Arterioles -> Metarterioles

Metarterioles -> Capillaries

Capillaries -> Venules

*A band of smooth muscle, called the precapillary sphincters, precedes the capillaries - they function like switches to determine blood flow to the capillary bed

* lipid-soluble substances pass through the endothelial cells

* small water-soluble substances pass through the pores (size of pores depends on organ, brain has small pores while liver has large ones)

* exchangeable proteins are moved across by vesicular transport

in general gases and solutes exchange across the capillary wall by simple diffusion

J = DA (Dc / Dx)

J = net rate of diffusion in moles or grams/unit time

D = Diffusion coefficient of the diffusing solute in the membrane, the larger the molecule and the more viscous the medium the smaller the diffusion coefficient

A = Surface area of the membrane, the greater the surface area the greater the rate of diffusion

Dc = concentration difference across the membrane, the greater the concentration gradient the greater the rate of diffusion

Dx = Thickness of the membrane, the thicker the membrane the slower the rate of diffusion

*The driving force of the fluid movement across capillary wall is due to hydrostatic and osmotic pressures

*Those pressures are called Starling pressures or forces

*Fluid movement driven by the sum of hydrostatic and effective osmotic pressures

*Effective osmotic pressure due to plasma protein is called oncotic pressure

Pc = capillary hydrostatic pressure (mmHg)

Pi = interstitial hydrostastic pressure (mmHg)

πc = capillary oncotic (colloid osmotic) pressure (mmHg)

πi = interstitial oncotic (colloid osmotic) pressure (mmHg)

Kf = hydraulic conductance (water permeability) ml/min . mmHg

Jv = Kf x [(Pc - Pi) - (πc - πi)] (ml/min)

*Is the water permeability of the capillary wall

*Determines the magnitude of the fluid movement for a given pressure difference

*Depends on the anatomical characteristics of the capillary wall - e.g. size of the cleft, fenestration of the wall

*Is lowest in the cerebral capillaries and highest in glomerular capillaries

*Increases in capillaries injury - e.g. toxins or burn

* increased due to arteriolar dilation and venous constriction

* decreased due to arteriolar constriction (e.g. hemorrhage)

*increased due to loss of fluid without loss of protein (e.g. dehydration or diarrhea)

*decreased due to less concentrated plasma protein (e.g. liver and renal diseases, infusion of saline instead of plasma or whole blood for bleeding compensation)

more negative thoracic pressure -> decreased in interstitial hydrostatic pressure -> increased filtration (e.g. respiratory distress syndrome)

*increased due to impaired lymphatic system (e.g. tumor in the lymph node

*increased capillary permeability (e.g. burns)

* they lie in the interstitial fluid, close to the vascular capillaries

*posses one-way flap valves, which permit interstitial fluid and protein to enter, but not leave, the capillaries

*lymphatic capillaries merge into larger lymphatic vessels and eventually into the largest lymphatic vessel, the thoracic duct

*lymphatic vessels have a smooth muscle wall, which has contractile ability

normally

*the rate of lymph flow is ~ 1-3 L/day

*the rate of blood flow is ~ 720 L/day

arteriolar dilation

venous constriction

increased venous pressure

heart failure

ECF volume expansion

NOTE - increased capillary hydrostatic pressure leads to edema

decreased plasma concentration

severe liver failure (failure to synthesize protein)

protein malnutrition

nephrotic syndrome (loss of protein in urine)

NOTE - decreased capillary oncotic pressure leads to edema

burn

inflammation (release of histamine; cytokines)

NOTE - increased Kf leads to edema

standing (lack of skeletal muscle compression of lymphatics)

removal or irradiation of lymph nodes

parasitic infection of lymph nodes (e.g. filariasis)

 These cause edema