Microcirculation And Lymphatics Flashcards
Small arterioles characteristics
. Highly innervated by ANS
. Contain vascular smooth muscle
. Site of resistance to blood flow by altering diameter of lumen
Terminal arterioles
. Less innervation by ANS than small arterioles
. Greater control by local mediators
. Contain vascular smooth muscle
. Site of resistance to blood flow by altering diameter of lumen
. Divide to give rise to capillaries
Metarterioles
. Act as thoroughfare vessels form small arterioles to venules
. Capillaries branch from metartioles
. Precapillary sphincters at origin
. Blood flow can bypass capillaries by going through metarterioles
Capillaries
. Major site for metabolic exchange bc of slowest flow and largest surface area
. Flow trough capillaries regulated by autonomic neural and local metabolic mechanisms
Postcapillary venules
. Some gas exchange may occur
General characteristics of capillary blood flow
. Low velocity . Intermittent (vasomotion) . Direction given by pressure gradients . Non uniform . Rouleaux formation
non-nutritive flow
. Most flow through metarterioles
. Bypasses capillaries
. Little metabolic exchange
Nutritive flow
. Active tissue requires greater nutritive flow
. Arterioles and precapillary sphincters dilate
. Inc. blood flow through capillaries to promote optimal exchange of nutrients and waste
. Arteriolar radius determines overall blood flow flow to particular vascular bed
. Precapillary sphincters determines which capillaries are perfused
Continuous capillaries
. Present in muscle, skin, lung, fat, CT, and neural tissue
. Contain intercellular clefts btw adjacent endothelial cells
. Hydrophilic molecules must pass through these clefts or small pores
. Clefts are absent in BBB as well as fenestrae so exchange of hydrophilic substances takes place by transporters or pinocytosis
Fenestrated capillaries
. Surround epithelia
. Present in kidneys, intestines, and endocrine glands
. Contain fenestrae
. Enhances permeability to small hydrophilic molecules
Discontinuous capillaries
. Present in sinusoids
. Wide gaps btw adjacent endothelial cells
. Permeable to large and small, hydrophilic molecules
. Gaps include basement membrane
Starling forces
. Influence transmembrane fluid exchange
. Osmotic and hydrostatic pressure
Diffusion
. Occurs through capillary wall and through pores
. Lipophilic substances pass directly through capillary cells
. Diffusion of hydrophilic substances is limited to pores
. Small molecules (Na, glucose) diffuse easily due to their small radius to pore size
. Large molecules diffuse poorly since they are larger than the pore size
. surface area for diffusion of lipophilic substance is greater than the SA for hydrophilic substances
. Blood remains in capillary from 0.5-1.0s which is enough time for molecules to equilibrate
Ways to regulate diffusion
. Capillary recruitment: inc. SA and reduces difffusion difference
. Inc. in conc. Gradient: metabolically active tissue uses substance to conc. Gradient btw plasma and cell inc.
. Inc. in blood flow: greater delivery of substance maintains the conc. Gradient down capillary
Hydrostatic pressure
. Pressure exerted by liquid as result of potential energy
. Equal in all directions
. Pressure at points lying in same horizontal plane are equal
. Inc. w/ depth under the free surface
Capillary hydrostatic pressure (Pc)
. Depends on ratio of pre and post-capillary resistances
. Favors filtration
. Average value is +30-+40 mmHg
Determinants of capillary hydrostatic pressure
. Arterial pressure: smaller effect bc of significant pre-capillary resistance
. Venous pressure: larger effect
. Ratio of post/pre-capillary resistance vessels (dec. post/pre ration dec. pressure, opposite if inc. post/pre ratio)
Osmotic pressure
. Hydrostatic pressure that results from water movement
. Due to plasma proteins that can’t ;eave capillary lumen due to their low permeability (oncotic pressure)
. Proteins exert +25-+30 mmHg of osmotic pressure
. Factors absorption
Van’t Hoff’s Law
. Actual osmotic pressure generated by charged solute differs significantly from that predicted by the conc. And number of ions formed by dissoc. Of a solute in solution
. Due to electrostatic interaction w/ ions like Na
T/F based on concentration, albumin contributed disproportionately large percentage of plasma oncotic pressure
T
Gibbs’Donnan equilibrium
. Inc. osmotic effects of albumin
. Presence of impermeable and neg. charged protein like albumin in intravascular space creates conditions for this
Transcapillary fluid exchange
. Movement of fluid, exchange of nutrients, out of and back into capillary
.
Net filtration pressure (NFP)
. NFP = (Pc-Pi) - (pi c - pi i)
. Represents balance of hydrostatic and osmotic forces
. Sign determines direction of fluid movement
. Net pos. Filtration pressure indicated filtration
. Net neg. filtration pressure indicates absorption
. Absolute value influences rate of fluid movement
Rate of fluid movement
. FM = Kf[(Pc-Pi) - (pi c - pi i)]
. Kf: filtration coefficient reflecting permeability of the capillaries int that organ bed
. Some capillary beds leaky (kidney, liveR)
. Some tight (brain)
Capillary permeability is influenced by ____
. Number and size of pores in endothelium
. Number of transporters in endothelium
. Number of perfused capillaries in organ bed
. Density of capillaries in organ bed
. Tightness of tight junctions
What occurs to capillary fluid movement when arterioles dilate?
Histamine release dilates arterioles -> inc. Pc -> inc. NFP -> favors filtration
What occurs to capillary fluid movement in vasoconstriction?
. SNS activates constriction -> dec. Pc -> dec. NFP -> favors absorption
What occurs in capillary fluid movement when plasma protein conc. Dec.
. Occurs form liver disease, burns
. Dec. oncotic pressure -> inc. NFP -> favors filtration
What occurs w. Capilllary fluid movement w/ administration of IV colloids?
. Occurs when treating anaphylactic shock
. Inc. oncotic pressure -> dec. NFP, favors absorption
What occurs to capillary fluid movement when Pv inc. from CHF?
. Inc Pc -> inc. NFP -> favors filtration
What occurs to capillary fluid movement in hypovolemia?
. Dec. Pa -> dec. Pc -> dec. NFP -> favors absorption
Lymphatic system
. Provides alternate route for fluids and proteins to flow from interstitial space back to cardiovascular system
. Preserves fluid balance by returning capillary ultraffiltrate and plasma proteins to venous system
. Approx. 8L/day returned back to blood
. Endothelial cell of lymph capillaries overlap forming valves to allow for entry but not exit of interstitial proteins
Major determinants of lymph flow
. Interstitial hydrostatic pressure (Pi)
. As Pi inc. from -6 to 0, flow of fluid into lymph capillaries inc.
. When Pi is over 0 the Max lymph flow is observed bc the high flow of fluid into lymph capillaries is opposed by external. Compression of larger lymph vessels by high Pi
Lymphatic pump
. Fluid fills terminal lympahwetic capillaries
. Smooth muscle in wall of lymph vessels contracts in response to stretch
. Contraction of fluid-filled segments forces lymph through valve into subsequent segment
. Very large lymph vessels (thoracic duct) the pump can generate pressure btw 50-100 mmHg
Precipitation factors of peripheral edema
. Reduction in plasma protein conc.
. Inc. in capillary hydrostatic pressure
. Inc. permeability of capillary membrane
. Lymphatic obstruction
Causes of edema
. CHF
. Mechanical obstruction of venous return
. Renal disease- loss of protein
. Liver disease: lack of protein synthesis
. Burn/anaphylaxis: inc. capillary permeability
. Obstruction of lymphatic due to filariasis
