Circulatory System Blood Vessels and Circulation Flashcards
6 Classes of Blood Vessels
- arteries
- arterioles
- capillaries
- venules
- veins
- anastomoses
arteries
carry blood away from heart
Branch and decrease in diameter
arterioles
Are smallest branches of arteries
Connect to capillaries
Capillaries
are smallest blood vessels
location of exchange between blood and interstitial fluid
Venules
Smallest veins
collect blood from capillaries
Veins
return blood to heart
Converge and increase in diameter
Anastomoses
Bypass connection between vessels
largest blood vessels to exit heart
pulmonary trunk and aorta
Pulmonary trunk
carries blood from right ventricle to pulmonary circulation
Aorta
carries blood from left ventricle
to systemic circulation
largest blood vessels to enter heart
superior and inferior vena cava
pulmonary vein
superior and inferior vena cava
carries blood to the right
ventricle from the systemic
circulation
pulmonary vein
carries blood to the left
ventricle from the
pulmonary circulation
The Smallest Blood Vessels
Capillaries
* Have small diameter and thin walls
* Chemicals and gases diffuse across walls
Arteries, veins, and capillaries
– Have different structures
– Have different functions
Walls have 3 layers
- tunica intima
- tunica media
- tunica externa
The Tunica Intima/Tunica interna
- Is the innermost layer
- Includes:
– the endothelial cell lining - Endothelium = simple squamous epithelial-like cells connected by tight junctions
– With basal lamina of loose connective tissue containing elastic fibers (elastin) - Arteries have internal elastic membrane
– extra layer of elastic fibers on the outer edge
Tunica Media
- Is the middle layer
- Contains smooth muscle cells in loose connective tissue with sheets of elastin
– Binds to inner and outer layers - Arteries have external elastic membrane
– extra layer of elastic fibers on the outer edge
Tunica Externa/Tunica adventitia
- Is outer layer
- Contains collagen rich external connective tissue sheath
- Infiltrated with nerve fibers and lymphatic vessels
- Large vessels contain vasa vasorum
- Arteries = more collagen, scattered elastic fiber bands
- Veins = extensive fiber networks, bundles of smooth muscle cells
Vasa Vasorum
- Small arteries and veins
- Found:
– in walls of large arteries and veins - Function:
– Supply cells of tunica media and tunica externa
Artery Characteristics
Elastic and muscular, thick walls
– Elasticity allows arteries to absorb pressure waves that come with each heartbeat
– Muscular layer allows contractility, change diameter
From heart to capillaries, arteries change
– from elastic arteries
– to muscular arteries
– to arterioles
Elastic Arteries
- Also called conducting arteries
- Diameter up to 2.5cm
- Elastin in all three tunics
– Elasticity evens out pulse force - Stretch (ventricular systole) and rebound (ventricular diastole)
- Not involved in systemic vasoconstriction
Muscular Arteries
- Also called distribution arteries
- Are medium-sized (most arteries)
- Transport blood to organs and tissues
- Diameter 10mm – 0.3mm
- More smooth muscle and less elastin in tunica media than elastic arteries
- Involved in systemic vasoconstriction via sympathetic stimulation
Vasoconstriction
The contraction
of arterial
smooth muscle
by the ANS
Vasodilatation
– The relaxation of
arterial smooth
muscle
– Enlarging the
lumen
Arterioles
- Also known as resistance vessels
- Connect blood supply to capillary beds
- Are small – diameters 300μm – 10μm
- All three tunics thin with few elastic fibers
- Involved in local vasoconstriction via endocrine or sympathetic stimulation
Arterial sense organs
- In walls of some major
vessels, sensory
structures monitor
blood pressure and
chemistry - Transmit information
to brainstem to
regulate heart rate,
blood vessel diameter,
and respiration
Health Problems with Arteries
aneurysm
arteriosclerosis
atherosclerosis
stroke
Aneurysm
– Pressure of blood
exceeds elastic
capacity of wall
– Causes bulge or
weak spot prone
to rupture
– Caused by chronic
high blood
pressure or
arteriosclerosis
Arteriosclerosis
– Variety of pathological conditions causing
changes in walls that decrease elasticity
(“thickenings”)
* Focal calcification = smooth muscle
degenerates, replaced by calcium salts
* Atherosclerosis (common type of
arteriosclerosis)
Atherosclerosis
lipid deposits
Stroke
cerebrovascular accident (CVA)
– Interruption of arterial supply to portion of brain due to embolism or atherosclerosis
– Brain tissue dies and function is lost
Capillaries
- Only vessels with thin enough wall structure to allow complete diffusion
– Designed to allow diffusion to/from the tissue - Diameter 8 μm
– Consists of tunica intima only
– endothelium + basal lamina - Human body contains 25,000 miles of capillaries
Capillary Structure
- Endothelial tube, inside thin basal lamina
- No tunica media
- No tunica externa
- Diameter is similar to red blood cell
Capillary Function
- Location of all exchange functions of cardiovascular system
- Materials diffuse between blood and interstitial fluid
Types of Capillaries
Continuous capillaries
Fenestrated capillaries
Sinusoids
Continuous capillaries
- Normal diffusion to all tissues; majority type
- Complete endothelium, tight junctions
- Functions:
- Permit diffusion of: water, small solutes, lipid-soluble materials
- Block: blood cells and plasma proteins
- e.g., the blood–brain barrier
Are found in all tissues except: epithelia and cartilage
Fenestrated capillaries
- Pores/fenestrations span endothelium
- High volume fluids or large solute transfer
- Permit rapid exchange of water and larger solutes between plasma and interstitial fluid
Fenestrated capillaries are found in
– choroid plexus
– endocrine organs
– kidneys
– intestinal tract
Sinusoids
– Gaps between endothelial cells
– Cell or large protein exchange
– Permit free exchange of water and large plasma proteins between blood and interstitial fluid
– Phagocytic cells monitor blood at sinusoids
Sinusoids are found in
– liver
– spleen
– bone marrow
– endocrine organs
Capillaries Networks
- Organized into Capillary bed or capillary plexus
- Connect 1 arteriole and 1 venule
- Not enough total blood to fill all capillaries at once
– Flow through capillary bed must be controlled based on need via precapillary sphincters
– Metarterioles (thoroughfare channels)
Capillary Sphincter
- Guards entrance to each capillary
- Opens and closes, causing capillary blood to flow in pulses
Anastomoses
- Bypass routes between vessels
– Bypass the capillary bed - Not present in retina, kidney, or spleen
- More common in veins
Veins vs. Arteries
- Are larger in diameter
- Have thinner walls
- Carry lower blood pressure
Veins
- Collect blood from capillaries in tissues and organs
- Return blood to heart
- Can serve as blood reservoir
- Thin walls but large lumens
- Thin tunica media = little smooth muscle or elastin
- Tunica externa = elastin and smooth muscle
- Tunica intima = valves to prevent back-flow
Three Vein Categories
- venules
- medium-sized veins- diameter 2-9 mm
- large veins- diameters up to 3 cm
venules
– very small veins
* Average diameter 20 μm
– collect blood from capillary beds
– Small venules lack tunica media
Vein Valves
- Valves = Folds of tunica intima
- Prevent blood from flowing backward
- Pressure from heart drives blood flow in arteries, but pressure in veins often too low to oppose gravity
- Compression pushes blood toward heart
– Skeletal muscle movement required to “squish” blood through veins
Valves in the Venous System
Valves in tunica
intima insure one
way movement
Health Problems with Veins
- Resistance to flow (gravity, obesity) causes pooling above valves, veins stretch out
– Varicose veins
– Hemorrhoids
Blood Reservoirs in
Venous System
- Venous system contains 65-70% total blood volume
- Can constrict during hemorrhage to keep volume in capillaries and arteries near normal
Blood flow
volume of blood flowing through a vessel in given period
– Total body flow = Cardiac output
Blood Pressure
force per unit area exerted on vessel by blood (mmHg)
– Blood flows from high pressure → low
Resistance
opposition to blood flow, friction
– Incr. blood viscosity = incr. resistance
– Incr. vessel length = incr. resistance
– Decr. vessel diameter = incr. resistance
Pressure (P)
– The heart generates P to overcome resistance
– Absolute pressure is less important than pressure gradient
The Pressure Gradient
The difference between pressure at the heart and pressure at peripheral capillary beds
Flow (F)
- Is proportional to the pressure difference (P)
- Is inversely proportional to resistance (R)
Vascular Resistance (R)
– Due to friction between blood and vessel walls
– Depends on vessel length, diameter and blood viscosity
* Adult vessel length is constant
* Vessel diameter varies by vasodilation and vasoconstriction
– R increases exponentially as vessel diameter decreases
Vasoconstriction
– Decr. Flow
– Incr. Blood Pressure
– Incr. Resistance
Vasodilation
– Incr. Flow
– Decr. Blood Pressure
– Decr. Resistance
Measuring Blood Pressure
- Blood pressure changes throughout body
– Greatest in arteries leaving heart, lowest in veins returning to heart
– Measured at brachial artery using
sphygmomanometer - Person’s BP measures
– Systolic pressure/diastolic pressure (from
ventricles)
– “Normal”: 120/80 mmHg
pressures in the systemic circuit
systolic pressure, diastolic pressure, pulse pressure
Systolic pressure
peak arterial pressure during ventricular systole
Diastolic pressure
minimum arterial pressure during diastole
Pulse pressure
difference between systolic pressure and diastolic pressure
Abnormal Blood Pressure
hypertension or hypotension
hypertension
abnormally high blood pressure
– Arterial pressure > 130/90 mmHg
– Causes incr. workload for heart
– Untreated = enlarged left ventricle requires more O2 heart can fail
hypotension
abnormally low blood pressure
– Caused by blood loss, dehydration, anemia
– No specific numerical criterion for hypotension
Mechanisms of Venous Return
pressure gradient
muscular compression of peripheral veins
the respiratory pump
cardiac suction
venous return
flow of blood back to the heart, is achieved by pressure gradient, muscular compression, respiratory pump, cardiac suction
Pressure gradient
Overall, venous pressure gradient toward heart
(venules ~13mmHg to venae cavae ~7mmHg)
Muscular compression of peripheral veins
Compression of skeletal muscles pushes blood toward heart (one-way valves)
The respiratory pump
– Inhaling decreases thoracic pressure
– Exhaling raises thoracic pressure
– Thoracic cavity action
Cardiac suction
– During contraction of the ventricles, valves are pulled downward and atrial space expands
– Slight suction draws blood into atria from venae cavae and pulmonary veins
Capillary Exchange
- Vital to homeostasis
- Functions to feed tissues and remove wastes
- Due to filtration and diffusion
- Dependent on good blood flow and pressure
capillaries move materials across capillary walls by:
- Diffusion
- Transcytosis
- Filtration
- Reabsorption
Diffusion
- Movement of ions or molecules:
– from high concentration to lower concentration
1. Small ions transit through endothelial cells
– e.g. Na+, K+, Ca2+, Cl−
2. Large ions & small organics pass between endothelial
cells or through fenestrated capillaries
– E.g. glucose, amino acids
3. Lipids pass through endothelial membrane
– e.g. steroid hormones
4. Large water-soluble compounds diffuse at fenestrated
capillaries
– e.g. in intestine
5. Large plasma proteins diffuse only at sinusoids
– e.g. in liver
Transcytosis
- Transcytosis is a vesicle-mediated transport
- Material picked up on one side of membrane by pinocytosis or
receptor-mediated endocytosis - Transport vesicles move material across epithelial cell
- Material discharged on other side by exocytosis
- Accounts for only a small fraction of solute exchange across the
capillary wall - Important for fatty acids, albumin, and some hormones such as
insulin
Filtration and Reabsorption
- Fluid filters out of arterial end of capillary and reabsorbed
osmotically at venous end of capillary
– Accelerates distribution of nutrients
– Flushes out toxins and pathogens - Balance between osmosis and hydrostatic pressure
- Hydrostatic pressure
- Colloid osmotic pressure (COP)
Hydrostatic pressure
drives fluid out of capillary
High on arterial end of capillary, low on venous end
Colloid osmotic pressure (COP)
draws fluid into capillary
Results from plasma proteins (albumin)—more in blood
Oncotic pressure = net COP (blood COP − tissue COP)
At arterial end of capillary
– fluid moves out of capillary
– into interstitial fluid
At venous end of capillary
– fluid moves into capillary
– out of interstitial fluid
Edema
Buildup of fluid in the tissues, due to:
– Too much filtration
– Less reabsorption
– blocked lymphatics
Total peripheral blood flow
equals cardiac output
Blood pressure overcomes
friction and elastic forces
to sustain blood flow
If blood pressure is too low:
– vessels collapse
– blood flow stops
– tissues die
If blood pressure is too high:
– vessel walls stiffen
– capillary beds may rupture
Cardiovascular Regulation
- Flow, BP, and resistance must be controlled to
insure delivery of nutrients and removal of
wastes in tissues - Changes blood flow to a specific area:
– at an appropriate time and area
– without changing blood flow to vital organs
3 Regulatory Mechanisms of cardiovascular regulation
- Autoregulation
- Neural Mechanism
- Hormonal Regulation
Autoregulation
ability of tissues to regulate
their own blood supply
– causes immediate, localized homeostatic adjustments
– Single capillary bed: action at a precapillary sphincter
- local vasodilators & local vasoconstrictors
Local vasodilators
(increase blood flow)
* Incr. CO2 or decr. O2
* Lactic acid, Incr. K+ or H+
* Inflammation: histamine, Nitric oxide (NO)
* Elevated temperature
Local vasoconstrictors
(decrease blood flow)
* Prostaglandins
* Thromboxanes
* Endothelins
Neural Mechanisms
Remote control of vessels by the central and autonomic
nervous systems
- cardiovascular (CV) centers
- baroreceptor reflexes
- chemoreceptor reflexes
Cardiovascular (CV) centers
– cardiac and vasomotor centers of medulla oblongata
– adjust cardiac output (CO) and peripheral resistance
– Cardiac Center
– Vasomotor Center
cardiac center
- Cardioacceleratory center: sympathetic = incr. CO
- Cardioinhibitory center: parasympathetic = decr. CO
vasomotor center
exerts sympathetic control
* Stimulates most vessels to constrict, but dilates vessels
in cardiac muscle to meet demands of exercise
Baroreceptor reflexes
- Respond to changes in blood pressure
- Trigger cardiovascular center
Chemoreceptor reflexes
- Respond to changes in blood and CSF CO2 and
O2, pH - Trigger respiratory and cardiac center
Hormonal regulation
- Antidiuretic Hormone (ADH)
- Angiotensin II
- Erythropoietin
- Atrial Natriuretic Peptides (ANP)
Antidiuretic Hormone (ADH)
– From pituitary gland in response to low blood volume
– Causes vasoconstriction and water conservation at kidney
Angiotensin II
– From kidney in response to low BP
– Causes:
* Na+ retention and K+ loss at kidney
* Stimulates release of ADH, stimulates thirst,
Stimulated CO
* Stimulates arteriole constriction
Erythropoietin
– From kidney in response to low O2
– Stimulates production and maturation of RBCs
Atrial Natriuretic Peptides (ANP)
– From atria in response to stretching/high BP
– Causes:
* Increase Na+ and H2O loss at kidney
* Reduced Thirst
* Blocks ADH release
* Stimulates vasodilation
Aging and the
Cardiovascular System
- Decreased hematocrit
- Increased blood clots (thrombus) formation
- Blood-pools in legs
– due to venous valve deterioration - Reduction in max cardiac output
- Increased arteriosclerosis
Blood flow to active vs. inactive tissues must be differentially controlled by
autoregulation, neural
regulation, and hormone release
Cardiac output (CO) cannot increase
indefinitely
