Blood Vessels

Blood Vessels

•   Delivery system of dynamic structures that begins and ends at the heart

•    Arteries: carry blood away from the heart; oxygenated except for pulmonary circulation and umbilical vessels of a fetus

•    Capillaries: contact tissue cells and directly serve cellular needs

•    Veins: carry blood toward the heart

 Structure of Blood Vessel Walls

•   Arteries and veins

•    Tunica intima, tunica media, and tunica externa

•   Lumen

•    Central blood-containing space

•   Capillaries

•    Endothelium with sparse basal lamina

Tunics

•   Tunica intima

•    Endothelium lines the lumen of all vessels

•    In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present

Tunics

•   Tunica media

•    Smooth muscle and sheets of elastin

•    Sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vessels

Tunics

•   Tunica externa (tunica adventitia)

•    Collagen fibers protect and reinforce

•    Larger vessels contain vasa vasorum to nourish the external layer

Elastic (Conducting) Arteries

•   Large thick-walled arteries with elastin in all three tunics

•   Aorta and its major branches

•   Large lumen offers low-resistance

•   Act as pressure reservoirs—expand and recoil as blood is ejected from the heart

Muscular (Distributing) Arteries and Arterioles

•   Distal to elastic arteries; deliver blood to body organs

•   Have thick tunica media with more smooth muscle

•   Active in vasoconstriction

Arterioles

•   Smallest arteries

•   Lead to capillary beds

•   Control flow into capillary beds via vasodilation and vasoconstriction

Capillaries

•   Microscopic blood vessels

•   Walls of thin tunica intima, one cell thick

•   Pericytes help stabilize their walls and control permeability

•   Size allows only a single RBC to pass at a time

Capillaries

•   In all tissues except for cartilage, epithelia, cornea and lens of eye

•   Functions: exchange of gases, nutrients, wastes, hormones, etc.

Capillaries

•            Three structural types

•           Continuous capillaries

•           Fenestrated capillaries

•           Sinusoidal capillaries (sinusoids)

Continuous Capillaries

•   Abundant in the skin and muscles

•    Tight junctions connect endothelial cells

•    Intercellular clefts allow the passage of fluids and small solutes

•   Continuous capillaries of the brain

•    Tight junctions are complete, forming the blood-brain barrier

Fenestrated Capillaries

•   Some endothelial cells contain pores (fenestrations)

•   More permeable than continuous capillaries

•   Function in absorption or filtrate formation (small intestines, endocrine glands, and kidneys)

Sinusoidal Capillaries

•   Fewer tight junctions, larger intercellular clefts, large lumens

•   Usually fenestrated

•   Allow large molecules and blood cells to pass between the blood and surrounding tissues

•   Found in the liver, bone marrow, spleen

Capillary Beds

•          Interwoven networks of capillaries form the microcirculation between arterioles and venules

•          Consist of two types of vessels

•         Vascular shunt (metarteriole—thoroughfare channel):

•         Directly connects the terminal arteriole and a postcapillary venule

Capillary Beds

•           True capillaries

•           10 to 100 exchange vessels per capillary bed

•           Branch off the metarteriole or terminal arteriole

Blood Flow Through Capillary Beds

•   Precapillary sphincters regulate blood flow into true capillaries

•   Regulated by local chemical conditions and vasomotor nerves

 Venules

•   Formed when capillary beds unite

•   Very porous; allow fluids and WBCs into tissues

•   Postcapillary venules consist of endothelium and a few pericytes

•   Larger venules have one or two layers of smooth muscle cells

Veins

•   Formed when venules converge

•   Have thinner walls, larger lumens compared with corresponding arteries

•   Blood pressure is lower than in arteries

•   Thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks

•   Called capacitance vessels (blood reservoirs); contain up to 65% of the blood supply

Veins

•            Adaptations that ensure return of blood to the heart

•           Large-diameter lumens offer little resistance

•           Valves prevent backflow of blood

•           Most abundant in veins of the limbs

•            Venous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

Vascular Anastomoses

•   Interconnections of blood vessels

•   Arterial anastomoses provide alternate pathways (collateral channels) to a given body region

•    Common at joints, in abdominal organs, brain, and heart

•   Vascular shunts of capillaries are examples of arteriovenous anastomoses

•   Venous anastomoses are common

Physiology of Circulation: Definition of Terms

•   Blood flow

•     Volume of blood flowing through a vessel, an organ, or the entire circulation in a given period

•   Measured as ml/min

•   Equivalent to cardiac output (CO) for entire vascular system

•   Relatively constant when at rest

•   Varies widely through individual organs, based on needs

Physiology of Circulation: Definition of Terms

•   Blood pressure (BP)

•    Force per unit area exerted on the wall of a blood vessel by the blood

•   Expressed in mm Hg

•   Measured as systemic arterial BP in large arteries near the heart

•    The pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas

Physiology of Circulation: Definition of Terms

•   Resistance (peripheral resistance)

•     Opposition to flow

•     Measure of the amount of friction blood encounters

•     Generally encountered in the peripheral systemic circulation

•   Three important sources of resistance

•     Blood viscosity

•     Total blood vessel length

•     Blood vessel diameter

Resistance

•   Factors that remain relatively constant:

•    Blood viscosity

•   The “stickiness” of the blood due to formed elements and plasma proteins

•    Blood vessel length

•   The longer the vessel, the greater the resistance encountered

Resistance

•   Frequent changes alter peripheral resistance

•   Varies inversely with the fourth power of vessel radius

•    E.g., if the radius is doubled, the resistance is 1/16 as much

Resistance

•   Small-diameter arterioles are the major determinants of peripheral resistance

•   Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance

•    Disrupt laminar flow and cause turbulence

Relationship Between Blood Flow, Blood Pressure, and Resistance

•   Blood flow (F) is directly proportional to the blood (hydrostatic) pressure gradient (DP)

•    If DP increases, blood flow speeds up

•   Blood flow is inversely proportional to peripheral resistance (R)

•    If R increases, blood flow decreases: F = DP/R

•   R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter

Systemic Blood Pressure

•   The pumping action of the heart generates blood flow

•   Pressure results when flow is opposed by resistance

•   Systemic pressure

•     Is highest in the aorta

•     Declines throughout the pathway

•     Is 0 mm Hg in the right atrium

•   The steepest drop occurs in arterioles

Arterial Blood Pressure

•   Reflects two factors of the arteries close to the heart

•    Elasticity (compliance or distensibility)

•    Volume of blood forced into them at any time

•   Blood pressure near the heart is pulsatile

Arterial Blood Pressure

•   Systolic pressure: pressure exerted during ventricular contraction

•   Diastolic pressure: lowest level of arterial pressure

•   Pulse pressure = difference between systolic and diastolic pressure

Arterial Blood Pressure

•   Mean arterial pressure (MAP): pressure that propels the blood to the tissues

MAP = diastolic pressure + 1/3 pulse pressure

•   Pulse pressure and MAP both decline with increasing distance from the heart

Capillary Blood Pressure

•   Ranges from 15 to 35 mm Hg

•   Low capillary pressure is desirable

•    High BP would rupture fragile, thin-walled capillaries

•    Most are very permeable, so low pressure forces filtrate into interstitial spaces

Venous Blood Pressure

•   Changes little during the cardiac cycle

•   Small pressure gradient, about 15 mm Hg

•   Low pressure due to cumulative effects of peripheral resistance

Factors Aiding Venous Return

•           Respiratory “pump”: pressure changes created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand

•           Muscular “pump”: contraction of skeletal muscles “milk” blood toward the heart and valves prevent backflow

•           Vasoconstriction of veins under sympathetic control         

Maintaining Blood Pressure

•   Requires

•    Cooperation of the heart, blood vessels, and kidneys

•    Supervision by the brain

Maintaining Blood Pressure

•   The main factors influencing blood pressure:

•    Cardiac output (CO)

•    Peripheral resistance (PR)

•    Blood volume

Maintaining Blood Pressure

•   F = DP/PR and CO = DP/PR

•   Blood pressure = CO x PR (and CO depends on blood volume)

•   Blood pressure varies directly with CO, PR, and blood volume

•   Changes in one variable are quickly compensated for by changes in the other variables

Cardiac Output (CO)

•   Determined by venous return and neural and hormonal controls

•   Resting heart rate is maintained by the cardioinhibitory center via the parasympathetic vagus nerves

•   Stroke volume is controlled by venous return (EDV)

Cardiac Output (CO)

•   During stress, the cardioacceleratory center increases heart rate and stroke volume via sympathetic stimulation

•    ESV decreases and MAP increases

Control of Blood Pressure

•   Short-term neural and hormonal controls

•    Counteract fluctuations in blood pressure by altering peripheral resistance

•   Long-term renal regulation

•    Counteracts fluctuations in blood pressure by altering blood volume

Short-Term Mechanisms: Neural Controls

•   Neural controls of peripheral resistance

•    Maintain MAP by altering blood vessel diameter

•    Alter blood distribution in response to specific demands

Short-Term Mechanisms: Neural Controls

•   Neural controls operate via reflex arcs that involve

•    Baroreceptors

•    Vasomotor centers and vasomotor fibers

•    Vascular smooth muscle

The Vasomotor Center

•   A cluster of sympathetic neurons in the medulla that oversee changes in blood vessel diameter

•   Part of the cardiovascular center, along with the cardiac centers

•   Maintains vasomotor tone (moderate constriction of arterioles)

•   Receives inputs from baroreceptors, chemoreceptors, and higher brain centers

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

•   Baroreceptors are located in

•    Carotid sinuses

•    Aortic arch

•    Walls of large arteries of the neck and thorax

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

•   Increased blood pressure stimulates baroreceptors to increase input to the vasomotor center

•    Inhibits the vasomotor center, causing arteriole dilation and venodilation

•    Stimulates the cardioinhibitory center

Short-Term Mechanisms:
Baroreceptor-Initiated Reflexes

•   Baroreceptors taking part in the carotid sinus reflex protect the blood supply to the brain

•   Baroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuit

Baroreceptor-Initiated Reflexes

Short-Term Mechanisms:
Chemoreceptor-Initiated Reflexes

•   Chemoreceptors are located in the

•    Carotid sinus

•    Aortic arch

•    Large arteries of the neck

Short-Term Mechanisms:
Chemoreceptor-Initiated Reflexes

•   Chemoreceptors respond to rise in CO₂, drop in pH or O₂

•    Increase blood pressure via the vasomotor center and the cardioacceleratory center

•   Are more important in the regulation of respiratory rate (Chapter 22)

Influence of Higher Brain Centers

•   Reflexes that regulate BP are integrated in the medulla

•   Higher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centers

Short-Term Mechanisms: Hormonal Controls

•   Adrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output

•   Angiotensin II, generated by kidney release of renin, causes vasoconstriction

Short-Term Mechanisms: Hormonal Controls

•   Atrial natriuretic peptide causes blood volume and blood pressure to decline, causes generalized vasodilation

•   Antidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BP

Long-Term Mechanisms: Renal Regulation

•            Baroreceptors quickly adapt to chronic high or low BP

•            Long-term mechanisms step in to control BP by altering blood volume

•            Kidneys act directly and indirectly to regulate arterial blood pressure

•           Direct renal mechanism

•           Indirect renal (renin-angiotensin) mechanism

Direct Renal Mechanism

•   Alters blood volume independently of hormones

•    Increased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BP

•    Decreased BP or blood volume causes the kidneys to conserve water, and BP rises

Indirect Mechanism

•   The renin-angiotensin mechanism

•    ¯ Arterial blood pressure ® release of renin

•    Renin® production of angiotensin II

•    Angiotensin II is a potent vasoconstrictor

•    Angiotensin II ® aldosterone secretion

•   Aldosterone ® renal reabsorption of Na⁺ and ¯ urine formation

•    Angiotensin II stimulates ADH release

Monitoring Circulatory Efficiency

•   Vital signs: pulse and blood pressure, along with respiratory rate and body temperature

•   Pulse: pressure wave caused by the expansion and recoil of arteries

•   Radial pulse (taken at the wrist) routinely used

Measuring Blood Pressure

•   Systemic arterial BP

•    Measured indirectly by the auscultatory method using a sphygmomanometer

•    Pressure is increased in the cuff until it exceeds systolic pressure in the brachial artery

Measuring Blood Pressure

•   Pressure is released slowly and the examiner listens for sounds of Korotkoff with a stethoscope

•   Sounds first occur as blood starts to spurt through the artery (systolic pressure, normally 110–140 mm Hg)

•   Sounds disappear when the artery is no longer constricted and blood is flowing freely (diastolic pressure, normally 70–80 mm Hg)

Variations in Blood Pressure

•   Blood pressure cycles over a 24-hour period

•   BP peaks in the morning due to levels of hormones

•   Age, sex, weight, race, mood, and posture may vary BP

Alterations in Blood Pressure 

•   Hypotension: low blood pressure

•    Systolic pressure below 100 mm Hg

•    Often associated with long life and lack of cardiovascular illness

Homeostatic Imbalance: Hypotension

•   Orthostatic hypotension: temporary low BP and dizziness when suddenly rising from a sitting or reclining position

•   Chronic hypotension: hint of poor nutrition and warning sign for Addison’s disease or hypothyroidism

•   Acute hypotension: important sign of circulatory shock

Alterations in Blood Pressure 

•   Hypertension: high blood pressure

•    Sustained elevated arterial pressure of 140/90 or higher

•   May be transient adaptations during fever, physical exertion, and emotional upset

•   Often persistent in obese people

Homeostatic Imbalance: Hypertension

•   Prolonged hypertension is a major cause of heart failure, vascular disease, renal failure, and stroke

•   Primary or essential hypertension

•    90% of hypertensive conditions

•    Due to several risk factors including heredity, diet, obesity, age, stress, diabetes mellitus, and smoking

Homeostatic Imbalance: Hypertension

•   Secondary hypertension is less common

•    Due to identifiable disorders, including kidney disease, arteriosclerosis, and endocrine disorders such as hyperthyroidism and Cushing’s syndrome

Blood Flow Through Body Tissues

•   Blood flow (tissue perfusion) is involved in

•    Delivery of O₂ and nutrients to, and removal of wastes from, tissue cells

•    Gas exchange (lungs)

•    Absorption of nutrients (digestive tract)

•    Urine formation (kidneys)

•   Rate of flow is precisely the right amount to provide for proper function

Velocity of Blood Flow

•   Changes as it travels through the systemic circulation

•   Is inversely related to the total cross-sectional area

•   Is fastest in the aorta, slowest in the capillaries, increases again in veins

•   Slow capillary flow allows adequate time for exchange between blood and tissues

Autoregulation

•   Automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time

•   Is controlled intrinsically by modifying the diameter of local arterioles feeding the capillaries

•   Is independent of MAP, which is controlled as needed to maintain constant pressure

Autoregulation

•            Two types of autoregulation

•           Metabolic

•           Myogenic

Metabolic Controls

•   Vasodilation of arterioles and relaxation of precapillary sphincters occur in response to

•    Declining tissue O₂

•    Substances from metabolically active tissues (H⁺, K⁺, adenosine, and prostaglandins) and inflammatory chemicals

Metabolic Controls

•   Effects

•    Relaxation of vascular smooth muscle

•    Release of NO from vascular endothelial cells

•   NO is the major factor causing vasodilation

•   Vasoconstriction is due to sympathetic stimulation and endothelins

Myogenic Controls

•   Myogenic responses of vascular smooth muscle keep tissue perfusion constant despite most fluctuations in systemic pressure

•   Passive stretch (increased intravascular pressure) promotes increased tone and vasoconstriction

•   Reduced stretch promotes vasodilation and increases blood flow to the tissue

Long-Term Autoregulation

•   Angiogenesis

•    Occurs when short-term autoregulation cannot meet tissue nutrient requirements

•    The number of vessels to a region increases and existing vessels enlarge

•    Common in the heart when a coronary vessel is occluded, or throughout the body in people in high-altitude areas

Blood Flow: Skeletal Muscles

•   At rest, myogenic and general neural mechanisms predominate

•   During muscle activity

•     Blood flow increases in direct proportion to the metabolic activity (active or exercise hyperemia)

•     Local controls override sympathetic vasoconstriction

•   Muscle blood flow can increase 10´ or more during physical activity

Blood Flow: Brain

•   Blood flow to the brain is constant, as neurons are intolerant of ischemia

•   Metabolic controls

•     Declines in pH, and increased carbon dioxide cause marked vasodilation

•   Myogenic controls

•     Decreases in MAP cause cerebral vessels to dilate

•     Increases in MAP cause cerebral vessels to constrict

Blood Flow: Brain

•   The brain is vulnerable under extreme systemic pressure changes

•    MAP below 60 mm Hg can cause syncope (fainting)

•    MAP above 160 can result in cerebral edema

Blood Flow: Skin

•   Blood flow through the skin

•    Supplies nutrients to cells (autoregulation in response to O₂ need)

•    Helps maintain body temperature (neurally controlled)

•    Provides a blood reservoir (neurally controlled)

Blood Flow: Skin

•   Blood flow to venous plexuses below the skin surface

•    Varies from 50 ml/min to 2500 ml/min, depending on body temperature

•    Is controlled by sympathetic nervous system reflexes initiated by temperature receptors and the central nervous system

Temperature Regulation

•   As temperature rises (e.g., heat exposure, fever, vigorous exercise)

•    Hypothalamic signals reduce vasomotor stimulation of the skin vessels

•    Heat radiates from the skin

Temperature Regulation

•   Sweat also causes vasodilation via bradykinin in perspiration

•    Bradykinin stimulates the release of NO

•   As temperature decreases, blood is shunted to deeper, more vital organs

Blood Flow: Lungs

•   Pulmonary circuit is unusual in that

•    The pathway is short

•    Arteries/arterioles are more like veins/venules (thin walled, with large lumens)

•    Arterial resistance and pressure are low (24/8 mm Hg)

Blood Flow: Lungs

•   Autoregulatory mechanism is opposite of that in most tissues

•    Low O₂ levels cause vasoconstriction; high levels promote vasodilation

•    Allows for proper O₂ loading in the lungs

Blood Flow: Heart

•   During ventricular systole

•    Coronary vessels are compressed

•    Myocardial blood flow ceases

•    Stored myoglobin supplies sufficient oxygen

•   At rest, control is probably myogenic

Blood Flow: Heart

•   During strenuous exercise

•    Coronary vessels dilate in response to local accumulation of vasodilators

•    Blood flow may increase three to four times

Blood Flow Through Capillaries

•   Vasomotion

•    Slow and intermittent flow

•    Reflects the on/off opening and closing of precapillary sphincters

Capillary Exchange of Respiratory Gases and Nutrients

•   Diffusion of

•     O₂ and nutrients from the blood to tissues

•     CO₂ and metabolic wastes from tissues to the blood

•   Lipid-soluble molecules diffuse directly through endothelial membranes

•   Water-soluble solutes pass through clefts and fenestrations

•   Larger molecules, such as proteins, are actively transported in pinocytotic vesicles or caveolae

Fluid Movements: Bulk Flow

•   Extremely important in determining relative fluid volumes in the blood and interstitial space

•   Direction and amount of fluid flow depends on two opposing forces: hydrostatic and colloid osmotic pressures

Hydrostatic Pressures

•   Capillary hydrostatic pressure (HP_c) (capillary blood pressure)

•    Tends to force fluids through the capillary walls

•    Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg)

•   Interstitial fluid hydrostatic pressure (HP_if)

•    Usually assumed to be zero because of lymphatic vessels

Colloid Osmotic Pressures

•   Capillary colloid osmotic pressure (oncotic pressure) (OP_c)  

•    Created by nondiffusible plasma proteins, which draw water toward themselves

•    ~26 mm Hg

•   Interstitial fluid osmotic pressure (OP_if)

•    Low (~1 mm Hg) due to low protein content

Net Filtration Pressure (NFP)

•   NFP—comprises all the forces acting on a capillary bed

•   NFP = (HP_c—HP_if)—(OP_c—OP_if)

•   At the arterial end of a bed, hydrostatic forces dominate

•   At the venous end, osmotic forces dominate

•   Excess fluid is returned to the blood via the lymphatic system

Fluid exchange

Circulatory Shock

•   Any condition in which

•    Blood vessels are inadequately filled

•    Blood cannot circulate normally

•   Results in inadequate blood flow to meet tissue needs

Circulatory Shock

•   Hypovolemic shock: results from large-scale blood loss

•   Vascular shock: results from extreme vasodilation and decreased peripheral resistance

•   Cardiogenic shock results when an inefficient heart cannot sustain adequate circulation

Circulatory Pathways

•   Two main circulations

•    Pulmonary circulation: short loop that runs from the heart to the lungs and back to the heart

•    Systemic circulation: long loop to all parts of the body and back to the heart

Differences Between Arteries and Veins

Developmental Aspects

•   Endothelial lining arises from mesodermal cells in blood islands

•   Blood islands form rudimentary vascular tubes, guided by cues such as vascular endothelial growth factor

•   The heart pumps blood by the 4th week of development

Developmental Aspects

•   Fetal shunts (foramen ovale and ductus arteriosus) bypass nonfunctional lungs

•   Ductus venosus bypasses the liver

•   Umbilical vein and arteries circulate blood to and from the placenta

Developmental Aspects

•   Vessel formation occurs

•    To support body growth

•    For wound healing

•    To rebuild vessels lost during menstrual cycles

•   With aging, varicose veins, atherosclerosis, and increased blood pressure may aris