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List the layers of a blood vessel from the outermost to the innermost layer.
Tunica Externa (Adventitia)
Tunica Media
Tunica Intima
Lumen
Tunica Intima
Innermost layer, simple squamous epithelium called endothelium
Cells fit together like tiles on a floor & minimizes friction w/ blood
Tunica Media
Middle layer thicker in arteries than in veins
Made of smooth muscles, elastic fibers, & is responsible for changing diameter of vessels
Contains Vasoconstriction & Vasodilation
Vasoconstriction
decreases the diameter of vessel, increasing pressure & reducing blood flow
Vasodilation
increases the diameter of vessel, decreasing pressure & increasing blood flow
Tunica Externa (Adventitian)
A sheath of connective tissue on surface of vessel, containing collagen & elastic fibers for protection & anchoring of vessel
Lumen
Central space within vessel which blood flows
Larger in a vein than an artery
The thicker walls & smaller diameters give atrial lumens a more rounded appearance in cross section than the lumens of veins
Arteries
a vessel that conducts blood away from the heart
They have relatively thick walls to withstand the high pressure of blood from heart
Types: Elastic, Muscular, Arterioles
Elastic Arteries
These are the thick walled, large diameter, arteries near the heart
They expand when the ventricles contract, then recoil to maintain blood pressure while the ventricles relax (closest to the heart)
Muscular Arteries
Typically ranges from 0.1 mm to 10 mm
Elastic arteries give way to these arteries (further away from the heart)
These are most of the smaller arteries in rest of the body
The thick tunica media allows muscular arteries to undergo vasoconstriction and vasodilation
Decreased quantity of elastic fibers limits ability to expand
Arterioles
Very small arteries that lead to a capillary, 0.3 mm to 10um (micrometer) in diameter
They control how much blood is going to a tissue
Almost shut off blood supply or increase blood supply
(For example, blushing involves vasodilation of arterioles in the face)
Capillaries
Are microscopic, they are the smallest blood vessels
Walls are only an endothelial layer, surrounded by a basement membrane with occasional smooth muscle fibers
Very thin-walled and narrow, RBCs pass through in single file
Site of exchange of gases and other substances between the blood and the surrounding cells and their tissue fluid
Types: Continuous, Fenestrated, & Sinusoidal
Remember that arteries and veins are only a transport system
Continuous Capillaries
Found in skin and muscles
These the most common type
Uninterrupted lining of endothelial cells form tight junctions
Contains very small gaps that allow substances to leak through
i) metabolic products (glucose, water)
ii) small hydrophobic molecules (gases and hormones)
iii) some leukocytes
Fenestrated Capillaries
Walls of these capillaries have pores
Permits greater permeability of fluids and larger molecules
Found in areas where absorption or secretion takes place
Ex: in the intestines and kidneys and endocrine structures, like the hypothalamus, pituitary, pineal gland, and thyroid glands
Sinusoidal Capillaries
Least common type of capillaries
Have extensive intercellular gaps and incomplete basement membranes
Very large openings allow for the passage of the large molecules, including plasma proteins and even cells
Found in liver, bone marrow, lymph tissue, some endocrine glands
Very leaky capillaries and blood flow is very slow through them
Veins
A blood vessel that conducts blood toward the heart
Usually, but not always, contain un-oxygenated blood
They are thin-walled vessels, with large and irregular lumens
Contain back-flow valves that maintain the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries
Low blood pressure
contains Venules
Venules
Are extremely small veins, generally 8-100 um in diameter
Very porous to permit the movement of WBCs & fluid
Multiple of these join to form veins
Blood Flow
Refers to the movement of blood through a vessel
Actual volume of blood flowing through a vessel, organ, or body per unit of time, ml/min
Blood Pressure
Force per unit area exerted by the blood on the walls of a blood vessel, mm Hg
Resistance
Opposition to flow, a measure of the amount of friction the blood encounters as it flows through vessels
What factors affect Blood Flow
Cardiac Output
Increase: ↑ BP & flow (more blood from heart)
Decrease: ↓ BP & flow
Compliance
Increase: ↓ resistance, ↑ flow
Decrease: ↑ resistance (less elastic), ↓ flow
Blood Volume
Increase: ↑ BP & flow
Decrease: ↓ BP & flow
Viscosity
Increase: ↑ resistance, ↓ flow
Decrease: ↓ resistance, ↑ flow
Vessel Diameter
Increase: ↓ resistance, ↑ flow (vasodilation)
Decrease: ↑ resistance, ↓ flow (vasoconstriction
Total Blood Vessel Lenghth
What Factors Increase Resistance in Blood Vessels
↑ Blood viscosity
Increases resistance
↑ Total blood vessel length
Increases resistance
↓ Vessel diameter
decreases ressitance
↓ Arterial compliance
decreases ressitance
Plaque buildup or vascular disease
Cardiac Output
How much blood flows from ventricles
Anything that causes output to increase, by elevating heart rate and/or stroke volume, will raise blood pressure and increase flow
Increase: ↑ BP & flow (more blood from heart)
Decrease: ↓ BP & flow
Compliance
The ability of any compartment to expand and accommodate increased.
(Ex: a balloon is compliant and a metal pipe is not)
Arteries with greater compliance are able to expand to accommodate surges in blood flow, without increased resistance or blood pressure
Veins are more compliant than arteries
Vascular disease reduces compliance, because the arteries stiffen, increasing resistance to blood flow
The result of reduced compliance is more turbulence and higher pressure within the vessel, reducing blood flow
Increase: ↓ resistance, ↑ flow
Decrease: ↑ resistance (less elastic), ↓ flow
Blood Volume
As blood volume increases, blood pressure and flow increase
Increase: ↑ BP & flow
Decrease: ↓ BP & flow
Blood Viscosity
Is the thickness of a fluid which affects its ability to flow. Ex: water is less viscous than molasses and flows faster
The more viscous the blood, the greater the resistance and less the flow
The two primary components of blood viscosity are the formed elements and plasma proteins
Increase: ↑ resistance, ↓ flow
Decrease: ↓ resistance, ↑ flow
Blood Vessel Diameter
Not constant
Fluid close to the walls encounters more resistance
Smaller vessels have more resistance because more fluid contracts the sides of the vessel
Capillaries cause the most resistance because they have the smallest diameters
Large diameter vessels near the heart have very little resistance due to vessel diameter, because relatively little of the blood is in contact with the sides
Increase: ↓ resistance, ↑ flow (vasodilation)
Decrease: ↑ resistance, ↓ flow (vasoconstriction
Total Blood Vessel Length
Longer the length of a vessel the greater the resistance and lower the flow
The total length of the circulatory system is relatively constant
Flow is greatest at output side of the heart
Flow is zero at input side of the heart
How is the body designed to aid blood movement in the relatively low pressure
Venous Valves: Prevent backflow
Respiratory Pump: Inhalation increases ↑ abdominal pressure increases, pushing blood to heart
Muscular Pump: Skeletal muscles contract to push blood upward toward heart
Respiratory Pump
Inhaling increases abdominal pressure, squeezing the abdominal blood vessels and forcing blood to the heart
Valves in the veins prevent backflow during exhalation
Muscular Pump
Skeletal muscles contract and relax, and they squeeze the blood towards the heart
The muscular pump is the most important of these two pumps
How to maintain Blood Pressure
Cardiac output - by increasing or decreasing heart rate
Peripheral resistance - by changing the diameter of blood vessel
Blood volume - by increasing or decreasing the volume of the blood
Vasomotor Center
located in the medulla
Controls blood vessel diameter
Transmits action potentials to the smooth muscles of the arterioles
These smooth muscles are always in a slight state of constriction
Thus they can always either increase or decrease in diameter
Increasing the number of action potentials causes muscles to constrict, decreasing diameter of the vessel
Decreasing the number of action potentials causes muscles to relax, increasing diameter of the vessel
Barorecptors
These are pressure receptors
Located in the aorta and most of the large elastic arteries
Increased arterial blood pressure causes these receptors to send action potentials to the vasomotor center
A decrease in blood pressure causes the opposite reaction
System functions to protect against rapid changes in blood pressure
Like when you stand up too quickly
↑ BP increases→ sends signals to vasomotor center to ↓ vessel resistance and lower BP
↓ BP decreases → opposite effect
Chemoreceptors
When O2 and CO2 levels rise and pH decreases
Chemoreceptors in the aortic arch and neck send action potentials to the vasomotor center
Vasoconstriction occurs
Increased blood pressure speeds up the flow of blood to the heart and lungs
↓ O₂ / ↑ CO₂ → trigger vasoconstriction to raise BP and speed up circulation
Epinephrine and Norepinephrine
Secreted by adrenal medulla as part of the fight or flight response
Effect heart by increasing cardiac output
Causes vasoconstriction in arterioles
Vasodilatation in large veins, cardiac, and skeletal muscle
Overall increases blood pressure
↑ BP → ↑ cardiac output + vasoconstriction (arterioles)
Angiotensin II Hormone
Effects peripheral resistance to causes vasoconstriction in arterioles
Increases blood pressure
↑ BP → Strong vasoconstrictor
Antidiuretic Hormone (ADH)
Stimulates kidney to conserve water and increase blood volume
Effects peripheral resistance when there is extreme blood loss
Causes vasoconstriction in arterioles
↑ BP → Retains water + vasoconstriction during blood loss
Nitric Oxide
Vasodilation
Very short lived
Released in response to high rates of blood flow
May be a major factor involved in maintaining blood pressure
↓ BP → Short-lived vasodilation
Alcohol
Causes drop in blood pressure
Inhibits antidiuretic hormone release
Depresses the vasomotor control center and causes vasodilatation
↓ BP → Inhibits ADH + vasodilation