they dilate & become sinusoidal capillaries

2. hypothalamohypophyseal portal system- between hypothalamus & anterior pituitary gland (endocrine control)

3. renal nephron portal system- within urine-forming structures of kidneys

Valves (not in capillaries)

• found in all veins > 2 mm in diameter

• folds of tunica intima form 2 overlapping flaps

• more valves in veins of lower extremities than in veins of upper extremities

• Veins that have diameters greater than 2 mm contain valves, which allow blood to flow toward the heart, but not in the opposite direction 

• The valves consists of folds in the tunica intimate that form 2 flaps shaped like the semilunar valves of the heart 

• the two folds overlap in the middle of the vein so that, when blood attempts to flow in a reverse direction, the valves occlude, or block, the vessel 

• medium veins contain many valves, and the number of valves is greater in veins of the lower limbs than in veins of the upper limbs 

Disorders of Veins

• varicose veins- valves of lower limbs become incompetent (too much stretch)

  • results in venous pressure & edema in lower limbs

    • possible hemorrhoids

  • Result when the veins of the lower limbs are stretched to the point that the valves become incompetent. Because of the stretching of the vein walls, the flaps of the valves no longer overlap to prevent the back flow of blood 

  • stagnant blood flow can lead to clots

  • causes: genetics, pregnancy, standing for long periods

• phlebitis- inflammation of veins

  • if severe can lead to large areas of stagnant blood

• gangrene- tissue death from loss of blood supply

  • can result from prolonged phlebitis

Veins of the lower limbs are subject to certain disorders 

• Some people have genetic propensity to develop varicose veins 

• the condition is further encouraged by activities that increase the pressure in the veins. Ex: pregnancy, standing in place for prolonged periods 

Vaso Vasorum

• small vessels that supply nutrients to arteries & veins > 1 mm whose diameter too large for diffusion of nutrients from lumen

• penetrate from exterior & form capillary network in tunica externa & tunica media

• For arteries and veins greater than 1 mm in diameter, nutrients cannot diffuse from the lumen of the vessel to all the layers of the wall 

• Therefore, nutrients are supplied to the blood vessel walls by way of small blood vessels called the vaso vasorum, which penetrate from the exterior of the vessel to form a capillary network in the tunica externa and tunica media 

Neural Innervation of Blood Vessels

• unmyelinated sympathetic nerve fibers form plexuses in tunica adventitia

  • Nerve terminals containing NT vesicles project among the smooth muscle fibers of the tunica media. Synapses consist of several enlargements of each of the nerve fibers among the smooth muscle fibers. Sympathetic stimulation causes blood vessels to constrict. The smooth fibers of blood vessels act to some extent in unison. Gap junctions exist between adjacent smooth muscle fibers; as a consequence, stimulation of a few smooth muscle fibers in the vessel wall results in constriction of a relatively large segment of the blood vessel

• small arteries & arterioles are most innervated

  • to greater extent than other blood vessel types

  • vasoconstriction

• vessels of penis & clitoris innervated by parasympathetic fibers

  • vasodilation

• a few myelinated sensory fibers innervate certain vessels

  • act as baroreceptors to monitor stretch & ∆BP

Hemodynamics

interrelationship between:

• flow

• pressure

• resistance

control mechanisms that regulate blood pressure & blood flow are critical to function of circulatory system

Flow in vessels

1. laminar flow- streamlined (Fluid, including blood, tends to flow through long, smooth-walled tubes in a streamlines fashion called laminar flow)

• interior blood vessel is smooth & of equal diameter along length

• fluids behave like they are composed of concentric rings: The movement of these layers is not the same because of the effect of resistance 

  • outermost layer flows slowest & center flows fastest

• The layer nearest the wall of the tube experiences the greatest resistance to flow because it moves against the stationary wall. The innermost layers slip over the surface of the outermost layers and experience less resistance to movement. Thus, flow in a vessel consists of movement of concentric layers, with the outer layer moving mostly slowly and the layer at the center moving most rapidly 

2. turbulent flow- interrupted (laminar flow is interrupted and becomes turbulent flow when…)

• rate of flow exceed critical velocity pr

• fluid passes a constriction, sharp turn, rough surface

• partially responsible for heart sounds

• turbulence sounds are not normal in arteries

  • probably due to some constriction

    • Sounds caused by turbulent blood flow in arteries are not normal and usually indicate that the artery is abnormally constricted

  • increases probability of thrombosis

• Turbulent flow is caused by numerous small currents flowing at an angle to the long axis of the vessels. These small currents result in flowing whorls or eddy currents in the blood vessel. Vibrations of the liquid and blood vessel walls during turbulent flow cause the sounds heard when blood pressure is measured using a blood pressure cuff. Turbulent flow is also common as blood flows past the valves in the heart and is partially responsible for the heart sounds. Turbulent flow of blood through vessels occurs primarily in the heart and to a lesser extent where arteries branch 

Laminar & Turbulent Flow

(A) in laminar flow, fluid flows in long, smooth-walled tubes as if it were composed of a large number of concentric layers. (B) turbulent flow is caused by numerous small currents flowing crosswise or obliquely to the long axis of the vessel, resulting in flowing whorls and eddy currents 

Blood Pressure

• measure of force exerted by blood against blood vessel walls

• blood moves through vessels because of blood pressure

• measured directly using cannula into blood vessel or indirectly using auscultatory method (health professionals most often use the auscultatory method to measure blood pressure)

  • sphygomomanometer & stethoscope to listen for Korotkoff sounds- turbulent flow in arteries as pressure released from blood pressure cuff

  • pressure during 1st sound: systolic

  • pressure where sound disappears: diastolic

• An instrument called a mercury (Hg) manometer measures blood pressure in millimeters of mercury (mm Hg)

• A blood pressure of 100 mm Hg is great enough to lift a column of mercury 100 mm

• blood pressure can be measured directly by inserting a cannula (tube) into a blood vessel and connecting a manometer or an electronic pressure transducer to it. Electronic transducers are very sensitive and can precisely detect rapid fluctuations in pressure 

• placing a catheter into a blood vessel or into a chamber of the heart to monitor pressure changes is possible but not appropriate for routine clinical examinations 

Auscultatory Blood Pressure Measurement

Allows medical professionals to measure arterial blood pressure 

Auscultatory Blood Pressure Measurement 2

1. blood pressure cuff connected to sphygmomanometer wrapped around patient’s arm just above elbow → stethoscope places over brachial artery

Some sphygmomanometers have mercury manometers, and others have digital manometers, but they all measure pressure in terms of millimeters of mercury 

2. blood pressure cuff inflated until brachial artery is completely collapsed (no sounds can be heard through the stethoscope bc blood flow through the constricted area is blocked)

3. pressure in cuff gradually lowered & blood flows through constricted area during systole (As soon as it declines below the systolic pressure, blood flows through the constricted area during systole (contraction of the ventricles)

• blood flow is turbulent and produces vibrations in blood & surrounding tissues that can be heard through stethoscope (Korotkoff sounds)

  • Sounds heard over an artery when blood pressure is determined by auscultatory method; caused by turbulent flow of blood 

• pressure at which a Korotkoff sounds 1st heard is systolic pressure

Auscultatory Blood Pressure Measurement 3

4. as pressure in blood pressure cuff is lowered more Korotkoff sounds change tone & loudness

5. when pressure has dropped until continuous laminar blood flow reestablished sound disappears

• pressure at which continuous laminar flow is reestablished is diastolic pressure

  • This method for determining systolic and diastolic pressures is not entirely accurate, but its results are within 10% of methods that more direct 

Blood Flow = CO

• rate of flow through tube expressed as volume that passes a specific point per unit of time

  • ex: cardiac output at rest is 5 L/min, thus blood flow through aorta is 5 L/min (when a person is resting)

• flow= (P1-P2/R) or F=∆P/R

  • P1 & P2 are pressures in vessel at points 1&2

  • R is resistance to flow (blood always flows from area of higher pressure to area of lower pressure)

  • directly proportional to ∆P & inversely proportional to R

  • viscocity & vessel diameter most important in resistance

    • length of vessel not as critical for R

• The rate at which a liquid, such as blood, flows through a tube can be expressed as the volume that passes a specific point per unit of time. Blood flow is usually reported in either milliliters (mL) or liters (L) per minute. Cardiac output: volume of blood pumped by the heart per minute; also called minute volume. The rate of blood flow is influenced by pressure differences within the vessel and resistance to flow.

• Where P1 and P2 are the pressures in the vessel at points one and two, and R is the resistance to flow 

• Blood always flows from an area of higher pressure to an area of lower pressure; the greater the pressure difference, the greater the rate of flow 

• for example, the average blood pressure in the aorta (P1) is greater than the pressure in the vessels of the relaxed right atrium (P2). Therefore, blood flows from the aorta to tissues and from tissues to the right atrium 

• this is dependent on the pumping action of the heart maintaining a pressure gradient throughout the circulatory system 

• if the heart should stop contracting, the pressure in the aorta would become equal to that in the right atrium, and blood would no longer flow 

• the flow of blood, resulting from a pressure difference between the two ends of a blood vessel, is opposed by a resistance to flow 

• as such, the degree of blood flow is inversely related to the amount of resistance 

• another way to state this is that as resistance increases, blood flow decreases; conversely, as the resistance decreases, blood flow increases 

• resistance is affected by several factors including blood viscosity, vessel length, and vessel diameter 

• the viscosity of blood changes slowly 

• a small change in the diameter of a vessel dramatically changes the resistance to flow, and therefore the amount of blood that flows through it 

• vasoconstriction decreases the diameter of a vessel, which causes a greater resistance to flow and, overall, reduced blood flow through the vessel 

• conversely, vasodilation increases the diameter of a vessel, which causes a lower resistance to flow and greater blood flow through the vessel 

• major changes in blood flow through blood vessels are produced by changes in blood pressure and blood vessel diameter 

• during exercise, heart rate and stroke volume increase, causing blood pressure in the aorta to increase 

• in addition, blood vessels in skeletal muscles vasodilate, and resistance to flow decreases 

• as a consequence, a dramatic increase in blood flow through blood vessels in exercising skeletal muscles occurs 

Poiseullie’s Law (F= ∆P/R)

• flow dec. when resistance inc. & vice versa

• since resistance is proportional to blood vessel diameter constriction of blood vessel inc. resistance & thus dec. flow

• during exercise:

  • heart beating w/ greater force inc. pressure in aorta

  • capillaries to skeletal muscle inc. diameter, dec. resistance & inc. flow

  • in aorta flow can go from 5 L/min to 25 L/min

Viscosity

• measure of resistance of liquid to flow

• R directly proportion to flow

• as viscosity inc. pressure required to flow inc.

• blood viscosity influenced largely by hematocrit

  • as hematocrit inc. viscosity inc. logarithmically

• dehydration &/or uncontrolled production of RBCs can lead to inc. viscosity which inc. workload on heart

• As the viscosity of a liquid increases, the pressure required to force it to flow also increases 

• The viscosity of liquids is commonly determined by considering the viscosity of distilled water as 1 and then comparing the viscosity of other liquids with that 

• using this procedure, whole blood has a viscosity of 3.0-4.5, which means that about three times as much pressure is required to force whole blood through a given tube at the same rate as forcing water through the same tube 

• the viscosity of blood is influenced largely by hematocrit, which is the percentage of the total blood volume composed of RBCs

• as hematocrit increases, the viscosity of blood increases logarithmically 

• blood with a hematocrit of 45% has a viscosity about three times that of water, whereas blood with a very high hematocrit of 65% has a viscosity about seven to eight times that of water 

• the plasma proteins have only a minor effect on the viscosity of blood, but dehydration or uncontrolled production of RBCs can increase the hematocrit and the viscosity of blood substantially. Viscosity above the normal range increases the workload on the heart. If this workload is great enough, heart failure can result 

Critical Closing Pressure & Laplace’s Law

Critical closing pressure: pressure at which a blood vessel collapses & blood flow stops

• a concern during circulatory shock

Laplace’s Law

• force acting on blood vessel wall is proportional to diameter of vessel x blood pressure

• F = D × P

  • as diameter of a vessel ↑, force on wall ↑

  • weakened part of a vessel wall bulges out & is an aneurysm

Vascular Compliance

Compliance: tendency for blood vessel volume to ↑ as blood pressure ↑

• more easily vessel wall stretches = greater compliance

• venous system has a large compliance (24 x greater than that of arteries)

• acts as a blood reservoir

Distribution of Blood Volume in Blood Vessels

Physiology of the Systemic Circulation

• as diameter of vessels ↓:

  • total cross-sectional area ↑ & velocity of blood flow ↓

  • aorta has cross-sectional area = 5 cm²

  • total cross-sectional area of all capillaries = 2500 cm²

  • like a stream that flows rapidly through a narrow gorge but slowly through a broad plain

Pressure & Resistance

• bp avg. 100 mm Hg in aorta & drops to 0 mm Hg at right atrium

• ↑ resistance to flow as cross-sectional area ↑

• greatest drop in pressure in arterioles

• no large fluctuations in capillaries or veins

• muscular arteries & arterioles capable of constricting or dilating in response to autonomic & hormonal stimulation

Pressure & Resistance 2

• bp ↓ as you get further from heart

• greater resistance = more pressure ↑ because diameter of vessels ↓

• R also affects speed at which pressure changes

  • blood moves fastest in aorta & large arteries & slowest in capillaries

Pulse Pressure

• difference between systolic & diastolic pressures

• ↑ when SV ↑ or vascular compliance ↓

  • compliance tends to ↓ w/ age (arteriosclerosis) & pressure rises

• can be used to take pulse to determine HR & rhythmicity

• most frequent site used is in carpus w/ radial artery (radial pulse)

Blood Pressure in the Major Vessel Types

Capillary Exchange

diffusion is most important means of exchange

• lipid-soluble substances cross capillary walls diffusing through PM (O₂, CO₂, steroid hormones, fatty acids)

• water-soluble substances diffuse through intercellular spaces or through fenestrations of capillaries (glucose & amino acids)

blood pressure, capillary permeability & osmosis affect movement of fluid from capillaries

fluid moves out of capillaries at arterial end

most fluid returns to capillaries at venous end

• that which remains in tissues is picked up by lymphatic system & returned to venous circulation

Capillary Exchange 2

Capillary Exchange 3

Venous return (preload)

• ↑ w/ an ↑ in: blood volume, venous tone, compression of veins, arteriole dilation

1. Muscular pump: contraction of skeletal muscles “milks” blood back toward heart; valves prevent backflow

2. Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand

3. Sympathetic venoconstriction: under sympathetic control, smooth muscles constrict, pushing blood back toward heart (venous tone)

Blood Pressure and the Effect of Gravity

• while standing position hydrostatic pressure caused by gravity ↑ bp below heart & ↓ bp above heart

• ~15 to 20% of total blood volume passes through walls of capillaries of lower limbs during 15 minutes of standing still

  • results in edema

• muscular movement improves venous return

Control of Blood Flow in Tissues

1. local control: in most tissues F is proportional to metabolic needs

2. Nervous System: responsible for routing blood flow & maintaining bp

3. hormonal control: sympathetic APs stimulate epinephrine & norepinephrine

Changes in MAP

• MAP: slightly ˂ avg of systolic & diastolic pressures bc diastole lasts longer than systole

• ∆M A P during lifetime:

  • ~70 mm Hg at birth

  • 100 mm Hg from adolescence to middle age

  • 110 mm Hg in healthy older individuals

• MAP = C O × P R or M A P = HR × SV × P R

• when HR, SV or PR ↑ = MAP ↑

• circulatory shock: inadequate F throughout body due to failure to maintain normal bp (can lead to death)

• if bp too high heart & blood vessels may be damaged

Change in MAP 2

• SV is affected by venous return (EDV)

• HR is maintained by medullary centers

• R is affected mostly by vessel diameter

Short-Term Regulation of BP

• baroreceptor reflexes: ∆bp = ∆PR, ∆HR & ∆SV

• adrenal medullary mechanism: results from substantial ↑ in sympathetic stimulation of heart & blood vessels (↓ in bp; ↑ in exercise; stress)

  • epinephrine & norepinephrine mimic sympathetic stimulation

• chemoreceptor reflexes: sensitive to oxygen, CO₂, & pH levels of blood

• central nervous system ischemic response: results from lack of blood flow to medulla oblongata

  • vasomotor neurons stimulate vasoconstriction to ↑ bp

PIC

Summary of Short-Term Regulation of Blood Pressure

• in most circumstances throughout the day, baroreceptor reflex is most important for maintaining bp

• adrenal medullary mechanism plays a role during exercise & emergencies

• chemoreceptor mechanism more important when blood [O₂] ↓:

  • high altitudes or ↑ [CO₂] or ↓ pH

• CNS ischemic response activated only in emergency conditions when brain receives too little O₂

Long-Term Regulation of Blood Pressure

• regulation of blood concentration & volume by kidneys

• movement of fluid across wall of blood vessels

• alterations in volume of blood vessels

some mechanisms begin to respond in minutes & continue to functions for hours, days or longer

Direct Regulation of Blood Volume by Kidneys

• kidneys regulate bp by altering blood volume through production of urine

• as blood volume & MAP ↑, urine volume ↑

  • ↑ output of urine = ↓ blood volume & bp 

• ↓ blood volume & bp = ↓ urine production

  • ↓ urine output ↑ blood volume & bp

Renin-Angiotensin-Aldosterone Mechanism

Renin-Angiotensin-Aldosterone Mechanism 2

• secretion of renin dependent on ∆bp:

  • renin released when ↓ bp

  • renin release ↓ when  ↑ bp

• angiotensin-converting enzyme (ACE) inhibitors are class of drugs used to treat hypertension

2. Antidiuretic Hormone (Vasopressin) Mechanism

3. Atrial Natriuretic Mechanism

• Atrial natriuretic hormone (A N H): released from cardiac muscle cells in atria of heart when ↑ venous return

1. acts on kidneys to ↑ rate of urine production & Na⁺ loss in urine

2. dilates arteries & veins

• loss of water & Na⁺ in urine → blood volume ↓ , venous return ↓

• vasodilation → PR ↓

• both cause a drop in bp

4. Fluid Shift Mechanism

• occurs in response to small ∆bp in capillaries

  • as bp ↑ some fluid is forced from capillaries into interstitial spaces

  • as bp ↓ interstitial fluid moves into capillaries & resists further bp ↓

  • interstitial volume acts as reservoir & is in equilibrium w/ large volume of intercellular fluid

• begins to act immediately but requires hours to reach full effect

5. Stress-Related Response

• vasomotor adjustment of blood vessel to respond to ∆ blood volume

  • when blood volume suddenly ↓ & pressure ↓ smooth muscles contract & vice versa

• most effective when ∆bp occur over many minutes

Summary of Long-Term Pressure Control Mechanisms

Control of Blood Flow to Maintain Tissue Perfusion

angiogenesis – production of new vessels to meet tissue needs

Control of Blood Flow via Arteriolar Smooth Muscle

Blood Flow to Skeletal Muscles

• varies w/ fiber type & activity

• at rest: myogenic & neural mechanisms predominate

  • F maintained at ~1 L / min

• activity/exercise: flow ↑ proportionally to metabolic activity

  • F can ↑ 10-fold (~10 L / min)

Blood Flow to Brain

• must be constant (~750 mL/min) since neurons intolerant of ischemia

• metabolic controls

  • vasodilation in response to ↓ pH or ↑ CO₂

• myogenic controls

  • ↓ MAP = dilation of cerebral vessels

  • ↑ MAP = constriction of cerebral vessels

• MAP < 60 mm Hg can lead to syncope

• MAP > 160 mm Hg can lead to cerebral edema

Circulatory Shock

inadequate blood flow that causes tissue damage

1. Hypovolemic shock – result of reduced blood volume

• severe bleeding; burns; diarrhea, vomiting, dehydration

2. Neurogenic (Vascular) shock – results from vasodilation

• emotional stress; anesthesia

3. Anaphylactic shock – result of allergic response

• histamine dump causes ↑ capillary permeability

4. Septic shock – caused by infection & toxins

• ↓ HR & ↑ capillary permeability

5. Cardiogenic shock - ↓ in CO

• MI; fibrillations & arrhythmias; electrical shocks

Developmental Aspects & Aging

• Mesoderm → endothelial lining

• Fetal shunts:

  • foramen ovale; ductus arteriosus; ductus venosus

• Umbilical veins & arteries

• vessel formation:

  • as needed; wound healing; uterine

• Aging:

  • hypertension; atherosclerosis; varicose veins

Aging of the Arteries

• Arteriosclerosis: degenerative changes in arteries making them less elastic

• Atherosclerosis: deposition of plaque on walls

  • effects medium & larger arteries (including coronary arteries)

Hypotension & Hypertension

• Hypotension: low bp (systolic < 100 mm Hg)

  • orthostatic – temporary (head rush after standing)

  • chronic – poor nutrition; Addison’s disease; hypothyroid

  • acute – sign of circulatory shock

• Hypertension: high bp (systolic > 160 mm Hg)

  • transient: fever; physical exertion; emotional stress

  • chronic: damage to vessels, baroreceptors, heart muscle or vascular organs

    • primary – heredity, stress, smoking

    • secondary – result of other disorders

Extrinsic Factors Affecting BP

• age

• sex

• weight

• physical activity

• ethnicity (hereditary factors)

• mood

• posture

• socioeconomic status (access to nutrition)