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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