Microcirculation And Lymphatics Flashcards by Emily Mosher

Small arterioles characteristics

. Highly innervated by ANS
. Contain vascular smooth muscle
. Site of resistance to blood flow by altering diameter of lumen

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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

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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

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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

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Postcapillary venules

. Some gas exchange may occur

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General characteristics of capillary blood flow

. Low velocity 
. Intermittent (vasomotion) 
. Direction given by pressure gradients 
. Non uniform 
. Rouleaux formation

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non-nutritive flow

. Most flow through metarterioles
. Bypasses capillaries
. Little metabolic exchange

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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

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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

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Fenestrated capillaries

. Surround epithelia
. Present in kidneys, intestines, and endocrine glands
. Contain fenestrae
. Enhances permeability to small hydrophilic molecules

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Discontinuous capillaries

. Present in sinusoids
. Wide gaps btw adjacent endothelial cells
. Permeable to large and small, hydrophilic molecules
. Gaps include basement membrane

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Starling forces

. Influence transmembrane fluid exchange

. Osmotic and hydrostatic pressure

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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

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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

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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

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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

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