Chapter 20 Blood Vessels and Circulation

arteries

carry blood away from the heart to capillaries; have thicker tunica media and narrower lumen than veins

Have more elastic and collagen fibers (spring back to shape)

capillaries

microscopic vessel through which exchanges take place between the blood and cells of the body; found between arteries and veins; contain only tunica intima - allows for rapid gas and nutrient exchange

veins

carry blood back to the heart; have thicker tunica externa and larger lumen than arteries

Have less elastic and collagen fibers

companion vessels

arteries and veins that lie next to each other serving the same body region

lumen

space within a tubular part or organ, such as the space within a blood vessel

tunica intima

the innermost layer of a blood vessel; made primarily of simple squamous epithelium

tunica media

The middle and thickest layer of tissue of a blood vessel wall, composed of elastic tissue and smooth muscle cells that allow the vessel to expand or contract in response to changes in blood pressure and tissue demand

vasodilation

widening of blood vessel lumen diameter

vasoconstriction

narrowing of blood vessel lumen diameter

tunica externa

outer layer of a blood vessel which connects/anchors it to surrounding tissues

types of arteries

elastic (conducting), muscular (distributing), arterioles

elastic (conducting) arteries

largest arteries; conduct blood from heart to muscular arteries; large proportion of elastic fibers

muscular (distributing) arteries

medium sized arteries; distribute blood to specific body regions; most name arteries are this type

arterioles

smallest arteries; smooth muscle usually somewhat constricted (vasomotor tone)

vasomotor tone

produced by constant action of sympathetic vasoconstrictor nerves; blood vessels typically slightly constricted

types of capillaries

continuous, fenestrated, sinusoid

continuous capillaries

capillaries with cells joined together by tight junctions forming continuous lining; small intercellular clefts allow passage of smaller molecule through vessel wall; most common type of capillary

fenestrated capillaries

capillaries with cells joined together by tight junctions forming continuous lining, but cells have fenestrations (pores); small plasma proteins able to pass though pores; found where much fluid transport happens (intestines and kidneys)

sinusoids

capillaries where the endothelial cells form an incomplete lining with large gaps; allow transport of large substances (formed elements and large proteins); found in bone marrow, spleen, and some endocrine glands

capillary beds

network of capillaries that function together; fed by metarteriole

metarteriole

Short vessels that link arterioles and capillaries

precapillary sphincter

band of smooth muscle that adjusts the blood flow into each capillary

vasomotion

Contraction and relaxation cycle of capillary sphincters; causes blood flow in capillary beds to constantly change routes

perfusion

Circulation of blood within an organ or tissue in adequate amounts to meet current needs of the cells; amount of blood entering capillaries per unit time per gram of tissue (mL/min/g)

venules

small vessels that transfer blood from the capillaries; merge to form veins

small and medium-sized veins

companion vessels with muscular arteries; contain valves

large veins

companion vessels with elastic arteries; contain valves

blood reservoir

at rest systemic veins contain 55% of blood in circulation; blood moved into circulation via vasoconstriction and shifted back through vasodilation

simple vessel pathway

one major end artery delivers blood to an organ or region; one end artery branches into arteries that branch into arterioles, each arteriole feeding into a capillary bed; the capillary bed is drained by a venule; venule merge into one major vein; ex. splenic artery to spleen to splenic vein

end artery

a single artery that is the only source of blood for an organ

alternative vessel pathways

arterial anastomosis, venous anastomosis, arteriovenous anastomosis; and portal system

arterial anastomosis

two or more arteries converge to supply the same region; ex. superior and inferior epigastric arteries suppling abdominal wall

venous anastomosis

two or more veins drain same body region; ex. basilic, brachial, and cephalic veins drain upper limb

arteriovenous anastomosis

artery flows directly into vein bypassing capillaries; allows areas to be bypassed if body hypothermic; ex. fingers, toes, ears

portal system

two capillary beds in sequence; path artery → capillary bed → portal vein → capillary bed → vein; ex. hepatic portal system and hypothalamo-hypophyseal portal system

total cross-sectional area

sum of diameters of all vessels of a certain type; as area of the vessels increases, the velocity of flow decreases.; are greatest in capillaries so blood flow is slowest, which allows time for exchange of gases and nutrients

blood flow velocity

speed of blood movement through vessels

capillary exchange

the movement of substances into and out of capillaries (between blood and interstitial fluid; processes include diffusion, vesicular transport, and bulk flow

diffusion

movement of molecules from an area of higher concentration to an area of lower concentration; small solute diffuse through cells or intercellular clefts and large molecules diffuse through fenestrations or gaps in sinusoids

vesicular transport

transport of substances through cell in membrane bound sac (vesicle); occurs both side of capillary (into/out of blood vs into/out of interstitial fluid)

pinocytosis

A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes

exocytosis

a process by which the contents of a cell vesicle are released to the exterior through fusion of the vesicle membrane with the cell membrane

bulk flow

movement of a fluid due to a difference in pressure between two locations - from high pressure to low pressure

filtration

a process that separates materials based on the size of their particles; fluid moving out of blood - small molecules pushed out with fluid, but large stuck inside vessel; occurs on arterial end of capillary

reabsorption

process by which water and dissolved substances are taken back into the blood; occurs on venous end of capillary

hydrostatic pressure (HP)

the force exerted by a fluid against the walls of its container

blood hydrostatic pressure (HPb)

Force exerted per unit area by blood on vessel wall; promotes filtration from capillary

interstitial fluid hydrostatic pressure (HPif)

force of interstitial fluid on outside of blood vessel; close to zero in most tissues

net hydrostatic pressure

difference between blood and interstitial fluid hydrostatic pressures; equals HPb - HPif

colloid osmotic pressure (COP)

the pull on water due to the presence of proteins (colloid)

blood colloid osmotic pressure (COPb)

draws fluid into blood due to blood proteins (e.g., albumins); promotes reabsorption (opposes the dominant hydrostatic pressure)' clinically called oncotic pressure

Interstitial fluid colloid osmotic pressure (COPif)

draws fluid into interstitial fluid; relatively low due to few proteins

net colloid osmotic pressure

difference between blood and interstitial fluid osmotic pressures; equals COPb - COPif

Net Filtration Pressure (NFP)

the difference between net hydrostatic pressure and net osmotic pressure; equals (HPb - HPif) - (COPb - COPif)

NFP at arterial end of capillary

high; favors filtration

NFP at venous end of capillary

low; favors reabsorption

lymphatic system

the network of vessels that picks up excess fluid from interstitial fluid (that isn't reabsorbed), filters it, and returns it to venous circulation

local blood flow

the blood delivered locally to the capillaries of a specific tissue; dependent on: degree of vascularity, myogenic response, local regulatory factors, and total blood flow

degree of vascularization

the extent of blood vessel distribution within a tissue, determines the potential ability of blood delivery; metabolically active tissues have more vessels

angiogenesis

formation of new blood vessels to increase potential perfusion; seen in muscle in response to increase aerobic training (over weeks/months)

regression

return to previous state of blood vessels; for example when someone that has been doing aerobic training stops training for an extended period (weeks/months)

myogenic response

intrinsic ability of vascular smooth muscle to respond to pressure changes; stretch in arteriole wall due to increased blood pressure causes smooth muscle to contract - returns local flow to original levels; less stretch in wall due to decreased pressure causes smooth muscle to relax - returns flow to original levels

local regulatory factors

alter blood flow based on tissue needs such as when more metabolically active or when tissue is damaged; primarily accomplished with vasoactive chemicals (vasodilators and vasoconstrictors)

vasodilators

in local flow, substances that dilate arterioles and relax pre-capillary sphincters; increases flow to capillary beds

vasoconstrictors

in local flow, substances that constrict arterioles and contract pre-capillary sphincters; decreases flow to capillary beds

autoregulation

the ability of tissues to regulate their own blood supply; when tissue activity perfusion can be inadequate and activity increases substances produced (CO₂, lactic acid H⁺, K⁺) - these substances act as vasodilators; vessels constrict when perfusion has increased due to negative feedback

reactive hyperemia

transient increase in organ blood flow that follows a brief period of disruption

blood flow changes due to tissue damage

inflammation causes release of histamine and bradykinin to cause vasodilation; damage to tissues can cause release of leukotrienes and thromboxanes to cause vasoconstriction - to reduce blood loss

total blood flow

amount of blood transported through the cardiovascular system per unit time; equal to cardiac output; can increase with exercise; proportional to pressure gradient divided by resistance (Pressure/Resistance); increases as pressure increases; decreases as resistance increases

blood pressure

the pressure that is exerted by the blood against the walls of blood vessels; highest in the arteries where it fluctuates between two pressures; typically recorded as systolic/diastolic pressures; lowest in veins

blood pressure gradient

the change in blood pressure from one end of a blood vessel to its other end; driving force to move blood through vasculature; increased by increased cardiac output

arterial blood pressure

measure of the pressure exerted by the blood as it flows through the arteries; pulses with cardiac cycle

systolic pressure (SP)

blood pressure in the arteries during contraction of the ventricles (during systole); highest pressure generated; recorded as upper number in blood pressure ratio

diastolic pressure (DP)

blood pressure in the arteries during relaxation of the ventricles (during diastole); lowest pressure generated; recorded as lower number in blood pressure ratio

pulse pressure (PP)

pressure added to the arteries due to contraction of the heart; equals difference between systolic and diastolic pressures (SP- DP)

pulse

beat of the heart as felt through the walls of the arteries

mean arterial pressure (MAP)

average arterial blood pressure across entire cardiac cycle; provides index of perfusion (<60 may indicate insufficient blood flow); equal to diastolic pressure + 1/3 of pulse pressure (DP + (1/3*PP))

capillary blood pressure

high enough for exchange of substances, low enough to not damage tissues; higher on arterial end (favors filtration) and lower on venous end (favors reabsorption)

venous return

The amount of blood returned to the heart by the veins; depends on pressure gradient, skeletal muscle pump, respiratory pump, and valves

skeletal muscle pump

pumping effect of contracting skeletal muscles on blood flow through underlying vessels

vein valves

prevent backflow of blood in veins

respiratory pump

cycling pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand; in inspiration, abdominal pressure increases and blood forced up from abdominal area into thoracic area; in expiration, abdominal pressure decreases and blood from lower limbs allowed into abdominal veins (like when taking kink out of hose) - thoracic pressure also increases causing blood to be pushed into heart from this area

resistance

friction the blood encounters as it moves through the vessels; due to contact between blood and vessel wall; opposes blood flow; affected by viscosity, vessel length, and lumen size

blood viscosity

resistance of fluid to its flow; greater thickness is greater viscosity, which raises resistance; depends on percentage of particles in fluid - increases with increases in plasma proteins or formed particles; decreases with anemia; increases with dehydration

vessel length

the farther liquid travels through a tube, the more cumulative friction/resistance it encounters; typically remains constant but can changes with weight changes; pressure and flow also decline with distance

vessel radius

half the diameter of the lumen of a vessel; smaller lumen has more resistance, less flow

relationship between flow, pressure, and resistance

flow = pressure/resistance; as pressure increases, flow increases (as pressure decreases flow decreases); as resistance increases, flow decreases (as resistance decreases, flow increases)

autonomic reflexes of blood pressure

regulate blood pressure short-term; components: change in blood pressure/stretch of vessel (stimulus) → baroreceptors (sensory receptors) → sensory neurons → cardiovascular center (control center) → motor neurons of ANS → heart and blood vessels (effectors)

blood pressure primarily involves baroreceptor reflex, but can be influenced by chemoreceptor reflex

baroreceptor reflex

the primary reflex pathway for homeostatic control of blood pressure

decreased blood pressure (reduced stretch of vessel wall) → reduced baroreceptors activation → increase in sensory nerve signals → activation of cardioacceleratory center and vasomotor center in brain; inhibition of cardioinhibitory center → increase in sympathetic nerve signals and decrease in parasympathetic nerve signals → increased heart rate, increased stroke volume, and increased vasoconstriction

increased blood pressure (increased stretch of vessel wall) → increased baroreceptors activation → decreased sensory nerve signals → activation of cardioinhibitory center and decreased activation of cardioacceleratory center and vasomotor center→ increase in parasympathetic nerve signals and decrease in sympathetic nerve signals → decreased heart rate, decreased stroke volume, and decreased vasoconstriction

vasomotor center

Clusters of neuron cells bodies located in medulla oblongata that control blood vessel diameter; acts together with cardiac centers to integrate blood pressure control; sends only sympathetic signals

chemoreceptor reflexes

respond to changes in chemical composition of blood, particularly pH and dissolved gases; high carbon dioxide and low pH stimulate chemoreceptors and vasomotor center - increases BP and shift blood to lungs to expire carbon dioxide which helps raise blood pH

blood volume

the total amount of blood circulating within the body; influenced by fluid intake, output, and retention; can be influenced by hormones

hormonal regulation of blood pressure

used to help maintain blood pressure long-term

-Renin-angiotensin system

-Aldosterone

-Antidiuretic hormone (ADH)

-Atrial natriuretic peptide (ANP)

renin-angiotensin system

low blood pressure or sympathetic activated → renin (kidneys) released → inactive angiotensinogen (release from liver into blood) activated into angiotensin I → angiotensin-converting enzyme (ACE) converts angiotensin I to the activated angiotensin II → angiotensin II causes vasoconstriction, stimulates thirst and reduced urine output to increase volume, and stimulates release of ADH and aldosterone → raises blood pressure

antidiuretic hormone (ADH)

hormone produced by hypothalamus and stored/released from the posterior pituitary gland; release stimulated by increased blood concentration or angiotensin II; increases water reabsorption in kidneys (decreases urine output), stimulates thirst, and causes vasoconstriction to raise blood pressure

aldosterone

released from adrenal cortex in response to decreases in sodium levels or angiotensin II; increases absorption of sodium ion and water (decreases urine output)

atrial natriuretic peptide (ANP)

hormone secreted from atrial cells of the heart in response to stretch of atrial walls due to an increase in blood volume; stimulates vasodilation and increases urine output

sphygmomanometer

blood pressure cuff; typically used to measure blood pressure at the brachial artery

hypertension

chronically elevated blood pressure; may lead to damage in blood vessel walls

atherosclerosis

condition in which fatty deposits called plaque build up on the inner walls of the arteries