Circulation Flashcards
Blood flow through the body
-heart to large arteries, to smaller arteries, arterioles, capillaries, venules to veins to the vena cava and back to the heart
Tunica intima
- endothelium
- subendothelial layer
- internal elastic membrane
tunica media
- (smooth muscle and elastic fibers)
- external elastic membrane
Tunica externa
- collagen fibers
- vasa vasorum
Arteries
- thick walls with elastic tissue and smooth muscle and collagen fibres
- inner surface of artery is lined with endothelial cells;
i) secrete substances that allow arteries to contract or dilate (ex. NO results in smooth muscles relaxation)
ii) regulate transport of nutrients and waste products
iii) hormone synthesis
Arterioles
- large amount of smooth muscle for walls; some elastic tissue; less collagen fibres
- inner surface lined with endothelial cells
- aerteriolar smooth muscle possess a large degree of spontaneous activity; myogenic tone
- myogenic tone sets a baseline level of contraction
- increase in contractile force above the vessel’s myogenic tone results in vasoconstriction while decrease results in vasodilation
- arterioles are the primary vessels involved in pressure flow/regulation*
Changes in myogenic tone
(i) local controls:
a) active (reactive) hyperaemia; increased metabilic activity causes a release of a variety of chemicals that result in vasodilation
b) flow autoregulation: decreased BP results in decreased oxygen delivery and decreased waste removal which stimulates the release of a variety of chemicals that produce vasodilation
(ii) extrinsic control:
- sympathetic nervous system is the primary neural regulator; sympathetic neurons release norepinephrine results in vasoconstriction: alpha-1-adrenegic, beta-2-adrenergic
- decreased sympathetic stimulation results in vasodilation
Alpha-1-adrenergic
receptors on the smooth muscle of arterioles when bound to norepinephrine= vasoconstriction
Beta-2-adrenergic
- receptors on the smooth muscle of arterioles that bind to norepinephrine result in vasodilation
- this effect is usually overwhelmed by the alpha-1 response
Parasympathetic NS regulation of vascular resistence
- plays a very minor role
- vasodilation can occur with the parasympathetic stimulation
- control is indirect
capillaries
- 0.006-0.010 mm in diameter
- thin-walled tube of endothelial cells, 1 layer thick resting on basement membrane
- arteriole drainage first goes through metarterioles and then per-capillary sphincters
- the structure of the capillaries is different depending on the location
types of capillaries
continuous: tight junctions (fat, muscle, NS)
fenestrated: pores (endocrine glands, liver)
discontinuous: Large gaps (liver, bone marrow, spleen)
- space between the endothelial cells can be altered through the release of Histamine
capillary regulation of nutrient exchange
- nutrients diffuse from the blood across capillary walls into cells
- metabolic end products move across cell membranes into capillaries
- capillary wall is a filter
- solute concentrations are higher inside the capillaries in the interstitial spaces and water tends to move into the capillaries
Starling hypothesis
-deals with the movement of water into or out of the capillaries
-plasma proteins are a key factor in preventing fluid loss from the capillaries= plasma protein oncotic pressure
Interstitial oncotic pressure; proteins/particles within the interstitium
capillary hydrostatic pressure: fluid pressure within the interstitium
-interstitial hydrostatic pressure: fluid pressure within the interstitium
Direction of water movement within arteries
1) high venous pressure: will result in back pressure into the capillaries, high capillary pressures will push H20 out of the capillaries into the interstitium
2) high arterial pressures:will increase blood delivery to the capillaries, increase blood volume in the capillaries will increase capillary pressures and push H20 into the interstitium
Starling equation
k[(pc+ pi(i))-(pi + pi (p))]
- pc is capillary hydrostatic pressue
- pi= interstitial hydrostatic pressure
- pi p=plasma protein oncotic pressure
- pi i= interstitial fluid oncotic presssure
- under normal conditions, equation is negative and absorption occurs
- if the equation is postive, filtration occurs and edema forms
Changes in Pc
1) increased venous pressure in pulmonary veins can increase
2) increased arterial pressures can increase it
Veins and venules
- thin-walled and more compliant than arteries
- contain some smooth muscle
- also contain some elastic fibers and collagen
- large veins contain smooth muscle
- smooth muscle veins innervated by sympathetic neurons
- peripheral veins contain valves; pressure valves close in response increased venous pressure
vein blood return to the heart
- 3 mechanisms
(a) skeletal-muscle contraction which compress teh veins forcing blood towards the heart
(b) respiration: inspiration causes an increase in abdominal pressure compressing veins while a decrease in thoracic pressure occurs, lowering the pressures in the right atrium
(c) smooth muscle in the larger veins contracts with signals from the sympathetic NS squeezing through the blood stream
