Anatomy 2 Ch 21

arteries

always carry blood away from heart

arterioles

smaller branches from arteries

capillaries

smallest vessels; where exchange of materials takes place

venules

smaller branches that merge into veins; carry blood to veins

veinss

always return blood to the heart

tunica interna

aka tunica intima

innermost layer

simple squamous epithelium

direct contact with lumen (interior opening)

tunica media

middle layer

smooth muscle and elastic fibers

smooth muscle allows vasoconstriction and vasodilation (controls diameter of vessels)

tunica externa

outermost layer

elastic and collagen fibers

adjacent to surrounding tissue

layers

tunics

artery tunica media

thick, muscle allows diameter control, elastic fibers allow high compliance (walls stretch and expand in response to small increase in pressure)

compliance

ability for tissue to stretch to accommodate pressure and go back to its original shape

elastic arteries

* conducting arteries
* large diameter
* more elastic fibers, less smooth muscle (more compliance closer to heart)
* function as pressure reservoirs
* ex aorta and pulmonary trunk

muscular arteries

* distributing arteries
* medium diameter
* more smooth muscle, fewer elastic fibers
* distribute blood to various parts of the body (instrumental in controlling blood flow to body and adjusting blood pressure)
* ex radial artery

pressure reservoir

blood ejected from the heart enters elastic arteries, their walls stretch to accommodate the surge of blood. As this occurs and the elastic fibers stretch, they store mechanical energy and function as a pressure reservoir. When they recoil and their stored energy propels the blood forward even though the ventricles are relaxed

anastomosis

* when the branches of 2 or more arteries supplying the same area unite (may also occur between veins, arterioles and venules aka net arteriole)
* provides alternative routes if there is a blockage (collateral circulation)

end arteries

do not anastomose

no alternative route for circulation → obstruction leads to necrosis

happens a lot with coronary arteries so if a vessel gets blocked heart suffers ischemia and eventually infarction and dies

necrosis

tissue death

arterioles

* microscopic arteries that lead to capillaries

metarteriole

end of artery that tapers toward capillary junction

precapillary sphincter

distal-most muscle cell at capillary junction (found between net arteriole and capillary bed to control blood flow)

regulates blood flow

thoroughfare channel

distal end of metarteriole with no precapillary sphincter

allows circulation from arteriole directly to venule; bypasses capillaries

capillaries

* microscopic, thin-walled vessels
* tunica interna is simple squamous epithelium
* lack tunica media and tunica externa
* flow is controlled by precapillary sphincters and is intermittent (vasomotion)
* where most exchange of materials takes place
* permeability varies from one tissue to the next
* the pattern of capillary density also varies from one body part to the next
* high count in kidney and liver

continuous capillary

* most capillaries are tis
* found in CNS, lungs, muscle tissue, skin

fenestrated capillaries

* pores allow some smaller proteins such as hormones to enter circulation
* found in kidneys, villi of small intestines, choroid plexuses in brain ventricles, most endocrine glands

Sinusoid capillaries

* wider and more winding than other capillaries
* can allow proteins or even blood cells to enter bloodstream from tissues (this is how RBCs enter bloodstream in red bone marrow)
* contain special lining cells adapted to function of the tissue
* found in liver, spleen, anterior pituitary, parathyroids, adrenals

venules

* thin-walled
* formed by merging capillaries
* drain blood from capillaries into veins
* distensible walls and act as blood reservoirs

postcapillary venules

* nutrient/waste exchange
* WBC diapedesis
* lead to muscular venules

muscular venules

have smooth muscle

veins

* always return blood to heart
* increase in size as move further from venules
* thinner walls and larger lumen than arteries (tunica media is thin and tunica externa is thick)
* one-way valves to prevent backflow (always point toward heart and found in limbs)
* not designed to withstand high pressure
* situated between skeletal muscles so that movement aids circulation

blood distribution at rest

elastic artery size

largest arteries in the body

artery tunica interna

well-defined internal elastic lamina

muscular artery size

medium-sized arteries

elastic artery tunica media

thick and dominated by elastic fibers; well-defined external elastic lamina

elastic artery tunica externa

thinner than tunica media

elastic artery function

conduct blood from heart to muscular arteries

muscular artery tunica media

thick and dominated by smooth muscle; thin external elastic lamina

muscular artery tunica externa

thicker than tunica media

muscular artery function

distribute blood to arterioles

arteriole size

microscopic (15-300 um in diameter)

arteriole tunica interna

thin with a fenestrated internal elastic lamina that disappears distally

arteriole tunica media

one or two layers of circularly oriented smooth muscle; distalmost smooth muscle cell forms a precapillary sphincter

arteriole tunica externa

loose collagenous connective tissue and sympathetic nerves

arteriole function

deliver blood to capillaries and help regulate blood flow from arteries to capillaries

capillary size

miscroscopic; smallest blood vessels (5-10 um in diameter)

capillary and venules tunica interna

endothelium and basement membrane

capillary and postcapillary venule tunica media

none

capillary tunica externa

n/a

capillary function

permit exchange of nutrients and wastes between blood and interstitial fluid; distribute blood to postcapillary venules

postcapillary venule size

microscopic (10-50 um in dimater)

venule tunica externa

sparse

postcapillary venule function

pass blood into muscular venules; permit exchange of nutrients and wastes between blood and interstitial fluid and function in white blood cell emigration

muscular venules size

micrscopic (50-200 um in diamter)

muscular venule tunica media

one or two layers of circulary oriented smooth muscle

muscular venule function

pass blood into vein; act as reservoirs for accumulating large volumes of blood (along with postcapillary venules)

veins size

range from .5mm to 3 cm in diamter

vein tunica interna

endothelium and basement membrane; no internal elastic lamina, contain valves; lumen much larger than in accompanying arteryc

vein tunica media

much thinner than in arteries; no external elastic lamina

vein tunica externa

thickest of the three layers

vein function

return blood to heart, facilitated by valves in limb veins

diffusion

* movement of substances from areas to greater to lower concentration
* important for solute exchange between blood and ISG
* lipid soluble substances: O2, CO2, and steroid hormones - simple diffusion through endothelium of capillary wall
* fenestrations allow larger, water soluble molecules such as NaCl and other nutrients
* sinusoids allow large proteins, such as fibrinogen, and RBCs to cross membrane

Transcytosis

* endocytosis on one side - moves across endothelium of capillary wall - exocytosis to other side
* large lipid-insoluble molecules
* how insulin (small protein) enters blood
* how certain antibodies pass from maternal circulation to fetal circulation

Bulk Flow

* large numbers of particles or ions in a fluid move together in the same direction
* occurs from an area of higher pressure to an area of lower pressure, and it continues as long as a pressure difference exists
* important for regulating relative volumes of blood and ISF

filtration

* how materials move from capillary to interstitial space
* pressure (hydrostatic) driven movement of fluids and solutes from blood to ISF
* occurs more at arterial end than venous end of capillaries
* blood hydrostatic pressure and interstitial fluid osmotic pressure promote filtration

interstitial fluid

ISF

tissue fluid

hydrostatic pressure

BHP

force exerted by the blood confined within blood vessels or heart chambers

higher at the arterial end of the capillary than it is at the venous end

interstitial osmotic pressure

IFOP

form of osmotic pressure exerted by proteins in the blood plasma or interstitial fluid

blood colloid osmotic pressure

BCOP

pressure created by the concentration of colloidal proteins in the blood; mostly albumins

Reabsorption

* how fluid returns to capillaries from ISF
* pressure (osmotic) driven movement of fluids and solutes from ISF back t blood
* IFHP and BCOP promote reabsorption

Starling’s Law of the Capillaries

under normal conditions, the volume of fluid and solutes reabsorbed is almost as large as the volume filtered

NFP=(BHP+IFOP)-(BCOP+IFHP)

hemodynamics

forces involved in blood circulation

blood flow

volume of blood flowing through a tissue at any given time period

factors affecting blood flow

Cardiac output

pressure difference: greater difference = greater blood flow

resistance: higher resistance = lower blood flow

Cardiac output

CO = HR x SV

CO = MAP / R

Blood Pressure

* hydrostatic pressure exerted on vessel walls
* highest in aorta and large systemic arteries, gets lower further away from heart
* the higher the BP, the higher the blood flow
* changes in blood volume affect BP
* created by ventricular contraction and ejection
* measured as systolic/diastolic
* higher the BP the greater the blood flow (highest in arteries closest to heart and lowest in veins closest to heart)

MAP

diastolic BP + 1/3(systolic BP - diastolic BP)

the average pressure in the arteries during 1 cardiac cycle (considered a better indicator of perfusion to vital organs than systemic BP is)

>60mmHg is enough to sustain organs in average person

Vascular Resistance

R

opposition of blood flow due to friction of blood against walls of vessels


1. size of lumen - smaller diameter = greater resistance, and vice versa
2. blood viscosity - higher viscosity (thickness) = greater resistance, and vice versa


1. total blood vessel length - resistance is directly proportional to the length of the vessel

SVR or TPR

total vascular resistance by all systemic blood vessels

* smallest vessels contribute most resistance
* controlled by arterioles

systemic vascular resistance

SVR

total peripheral resistance

TPR

venous return

* occurs due to pressure of left ventricular contraction
* valves
* skeletal muscle pump
* respiratory pump

skeletal muscle pump and respiratory pump

aid venous return due to compressing larger veins and aiding blood flow

leaky tricuspid valve hinders return due to blood regurgitation as ventricles contract

leads to systemic return backup

velocity of blood flow

* inversely related to the total cross-sectional area
* there are many more capillaries, therefore much more cross-sectional area, this means much lower velocity of blood flow
* velocity is highest in arteries close to heart, lowest in capillary beds

systolic pressure

highest arterial pressure measured during ventricular systoled

diastolic pressure

lowerst arterial pressure measured during ventricular diastole

resting BP

110/70 mmHg

Neural regulation

* baroreceptor reflexes
* chemoreceptor reflexes
* hypoxia, hypercapnia, acidosis
* hormonal reflexes
* autoregulation
* found in aorta and large arteries in chest and neck

CV center

group of neurons that regulate heart rate, contractility and blood vessel diameter

baroreceptor reflexes

pressure sensitive

monitor stretching of walls in blood vessels and atria

chemoreceptor reflexes

chemical sensitive

carotid bodies

aortic bodies

O2, CO2, H+

hypoxia, hypercapnia, acidosis

CV center increases sympathetic stimulation → vasoconstriction arterioles and veins

increases BP

Hormonal regulation

* RAA system
* renin-angiotensin-aldosterone
* increases BP
* Epinephrine/norepinephrine
* increases BP
* ADH
* Increases BP
* ANP
* decreases BP

autoregulation

* ability of a tissue to automatically adjust its own blood flow to match its metabolic demand for delivery of oxygen and nutrients and removal wastes
* physical and chemical stimuli can lead to autoregulation

duration of blood travel

one minute to travel throughout body and back to your heart; shorter time for heart to brain

varicose veins

* formed when venous valves become weak or damaged
* veins are dilated and twisted in appearance
* dark blue or purple color
* veins protrude above the surface of the skin and can lead to pain, burning and spasms

spider veins

* dilated venules close to the skin, especially in the lower limb and face
* appear red, blue or purple, resembling a spider web
* accompanied by pain and discomfort in the affected area

Shock

failure of cardiovascular to deliver enough Os and nutrients to meet cellular metabolic needs

hypovolemic shock

* low blood volume
* hemorrhage, loss of fluids, diabetes mellitus

cardiogenic shock

* poor heart function