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Capillaries
Connect small arteries and small veins
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Vessel Wall: Tunica Interna
- Inner lining of blood vessel
- Layer of epithelial cells
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Tunica Interna Functions
- Selective permeable barrier
- Secretes chemicals that affect dilation/vasoconstriction of vessel
- Repels platelets to avoid clot formation
- When tissue around vessel is inflamed, it produces cell-adhesion molecules to recruit leukocytes
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Tunica Media
- Middle layer
- Made of smooth muscle, collagen, and elastic tissue
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Tunica Media Functions
- Strengthens vessel to prevent rupture
- Vasomotion: change in dilation of blood vessel
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Tunica Externa
- Outermost layer
- Loose connective tissue
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Tunica Externa Function
Anchors vessel to something
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Conducting Arteries
- Biggest arteries
- Aorta, pulmonary trunk, common carotid, subclavian, common iliac
- Elastic tissue between wall layers
- Expand during (ventricular) systole recoil during diastole
- Decreases fluctuations in blood pressure
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Distributing Arteries
- Carry blood to specific organs
- Brachial, femoral, renal, splenic
- Smooth muscle makes up most of wall
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Arterioles
- Smalled arteries
- Control amount of blood to various organs
- Metarterioles: link arterioles to capillaries
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Aneurysm
- Thin-walled bulging sac in artery wall
- Often in abdominal aorta, renal arteries, and arterial circle surrounding hypothalamus
- Often results from hypertension and/or atherosclerosis
- Sometimes pain
- Possibility of hemorrhage
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Capillaries
Site of gas, nutrient, hormone, waste transfer between blood and tissue
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Continuous Capillaries
- Most common tissues
- Endothelial cells have tight junctions, allow for passage of glucose, oxygen
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Fenestrated Capillaries
- Kidneys, endocrine glands, small intestine
- Endothelial cells have lots of holes to allow for rapid absorption/filtration
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Sinusoids
- Liver, bone marrow, spleen
- Blood-filled spaces with large fenestrations to allow proteins, clotting factots, new blood cells to enter circulation
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Capillary Beds
Network of capillaries supplied by metarteriole
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Precapillary Sphincter
Regulates whether bed is perfused with blood
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Sphincter Open
Blood Flows Through
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Sphincter Closed
Blood bypasses capillaries through thoroughfare channel
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Veins vs Arteries
- Greater Capacity of blood
- Thinner walls, less muscular, and elastic tissue
- Collapse when empty, expand when blood arrives
- Blood is under little pressure
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Venules
- Smallest Veins
- Porous, so exchange fluid with surrounding tissue
- Site of most leukocyte emigration
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Medium Veins
- Thin walls
- Tunica interna forms venous valves
- Examples: radial, ulnar, saphenous veins
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Venous Valves
- Muscle contraction pushes blood through valves to help get it up to the heart
- Failure of these valves to operate causes varicose veins (blood pools in lower legs)
- Hemorrhoids are varicose veins of anal canal
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Large Veins
- Some smooth muscle in all wall layers
- Venae cavae, pulmonary veins, internal jugular veins, renal veins
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Simple Pathway
1 Capillary Bed
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Portal System
2 Capillary Beds
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Arteriovenous Anastomosis
Shunt
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Anastomosis
Many branches
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Blood Pressure
- Force exerted against vessel wall
- Measured at brachial artery with sphygmomanometer
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2 Components of Blood Pressure
Systolic and Diastolic pressure
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Systolic Pressure
Peak bp taken during ventricular systole
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Diastolic Pressure
Minimum bp taken during ventricular diastole
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Normal BP
Around 120/75 mm Hg
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Pulse Pressure
Difference between systolic and diastolic pressure
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Mean Arterial Pressure (MAP)
- Mean blood pressure throughout cycle
- Best predictor of edema, fainting, atherosclerosis, kidney failure, aneurysm
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Elasticity of Arteries
- Ability of arteries to stretch and recoil prevents excessive bp
- Lose elasticity with age, which increases bp
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What determines blood pressure?
- Determined by cardiac output, total blood volume, peripheral resistance
- Resistance determined by blood viscosity, vessel length, vessel radius
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Hypertension
- High bp
- Resting bp greater than 140/90
- Can weaken small arteries and cause aneurysms
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Hypotension
Caused by blood loss, dehydration, anemia
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Autoregulation
Tissues can regulate vasodilation/vasoconstriction
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Vasoactive Chemicals
Platelets, endothelial cells, perivascular tissue affect vasomotion
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Reactive Hyperemia
If blood flow is impacted (blocked), flow increases to compensate
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Angiogenesis
Growth of new blood vessels
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Neural Control of BP
- Vasomotor center in medulla controls sympathetic regulation of blood vessels
- Stimulates vessels to constrict EXCEPT those in skeletal and cardiac muscles
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Angiotensin II
Vasoconstrictor hormone released by the kidneys
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Aldosterone and ADH
Promotes water retention so increases blood volume and pressure
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Epinephrine and Norepinephrine
- Vasoconstriction of most blood vessels
- Vasodilation of skeletal and cardiac muscle vessels
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Diffusion
- Chemicals pass passively between blood and tissues
- Steroid hormones, O2, CO2, go through plasma membranes of epithelial cells
- Glucose and electrolytes pass through pores between cells
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Transcytosis
- Endothelial cells pick up chemicals in vesicles and dump them out on other side
- Fatty acids, albumin, some non-steroid hormones
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Filtration and Reabsoption
- Fluid filters out of the arterial end of capillary and reenters at venous end
- Delivers materials to cell and picks up waste
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Mechanisms of Venous Return
- Pressure gradient
- Gravity drains blood from head and neck
- Skeletal muscle pumps: contracting muscle squeezes blood out
- Thoracic Pump: inhalation increases abdominal pressure to force blood up
- Cardiac Suction
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Pulmonary Circulation
- Pulmonary arteries branch and go to lobes of the lungs
- 3 lobes/branches on right, 2 on left
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Pulmonary Capillaries
- Capillary beds form around alveoli
- Site of gas exchange with air and lungs
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Ascending Aorta
Right and Left coronary arteries
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Aortic Arch
- Brachiocephalic: right carotid (supplying right side of head) and right subclavian (supplying right shoulder and arm)
- Left Common Carotid: left side of head
- Left Subclavian: left shoulder and arm
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Descending Aorta
- Thoracic Aorta: above diaphragm
- Abdominal Aorta: below diaphragm
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Arteries of Head and Neck
- Common carotid divides into internal and external
- External carotid supplies most of external head (everything but brain)
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Arteries of the Brain
- Vertebral arteries combine to form basilar artery
- Basilar artery and internal carotid arteries form Circle of Willis
- Anterior, middle posterior cerebral arteries
- Superior, anterior, posterior cerebellar arteries
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Blood Flow to Brain
- Needs about 700 ml/min
- Seconds of deprivation cause loss of consciousness
- 4-5 minutes of loss cause irreversible brain damage
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Transient Ischemia Attack (TIA)
- Brief episode of cerebral ischemia
- Dizziness, loss of vision, weakness, paralysis, headache, aphasia
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Cerebral Vascular Accident (stroke)
- Ischemia that causes death of brain tissue
- Caused by atherosclerosis, thrombosis, or ruptured aneurysm
- Blindness, paralysis, loss of sensation, loss of speech, sometimes death
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Veins of the Brain
- Blood from brain flows into dural sinuses
- Sinuses connect to internal jugular vein
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Veins of Head and Neck
- External jugular vein receives blood from external parts of head
- Arm drained by subclavian vein
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Arteries in the thorax
- Thoracic aorta supplies viscera and body wall
- Esophagus, lungs, spinal cord, vertebrae, diaphragm, back muscles, intercostal muscles
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Right Subclavian
- Overlaps with thoracic aorta
- Diaphragm, breast, pericardium, ribs, intercostal muscles, shoulder and pectoral muscles, some arm muscles
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Thoracic Veins
Superior vena cava receives blood from upper thorax, head, neck and arms
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Abdominal Aorta
- Branches supply lower trunk, adrenal gland, intestine, gonads, abdominal wall, spinal cord, sacrum, coccyx
- Celiac trunk: supplies stomach, liver, pancreas and spleen
- Mesenteric Arteries: supplies intestines, pancreas, colon, rectum
- Renal Arteries: kidneys
- Iliac arteries: supply legs
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Arteries in the Arm
Arise from subclavian artery
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Veins in the Arm
- Merge with jugular veins to form superior vena cava
- Superficial veins carry more blood and are often visible
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Arteries of the Legs
- Arise from iliac arteries
- External iliac becomes femoral artery
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Veins of the Legs
Iliac veins drain blood from legs into inferior vena cava
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