Blood Vessels Anatomy

Blood Vessels

• The delivery system of dynamic structures begins and ends at the heart.

Arteries

• Arteries carry blood away from the heart
• Blood in arteries is oxygenated except for pulmonary circulation and umbilical vessels of a fetus

Capillaries

• Serve cellular needs directly because they contact tissue cells
• Exchange substances between blood and tissues

Veins

• Veins carry blood from capillaries toward the heart

Structure of Blood Vessel Walls

• The lumen is the central blood-containing space.
• Arterial and venous walls consist of three layers (tunics): tunica intima, tunica media, and tunica externa.

Tunica Intima

• This is the innermost layer of the vessel wall.
• It consists of endothelium (simple squamous epithelium) which lines the lumen of all vessels.
• It is continuous with the endocardium, and its slick surface reduces friction.
• A subendothelial layer of connective tissue basement membrane is present in vessels larger than 1 mm.

Tunica Media

• This is the middle layer of the vessel.
• It is made of smooth muscle and sheets of elastin.
• Sympathetic nerve fibers control vasoconstriction and vasodilation, influencing blood flow and blood pressure.

Tunica Externa (Tunica Adventitia)

• This is the outermost layer of the vessel wall.
• It consists of collagen fibers that protect, reinforce, and anchor the vessel to surrounding structures.
• It contains nerve fibers, lymphatic vessels, and the vasa vasorum in larger vessels to nourish the external layer.

Arterial System: Elastic (Conducting) Arteries

• These are large, thick-walled arteries with elastin in all three tunics.
• Examples are the aorta and its major branches
• The large lumen offers low resistance.
• They act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart.

Arterial System: Muscular (Distributing) Arteries

• Distal to elastic arteries and deliver blood to body organs.
• Features include a thick tunica media with more smooth muscle and active vasoconstriction.
• There is elastic tissue in two layers: the internal and external elastic lamina.
• Examples include the brachial and coronary arteries

Arterioles

• These are the smallest arteries leading to capillary beds.
• They control flow into capillary beds through vasodilation and vasoconstriction

Capillaries

• These are microscopic blood vessels with thin tunica intima walls.
• In the smallest ones, one cell forms the entire circumference.
• A diameter allows only a single RBC to pass at a time.
• The walls are stabilized by pericytes and control permeability
• All tissues contain them except cartilage, epithelia, the cornea, and the lens of the eye.
• They provide direct access to almost every cell.
• They facilitate the exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.

Continuous Capillaries

• They are the least permeable and most common
• Abundant in the skin and muscles, tight junctions connect endothelial cells with incomplete seals
• Intercellular clefts allow the passage of fluids and small solutes, but not large particles
• Unique continuous capillaries in the brain have tight junctions that form the blood-brain barrier.

Fenestrated Capillaries

• Some endothelial cells contain pores (fenestrations)
• More permeable than continuous capillaries
• Involved in absorption or filtrate formation in small intestines, endocrine glands, and kidneys

Sinusoid Capillaries (Sinusoids)

• These capillaries have fewer tight junctions, are usually fenestrated, have larger intercellular clefts, and large lumens.
• They feature an incomplete or absent basement membrane, allowing modification.
• Openings allow the passage of large molecules and blood cells between blood and surrounding tissues
• They are found only in the liver, bone marrow, spleen, and adrenal medulla.
• Macrophages may be present in the lining to destroy bacteria

Capillary Beds

• These are interwoven networks of capillaries between arterioles and venules, representing microcirculation.
• Blood flows from an arteriole to a venule
• A terminal arteriole leads into a metarteriole, continuous with a thoroughfare channel
• A thoroughfare channel leads to a postcapillary venule draining the bed
• A vascular shunt such as a metarteriole - thoroughfare channel directly connects the terminal arteriole and postcapillary venule.
• True capillaries consist of 10 to 100 exchange vessels per capillary bed, branching off the metarteriole or terminal arteriole.

Precapillary Sphincters

• They regulate blood flow into true capillaries
• Blood may go into true capillaries or to the shunt.
• The flow is regulated by local chemical conditions and vasomotor nerves

Venous System: Venules

• Venules form when capillary beds unite.
• The smallest postcapillary venules are very porous, allowing fluids and WBCs into tissues.
• These consist of endothelium and a few pericytes.
• Larger venules have one or two layers of smooth muscle cells and merge to form veins.

Veins

• Veins form when venules converge.
• Their pressures are lower than arteries and lumen diameter are larger than those of corresponding arteries
• Small and medium-sized veins often are companion vessels with muscular arteries.
• Largest veins travel with elastic arteries
• Veins have a thin tunica media and thick tunica externa of collagen fibers and elastic networks.
• They are called capacitance vessels (blood reservoirs), containing up to 65% of the blood supply.

Veins: Adaptations for Returning Blood to the Heart

• Large-diameter lumens offer little resistance.
• Venous valves prevent blood backflow and are most abundant in veins of limbs
• Venous sinuses are specialized, flattened veins with extremely thin walls, including the coronary sinus of the heart and dural sinuses of the brain.

Vascular Anastomoses

• These are interconnections of blood vessels.
• Arterial anastomoses provide alternate pathways (collateral channels).
• They are common at joints and in abdominal organs, the brain, and the heart
• They do not exist in the retina, kidneys, or spleen.
• Vascular shunts of capillaries are arteriovenous anastomoses and venous anastomoses are abundant, with occluded veins rarely blocking blood flow.

Physiology of Circulation: Blood Flow

• Volume of blood flowing through vessel, organ, or entire circulation in given period.
• Measured in ml/min
• Equivalent to cardiac output (CO) for the entire vascular system
• Varies widely through individual organs based on needs.

Physiology of Circulation: Blood Pressure (BP)

• The 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 to keep blood moving from higher to lower pressure areas.

Resistance

• Friction blood encounters and includes opposition to flow.
• Due to the contact between blood and the vessel wall.

Peripheral Resistance

• The amount of friction blood encounters with vessel walls, especially during peripheral (systemic) circulation.

Three Important Sources of Resistance

• Blood viscosity
• Total blood vessel length
• Blood vessel diameter (lumen size)

Blood Viscosity

• Resistance of fluid to its flow
• Increased blood viscosity is caused by "stickiness" of blood

Blood Vessel Length

• Longer vessels have a greater resistance encountered
• Weight gain is associated with angiogenesis (increased resistance) and weight loss with vessel regression (decreased resistance)

Blood Vessel Diameter

• The vessel diamater of a blood cell has the greatest influence on resistance
• Laminar flow refers to different flow rates within the vessel.

Resistance and Blood Flow

• Varies inversely with the fourth power of vessel radius (F α r4 )
• Vasoconstriction increases resistance
• Vasodilation decreases resistance.

Relationship Between Blood Flow, Blood Pressure, and Resistance

• Blood flow (F)is directly proportional to the blood pressure gradient (∆P) divided by resistance (R): F = ∆P/R.
• If ∆P increases, blood flow speeds up (systemic blood pressure gradient = ∆P).

Blood Flow and Peripheral Resistance

• Blood flow is inversely proportional to peripheral resistance (R): F = ∆P/R.
• If R increases, blood flow decreases
• Changes in blood vessel diameter are more important in influencing local blood flow.
• Resistance can be elevated by: increasing blood viscosity, increasing vessel length, or decreasing vessel lumen diameter.

Systemic Blood Pressure

• Highest in the aorta and declines throughout the pathway
• 0 mm Hg in the right atrium

Arterial Blood Pressure Factors

• Reflects two factors: elasticity (compliance or distensibility) and the volume of blood forced into them at any time
• Pressure near the heart is pulsatile.

Systolic Pressure

• The pressure exerted in the aorta during ventricular contraction.
• Averaging 120 mm Hg in a normal adult

Diastolic Pressure

• The lowest level of aortic pressure

Pulse Pressure

• The difference between systolic and diastolic pressure, and is the throbbing of arteries (pulse).

Mean Arterial Pressure (MAP)

• The Mean arterial pressure is the pressure that propels blood to tissues.
• It is calculated by adding diastolic pressure and 1/3 pulse pressure
• Pulse pressure and MAP both decline with increasing distance from the heart.
• Example: BP = 120/80; MAP = 93 mm Hg

Capillary Blood Pressure

• Capillary blood pressure ranges from 17 to 35 mm Hg
• Low capillary pressure is desirable
• High BP would rupture fragile, thin-walled capillaries
• Most are permeable so the low pressure supports filtrate of fluid into interstitial spaces

Venous Blood Pressure

• Venous pressure changes little during the cardiac cycle
• Venous pressure has a small pressure gradient,about 15 mm Hg
• The low pressure gradient is due to the cumulative effects of peripheral resistance
• The energy of blood pressure is lost as heat during each circuit.
• Venous return depends on the pressure gradient, skeletal muscle pump, and respiratory pump.

Muscular Pump

• The contraction of skeletal muscles "milks" blood toward the heart and valves prevent backflow.

Respiratory Pump

• Pressure changes during breathing and moves blood toward the heart by squeezing abdominal veins as thoracic veins expand

Venoconstriction

• Venoconstriction under sympathetic control pushes blood toward the heart

Maintaining Blood Pressure

• Requires cooperation of the heart, blood vessels, and kidneys, supervised by the brain
• Main factors influencing blood pressure include cardiac output (CO), peripheral resistance (PR), and blood volume

Blood Pressure

• Is cardiac output times ( = ) peripheral resitance
• Changes in one variable are quickly compensated for by changes in other variables

Cardiac Output (CO)

• The product of Stroke Volume (SV) and Heart Rate (HR)
• A normal cardiac output rate is 5.0-5.5 L/min
• Determined by venous return, and neural and hormonal controls
• Resting heart rate is maintained by a cardioinhibitory center through parasympathetic vagus nerves
• Stroke volume is controlled by venous return (EDV).

The influence of Stress on Cardiac Output

• 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 blood pressure fluctuations by altering peripheral resistance and CO.
• Long-term renal regulation counteracts blood pressure fluctuations by altering blood volume.

Short-term Mechanisms: Neural Controls

• Neural controls of peripheral resistance maintain MAP by altering blood vessel diameter.
• Alter blood distribution to organs in response to specific demands.
• Neural controls operate via reflex arcs involving baroreceptors, the cardiovascular center of the medulla, vasomotor fibers to the heart and vascular smooth muscle, and input from chemoreceptors and higher brain centers.

The Cardiovascular Center

• Medulla neurons oversee changes in cardiac output and pressure
• Two aunomotic nuclei lie in or comprise the cardiac centers and vasomotor center
• The Vasomotor center sends impulses via sympathetic efferents to blood vessels, moderate constriction called vasomotor tone
• The Cardiac center influences blood pressure to have less effect on diameter

Baroreceptor Reflexes

• Are nerve endings that respond to stretch of the vessel wall
• Located in Wallsof large arteries of neck and thorax

If blood pressure increases

• Increased blood pressure stimulates baroreceptors to increase input to vasomotor center.
• Inhibits vasomotor and cardioacceleratory centers, causing arteriole dilation and venodilation
• Stimulates cardioinhibitory center, decreasing blood pressure

A decrease in blood pressure due to

• Arteriolar vasodilation
• Venodilation
• Decreased cardiac output

Chemoreceptor Reflexes

• Increases in blood or drop in pH or O2 cause increased blood pressure
• Receptors are in aortic arch and large arteries of neck

Short-term Mechanisms of Cardia Output

• Signaling of cardioacceleratory center causes an increase in cardiac output
• Signaling of vasomotor center causes an increase in vasoconstriction

Influence of Higher Brain Centers

• Hypothalamus and cerebral cortex modify arterial pressure via relays to medulla
• Hypothalamus increases blood pressure during stress
• Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature

Short-term Mechanisms: Hormonal Controls

• Short term regulation via changes in peripheral resistance
• Long term regulation via changes in blood volume

Cause increased blood pressure

• Release of epinephrine and norepinephrin from adrenal gland increases cardiac output and vasoconstriction
• Angiotensin II stimulates vasoconstriction
• High ADH levels cause vasoconstriction

Causes Lowered Blood Pressure

• Atrial natriuretic peptide causes decreased blood volume by antagonizing aldosterone

Long-term Mechanisms: Renal Regulation

• Baroreceptors quickly adapt to chronic high or low BP and become ineffective.
• Long-term mechanisms control BP by altering blood volume via the kidneys, which regulate arterial blood-pressure.

Direct Renal Mechanism

• Alters blood volume independently of hormones
• Increased BP or blood volume causes elimination of more urine, reducing BP
• Decreased BP or blood volume causes kidneys to conserve water, and BP rises.

Indirect Mechanism

• Renin-angiotensin-aldosterone mechanism
• Decreased Arterial blood pressure increases release of renin
• Renin catalyzes conversion of angiotensinogen from liver to angiotensin I.
• Angiotensin converting enzyme (ACE), converts angiotensin I to angiotensin II.
• Angiotensin II raises blood pressure: this is controlled by kidney

Monitoring Circulatory Efficiency

• Vital signs, including pulse and blood pressure, along with respiratory rate and body temperature
• Pulse is the pressure wave caused by the expansion and recoil of arteries
• Radial pulse, which is routinely taken on the wrist
• Pressure points where arteries close to body surface, arteries that can be compressed to stop blood flow

Measuring Blood Pressure Method

• Measured indirectly by auscultatory method using a sphygmomanometer (blood pressure cuff).
• Increased pressure in the cuff temporarily stops blood flow in the brachial artery
• Pressure is slowly released and examiner listens for (Korotkoff) blood flow sounds using a stethoscope.

Systolic pressure

• Normally less than 120 mm Hg, is measured when blood spurts through, at which sounds first occur

Diastolic pressure

• Normally less than 80 mm Hg, is measured when blood flows freely, which is also when sounds disappear because the artery is no longer constricted

Velocity of Blood Flow

• Speed changes as travel through systemic circulation
• Inversely related to total cross-sectional area
• Fastest in the aorta, slowest in capillaries, increases in veins

Long-term Autoregulation & Blood Flow

• occurs when short-term autoregulation cannot meet tissue nutrient requirements
• Can cause angiogenesis can occur, which includes increased number of vessels to region and existing vessels enlarge.
• Common in heart when coronary vessel occluded, or throughout body in people in high-altitude areas

Long-term Autoregulation: Examples

Some examples:

• In skeletal muscle in response to aerobic training
• In adipose tissue with weight gain
• In coronary vessels in response to gradual blockage

Blood Flow Control

• The heart, blood vessels, and kindeys all regulate blood flow
• Cardiac ouput is often reffered to as (CO)

Blood Flow Through Capillaries

• The vasomotor system is a slow, intermittent mechanism
• Vasomotion reflects the on/off system of blood-flow by opening and closing of precapillary sphincters

Blood Flow Through Capillaries & Sphinctors

• O2 and nutrients diffuse from blood to tissues.
• CO2 and metabolic wastes diffuse from tissues to blood
• Lipid-soluble molecules diffuse directly through endothelial membranes
• Water-soluble solutes pass through intercellular clefts and fenestrations
• Larger molecules, like proteins, are actively transported in pinocytotic vesicles or caveolae.

Fluid Movements

• Fluids flow down pressure gradient (bulk flow)
• Fluid leaves capillaries at arterial end and most returns to blood at venous end, determining relative fluid volumes in blood and interstitial space
• Direction and amount of fluid flow relies upon two opposing forces: hydrostatic and colloid osmotic pressures

Filtration

• What occurs when fluid runs out of blood
• Fluid and small solutes flow easily through capillary's openings, while large solutes are blocked
• Only occurs on arterial end of capillary

Reabsorption

• Is when fluid goes into blood
• Only occurs on venous end

Hydrostatic Pressures: Capillary Hydrostatic Pressure (HPc)

• This is also referred to as capillary blood pressure
• Tends to push fluids down and throught capillary walls
• Greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg).

Hydrostatic Pressures and Fluids

• Pressure that would push fluid into vessel
• Usually assumed to be zero because of lymphatic vessels

Colloid Osmotic Pressures: Capillary Colloid Osmotic Pressure (OPc)

• This is also referred to as oncotic pressure
• Nondiffusible plasma proteins are created during this process, creating around 26 mm of water which is then pulled towards themselves

Interstitial Fluid Osmotic Pressure (OPif)

• Interstitial Fluid Osmotic Pressure has a low (~1 mm Hg) protein content when compared to other components

Hydrostatic-Osmotic Pressure Interactions & Net Filtration Pressure (NFP)

• This Net Filtration mechanism comprises all forces acting on capillary bed
• In this NFP, Net fluid flow is released at arterial end and absorbed in at venous end.
• NFP = (HPc-HPif)-(OPc-OPif)
• Fluid that is not returned is later absorbed again through the Lymphatic system

Two Main Circulations: Pulmonary Circuit

• Short loop that runs from the heart to the lungs and back to the heart

Two Main Circulations: Systemic Circulation.

• Long loop to all parts of the body and back to the heart