Ch 21: Blood Vessels

Cardiovascular system has 5 classes of blood vessels

-*Arteries* carry blood away from the heart -As they enter peripheral tissues, they branch repeatedly and decrease in diameter -Smallest arterial branches are called *arterioles* -From there, they move into *capillaries*, where diffusion takes place between blood and interstitial fluid -From capillaries, blood enters small *venules* -Venules unite to form larger *veins* that return blood to the heart

Sinusoids

-*Sinusoidal capillaries* -Resemble fenestrated capillaries that are flattened and irregularly shaped -They are fenestrated, have gaps between adjacent endothelial cells, and the basement membrane is either thinner or absent -Permit free exchange of water and solutes as large as plasma proteins between blood and interstitial fluid -Blood flow is slow allowing time for exchange across sinusoidal walls -Occur in the liver, bone marrow, spleen, and many endocrine organs--plasma proteins and phagocytic cells

The sympathetic nervous system can change the diameter of the arterial walls

-*Vasoconstriction*: when SNS is stimulated, arterial smooth muscles contract, constricting the artery -*Vasodilation*: when smooth muscles relax, diameter of the lumen increases -They both affect the 1) afterload of the heart, 2) peripheral blood pressure, and 3) capillary blood flow

Nervous system adjusts

-Cardiac output and peripheral resistance to maintain adequate blood flow to vital tissues and organs -Centers responsible: cardiac centers and vasomotor center of the medulla oblongata

Net Filtration Pressure (NFP)

-Difference between the net hydrostatic pressure and the net osmotic pressure -Net filtration pressure (NFP) = Net hydrostatic pressure (CHP-IHP) - Net colloid osmotic pressure (BCOP-ICOP) *Positive value means fluid will move out of capillary *Negative value means fluid will move into the capillary

Under normal circumstances, the heart pumps blood into the aorta at the same rate at which blood arrives at the right atrium

-Meaning that when BP rises at right atrium, blood is arriving at heart faster than it is being pumped out -Atrial baroreceptors correct the situation by stimulating CV centers to increase CO until backlog of venous blood is removed--then atrial pressure returns to normal

Antidiuretic Hormone (ADH)

-Released at posterior lobe of pituitary gland in response to decrease in blood volume, to an increase in the osmotic concentration of the plasma, or to circulating angiotensin II -Brings about peripheral vasoconstriction that elevates blood pressure -This hormone stimulates water conservation by the kidneys, thus preventing a reduction in blood volume that would further reduce blood pressure

Medium-Sized Veins

-Tunica media is thin and contains few smooth muscle cells -Thickest layer of medium-sized vein is the tunica externa, which contains longitudinal bundles of elastic and collagen fibers

Hormones important in regulating cardiovascular function (long-term regulation)

1) Antidiuretic hormone (ADH) 2) Angiotensin II 3) Erythropoietin (EPO) 4) Natriuretic peptides (ANP and BNP)

Diffusion occurs most rapidly when

1) Distances are short 2) Concentration gradient is large 3) The ions or molecules involved are small

Light Exercise

3 interrelated changes take place: -*Extensive vasodilation occurs* as skeletal muscles consume oxygen more quickly, peripheral resistance decreases, blood flow through capillaries increases, and blood enter venous system at faster rate -*Venous return increases* as skeletal muscle contractions squeeze blood along peripheral veins and faster breathing pulls blood into venae cavae by respiratory pump -*CO rises* primarily in response to rise in venous return and atrial stretching -This regulation by venous feedback produces increase in CO -Increase supports accelerated blood flow to skeletal muscles, cardiac muscle, and skin -Flow to skeletal and cardiac muscles increases as arterioles and precapillary sphincters dilate in response to local factors -Flow to skin increases response to rise in body temperature

Capillary blood pressure declines as blood flows from the arterial end to the venous end of a capillary

As a result, the rates of filtration and reabsorption gradually change as blood passes along the length of a capillary

Coronary Spasms

Can temporarily restrict coronary circulation and produce symptoms of angina

Vessel characteristics change gradually with distance from the heart

Ex. Largest muscular arteries contain a lot of elastic tissue, smallest resemble heavily muscled arterioles

Any condition that affects hydrostatic or osmotic pressures in the blood or tissues will shift the balance between hydrostatic and osmotic forces

Examples: -If hemorrhaging occurs, both blood volume and blood pressure decrease--this reduction in CHP lowers the NFP and increases amount of reabsorption which results in decrease in volume of interstitial fluid and increase in circulating plasma volume (*recall of fluids*) -If dehydration occurs, plasma volume decreases due to water loss, and concentration of plasma proteins increases--increase in BCOP accelerates reabsorption and recall of fluids that delays the onset and severity of clinical signs and symptoms -If CHP rises or BCOP declines, fluid moves out of blood and builds up in peripheral tissues--*edema*

Vasodilators

Factors that promote the dilation of precapillary sphincters

Effects on the heart result from release of

NE by sympathetic neurons innervating the SA node, AV node, and general myocardium

Systolic Pressure

Peak blood pressure measured during ventricular systole

CV performance improves significantly with training

-Trained athletes have larger hearts and greater stroke volumes than nonathletes -Greater stroke volume people have slower heart rate too--meaning can maintain normal blood flow to peripheral tissues at low heart rate but can increase CO levels to 50% higher than nonathletes -Trained athlete can tolerate sustained levels of activity

A general sympathetic activation stimulates the cardioacceleratory and vasomotor centers

As a result cardiac output and blood pressure increase

Resistance (R)

Force opposing blood flow

Vasoconstriction results from release of NE by sympathetic neurons

Increases peripheral resistance

Anastomosis

Joining of blood vessels

Regulatory mechanisms focus on controlling cardiac output and blood pressure to restore adequate blood flow after blood pressure decreases

Mechanisms: -Autoregulation -Neural mechanisms -Endocrine mechanisms

Diastolic Pressure

Minimum blood pressure at the end of ventricular diastole

Cardiovascular Regulation

Purpose is to ensure blood flow changes occur: 1) At an appropriate time 2) In the right area 3) Without drastically changing blood pressure and blood flow to vital organs

Increased cardiac output and increased peripheral resistance work together to

Raise blood pressure

When parasympathetic division is activated, the cardioinhibitory center is stimulated

Result is a decrease in cardiac output--parasympathetic activity does not directly affect vasomotor center, but vasodilation takes place as sympathetic activity declines

Decrease in cardiac output reflects primarily a reduction in heart rate due to

The release of acetylcholine at the SA node

Special Circulation

Vascular supply through organs in which blood flow is controlled by separate mechanisms--*blood flow to brain, heart, and lungs*

Aging affects the blood, heart, and blood vessels

-*Age-related changes in blood* may include decreased hematocrit; constriction or blockage of peripheral veins by stationary blood clot called *thrombus*, which can be detached and pass through heart and wedged in small artery causing *pulmonary embolism*; pooling of blood in veins of legs because valves aren't working -*Age-related changes heart* include reduction in maximum CO, changes in activities of nodal and conducting cells, reduction in elasticity of cardiac (fibrous) skeleton, progressive atherosclerosis that can restrict coronary circulation, replacement of damaged cardiac muscle cells by scar tissue -*Age-related changes in blood vessels* may be linked to artheriosclerosis: inelastic walls of arteries become less tolerant of sudden pressure increases, can lead to aneurysm, calcium salts can be deposited on weakened vascular walls, increasing risk of stroke or myocardial infarction, lipid deposits in tunica media associated with damaged endothelial lining and calcium slats can form atherosclerotic plaques, thrombi can form at atherosclerotic plaques

Capillary Bed

-*Capillary plexus* -Capillaries function not as individual units, but as part of interconnected network called capillary bed -Contains several direct connections between arterioles and venules =Wall in first part of passageway contains smooth muscle that can change its diameter--*metarteriole or precapillary arteriole* =Rest of passageway resembles typical capillary in structure--*thoroughfare channel* -More than one artery may supply blood to capillary bed--*collaterals* (multiple arteries) =Collaterals fuse before giving rise to arterioles--*arterial anastomosis*

Elastic Arteries

-*Conducting arteries* because they carry large volumes of blood away from the heart -Large vessels: pulmonary trunk, aorta, as well as their major branches -Walls are not very thick but extremely resilient (flexible, strong) because tunica media contains high density of elastic fibers and few smooth muscle cells -*Elastic rebound*: expands during ventricular systole (cushions sudden rise in pressure), and elastic fibers recoil during ventricular diastole (slows drop in pressure)--this helps make blood flow continuous =This function is important because blood pressure is reason for blood flow--greater fluctuations in pressure, greater changes in blood flow =Elasticity helps with the fluctuations so when they reach the arterioles, pressure fluctuations have disappeared--blood flow is continuous

Muscular Arteries

-*Medium-sized arteries* or *distribution arteries* -They distribute blood to the body's skeletal muscles and internal organs -Most vessels of arterial system are muscular arteries -Thick tunica media, contains more smooth muscle cells -Ex: external carotid arteries, brachial arteries, mesenteric arteries, femoral arteries -*Pressure points*: places in the body where muscular arteries can be pressed against deeper bones to reduce blood flow and control severe bleeding =Major arterial pressure points: common carotid, radial, brachial, femoral, popliteal, posterior tibial, and dorsal pedal

Arterioles

-*Resistance vessels* -Poorly defined tunica externa -In larger arterioles, tunica media consists of 1 or 2 layers of smooth muscle cells -In smaller arterioles, tunica media contains scattered smooth muscle cells that do not form complete layer -Diameters of smaller muscular arteries and arterioles change in response to local conditions, sympathetic, or endocrine stimulation--vasodilate/vasoconstrict =Changes in diameter affect amount of force required to push blood around cardiovascular system =More pressure is required to push blood through constricted vessel than through dilated one

Differences between Arteries and Veins

-*Vessel walls*: artery walls are thicker than those of veins =Tunica media of artery contains more smooth muscle and elastic fibers than does that of a vein--helps resist arterial pressure from heart -Arteries have larger tunica media, and veins have larger tunica externa -*Vessel lumen*: artery lumen looks smaller than those of veins =Artery lumen has a circular shape, veins have lumen that collapse--look flattened or grossly distorted -*Vessel lining*: endothelial lining of artery cannot contract, so when artery constricts, its endothelium becomes folded and sectioned arteries have pleated appearance--lining of vein lacks these folds -*Extra*: arteries keep they cylindrical shape, veins collapse =Arteries are more resilient: when stretched they keep their shape and lengthen, when released they snap back--veins cannot tolerate that distortion without tearing or collapsing =Veins contain valves

Blood Viscosity

-*Viscosity*: resistance to flow caused by interaction among molecules and suspended materials in a liquid -Blood is 5x more viscous than water due to plasma proteins and blood cells -Viscosity of blood remains stable

Distribution of Blood

-30-35% of blood is in the heart, arteries, capillaries in the pulmonary and system circuits -65-70% of blood is in the venous system

Hypertension

-Abnormally high blood pressure -Much more common -Systolic BP range :140-159 -Diastolic BP range: 90-99 -BP less than 120/80 is normal -Increases workload on the heart, and left ventricle gradually enlarges -More muscle mass means greater demand for oxygen -Increased arterial pressures place physical stress on walls of blood vessels -Stress promotes development of arteriosclerosis, increases risk of aneurysms, heart attacks, and strokes

Hypotension

-Abnormally low blood pressure -Tend to come from overly aggressive drug treatment for hypertension

Local Vasodilators

-Act at the tissue level to accelerate blood flow through their tissue of origin -Work by relaxing the smooth muscle cells of precapillary sphincters -Control blood flow within single capillary bed -In high concentrations, these factors also affect arterioles, increasing or decreasing blood flow to all capillary beds in given area -Ex: =Decreased tissue oxygen levels or increased CO2 levels =Lactic acid or other acids generated by tissue cells =Nitric oxide (NO) released from endothelial cells =Rising concentrations of potassium ions or hydrogen ions in the interstitial fluid =Chemicals released during local inflammation, including histamine and NO =Elevated local temperature

Local Vasoconstrictors

-Aggregating platelets and damaged tissues produce compounds that stimulate precapillary sphincters to constrict -Ex. prostaglandin and thromboxanes -Control blood flow within single capillary bed -In high concentrations, these factors also affect arterioles, increasing or decreasing blood flow to all capillary beds in given area

Importance of Capillaries

-All chemical and gaseous exchange between blood and interstitial fluid takes place across capillary walls -Cells rely on capillary diffusion to obtain nutrients and oxygen and to remove metabolic wastes--CO2 and urea

Angiotensin II

-Appears in blood when specialized kidney cells (*juxtaglomerular cells*) release renin in response to fall in renal blood pressure -Renin converts angiotensinogen to angiotensin I -Angiotensin-converting enzyme (ACE) modifies angiotensin I to angiotensin II -More effective on blood pressure than NE

Arterial Blood Pressure

-Arterial pressure is important because it maintains blood flow through capillary beds -To do this, it must always be high enough to overcome the peripheral resistance -It's not constant -It rises during ventricular systole and falls during ventricular diastole -Blood pressure is recorded as systolic pressure/diastolic pressure (ex. 120/80) -*Pulse*: rhythmic fluctuation in pressure that accompanies each heartbeat -*Pulse pressure*: difference between systolic and diastolic pressures -To report single blood pressure value, we use the *mean arterial pressure (MAP)* =Calculated by adding 1/3 of pulse pressure to diastolic pressure =*MAP = diastolic pressure + (pulse pressure/3)*

Elastic Rebound

-As systolic pressure rises, arterial walls stretch--the expansion allows arterial system to accommodate some of the blood provided by ventricular systole -As diastole begins and BP falls, arteries recoil to their original dimensions -Some blood is forced back toward left ventricle in order to close the aortic valve and help drive additional blood into the coronary arteries -Most of the push from elastic rebound forces blood toward the capillaries--maintains blood flow along the arterial network while left ventricle is in diastole

Respiratory Pump

-As you inhale, your thoracic cavity expands, reducing the pressure within the pleural cavities =This drop in pressure pulls air into your lungs =It also pulls blood into the inferior vena cava and right atrium from smaller veins -As you exhale, thoracic cavity decreases in size, internal pressure rises, forcing air out of your lungs and pushing venous blood into the right atrium

Tissue Perfusion

-Blood flow through tissues -Factors affecting it: 1) Cardiac output 2) Peripheral resistance 3) Blood pressure

Venous Valves

-Blood pressure in venous system is less than in arterial system--and falls as it continues along -BP in venules and medium-sized veins is so low that it cannot overcome force of gravity -*Valves*: folds of tunica intima that project from the vessel wall and point in the direction of blood flow =Permit blood flow in one direction only -Venous valves prevent blood from moving back toward capillaries -Any movement or contraction (ex. of leg muscles) squeezes blood towards the heart -If walls of veins near valves weaken or become stretched and distorted, valves may not work properly leading to blood pooling in the veins and vessels become grossly distended--may cause *varicose veins* or even *hemorrhoids*

Aneurysm

-Bulge in the weakened wall of an artery -When local arterial pressure exceeds the capacity of the elastic components of the tunics

Heavy Exercise

-CO increases toward maximal levels -Major changes in peripheral distribution of blood improve blood flow to active skeletal muscles -Blood races between skeletal muscles and lungs and heart--vasomotor center restricts blood flow to "nonessential" organs -Skin perfusion increases because body temperature continues to climb -Only blood supply to brain remains unaffected

Pressure and resistance determine blood flow

-Capillary blood flow is determined by the interplay between *pressure (P)* and *resistance (R)* -To keep blood moving, heart must generate enough "pressure" to overcome the "resistance" to "blood flow" in the pulmonary and systemic circuits -*Flow (F)* is directly proportional to pressure (increased pressure = increased flow), and inversely proportional to resistance (increased resistance = decreased flow) =Pressure *gradient*: difference in pressure from one end of vessel to other--largest is between base of aorta and proximal ends of peripheral capillary beds -F ∝ ΔP/R

Natriuretic Peptides

-Cardiac muscle cells in wall of right atrium of heart produce atrial natriuretic peptide (ANP) in resposne to excessive stretching during diastole -Ventricular muscle cells exposed to stimuli produce brain natriuretic peptide (BNP) -These peptide hormones reduce BV and BP =They do so by: 1) Increasing sodium ion excretion by kidneys 2) Promoting water losses by increasing volume of urine produced 3) Reducing thirst 4) Blocking release of ADH, aldosterone, E, NE 5) Stimulating peripheral vasodilation =As BV and BP decrease, stresses on walls of heart are removed and natriuretic peptide production ceases

Autoregulation

-Causes immediate, localized homeostatic adjustments--like precapillary sphincters opening and closing in response to chemical changes in interstitial fluids -If it fails to normalize conditions at tissue level, neural mechanisms and endocrine factors are activated

Veins

-Collect blood from all tissues and organs and return it to the heart -Walls can be thinner than arteries because blood pressure in veins is lower, but larger in diameter -Classify veins on size

Cardiovascular (CV) Center

-Composed of the cardioinhibitory center, cardioacceleratory center, and the vasomotor center -*Cardioacceleratory center* increases CO through sympathetic innervation -*Cardioinhibitory center* decreases CO through parasympatheticc innervation -*Vasomotor center* contains 2 types of neurons: a large group responsible for widespread vasoconstriction and smaller group responsible for vasodilation of arterioles in skeletal muscles and brain

Fenestrated Capillaries

-Contain pores that penetrate the endothelial lining -Pores allow rapid exchange of water and solutes between blood and interstitial fluid -Ex. choroid plexus , blood vessels of endocrine organs -Found along intestinal tract and at filtration sites in kidneys -Number of pores and permeability characteristics may vary at each region

Long-Term Restoration of Blood Volume

-Decrease in capillary blood pressure triggers recall of fluids from interstitial spaces -Aldosterone and ADH promote fluid retention and reabsorption at kidneys, preventing further reductions in BV -Thirst increases, digestive tract absorbs additional water--intake of fluid increases BV and replaces interstitial fluids borrowed at capillaries -EPO targets bone marrow--stimulates maturation of RBCs which increases BV and improves oxygen delivery to peripheral tissues

Net Hydrostatic Pressure

-Difference between the pressure inside the capillary wall and the hydrostatic pressure outside the capillary -Tends to push water and solutes out of capillaries and into the interstitial fluid

Vessel Diameter

-Differences in diameter have more significant effects on resistance than do differences in length -Smaller diameter = more friction -Vessel diameter can change through vasoconstriction or vasodilation

Important Processes of Capillary Exchange

-Diffusion -Filtration -Reabsorption

Arteriovenous Anastomoses

-Direct connections between arterioles and venules -When it's dilated, blood bypasses the capillary bed and flows directly into the venous circulation -Regulated primarily by sympathetic innervation under control of cardiovascular center of medulla oblongata

Blood Flow to the Heart

-Each time heart contracts, it squeezes coronary vessels, so blood flow is reduced -When workload on heart increases, local factors dialte coronary vessels and increase blood flow -E released during sympathetic stimulation promotes vasodilation of coronary vessels, and also increases heart rate and strength of cardiac contractions--result: coronary blood flow increases while vasoconstriction occurs in other tissues

From Arteries to Veins

-Elastic artery -Muscular artery -Arteriole -Continuous Capillary -Fenestrated Capillary -Venule -Medium-sized vein -Large vein

Endocrine Mechanisms

-Endocrine system releases hormones that enhance short-term adjustments and direct long-term changes in cardiovascular performance

Regular exercise has beneficial effects

-Exercise lowers cholesterol by stimulating enzymes that help move low-density lipoproteins (LDLs or bad cholesterol) from the blood to liver where it is converted to bile and excreted from body -Exercise increases size of lipoprotein particles that carry cholesterol, making it harder for small proteins to lodge in vessel walls -High cholesterol level is major risk factors for atherosclerosis--leads to CV disease and strokes -Healthy lifestyle can reduce stress, lower BP, and slow formation of plaques

Blood Flow to the Lungs

-Extensive capillary network surrounds each alveolus -Local responses to oxygen levels within individual alveoli regulate blood flow through the lungs

Vascular Resistance

-Forces that oppose blood flow in the blood vessels -Largest component -Most important factor in vascular resistance is friction between blood and vessel walls =Amount of friction depends on 2 factors: *vessel length and vessel diameter*

Arterial Anastomosis

-Fusion of 2 collateral arteries (into arterioles) that supply a capillary bed -If one artery is blocked, capillary circulation will continue

Precapillary Sphincter

-Guards entrance to each capillary -Contraction of smooth muscle cells of this sphincter narrows the capillary entrance--reducing or stopping blood flow -When one sphincter contracts, blood is diverted to other branches of network -When sphincter relaxes, entrance dilates, blood flows into the capillary

Turbulence

-High flow rates, irregular surfaces, and sudden changes in vessel diameter upset smooth flow of blood by creating eddies and swirls--turbulence -Increases resistance and slows blood flow -Occurs when blood flows between atria and ventricles -Develops in large arteries, like aorta, when cardiac output and arterial flow rates are very high -Turbulence seldom occurs in smaller vessels unless their walls are damaged

Vessel Length

-Increasing blood vessel length increases friction -Longer vessel, greater the surface area in contact with blood -Can change due to growth, gain, or loss in weight

Erythropoietin (EPO)

-Kidneys release EPO if BP falls or if oxygen content of blood becomes abnormally low -Acts directly on blood vessels, causing vasoconstriction, thereby increasing BP -EPO stimulates production and maturation of RBCs -Cells increase volume and viscosity of blood and improve its oxygen-carrying capacity

Mean arterial pressure (MAP) and pulse pressure become smaller as distance from heart increases

-MAP declines as arterial branches become smaller and more numerous -BP decreases as it overcomes friction and produces blood flow -Pulse pressure lessens due to cumulative effects of elastic rebound along arterial system--ex. loud shout followed by echoes =Pressure surge following ventricular ejection is the shout, it is reflected by wall of aorta, echoing down arterial system until it disappears at level of arterioles--when blood reaches precapillary sphincter there are no pressure fluctuations and BP is steady

Atrial Baroreceptors

-Monitor blood pressure at the end of the systemic circuit--at the venae cavae and the right atrium -*Atrial reflex*: responds to a stretching of the wall of the right atrium

Chemoreceptor Reflexes

-Monitor changes in chemical composition of arterial blood -Respond to changes in carbon dioxide, oxygen, or pH levels in blood and cerebrospinal fluid (CSF) -Chemoreceptors involved are sensory neurons -Located in carotid bodies and aortic bodies--these receptors monitor composition of arterial blood -Chemoreceptors located on ventrolateral surfaces of MO monitor the composition of CSF

Cardiovascular center detects changes in tissue demand by

-Monitoring arterial blood, especially its blood pressure, pH, and concentrations of dissolved gases -Baroreceptor and chemoreceptor reflexes -These reflexes are regulated through a negative feedback loop: the stimulation of a receptor by an abnormal condition leads to a response that counteracts the stimulus and restores normal conditions

In a crisis, sympathetic activation occurs, and its effects are enhanced by release of both

-NE and E from adrenal medullae -Net effect is an immediate increase in heart rate and stroke volume, and a corresponding increase in cardiac output

Diffusion

-Net movement of ions or molecules from an area where their concentration is higher to an area where their concentration is lower -*Concentration gradient*: difference between the high and low concentrations represents this--diffusion eliminates this

Reabsorption

-Occurs as result of *osmosis* (diffusion of water across selectively permeable membrane that separates 2 solutions of differing solute concentrations) -*Osmotic pressure (OP)*: an indication of the force of osmotic water movement--represents the pressure that must be applied to prevent osmotic movement across a membrane =Higher solute concentration of solution, greater the solution's osmotic pressure -Presence of suspended proteins that cannot cross capillary walls creates *blood colloid osmotic pressure (BCOP)*--the largest driving force for pulling fluid from the interstitial spaces back into the capillaries

Capillaries

-Radiate through CT and branch beneath the basement membrane of epithelia -Only blood vessels whose walls permit exchange between blood and surrounding interstitial fluids =Exchange can be quick because capillary walls are thin and diffusion distances are short =Blood flow through capillaries are slow--more time for diffusion or active transport of materials across capillary walls -Structure of capillaries allows for a two-way exchange of substances between blood and interstitial fluid -Structure: endothelial tube inside a thin basement membrane--no tunica media nor tunica externa--also diameter is close to that of RBC -2 types of capillaries: *continuous and fenestrated*

Total Peripheral Resistance

-Resistance of the entire cardiovascular system -Factors: *vascular resistance, blood viscosity, and turbulence*

Neural Mechanisms

-Respond to changes in arterial pressure or blood gas levels sensed at specific sites -When those changes occur, cardiovascular center of autonomic nervous system adjusts cardiac output and peripheral resistance to maintain blood pressure and ensure adequate blood flow

Baroreceptor Reflexes

-Respond to changes in blood pressure -Specialized receptors that monitor the degree of stretch in the walls of expandable organs -Adjust cardiac output and peripheral resistance to maintain normal arterial pressures -Located: carotid sinuses, expanded chambers near bases of internal carotid arteries of neck; aortic sinuses, pockets in walls of the ascending aorta adjacent to the heart; and wall of right atrium -Aortic baroreceptors monitor blood pressure within the ascending aorta--any changes trigger the *aortic reflex* which adjusts blood pressure to maintain adequate blood pressure and blood flow through the systemic circuit -Carotid sinus baroreceptors trigger reflexes that maintain adequate blood flow to the brain--extremely sensitive because blood flow to the brain must remain constant

Vasomotion

-Rhythmic changes in vessel diameter -Precapillary sphincter alternately conducts and relaxes--so flow varies -Blood flow in capillaries occur in pulses rather than as a steady and constant stream -Net effect is that blood may reach venules by one route now and different route later -Controlled locally by changes in the concentrations of chemicals and dissolved gases in the interstitial fluids =Ex. when dissolved oxygen concentrations decrease within a tissue, capillary sphincters relax, so blood flow to area increases

Vasa Vasorum

-Small blood vessels that supply the smooth muscle cells and fibroblasts of the tunica media and tunica externa -A network of small blood vessels that supply large blood vessels -Located in the Tunica externa

Venules

-Smallest venous vessels -Collect blood from capillary beds -Venules smaller than 50 um lack a tunica media

Large Veins

-Superior and inferior venae cavae and their branches -All large veins have all 3 layers -Slender tunica media is surrounded by thick tunica externa composed of mixture of elastic and collagen fibers

Continuous Capillaries

-Supply most regions of the body -Basement membrane wraps--Endothelium is a complete lining--several or one endothelial cell encircles the lumen -Located in all tissues except epithelia and cartilage -Permit the water, small solutes, and lipid-soluble materials to diffuse into interstitial fluid -Prevent loss of blood cells and plasma proteins -*Bulk transport*: movement of materials by endocytosis or exocytosis at the inner endothelial surface -In specialized continuous capillaries, endothelial cells are bound together by tight junctions--these have very restricted permeability

Plasma proteins in capillary blood create capillary colloid osmotic pressure

-The *net capillary colloid osmotic pressure* tends to pull water and solutes into a capillary from the interstitial fluid -The net colloid osmotic pressure is the difference between: 1) *Blood colloid osmotic pressure (BCOP)*: 25 mm Hg 2) *Interstitial fluid colloid osmotic pressure (ICOP)*: varies and is low because the interstitial fluid in most tissues contains negligible quantities of suspended proteins

Muscular Compression

-The contractions of skeletal muscles near a vein compress it, helping to push blood toward the heart -Valves help ensure blood flows in one direction -*Fainting*: can be caused by reduction in venous return because leads to fall in CO which reduces blood supply to the brain

Angiogenesis

-The formation of new blood vessels from pre-existing vessels and occurs under the direction of *vascular endothelial growth factor (VEGF)* -Occurs in the embryo as tissues and organs develop -May also occur in response to factors released by cells that are *hypoxic* (oxygen-starved) -Important in cardiac muscle, takes place in response to constricted or occluded vessel

Filtration

-The removal of solutes as a solution flows across a porous membrane -Solutes too large to pass through the pores are filtered out of the solution -Driving force for filtration is *hydrostatic pressure*--pushes water from area of higher pressure to area of lower pressure -*Capillary filtration*: water and small solutes are forced across a capillary wall, leaving larger solutes and suspended proteins in the bloodstream =The solute molecules that leave the bloodstream are small enough to pass between adjacent endothelial cells or through the pores in a fenestrated capillary =Filtration takes place primarily at the arterial end of a capillary, where *capillary hydrostatic pressure (CHP)* is highest

Vasomotor Tone

-The sympathetic vasoconstrictor nerves are always active, producing a significant vasomotor tone -This vasoconstrictor activity normally keeps the arterioles partially constricted -Under maximal stimulation, arterioles constrict to about half resting diameter -Can control arterial blood pressure very effectively by making modest adjustments in vessel diameters

Blood Flow to the Brain

-Total blood flow to brain remains constant, but blood flow to specific regions of brain changes from moment to moment -Changes occur in response to local changes in composition of interstitial fluid that accompany neural activity--reading, writing, etc. will increase blood flow to those regions -Brain receives arterial blood through 4 arteries -Interruption of flow in one of these large vessels doesn't reduce blood flow to brain as whole because these arteries form anastomoses -Plaque or blood clot may still block small artery, and weakened arteries may rupture--temporarily or permanently shut off blood flow to localized area of brain, damaging, or killing dependent neurons

Valsalva Maneuver

-Trying to exhale forcefully with closed lips and nostrils so no air can leave lungs and pressure in thoracic cavity rises sharply -This action causes reflexive changes in blood pressure and cardiac output due to increased intrathoracic--which impedes venous return to the right atrium

Typical Vein

-Usually flattened or collapsed, with relatively thin wall -*Tunica intima*: often smooth -*Tunica media*: thin, dominated by smooth muscle cells and collagen fibers -*Tunica externa*: collagen and elastic fibers and smooth muscle cells

Typical Artery

-Usually round, with relatively thick wall -*Tunica intima*: outer margin of tunica intima contains thick layer of elastic fibers called *internal elastic membrane* =Endothelium is usually rippled, due to vessel constriction -*Tunica meda*: thickest layer =Dominated by smooth muscle cells and elastic fibers =Separated from tunica externa by thin band of elastic fibers called *external elastic membrane* -*Tunica externa*: contains collagen fibers with scattered bands of elastic fibers

Capacitance Vessels

-Veins that expand easily -*Capacitance*: relationship between the volume of blood a vessel contains and the blood pressure -High capacitance: when vessel expands easily at low pressures -Low capacitance: when vessel expands only at high pressures

Venous Pressure and Venous Return

-Venous pressure, although low, determines venous return -Venous return directly affects cardiac output -Veins offer little resistance, so pressure declines very slowly as blood moves through venous system -As blood moves toward the heart, veins become larger, and resistance drops, and velocity of blood flow increases -2 factors assist low venous pressures in propelling blood toward your heart: *muscular compression of peripheral veins and respiratory pump*

Relationships among vessel diameter, cross-sectional area, blood pressure, and blood velocity within the systemic circuit

-Vessel diameter: diameters get smaller towards capillaries, but veins have larger diameters than arteries -Cross-sectional area: combined cross-sectional area of all the vessels -Pressures: BP declines rapidly as arteries branch, and venous pressures are low -Velocity of blood flow: velocity of blood flow decreases from aorta to capillaries, but increases from capillaries to venae cavae

Factors that contribute to Net Hydrostatic Pressure

1) *Capillary hydrostatic pressure (CHP)*: ranges from 35 mm Hg at the arterial end of a capillary to 18 mm Hg at the venous end 2) *Interstitial fluid hydrostatic pressure (IHP)*: measurements of IHP have yielded very small values that differ from tissue to tissue--from +6 mm Hg in the brain to -6 mm Hg in subcutaneous tissues =A positive IHP opposes CHP, and the tissue hydrostatic pressure must be overcome before fluid can move out of a capillary =A negative IHP assists CHP, and additional fluid will be pulled out of the capillary =Average IHP is 0 mm Hg--net hydrostatic pressure is equal to CHP

Vasomotor center controls activity of sympathetic motor neurons

1) *Control of Vasoconstriction*: neurons innervating peripheral blood vessels in most tissues are *adrenergic* meaning they release neurotransmitter NE--which stimulates smooth muscles in the walls of arterioles, producing vasoconstriction 2) *Control of Vasodilation*: vasodilator neurons innervate blood vessels in skeletal muscles and in brain -The stimulation of these neurons relaxes smooth muscle cells in walls of arterioles, producing vasodilation -Relaxation is triggered by appearance of NO -Vasomotor may control NO release indirectly or directly =Most common vasodilator synpases are *cholinergic*--axon terminals release ACh which stimulates endothelial cells in the area to release NO, causing vasodilation =Other vasodilator synapses are *nitroxidergic*--their axon terminals release NO as a neurotransmitter =NO has an immediate and direct relaxing effect on the vascular smooth muscle cells in the area

If serious hemorrhaging (blood loss) occurs, vasomotor center of MO stimulates sympathetic nerves that innervate smooth muscle cells in the walls of medium-sized veins which has 2 effects

1) *Systemic veins constrict*: process is called *venoconstriction*--decreases amount of blood within the venous system, increasing volume within the arterial system and capillaries -Venoconstriction can keep blood volume within the arteries and capillaries at near-normal levels despite a significant blood loss 2) *The constriction of veins in the liver, skin, and lungs redistributes a significant proportion of the total blood volume* -As a result, blood flow to delicate organs and to active skeletal muscles can be increased or maintained after blood loss -*Venous reserve*: amount of blood that can be shifted from veins in the liver, skin, and lungs to the general circulation--20% of total blood volume

Blood Vessels have 3 Layers

1) *Tunica intima*: inner layer of blood vessel -Includes the endothelial lining and surrounding layer of connective tissue with variable number of elastic fibers 2) *Tunica media*: middle layer of blood vessel -Contains concentric sheets of smooth muscle tissue in a framework of loose connective tissue -Collagen fibers bind the tunica media to the tunica intima and externa -Smooth muscle cells of this layer encircle the endothelium that lines the lumen of the blood vessel--contract = decreased vessel diameter, relax = diameter increases 3) *Tunica externa*: outer layer of blood vessel -Connective tissue sheath -Connective tissue fibers of this layer typically blend into those of adjacent tissues, stabilizing and anchoring the blood vessel

Different substances diffuse across capillary walls by different routes

1) *Water, ions, and small organic molecules (glucose, amino acids, urea)* can usually enter/leave bloodstream by diffusion between adjacent endothelial cells or through the pores of fenestrated capillaries 2) *Many ions, including sodium, potassium, calcium, and chloride* can diffuse across endothelial cells by passing through channels in plasma membranes 3) *Large water-soluble compounds* are unable to enter/leave the bloodstream except at fenestrated capillaries, such as those of the hypothalamus, kidneys, many endocrine organs, and the intestinal tract 4) *Lipids, such as fatty acids and steroids, and lipid-soluble materials, including soluble gases such as oxygen and carbon dioxide* can cross capillary walls by diffusion through the endothelial plasma membranes 5) *Plasma proteins* are normally unable to cross the endothelial lining anywhere except in sinusoids, such as those of the liver, where plasma proteins enter the bloodstream

When blood pressure rises, the increased output from the baroreceptors alters activity in the CV center and produces 2 major effects

1) A decrease in CO, due to parasympathetic stimulation and inhibition of sympathetic activity 2) Widespread peripheral vasodilation, due to inhibition of excitatory neurons in vasomotor center

When blood pressure falls below normal, baroreceptor output is reduced accordingly--which has 2 major effects working together to raise blood pressure

1) An increase in cardiac output, through the stimulation of sympathetic innervation to the heart--results from stimulation of the cardioacceleratory center and is accompanied by an inhibition of the cardioinhibitory center 2) Widespread peripheral vasoconstriction, caused by the stimulation of sympathetic vasoconstrictor neurons by the vasomotor center

Filtration and Reabsorption has 4 important functions

1) Ensures the plasma and interstitial fluid are in constant communication and mutual exchange 2) Accelerates distribution of nutrients, hormones, and dissolved gases throughout tissues 3) Assists in the transport of insoluble lipids and tissue proteins that cannot enter the bloodstream by crossing capillary walls 4) Has a flushing action that carries bacterial toxins and other chemical stimuli to lymphatic tissues and organs responsible for providing immunity to disease

4 Functions of Angiotensin II

1) Stimulates adrenal production of aldosterone, causing Na+ retention and K+ loss by kidneys 2) Stimulates secretion of ADH, in turn stimulating water reabsorption by kidneys and complementing effects of aldosterone 3) Stimulates thirst, resulting in increased fluid consumption 4) Stimulates CO and triggers constriction of arterioles, increasing systemic blood pressure

3 Types of Cardiovascular Pressures

1. *Blood pressure (BP)*: arterial pressure (mm Hg) 2. *Capillary Hydrostatic Pressure (CHP)*: force exerted by a fluid pressing against a wall =Capillary pressure: pressure within the capillary walls 3. *Venous pressure*: pressure within the venous system =Quite low -*Circulatory pressure*: difference in pressure across the entire systemic circuit averages about 100 mm Hg -For circulation to occur, circulatory pressure has to overcome *total peripheral resistance* (resistance of entire cardiovascular system)

Widespread vasodilation lowers peripheral resistance, and combined with a reduction in cardiac output, leads to

A fall in blood pressure to normal levels

Short-Term Elevation of Blood Pressure

As soon as pressures start to decrease, short-term responses appear to elevate BP and improve peripheral blood flow: -Initial neural response, carotid and aortic reflexes increase CO and cause peripheral vasoconstriction -Combo of stress and anxiety stimulates sympathetic nervous system headquarters in hypothalamus which triggers increase in vasomotor tone, constricting arterioles and raising BP--at same time venoconstriction mobilizes venous reserve and quickly improves venous return -Short-term hormonal effects occur--sympathetic activation causes adrenal medullae to secrete E and NE--which increase CO and extend peripheral vasoconstriction, in addition release of ADH and production of angiotensin II enhance vasoconstriction as part of long-term response