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 wallsCause – coronary artery disease and strokes2 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. Communication between blood plasma and interstitial fluids2. Distribute nutrients, hormones, dissolved gases3. Aid in movement of insoluble lipids and tissue proteins4. 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. Hypotension BP lower than 90mm Hg2. Skin = pale, cool, moist3. Confusion/disorientation – drop in BP in brain4. Rise in heart rate/rapid, weak pulse5. Cessation of urination6. 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 – identicala. Example: right subclavian , axillary, brachial, and radial arteries

parallels left subclavian, axillary, brachial and radial arteries2. 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 hormonesa. Epinephrine and norepinephrine – suprarenal glands – stimulate

cardiac output and vasoconstrictionb. ADH – pituitary gland – vasoconstriction, kidneys conserve water

as a response to low blood volumec. 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 ADHiii. stimulates “thirst”iv. stimulates production of aldosterone – reabsorb water

from kidneysd. erythropoietin – hormone in kidneys activates bone marrow to

produce RBCe. 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%