Cardiovascular System: Blood Vessels and Circulation - Notes

Blood Vessels

• Blood travels in a closed system of vessels that begins and ends at the heart.

• The three major types of vessels are arteries, capillaries, and veins.

Arteries

• Carry blood away from the heart to the capillaries

• There are three types of arteries:

  • elastic

  • muscular

  • arterioles

Capillaries

• The smallest blood vessel

• Its thin walls allow movement of substances between blood and interstitial fluid; endothelium

  • Walls consisting of a thin tunica interna, one cell thick, no muscle or connective tissue

• Porous blood vessels for the exchange of substances between blood and tissues

• Allow only a single RBC to pass at a time

• Involved in gas exchange, nutrient-waste exchange, and delivery of various substances

Veins

Drain blood from the capillaries, transporting it back toward the heart

Structure of Blood Vessels

• Arteries and veins are composed of three tunics

  • Tunica interna

  • Tunica media

  • Tunica externa

• Capillaries are composed of endothelium with sparse basal lamina.

• Lumen is the central blood-containing space surrounded by tunics.

Tunica Interna

• Composed of a simple squamous epithelium layer that lines the lumen of all vessels

• Provides a smooth surface for substances (nitric oxide) to regulate contraction and relaxation within the tunica media

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

Tunica Media

• Smooth muscle and elastic fiber layer, regulated by the sympathetic nervous system

• Controls vasoconstriction/vasodilation of vessels

Tunica Externa

• Also known as tunica adventitia

• Outermost layer of the blood vessel

• Collagen fibers that protect and reinforce vessels

• Larger vessels contain vasa vasorum

Companion Vessels

Arteries and veins that supply the same body region and tend to lie next to one another

Elastic (Conducting) Arteries

• Largest arteries near the heart; the aorta and its major branches

• Vessel Diameters from 2.5 to 1 cm

  • Large lumen allows low-resistance conduction of blood

  • Contain elastin in all three tunics, little smooth muscle

  • Withstand and smooth out large blood pressure fluctuations

  • Transport blood to the muscular arteries

  • Allow blood to flow fairly continuously through the body

Muscular (Distributing) Arteries and Arterioles

• Medium-sized arteries

• Diameters from 1 cm to 0.3 mm

• Circumscribed sheets: Internal elastic lamina and external elastic lamina

  • Distal to elastic arteries; deliver blood to body organs

  • Have thick tunica media with more smooth muscle and less elastic tissue

  • Active in vasoconstriction

  • Carries blood to specific organs

Arterioles

• Smallest arteries; lead to capillary beds

• Diameter from 0.3 mm to 10 µm

  • 6 layers of smooth muscles

  • Control flow into capillary beds & hence tissues via vasodilation and constriction

• Contracted state called muscle tone

Vasomotor tone

• Contracted state in blood vessels

• Regulated by vasomotor center in the medulla oblongata

Three structural types of capillaries:

• Continuous

• Fenestrated

• Sinusoids

Continuous Capillaries

• Abundant in skin and muscles, and have:

  • Endothelial cells that provide an uninterrupted lining

  • Adjacent cells that are held together with tight junctions

  • Intercellular clefts of unjoined membranes that allow the passage of fluids

  • Form the blood-brain barrier

• Simple diffusion and Pinocytosis for materials passing through walls

Fenestrated Capillaries

• Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

• Characterized by:

  • Endothelium riddled with pores (fenestrations), gap junctions

  • Greater permeability to solutes and fluids than other capillaries, forms interstitial fluid

Sinusoids

• Highly modified, leaky, fenestrated capillaries with large, irregular-shaped lumens

• Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

• Allow large molecules (proteins & blood cells) to pass between the blood & surrounding tissues (more permeable)

• Blood flows sluggishly, allowing for modification in various ways

• In the liver, lined with macrophages for phagocytosis

Capillary Beds

• A microcirculation of interwoven networks of capillaries (10-100), consisting of:

  • Vascular shunts: metarteriole thoroughfare channel connecting an arteriole directly with a postcapillary venule

  • True capillaries: branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

Atherosclerosis

• Disease in which an artery wall thickens, leaving a smaller lumen for blood flow

• Result in coronary artery disease (CAD)/ peripheral artery disease (PAD)

Aneurysm

Ballooning of an artery due to a weakened vessel wall; it is susceptible to rupture, leading to severed bleeding

Venous System: Venules

• The smallest type of veins, from 8 to 100 micrometers in diameter

• Companion vessels with arterioles

• Allow fluids and WBCs to pass from the bloodstream to tissues

• Postcapillary Venules: drain blood from capillaries

  • Smallest venules, composed of endothelium

• Large venules have one or two layers of smooth muscle (tunica media) and thin tunica externa

Venous System: Veins

• Formed when venules converge

  • Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks

  • Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

Veins

• Veins have much lower blood pressure and thinner walls than arteries

• To return blood to the heart, veins have special adaptations:

  • Large-diameter lumens, which offer little resistance to flow

  • Valves (resembling semilunar heart valves), which prevent backflow of blood

Venous Sinuses

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

Blood Flow through Capillary Beds

• Precapillary sphincter

  • Ring of smooth muscle that surrounds each true capillary (at metarteriole junction)

  • Regulates blood flow into the capillary

• Blood flow is regulated by vasomotor nerves (ANS) and local chemical conditions, so it can either bypass or flood the capillary bed

Simple Pathway

• Artery delivers blood to organ and then branches into even smaller arteries to become arterioles

• Each arteriole feeds into a single capillary bed

• A venule then drains blood from the capillaries and merges with other venules to form ONE MAJOR VEIN that drains from the organ or body region

• Arteries can reach an organ is referred as end arteries

Example of simple pathway

Blood is transported to and from the spleen

Vascular Anastomoses

• Merging blood vessels, more common in veins than arteries

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

  • If one branch is blocked, the collateral channel can supply the area with adequate blood supply

• Thoroughfare channels are examples of arteriovenous anastomoses

Four alternative blood vessel pathways

1. Simple pathway: involves blood flowing through an arteriole to a capillary bed, and out of the capillaries through a venule.

2. Arterial anastomosis: includes two or more arteries converging to supply the same body region

3. Venous anastomosis: includes two or more veins draining the same body region

4. Arteriovenous anastomosis: is a shunt, bypassing a capillary bed by connecting an arteriole directly to a vein

• Portal system: blood flows through two capillary beds, with the two capillary beds separated by a portal vein.

Blood Flow Velocity

Rate of blood transported per unit and typically measured in cm per second

Relationship of Total Cross-Sectional Area and Velocity of Blood Flow

The greater the total cross-sectional area, the slower the blood flow

In which type of vessel is blood flow the slowest? The Fastest?

• Slowest: Capillaries

  • Allows sufficient time for capillary to exchange nutrients and gas

• Fastest: Arteries

Diffusion

Passive transport is when very small solutes (O2, CO2, glucose, ions) move from areas of higher concentration to areas of lower concentration

Vesicular transport

When endothelial cells use pinocytosis (like phagocytosis but with fluids instead of solids) to form vesicles, used for larger solutes like insulin

Bulk flow

refers to the movement of large amounts of fluids and their dissolved substances in one direction down a pressure gradient

Filtration and Reabsorption

• Filtration:

  • arterial end of the capillary

  • small solutes flow from blood to tisue

• Reabsorption:

  • arterial end of capillary

  • small solutes flow from tissue to blood

Hydrostatic pressure (HP_b)

• Pressure of blood against the capillary walls

• In capillaries, it promotes filtration

  • HP_b on the arterial end is 40 mm Hg (drops quickly) to below 20 mm Hg on venous end

• Tends to force fluids through capillary walls

• Is greater at the arterial end of a bed than at the venule end

Interstitial fluid

• Interstitial fluid hydrostatic pressure (HP_if)

• Very small and for simplicity’s sake is assumed to be close to zero

Colloids Osmotic Pressure (COP)

• Pressure exerted by the pull of water back into a tissue by tissue’s protein concentration

• Promotes reabsorption

Net Filtration Pressure (NFP)

• Difference between net hydrostatic pressure and net colloid osmotic pressure & determines a new gain or a new lot of fluids from the fluid

• NFP= (HP_b - HP_if) - (COP_b - COP_if)

• Positive Value = flitration

• Negative Value = reabsorption

Net Filtration Pressure (NFP) continue

• At the arterial end of a bed, hydrostatic forces dominate (fluids flow out)

• At the venous end of a bed, osmotic forces dominate (fluids flow in)

• More fluids enter the tissue beds than return to blood, and the excess fluid is returned to the blood via the lymphatic system

Lymphatic System

• Responsible for picking up excess fluid and returning it to the blood

• Reabsorb excess fluid → filter it → return to venous circulation

Explain the lymphatic system's role at the capillary bed

• Capillary only reabsorbs 75-85% of the fluid that has passed into the interstitial fluid.

• The other 15-20% is picked up by the lymphatic system, filtered, and returned to the blood.

Degree of Vasculariation

• It is the extent of blood vessel distribution within a tissue

  • High Vascularization - brain, skeletal muscle, heart, liver

  • Little Vascularization - tendons and ligaments

  • No Vascularization - epithelial tissue, cartilage, cornea and lens of ey

Myogenic Response

• Contraction and relaxation of smooth muscle within blood vessels in response to changes in stretch of the blood vessels

• Example: In response to higher blood pressure, the smooth muscle vasoconstricts to slow down blood flow, and vice versa for low blood pressure.

Vasodilator

• Causes smooth muscle relaxation by dilating arterioles and opens precapillary sphincters

• Decrease O2 & nutrient lvls

• Increased Co2, H+, K+. & lactate lvls

• Histamine, bradykinin, nitric oxide, prostaglandins

Chemicals the Decrease Blood Pressure: Vasodilator

• Atrial natriuretic peptide (ANP): Causes blood volume and pressure to decline

• Nitric oxide (NO): Has brief but potent vasodilator effects

• Inflammatory chemicals: Histamine, prostacyclin, and kinins are potent vasodilators

• Alcohol: Causes BP to drop by inhibiting ADH

Vasoconstrictors

• Causes smooth muscle contraction, which constrict arterioles and close precapillary sphincters

• Increase O2 and nutrient lvls

• Decrease CO2, H+, K+, & lactate lvls

• Leukotrienes, Thromboxanes, endothelins

Chemicals that Increase Blood Pressure: Vasoconstrictors

• Adrenal medulla hormones: Norepinephrine and epinephrine increase blood pressure

• Antidiuretic hormone (ADH): Causes intense vasoconstriction in cases of extremely low BP

• Angiotensin II: Kidney release of renin generates angiotensin II, which causes intense vasoconstriction and stimulates aldosterone secretion which enhances renal reabsorption and stimulates ADH release

• Endothelium-derived factors: Endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors

General relationship of total blood flow to local blood flow

• Total blood flow is same as cardiac output → rest average of 5.25 liters per minute

• Total blood flow has direct relationship and casual relationship with local blood flow

How is local blood flow dependent on total blood flow?

• Cardiac output decreases, total blood flow decreases, and less blood is available to tissue

• Vice versa

Blood Pressure (BP)

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

  • Measured in millimeters of mercury (mm Hg)

  • Measured in reference to systemic arterial BP in large arteries near the heart

• The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

Arterial Blood Pressure

• Reflects two factors of the arteries close to the heart:

  • Their elasticity (compliance or distensibility)

  • The amount of blood forced into them at any given time

• Blood pressure is elastic arteries near the heart is pulsatile (BP rise and falls); in response to systolic and diastolic pressure changes

Systolic Pressure

Pressure exerted on arterial walls during ventricular contraction

Diastolic Pressure

Lowest level of arterial pressure during a ventricular cycle

Pulse Pressure

• the difference between systolic and diastolic pressure

• greatest in aorta

• declines in muscular arteries and non-existent in arterioles

• measure of the elasticity and recoil of arteries

Arterial Blood Pressure (Clinical Monitoring)

• Vital signs: pulse and blood pressure, along w/ respiratory rate and body temp

• Taking a pulse:

  • Radial pulse (taken at the wrist): most routinely used, but there are other clinically important pulse points

• Pressure points: areas where arteries are close to the body surface

  • Can be compressed to stop blood flow in event of hemorrhaging

Capillary Blood Pressure

• Ranges from 35 mm Hg at the beginning of the capillary bed to 17 mm Hg at the end of the bed

• Low capillary pressure is desirable because:

  • High BP would rupture fragile, thin-walled capillaries

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

Compare and contrast blood pressure and blood pressure gradients in the arteries, capillaries, and veins

• Blood pressure is highest in arteries

• Lowest in Veins

• In between in capillaries

Explain the mechanisms that help overcome the small pressure gradient in veins to return blood to the heart

• Skeletal muscle pump moves blood through limbs

• Respiratory pump moves blood through the thoracic cavity

Venous Blood Pressure

• Steady and changes little during the cardiac cycle

• The pressure gradient in the venous system is only about 15-20 mm Hg; falls to almost 0mm Hg in vena cava

• A cut vein has even blood flow; a lacerated artery flows in spurts

Maintaining Blood Pressure

• The main factors influencing blood pressure are:

  • Cardiac output (CO)

  • Peripheral resistance (PR)

  • Blood volume

• BP = CO*PR

Blood Flow

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

  • Measured in ml per min.

  • In the entire vascular system it is equivalent to cardiac output (CO)

  • Relatively constant when at rest

  • Varies widely through individual organs, according to function

  • Determined by blood pressure & resistance

Resistance

• Any opposition to blood flow; increases pressure

  • Measure of the amount of friction blood encounters as it passes through vessels

  • Generally encountered in the systemic circulation

  • Referred to as peripheral resistance (PR)

Sources of Resistance

• Blood viscosity

• Total blood vessel length

• Blood vessel diameter

Blood Viscosity

• Thickness or “stickiness” of the blood

• Determined by the number of cells and amount of protein

Blood Vessel Length

The longer the vessel, the greater the resistance encountered

Blood Vessel Radius

• Most common way resistance is altered is because of vasoconstriction/vasodilation

• Greater width of vessels (arteries) increases laminar flow

• The difference between the rate of flow at the center vs the rate of flow at the periphery where friction occurs

Relationship between blood pressure gradient and resistance to total blood flow

• Direct relationship between the blood pressure gradient and total blood flow

• Inverse relationship between resistance and total blood flow

Why does blood pressure increases with increased resistance in the systemic circulation

More resistance = greater pressure gradient to overcome

ensure adequate perfusion = blood pressure increases

Relationship between Blood Flow, Blood Pressure, and Resistance Formula

• F=Δ P/R

  • Blood flow = F

  • Difference in blood pressure = Δ P

  • Resistance = R

Relationship between Blood Flow, Blood Pressure, and Resistance

• Blood flow ($F$ ) is directly proportional to blood pressure gradient ($\Delta P$).

  • If $\Delta P$ increases, blood flow speeds up.

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

  • If $R$ increases, blood flow decreases, so

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

Systemic Blood Pressure

• Pumping action = heart generating blood flow through vessels along a pressure gradient

  • high to lower pressure

• Pressure results when flow is opposed by resistance

Systemic Pressure

• Highest in the aorta

• Declines throughout the length of the pathway

• Is 0 mm Hg in the right atrium

• The steepest change in blood pressure occurs n the arterioes

Factors Aiding Venous Return

Venous BP alone is low to promote adequate blood return and is aided by the Respiratory pump, Muscular pump, & the sympathetic venoconstriction

Respiratory Pump

Pressure changes created during breathing suck blood toward the heart by squeezing local veins

Muscular Pump

Contractions of skeletal muscles “milk” blood toward the heart

Sympathetic Venoconstriction

• Smooth muscles constrict, pushing blood back toward the heart

  • Valves prevent backflow during venous return

  • Formed from tunica intima

Cardiac Output (CO)

• Determined by venous return and neural and hormonal controls

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

• Stroke volume is controlled by venous return (end diastolic volume, or EDV)

• Under stress, the cardioacceleratory center increases heart rate and stroke volume

  • The end systolic volume (ESV) decreases and MAP increases

Controls of Blood Pressure: Short-term

• Are mediated by the nervous system and bloodborne chemicals

• Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance and cardiac output

Controls of Blood Pressure: Long-term

Regulate blood voluem

Short-Term Mechanisms: Neural Controls

• Neural controls of peripheral resistance:

  • Alter blood distribution to respond to specific demands

  • Maintain MAP by altering blood vessel diameter

• Neural controls operate via reflex arcs involving:

  • Baroreceptors

  • Vasomotor centers of the medulla and vasomotor fibers

  • Vascular smooth muscle

Baroreceptors

• Significant types of sensory receptors in regulating blood pressure

• Specialized sensory nerve endings in blood vessel walls that respond to stretch

• Location: Carotid Sinuses and Aortic Arch in the tunica externa

Carotid Sinus Reflect

• Prevents major changes in blood supply to the brain; rapid response to short-term changes

• Mediated by baroreceptors in carotid sinus

• Regulates the activity of the CIC and the CAC

• Located within internal carotid artery

• Detects changes in blood pressure

• Monitors BP changes in head and neck

Aortic Sinus Reflex

• Regulates blood flow to the systemic blood vessels

• Action is similar to the carotid sinus reflex

Short-Term Mechanisms: Vasomotor Center

• Vasomotor center

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

  • Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles

Short-Term Mechanisms: Vasomotor Activity

• Sympathetic activity causes:

  • Vasoconstriction and a rise in blood pressure if increased

  • Blood pressure to decline to basal levels if decreased

• Vasomotor activity is modified by:

  • Baroreceptors (pressure-sensitive), chemoreceptors (O2, CO2, and H+ sensitive), higher brain centers, bloodborne chemicals, and hormones

Vasoactive

• Chemicals classified as either vasodilators or vasoconstrictors

• Alter local blood flow collectively

Angiogenesis

• Is the formation of new blood vessels

• Takes place:

  • As the number of vessels to a region increases

  • When existing vessels enlarge

  • When a heart vessel becomes partly occluded

  • Routinely in people in high altitudes, where oxygen content of the air is low

  • In response to exercise

Short-Term Mechanisms: Chemical & Neural Controls

• Blood pressure is regulated by chemoreceptor reflexes sensitive to oxygen, H+ and carbon dioxide

  • Prominent chemoreceptors are in the aortic arch, & the carotid and aortic bodies

• Reflexes that regulate blood pressure are integrated in the medulla

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

Mean Arterial Pressure (MAP)

• Working pressure; pressure that propels the blood to the tissues

• MAP = diastolic pressure + (1/3*pulse pressure)

• 70-110 mm Hg typically indicates good perfusion

Cardiovascular center

Vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter

Angiotensinogen

• Liver produces the plasma protein and continuously releases into the blood

• The kidney releases renin and converts it into angiotensin I

• Angiotensin I is converted to Angiotensin II by angiotensin-converting enzyme (ACE)

  • Associated with the capillary endothelium

• ACE found in high concentrations in pulmonary capillaries

• Conversion occurs in blood vessels of lungs

Long-Term Mechanisms: Renal Regulation

• Baroreceptors adapt to chronic high or low blood pressure

• Long-term mechanisms control BP by altering blood volume via kidney action

Direct renal mechanism:

• Increased BP stimulates the kidneys to eliminate water, thus reducing BP

• Decreased BP stimulates the kidneys to increase blood volume and BP

Indirect renal mechanism

• Renin release produces angiotensin II; leads to aldosterone and ADH release which leads to water reabsorption; also leads to vasoconstriction; also thirst sensation