A&P Lecture Chapter 19

Arteries

(carry blood away from the heart) • Also called efferent vessels

Veins

(carry blood to the heart) • Also called afferent vessels

Right atrium

(entry chamber) • Collects blood from systemic circuit • To right ventricle to pulmonary circuit

▪ Capillaries

(exchange substances between blood and tissues) • Interconnect smallest arteries and smallest veins

small sized veins

(venule)

• Large veins

- Contain all three vessel wall layers o Thin tunica media surrounded by thick tunica externa - Include superior and inferior venae cavae and tributaries

Vessel diameter

- Diameter has a much larger effect on resistance compared to vessel length. Resistance is inversely proportional to the 4th power of radius of blood vessel. o R = 1/r4 - Small change in diameter produces a large change in resistance - Vasomotor center controls peripheral resistance primarily by altering diameters of arterioles

• Peripheral resistance

- Resistance of the arterial system as a whole - Increases as vessels get smaller

Circulation pathway through circuits

- Right Atrium - Pulmonary Circuit - Left Atrium - Systemic Circuit

Venus pressure

- the force exerted by blood within the venous system. ▪ Blood pressure in veins is maintained by: • Valves • Muscular compression of peripheral veins; "muscle pumps." ▪ As blood moves toward the heart, vessels get larger, and resistance decreases

Blood pressure

- the hydrostatic pressure (force) exerted by blood on the walls of a blood vessel. • Pressure within the cardiovascular system as a whole - Arterial pressure is much higher than venous pressure o Must push blood greater distance through smaller vessels

Five general blood vessel classes

1. Arteries 2. Arterioles 3. Capillaries 4. Venules 5. Veins

Two regulatory pathways

1. autoregulation 2. central regulation

Distribution of body's blood

64% - systemic venous system 13% - systemic arterial system 9% - pulmonary circuit 7% - heart 7% - systemic capillaries

Shock

Acute cardiovascular crisis marked by: • Low blood pressure (hypotension) • Inadequate peripheral blood flow ▪ Most common causes are hemorrhaging and heart damage (as in heart attack) • Normal homeostatic mechanisms cannot compensate

▪ Mean arterial pressure (MAP)

Adding one-third of pulse pressure to diastolic pressure • Example: 90 + (120 - 90)/3 = 100 mm Hg

Possible variations in capillary exchange

Any condition that affects blood pressure or osmotic pressure of blood or interstitial fluid shifts balance of hydrostatic and osmotic forces ▪ Edema: when fluid moves out of the blood capillaries and builds up in the peripheral tissues.

• Hemangiogenic: blood vessel-forming tissue.

At day 13-15 of embryonic development, aggregates of cells scatter within the yolk sac forming blood islands. The islands give rise to Hematopoietic stem cells (form blood elements) and hemangioblasts (which form vessels).

Cardiovascular adjustments

At rest • Cardiac output = 5800 mL/min ▪ During light exercise • Three changes take place 1. Vasodilation occurs, peripheral resistance drops, and capillary blood flow increases 2. Venous return increases with skeletal muscle contraction; increased respiration creates negative pressure in thoracic cavity, drawing blood back (respiratory pump) 3. Cardiac output increases to 9500 mL/min o Primarily due to venous return

▪ Blood Flow regulation by

Autoregulation acts locally • Central regulation - Involves neural and endocrine mechanisms - Makes coordinated adjustments to heart rate, stroke volume, peripheral resistance, and venous pressure so cardiac output is sufficient

As filtration occurs, Blood Colloid Osmotic Pressure (BCOP) increases.

BCOP is blood plasma osmolarity. ▪ This occurs because as water leaves capillary, and plasma proteins remain, the tendency for water to return increases due to laws of diffusion and osmosis. ▪ CHP decreases along capillary length

▪ Precapillary sphincters

Bands of smooth muscle that contract and relax to control flow into the capillary bed

Pressure and blood flow in veins

Blood pressure in peripheral venules is <10 percent of that in ascending aorta (largest artery) ▪ Mechanisms are needed to maintain flow of blood in veins against force of gravity

Capillary hydrostatic pressure (CHP):

Blood pressure within the capillary bed

Blood vessels

Blood vessels conduct blood between the heart and peripheral tissues

Viscosity

Blood viscosity is about five times that of water - Due to cells and plasma proteins - Viscosity is normally stable o Disorders affecting hematocrit or plasma composition change viscosity and affect peripheral resistance

▪ Progressive shock

Blood volume drops by more than 35 percent - Despite sustained vasoconstriction and mobilized venous reserves: o Blood pressure remains low o Venous return reduced o Cardiac output inadequate 2. Low cardiac output reduces blood flow to heart - Damages myocardium - Leads to further reduction to cardiac output . Reduced cardiac output accelerates oxygen starvation in tissues - Local chemical changes promote intravascular clotting - Further restricts peripheral blood flow Local pH changes increase capillary permeability - Further reducing blood volume

At roughly two-thirds of the way along the capillary ▪ No net movement

Capillary hydrostatic pressure equals blood colloid osmotic pressure • NFP = CHP - BCOP = 0 - BCOP becomes negative by end of capillary which leads to reabsorption

Cardiovascular regulation is accomplished by adjusting:

Cardiac output regulation and Blood flow regulation

Vasculogenesis (continued) forms the following major vessel structures:

Cardinal veins, aortic arches, and dorsal aorta

▪ Irreversible shock

Carotid baroreceptors trigger vasomotor centers - Sympathetic output causes widespread vasoconstriction o Reduces peripheral circulation but increases brain blood flow temporarily Without treatment, blood pressure will again decline - Heart is now seriously damaged, and cardiac output declines Circulatory collapse - Occurs when arteriolar smooth muscles and precapillary sphincters cannot contract - Leads to: o Widespread vasodilation o Immediate and fatal decrease in blood pressure o Cessation of blood flow

▪ Pulmonary circuit

Composed of arteries and veins that transport blood between the heart and lungs • Begins at the right ventricle; ends at the left atrium

Typical capillary

Consists of a tube of endothelial cells with delicate basement membrane • Lacking both tunica media and tunica externa ▪ Average diameter = 8 µm • About the same as a single RBC

▪ Two major types of capillaries

Continuous capillaries 2. Fenestrated capillaries

Muscle Pump

Contraction of skeletal muscles

Process of maintaining blood flow in veins

Contraction of skeletal muscles squeezes veins (and the blood within them) up through valves to return to heart. Muscle pump.

Venoconstriction

Contraction of smooth muscle fibers in veins • Reduces the diameter of the veins and the amount of blood in the venous system ▪ This reduction in diameter of veins is the body's method of maintaining blood volume in the arterial system even with a significant blood loss • Controlled by the vasomotor center in the medulla oblongata • Sympathetic nerves stimulate smooth muscles in medium-sized veins

Capillary exchange

Diffusion is the net movement of substances from an area of higher concentration to lower concentration • Occurs most rapidly when: 1. Distances are short 2. Concentration gradient is large 3. Ions or molecules involved are small • Occurs continuously across capillary walls, but transport mechanism varies for different substances

Blood flow - Volume of blood flowing per unit of time through a vessel or group of vessels. Two factors:

Directly proportional to arterial pressure • More important than absolute pressure is the pressure gradient (Pressure difference (ΔP)) • Difference in pressure from one end of vessel to other 2. Inversely proportional to peripheral resistance • Largest gradient from base of the aorta and proximal end of capillary beds

Flow accelerates in venous system

Due to larger diameter vessels = lower resistance

1. Continuous capillary

Endothelium is a complete lining ▪ Located throughout body in all tissues except epithelia and cartilage ▪ Permits diffusion of water, small solutes, and lipid soluble materials • Prevents loss of blood cells and plasma proteins • Some selective vesicular transport ▪ Specialized continuous capillaries in CNS and thymus have endothelial tight junctions • Enables restricted and regulated permeability

Hemodynamics

Factors affecting Blood flow

At the arterial end of the capillary

Filtration: when capillary hydrostatic pressure causes water and small solutes to move across the capillary wall and enter the interstitial fluid. ▪ Predominates at the arterial end of the capillary • Capillary hydrostatic pressure (CHP) is highest near arteriole - Causes water and small solutes to enter interstitial fluid - CHP > BCOP = fluid forced out of capillary

Valves

Folds of tunica intima projecting from vessel wall and pointing in the direction of blood flow - Ensure one-way flow of blood toward heart

Vasculogenesis

Formation of the first blood vessels by precursor endothelial cells called hemangioblasts • Early embryonic development begins by forming blood islands in the yolk sac • Hemangioblasts in the center of blood island differentiate into hematopoietic stem cells - Give rise to all other blood cells (formed elements) • Hemangioblasts in periphery become angioblasts - Combine to form first blood vessels • Angioblasts remodel blood islands into primitive capillary networks and then into larger vessel networks

2. Vessel luminal diameter (diameter of lumen= opening)

Friction occurs between moving fluid layers o Layer closest to vessel wall is slowed most because of friction with endothelial surface o Effect gradually diminishes away from wall In smaller vessels, more fluid volume is near wall, so higher resistance - In larger vessels, central region unaffected, so lower resistance

1. Vessel length

Friction occurs between vessel walls and moving blood - Increase in vessel length = increased surface area = increased in friction or resistance - Largest change occurs between birth and maturity

Blood flow (continued) Changes in diameter

From aorta to capillaries: • Blood vessels diverge and branch • Decreasing diameter increases resistance = decreased pressure = decreased flow ▪ From capillaries to venae cavae: • Blood vessels converge to form larger vessels • Increasing diameter decreases resistance = increased flow

Changes in blood flow ▪ Highest flow in the aorta

Highest blood pressure, largest diameter

▪ Changes in blood pressure

Highest pressure at the aorta - Heart generates pressure of about 120 mm Hg - Aorta cross-sectional area 4.5 cm2

▪ Metarteriole, or precapillary arteriole

Initial segment of the connection passageway • Contains smooth muscle that can change the vessel's diameter and adjust flow rate

Tunica intima, or tunica interna

Innermost layer • Endothelial cells with connective tissue with elastic fibers - In arteries, outer margin has layer of elastic fibers (internal elastic membrane)

Capillary bed

Interconnected network of capillaries • Contains several connections between arterioles and venules May be supplied by more than one artery • Multiple arteries called collaterals • Fuse before giving rise to arterioles • Fusion is an example of arterial anastomosis - Anastomosis is joining of blood vessels • Allows continuous delivery of blood to capillary bed even if one artery is blocked or compressed

Autoregulation

Involves local changes in blood flow within capillary beds. • Regulated by precapillary sphincters in response to chemical changes in interstitial fluid - Vasodilators: Factors (local chemicals) that promote the dilation of blood vessels.

Elastic arteries

Large vessels close to the heart that stretch and recoil when heartbeats - Include pulmonary trunk, aorta, and branches

• Muscular arteries

Medium-sized arteries - Distribute blood to skeletal muscles and internal organs

▪ Thoroughfare channel

Most direct passageway through capillary bed

Capillaries

Only blood vessels to allow exchange between blood and interstitial fluid • Very thin walls allow easy diffusion

Vascular resistance

Opposition to blood flow in vessels - Largest component of total peripheral resistance - Primarily results from friction between blood and vessel walls - Amount of friction depends on two factors 1. Vessel length 2. Vessel diameter

Arterioles

Poorly defined tunica externa • Tunica media is only 1-2 smooth muscle cells thick

▪ Circulatory shock

Positive feedback loops beginning when blood loss >35 percent

Cardiac output regulation

Pressure difference (ΔP) drives the blood flow through tissue. Blood flows from regions of higher pressure to lower pressure. Greater the ΔP, the greater the blood flow. - Must generate enough pressure to force blood through miles of peripheral capillaries within tissues

2. Endocrine responses

Provide short-term and long-term regulation of cardiovascular function ▪ Utilize endocrine functions of: 1. The heart 2. The kidneys 3. The pituitary gland (antidiuretic hormone, or ADH)

▪ Capillary hydrostatic pressure (CHP) is the blood pressure within capillary beds

Provides driving force for filtration • Filtration is the size-selective movement of water and small molecules out of the bloodstream into interstitial fluid • Larger molecules (such as plasma proteins) remain in blood

Pulmonary circuit

Pulmonary arteries to pulmonary capillaries to pulmonary veins

Veins • Medium-sized veins

Range from 2 to 9 mm in internal diameter - Thin tunica media with smooth muscle cells and collagen fibers - Thickest layer is tunica externa with longitudinal and elastic fibers.

Left atrium

Receives blood from pulmonary circuit • To left ventricle to systemic circuit

Capillary Bed can be bypassed by arteriovenous anastomosis that directly connects arteriole to venule

Regulated by sympathetic innervation

❖Aortic arches

Series of arterial channels that will form the carotid arteries, aortic arch, and part of pulmonary arteries

❖Cardinal veins

Series of venous channels that will form the superior and inferior venae cavae

Venules

Small veins - Those smaller than 50 µm lack a tunica media and resemble expanded capillaries • Collect blood from capillaries

• Pressure drops at each branching in arterial system

Smaller, more numerous vessels produce more resistance, reducing pressure - At start of peripheral capillaries, pressure is 35 mm Hg - At the venules, pressure is 18 mm Hg

Slowest in the capillaries

Smallest diameter • Slow flow important to allow exchange between blood and interstitial fluid

Systemic circuit

Systemic arteries to systemic capillaries to systemic veins

Changes in arterial pressure ▪ Rises during ventricular systole

Systolic pressure: Peak pressure measured during systole - Declines during ventricular diastole • Diastolic pressure: Minimum pressure measured during diastole. • Commonly written with a "/" between pressures • Example: 120/90

Net filtration pressure (NFP)

The difference between CHP (which pushes water out) and BCOP (which draws water in). NFP = CHP - BCOP

Cardiac Output (CO)

The total blood pumped by the heart (ml/min.) Normally calculated by HR x SV. (Heart rate x stroke volume = cardiac output)

Homeostatic mechanisms

Tissue perfusion: The blood blow through tissues. • Blood flow must match changes in demand for oxygen and nutrients ▪ Two regulatory pathways 1. Autoregulation - Occurs at local level 2. Central regulation - Neural and endocrine control - Activated if autoregulation is ineffective

systemic circuit

Transports oxygenated blood to all organs and tissues • Begins at the left ventricle; ends at the right atrium

Total blood volume distribution

Uneven distribution among arteries, veins, and capillaries

Veins have valves. What if the valves don't work?

Valves permit blood flow in one direction and prevent backflow of blood toward capillaries If valves do not work properly, blood can pool in veins, causing distention and a range of effects Mild discomfort and cosmetic problems, as with varicose veins (in thighs and legs) - Painful distortion of adjacent tissues, as in hemorrhoids (form in the venous networks of the anal canal)

Blood vessel formation involves two process

Vasculogenesis 2. Angiogenesis.

Compensation mechanisms

When hemostasis fails to prevent significant blood loss, entire cardiovascular system adjusts to compensate ▪ Immediate problem is maintenance of blood pressure and peripheral blood flow ▪ Long-term problem is restoration of blood volume ▪ Mechanisms can cope with blood losses equal to ~30 percent of total blood volume

. Blood flow regulation

affecting distribution of blood within systemic and pulmonary circuits - Resistance to blood flow in specific blood vessels. The higher the resistance, the smaller the blood flow.

1. Baroreceptor reflexes (baro-, pressure) are

are receptors that respond to a change in blood pressure. ▪ Receptors are located in walls of: 1) Carotid sinuses 2) Aortic sinuses 3) Right atrium

Blood flow is determined by the interplay between

arterial pressure and peripheral resistance

Chemoreceptors monitor the chemical composition of the

blood and cerebrospinal fluid

The venous system has low pressures and contains almost two-thirds of the body's

blood volume

The heart pumps blood, in sequence, through the arteries

capillaries, and veins of the pulmonary and systemic circuits

The cardiovascular center makes extensive adjustments to

cardiac output and blood distribution during exercise

: Capillary exchange is a dynamic process that includes

diffusion, filtration, and reabsorption

The remaining blood is in the systemic capillaries

heart, and pulmonary circuit

resistance

is a force that opposes movement of blood

Venous return

is the amount of blood arriving at the right atrium each minute • On average, equal to the cardiac outp

Capillary exchange

is the exchange by diffusion of gases, minerals, nutrients, etc. between the blood and interstitial

• Capillary pressure

is the force exerted by blood against capillary walls. • Normally, it is very low • Allows plenty of time for capillary exchange

Concluding Statement: Large vessels are

nourished ("fed") by having small arteries and veins embedded in their walls. These small arteries and veins which function for nourishment are called the Vasa vasorum, "vessels of vessels

▪ Systemic venous system contains 64 percent

of total blood volume (~3.5 L) • Of that , ~1 L is in venous networks carrying blood from digestive organs to liver • Act as blood reservoirs

Blood colloid osmotic pressure (BCOP)

osmolarity of the blood plasma

Pulsing (contraction) of the heart generates

pressure

Flow through blood vessels is influenced by

resistance

Blood flow in capillaries is very

slow

Arteries and veins differ in the

structure and thickness of their walls

Capillary exchange is the bidirectional movement of substances between

the blood plasma and interstitial fluid across the capillary walls. ▪ Involves a combination of diffusion, osmosis, and filtration and reabsorption.

Capillary structure and capillary blood flow affect

the rates of exchange between the blood and interstitial fluid

Blood Flow

the volume of blood that flows through any tissue in a given time period (in mL/min.)

Systemic arteries contain 13 percent

total blood volume

three layers of arteries and veins

tunica intima, tunica media, tunica externa

: New blood vessels form through

vasculogenesis and angiogenesis

Pressure, resistance, and

venous return affect cardiac output

Vessel luminal diameter is the main source of resistance

within the cardiovascular system

❖Dorsal aorta

• Becomes the descending aorta

▪ During heavy exercise

• Cardiac output approaches maximal levels (~17,500 mL/min) • Major changes in peripheral blood distribution allow large increase in flow to skeletal muscles without overall decrease in systemic blood pressure - Increased flow to skeletal muscles - Increased flow to skin (promotes heat loss) - Reduced flow to digestive viscera and kidneys - Brain blood flow remains unchanged

▪ Vasomotion

• Cycles of contraction and relaxation

Net filtration pressure (NFP)

• Difference between capillary hydrostatic and blood colloid osmotic pressure • NFP = CHP - BCOP - Is positive at beginning of capillary o Filtration

▪ Pulse pressure

• Difference between systolic and diastolic pressure • Example: 120 - 90 = 30 mm Hg

Central regulation

• Involves both neural and endocrine mechanisms - Neural Mechanisms o Activation of cardioacceleratory center o Activation of vasomotor center (controls peripheral vasoconstriction) o Can increase cardiac output and reduce blood flow to nonessential or inactive tissues - Endocrine Mechanisms o Release of vasoconstrictor (primarily NE), producing long-term increases in blood pressure

Tunica media

• Middle layer • Contains concentric sheets of smooth muscle - Contraction causes a decrease in vessel diameter, or vasoconstriction - Relaxation causes an increase in vessel diameter, or vasodilation • Separated from the tunica externa by the external elastic membrane

Total peripheral resistance is the resistance of entire cardiovascular system to blood flow.

• Must be overcome by sufficient pressure from the heart in order for circulation to occur • Depends on three factors 1. Vascular resistance 2. Viscosity 3. Turbulence

Tunica externa, or tunica adventitia

• Outermost layer • Connective tissue sheath - In arteries, contains collagen and scattered elastic fibers - In veins, generally thicker than the tunica media o Contains networks of elastic fibers and bundles of smooth muscle cells • Anchors vessel to surrounding tissues

Viscosity

• Resistance to flow caused by interactions among molecules and suspended materials in a liquid • Low-viscosity fluids have lower resistance, so flow at low pressures - Example: water (viscosity 1.0) • High-viscosity fluids have higher resistance, so flow only at high pressures - Example: molasses (viscosity 300)

▪ Cardiovascular performance improves with training

• Trained athletes have bigger hearts and stroke volumes - Can maintain normal blood flow with lower heart rate (as low as 32 bpm) - Maximal cardiac output can be 50 percent higher than in non-athletes

Turbulence

• Type of fluid flow with eddies and swirls • Caused by high flow rates, irregular surfaces, and sudden changes in vessel diameter • Responsible for production of third and fourth heart sounds - Normal turbulent flow in the heart • Increased turbulence = increased resistance = slow blood flow

2. Fenestrated (fenestra, window) capillary

▪ Contains "windows," or pores, penetrating endothelial lining ▪ Permits rapid exchange of water and larger solutes ▪ Examples found in: • Choroid plexus of brain • Capillaries of hypothalamus, pituitary, pineal, and thyroid glands • Absorptive areas of intestinal tract • Kidney filtration sites

Angiogenesis: The growth of new blood vessels from preexisting vessels

▪ Dorsal aorta and cardinal veins formed by vasculogenesis ▪ Further growth of blood vessels occurs through angiogenesis

Hormonal response to high blood pressure and blood volume

▪ High blood volume stretches the heart wall during diastole • Triggers release of natriuretic peptides - Atrial natriuretic peptide (natrium, sodium + ouresis, urination), or ANP o Released from right atrial walls - Brain natriuretic peptide, or BNP o Released from ventricular muscle cells ▪ Decrease in blood volume and pressure leads to decreased stress on heart walls which leads to decreased production of natriuretic peptides

Hormonal response to low blood pressure and volume

▪ Immediate response • Release of epinephrine (E) and norepinephrine (NE) - Released from the adrenal medullae ▪ Other hormones important in the long-term response 1. Antidiuretic hormone (ADH) 2. Angiotensin II 3. Erythropoietin (EPO) 4. Aldosterone

At roughly two-thirds of the way along the capillary

▪ No net movement - because the Capillary hydrostatic pressure equals blood colloid osmotic pressure • NFP = CHP - BCOP = 0

At the venule end of the capillary.....

▪ Reabsorption predominates • Capillary hydrostatic pressure falls below blood colloid osmotic pressure - CHP < BCOP and water moves back into capillary Overall, more water leaves bloodstream then is reabsorbed • KEY POINT : Difference (about 3.6 L/day) enters the lymphatic vessels and is eventually returned to the venous system.

3. Sinusoids (sinusoidal capillaries)

▪ Resemble fenestrated capillaries that are flattened and irregularly shaped ▪ Commonly have gaps between endothelial cells ▪ Basement membrane is thin or absent ▪ Permit more water and solute (plasma proteins) exchange ▪ Occur in liver, bone marrow, spleen, and many endocrine organs

Chemoreceptor reflexes

▪ Respond to blood and Cerebrospinal fluid (CSF) changes in: • Carbon dioxide • Oxygen • pH ▪ Receptors located in: • Carotid bodies • Aortic bodies • Ventrolateral surface of medulla oblongata ▪ Stimulation of receptors triggers coordinated adjustment in cardiovascular and respiratory activity