Tunica intima
- Innermost layer
- Contains endothelium: runs uninterrupted through entire cardiovascular system
Internal, external elastic lamina
Present only in arteries
Tunica media
Contains smooth muscle fibers (vasoconstriction)
Tunica externa
Connective tissue that stabilizes blood vessels
Structure and function of arteries
- Need to handle larger amounts of pressure from the heart
- Have thicker walls, smaller lumens than veins
Elastic arteries aka:
Conducting arteries
Elastic arteries
- Includes aorta and its principal branches
- Have large lumens to collect large volumes of blood from the heart
Muscular arteries aka:
Distributing arteries
Muscular arteries
- Have the largest tunica media of any blood vessel type; possess a great ability to vasoconstrict
- Branch extensively to deliver blood to arterioles throughout the body
Arterioles
Smallest artery type
Largest arterioles
- Still contain all three tunics
- Resemble small muscular arteries
Tiniest aerterioles
Simply endothelium surrounded by 1-2 fiber layers of smooth muscle
___ of arterioles regulates blood flow into capillary beds
Diameter
Resistance vessels
Arterioles that change their diameter to allow more or less blood flow
Metabolic responses
Adjust blood flow based on specific demands of local tissue
- Ex: increased NO release during exercise
Myogenic responses
Occur reflexively within smooth muscle of tunica media
- Ex: increased pressure causes contraction
Capillaries
- Smallest blood vessel type, so erythrocytes flow in single file
- Site of gas and nutrient exchange: only tunic intima is present, and is permeable
Continuous capillaries
- Most common type
- Lining does not contain pores
- Exchange occurs through intercellular clefts and pinocytic vessels
Fenestrated capillaries
- Endothelial cells dotted with pores
- More readily allows passage of fluid and larger molecules
Sinusoid capillaries
- Have larger fenestrations and a discontinuous basement membrane
- Allow movement of entire cells through barrier
Organization of capillaries into capillary beds
- Precapillary sphincters
- Terminal arteriole
- Metarteriole
- True capillaries
- Thoroughfare channel
- Vascular shunt
- Postcapillary venule
Precapillary sphincters
Bands of smooth muscle that encircle each true capillary at its origin from a metarteriole
Terminal arteriole
Delivers blood to capillary bed
True capillaries
Vessels of exchange: branch off the metarteriole, converge upon the thoroughfare channel
Postcapillary venule
Drains blood away from capillary bed
Vascular shunt
Directly connects the terminal arteriole to the postcapillary venule
Thoroughfare channel
Arises from the metarteriole; returns blood to postcapillary venule
Metarteriole
Arises from the terminal arteriole
Venules
- Smallest of vessels carrying blood back to the heart
- Have all three tunics, but very thin
- Have large lumens: blood reservoirs or capacitance vessels
Venyles converge to form ___
Veins
Systemic veins- blood volume
64%
Systemic arteries- blood volume
13%
Systemic capillaries- blood volume
7%
Pulmonary circulation- blood volume
9%
Heart- blood volume
7%
Veins contain ___
Valves
Veins experience much lower pressure than ___
Arteries
To ensure blood returns to heart:
- Large lumens offer less resistance to blood flow
- Venous valves (specialized foldings of the tunica intima) ensure unidirectional blood flow
Large lumens offer ___ resistance to blood flow
Less
Venous valves (specialized foldings of the tunica intima) ensure ___ blood flow
Unidirectional
What are varicose veins?
Failure of venous valves, allowing blood to pool in peripheral veins of legs
Varicose veins- causes
Anything that impedes venous return (pregnancy, obesity, prolonged periods of standing)
Varicose veins- symptoms
Dilation and distension of veins, sometimes causing discomfort or pain
Blood flow (F)
Volume of blood moving through a vessel, tissue, organ or entire circulation per unit of time
Blood pressure (BP or P)
Force exerted onto a given area of the vessel wall by the blood contained within it, measured in mm Hg
Resistance (R)
Friction encountered by blood, impeding its flow
F =
Delta P/ R
Three variables influence resistance
- Blood viscosity
- Blood vessel length
- Blood vessel radius
Poiseuille's equation
Delta P = 8nLF/pi(r)4
Rearranging for F
F = pi(delta)(P)(r)4/8nL
Orignal equation
R = delta P/F
Substituting F with poiseuille's equation
R = 8nL/pi(r)4
Delta P
Difference in pressure between two points
n
Viscosity of the bluid
L
Length of the tube
F
Rate of flow
Pi
Mathematical constant
r
Radius of the tube
Total peripheral resistance (TPR)
- Used to describe forces impeding blood flow throughout the entire circulation
- Mainly due to veins, not arteries
Flow is normally ___, but abrupt changes in diameter can lead to ___ flow, increasing ___
Laminar; turbulent; resistance
Blood vessels exhibit ___
Compliance
- Ability to change structure in response to changes in pressure
Mean arterial pressure =
Diastolic blood pressure + Pulse pressure/3
Capillary blood pressure and exchange
- CHP
- IFHP
- BCOP
- IFCOP
CHP
Pressure exerted by blood onto capillary wall
- IFHP
- BCOP
Pressure opposing filtration
IFCOP
Pressure exerted by proteins in interstitial fluid
Net filtration pressure =
(CHP + IFCOP) - (IFHP + BCOP)
Pressure in veins is much ___ than arteries
Lower
Muscular pump assists venous return
During contraction, bulging muscles compress veins, forcing blood back towards the heart
Respiratory pump also assists venous return
During inspiration, intrathoracic pressure decreases, drawing blood towards the thoracic cavity
Blood pressure varies with ___ and ___
Cardiac output; peripheral resistance
MAP = CO x TPR
(1)
Increasing either cardiac output or peripheral resistance increases blood pressure
MAP = CO x TPR
(2)
CO = SV x HR
Increasing either stroke volume or heart rate increases cardiac output
Nervous control of blood pressure
Baroreceptors
Receptors (BP increased)
- Baroreceptors in various locations are stimulated
- More impulses sent to cardiovascular center
Control center (BP increased)
- Cardioinhibitory center is excited
- Cardioacceleratory and vasomotor centers are inhibited
Effectors (BP increased)
- Heart: HR and contractility decrease
- Blood vessels: vasodilation
- Lowered CO and R will lower BP
Receptors (BP decreased)
- Baroreceptors in various locations are inhibited
- Fewer impulses sent to cardiovascular center
Control center (BP decreased)
- Cardioacceleratory and vasomotor centers are excited
- Cardioinhibitory center is inhibited
Effectors (BP decreased)
- Heart: increased HR and contractility
- Blood vessels: vasoconstriction
- Increased CO and R will increase BP
Other methods of blood pressure control
- Chemoreceptors
- Higher brain centers
- Endocrine factors
Chemoreceptors
- Monitor oxygen, carbon dioxide, hydrogen ion contents of blood
- Objective: change cardiac output and blood pressure to meet varying metabolic needs of body
Higher brain centers
- Cerebral cortex and hypothalamus also relay information
- Communicate with the limbic system; cause of physical manifestation of emotions
Endocrine factors
- Antidiuretic hormone (ADH): aka. vasopressin; causes vasoconstriction
- Thromboxane and serotonin: cause vasoconstriction
- Epinephrine and norepinephrine: cause vasoconstriction
Aging effects on circulation
- Blood vessels stiffen and narrow due to loss of elasticity, decreased vessel compliance and accumulation of fatty deposits (plaques)
- Stiffened blood vessels cause heightened MAP, which further stiffens blood vessels -> vicious cycle
- Increased risk of myocardial infarction (MI)
- Cardio-protective estrogen effects are lost in post-menopausal women