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Collin County Community College
BIOL. 2402Anatomy & Physiology
WEEK 7
Capillary Exchange
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The capillary isthe primary pointexchange between theblood and theinterstitial fluid (ISF).
Intercellular cleftsassist the exchange.
There are many, many capillaries, so there is a huge amountof surface area for exchange between the blood and the ISF.
Capillary walls are a singleendothelial cell in thickness.
Blood flow in the Capillary beds
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Capillaries lack smooth muscle, but contraction/relaxation of circularsmooth muscle in upstream metarterioles and precapillary sphinctersdetermine the volume of blood each capillary receives.
Blood flow in the Capillary beds
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In other words, thesmaller a blood vessel,the faster the velocity at
constant flow .
Does this means thatblood velocity isextremely fast in
capillaries ?
Velocity of Blood flow
Velocity = Flow per crosssectional area
Blood flow in the Capillary beds
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The answer is of course NO , because there is an amazingbranching that occurs in the CVS.
Because of the branching, the sum of all the cross sectional areasof all blood vessels increases as we get closer to the capillaries.
Blood flow in the Capillary beds
In addition, due to that branching, flow decreases as well ineach new branching system (thus dropping BP as we moveaway from the BP source - the heart).
The result is that at the capillary level, blood velocity and bloodflow through each capillary is extremely low.
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Six balls in per minute mandates six balls out per minute.Therefore, the velocity of the balls in the smaller tubes is slower.
Velocity of Blood flow
Velocity = Flow per crosssectional area
Blood flow in the Capillary beds
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Blood flow in the Capillary beds
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There are many, many capillaries, each with slow-moving blood in it, resulting in adequate time and surface area for exchange between the capillary blood and the ISF.
Blood flow in the Capillary beds
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This low blood velocity in the capillaries isextremely important, because it allowsadequate time for exchange of nutrientsand gases within the capillary beds.
Blood flow in the Capillary beds
Exchange of nutrients and gasesis between the blood in thecapillary vessels and thesurrounding interstitial fluid.
The pathway for exchange is• via the the pores and clefts of
the capillary walls.• through the endothelial cells• vesicular transport
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• Flow of water and solutes from capillaries to interstitialspace
• Plasma and interstitial fluid are in constantcommunication
• Assists in the transport of lipids and tissue proteins• Accelerates the distribution of nutrients• Carries toxins and other chemical stimuli to lymphoid
tissues
Capillary Exchange
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Forces involved in Capillary Exchange
Hydrostatic Pressure
Osmotic (Oncotic) Pressure
• Hydrostatic pressure (blood pressure) in capillaries
• Hydrostatic pressure in Interstitial FluidPushes fluid out of the capillaries
Directs fluid back into the capillaries
• Osmotic Pressure in CapillariesAttracts fluid into the capillaries
• Osmotic Pressure in ISFPulls fluid out of the capillaries
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Pc
Pif
Pif is constant and doesn’t change much
Pc changes along the capillary bed and determines flow direction
Pc = outward force due to capillary pressure
Pif = inward force due to interstitial fluidpressure
Forces involved in Capillary Exchange
Hydrostatic Pressure Forces
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Πp and Πif are constant and don’t change much
Pc changes along the capillary bed and determines flow direction
Πp = inward force due to capillary osmoticpull
Πif = outward force due to interstitial fluidosmotic pull
Forces involved in Capillary Exchange
Oncotic Pressure Forces
ΠpΠif
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Pif
Πp
Πif
Net force = Pc Pif Πp Πif- - +
ΠpPif Πif are constant and don’t change much
Pc changes along the capillary bed and determines flow direction
28 mm Hg
3 mm Hg
0 mm Hg
Forces involved in Capillary Exchange
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Arterial end Venous end
Pc35 mm Hg
Pc15 mm Hg
Arterial end
Net force = Pc Pif Πp Πif- - +
Net force =
= 35 - 0 - 28 + 3 = + 10 mm HgVenous end
15 - 0 - 28 + 3
= - 10 mm Hg
Filtration Absorption
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Usually the outward net force(filtration ) balances the inwardnet force (absorption).
What if the net inward force isless than the net outward force ?
• Not all fluid returns to bloodstream and remains in interstitial fluid
• Excess fluid is re-directed into blood stream by means oflymphatic system
Forces involved in Capillary Exchange
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Forces involved in Capillary Exchange
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Imbalances in the forces may drive excess fluid into theinterstitial fluids and cause Edema• High blood pressure
• increases Arterial Pc more than Venous Pc• Liver problems
• liver makes most of blood proteins• less blood proteins, less oncotic inward pressure Πp
• Capillary damage• blood proteins leak into the ISF and increases Πif
Forces involved in Capillary Exchange
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Arterial endNet force = Pc Pif Πp Πif- - +
Net force =
= 35 - 0 - 28 + 3 = + 10 mm Hg
Venous end15 - 0 - 28 + 3
= - 10 mm Hg
Forces involved in Capillary Exchange
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Lymphatic System
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• Lymphatic capillaries
• Small lymphatic vessels (veins)
• Major lymph-collecting vessels
The lymphatic system has neither a heart nor arteries. Itsmicroscopic dead-end capillaries extend into most tissues,paralleling the blood capillaries.
Lymphatic System and Vessels
The lymphatic circulation is a drainage system. Its job inmaintaining fluid balance is to:
• collect excess interstitial fluid and return it to theblood (approximately 3 liters daily).
• return plasma proteins to the blood.
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Lymphatic System
• The lymphatic system also– Contains cells (mostly lymphocytes), tissues, and
organs responsible for defending the body• Thus the other function of the Lymphatic system
is to resist infection and disease by responding to– Invading pathogens such as bacteria or viruses– Abnormal body cells such as cancer cells– Foreign proteins such as toxins