The cardiovascular system: blood vessels and circulation
Objectives:
1. Distinguish among the types of blood vessels based on their structure and
function
2. Describe the control mechanisms that regulate blood flow through the
vessels
3. Explain how the activities of the cardiac,vasomotor,and respiratory centers
are coordinated to control blood flow through tissues
4. Explain how the cardiovascular system responds to the stresses of exercise
and hemorrhage
Function of the cardiovascular system which occurs at the capillary level :
chemical and gaseous exchange between the blood and interstitial fluid across
capillary walls
Blood flows OUT of capillary network  venules  veins  venae cavae(systemic
circuit) and pulmonary veins(pulmonary circuit)
Types of blood vessels
Structure of Vessel walls
The walls of arteries and veins: (lumen- open space inside BV)
1. Tunica intima: innermost lining – endothelium and CT with high % of elastic
fibers
2. Tunica media – smooth muscle tissue in a framework of collagen and elastic
fibers
3. Tunica externa – a sheath of CT around vessel – collagen fibers intertwine
with surrounding tissues to “anchor” the BV
Vasoconstriction and vasodilation – controlled by Autonomic system –
Arteries
1. Elastic arteries – LARGE, RESILIENT – diameters up to 2.5cm
Examples: aorta, pulmonary trunk including the aorta and pulmonary trunk
branches
Tunica media is DOMINATED by ELASTIC FIBERS not smooth muscle
Able to absorb the demand of the pressure changes during cardiac cycle
Notice all of the elastic fibers!
2. Muscular arteries – medium sized arteries(distribution arteries) – distribute
blood to skeletal muscle and internal organs
Diameter approximately - .4cm
Example: external carotid arteries
Less elastic fibers more smooth muscle cells
3 arterioles – internal diameter – 30um
Tunica media – 1-2 layers of smooth muscle cells
Arteriosclerosis – (skleros = hard) a hardening/thickening of arterial walls
Cause – coronary artery disease and strokes
2 forms
Focal calcification: loss of smooth muscle in tunica media replaced
by calcium deposits – result of aging and complications of diabetes
Atherosclerosis – lipid deposits in tunica media due to damage to
endothelial lining
Cause: increased blood cholesterol
(LDL and HDL – both formed in liver – not due to amount of triglycerides!
LDL shuttles cholesterol to peripheral tissues from liver; HDL shuttles
cholesterol from peripheral tissues to liver for storage. LDL can end up in
arterial “plaques” = “bad” cholesterol)
Monocytes in blood try to remove the excess cholesterol rich LDL, and
“fill-up”, slow down and attach to endothelial wall – vessel thickens =
atherosclerosis
CARDIOVASCULAR REGULATION INVOLVES AUTOREGULATION, NEURAL
MECHANISMS, AND ENDOCRINE RESPONSES
1. AUTOREGULATION: a change in tissue condition activates precapillary
sphincters to change pattern of blood flow
2. NEURAL MECHANISMS: respond to arterial pressure or blood gas levels
– autonomic
CAPILLARIES
Allow exchange of materials between blood and interstitial fluids
No tunica media; no tunica externa
Diameter = diameter of RBC = 8um
Water, small solutes, lipid-soluble materials easily diffuse
(exception : the capillaries of the choroid plexus of brain, hypothalamus, and
glomerulus of kidneys – small pores in wall to allow for protein diffusion)
Capillary beds: the functional structure of capillaries –
Single arteriole gives rise to dozens of capillaries which become venules
Precapillary sphincter – controls amount of blood entering capillary bed at
any given time – located at beginning of each capillary in capillary bed
Arteriovenous anastomosis – a vessel that connects an arteriole to a venule –
bypassing a capillary bed
“Insurance policy” – if an artery is blocked or compresses, others can
continue to supply blood to capillary bed preventing tissue damage
Common in brain, coronary circulation and others
VEINS
Collect blood from all tissues and organs and return to heart
Venules – smallest resemble capillary – very small – no tunica media
Medium size veins- resemble muscular arteries – 2mm-9mm diameter
Tunica media – layers of smooth muscle
Tunica externa – elastic and collagen fibers
Limbs – valves to prevent backflow of blood
Large veins – 2 venae cavae and their “tributaries” in thoracic and abdominopelvic
cavities - Tunica media thinner than tunica externa
Thin walls relative to arteries
Varicose veins / hemorrhoids = valves weaken, blood pools in vein
Blood pressure – arterial pressure
Systolic pressure – peak blood pressure(ventricular systole)
Diastolic pressure – minimum BP(ventricular diastole)
“normal” BP = 120/80
Pulse – rhythmic pressure which accompany each heartbeat
Pulse pressure – difference between SP and DP
Decreases as distance from heart increases
Measures 35mmHg when blood reaches precapillary sphincter
Forces acting against capillary walls
At anterior end of capillary filtration occurs – near venule, fluid is reabsorbed
movement is into and out of the lymphatic vessels
3.6 L of water and solutes flow through peripheral tissues into lymphatic
vessels and back into the bloodstream
4 functions of capillary exchange
1.
2.
3.
4.
Communication between blood plasma and interstitial fluids
Distribute nutrients, hormones, dissolved gases
Aid in movement of insoluble lipids and tissue proteins
Flush bacterial toxins and other stimuli to lymphoid tissues and organs of
immunity
Edema: abnormal accumulation of interstitial fluid
Pulmonary edema: congestive heart failure – Left ventricle can’t keep pace with
right ventricle – blood flow becomes “congested” in pulmonary circuit – results in
build up of fluid in lungs
“recall of fluids” – during blood loss or extreme dehydration – movement of water
from interstitial fluid into blood stream to increase blood volume
Shock – acute circulatory crisis marked by low blood pressure – “acute = sudden
in onset, severe in intensity, brief in duration”
Potentially fatal – as vital tissues are starved for oxygen
Circulatory shock – 30% reduction in total blood volume
Symptoms:
1.
2.
3.
4.
5.
6.
Hypotension BP lower than 90mm Hg
Skin = pale, cool, moist
Confusion/disorientation – drop in BP in brain
Rise in heart rate/rapid, weak pulse
Cessation of urination
Acidosis – due to lactic acid generation in oxygen deprived tissues
Types of:
Septic shock, such as toxic shock syndrome, and anaphylactic shock – all due to a
widespread uncontrolled vasodilation – similar in effect and consequences to
circulatory shock
THREE GENERAL FUNCTIONAL PATTERNS OF BV
1. Distribution of arteries and veins on left and right side of body – identical
a. Example: right subclavian , axillary, brachial, and radial arteries
parallels left subclavian, axillary, brachial and radial arteries
2. BV have multiple name changes as they pass different anatomical
boundaries: external iliac artery(trunk) becomes femoral artery(thigh)
3. Tissues and organs usually serviced by several arteries/veins
CARDIOVASCULAR REGULATION INVOLVES AUTOREGULATION, NEURAL
MECHANISMS, AND ENDOCRINE RESPONSES
1. AUTOREGULATION: a change in tissue condition activates precapillary
sphincters to change pattern of blood flow
2. NEURAL MECHANISMS: respond to arterial pressure or blood gas levels
– autonomic nervous system(cardiac center and vasomotor center of
medulla oblongata) adjust cardiac output and peripheral resistance to
maintain adequate blood flow
3. Endocrine mechanisms – release of hormones
a. Epinephrine and norepinephrine – suprarenal glands – stimulate
cardiac output and vasoconstriction
b. ADH – pituitary gland – vasoconstriction, kidneys conserve water
as a response to low blood volume
c. Angiotensin II – formed in blood activated by renin(enzyme) . in
response to low BP –
i. stimulates cardiac output and arterial constriction
ii. stimulates production of ADH
iii. stimulates “thirst”
iv. stimulates production of aldosterone – reabsorb water
from kidneys
d. erythropoietin – hormone in kidneys activates bone marrow to
produce RBC
e. atrial natriuretic peptide(ANP) – produced by cardiac muscle cells
in wall of right atria – reduces blood volume/BP by blocking
release of ADH, aldosterone, E, and NE
Exercise and cardiovascular system
1. extensive vasodilation – as rate of oxygen consumption in skeletal
muscles increases – increasing blood flow into capillaries
2. venous return increases – increased skeletal muscle contractions and
increased breathing blood pulled into “respiratory pump” of heart
3. cardiac output rises – due to increased venous return –
benefits of modest( jogging 5 miles/week) exercise program:
lower blood cholesterol
reduction in stress
lowers blood pressure
slows plaque formation
moderate exercise can cut risk of heart attack by 50%