When a person stands, hydrostatic pressure increases in the legs and decreases in the head (Fig. 17.28). The driving force for filtration is the difference between capillary hydrostatic pressure (Pc) and interstitial fluid pressure (Pi), which is also known as tissue pressure. Normally, Pi is close to zero, so Pc essentially represents the hydrostatic driving force.

The osmotic pressure of plasma proteins, specifically albumin, is the key factor that prevents fluid loss from capillaries. This osmotic pressure is called colloid osmotic pressure or oncotic pressure (πp). The total osmotic pressure of plasma is approximately 6000 mm Hg, while the oncotic pressure is only around 25 mm Hg. This relatively low oncotic pressure is important for fluid exchange across the capillary because plasma proteins are mainly confined to the intravascular space, whereas electrolytes have similar concentrations on both sides of the capillary endothelium. The actual magnitude of osmotic pressure is influenced by the permeability of solutes to water. The reflection coefficient (σ) indicates the relative impediment to the passage of a substance through the capillary membrane. Water has a reflection coefficient of 0, while albumin (which the endothelium is impermeable to) has a reflection coefficient of 1. Filterable solutes have reflection coefficients between 0 and 1. Additionally, different tissues may have different reflection coefficients for the same molecule, resulting in varying movement of solutes across the endothelial wall. The actual oncotic pressure of the plasma (πp) is defined by the equation (see also Equation 17.19).

The structure of capillaries can vary. In a cross-section of a capillary in a mouse ventricle, the capillary wall is formed by a single endothelial cell, with a pericapillary space occupied by a pericyte and a connective tissue cell. These cells form a functional complex. Plasmalemmal vesicles are prominent in vascular endothelium and play a role in the transport of substances across the blood vessel wall. Tight junctions typically form in small blood vessels, consisting of fusions between apposed endothelial cell surface membranes. In muscular arteries, which are found in papillary muscles, tight junctions similar to those in capillaries are present. However, extensive junctions resembling gap junctions in the intercalated disks between myocardial cells are often seen in arterial endothelium.

Illustration of an electron micrograph of a capillary in cross-section is shown in Fig. 17.27, depicting the junction of two discontinuous endothelial cells.
Fig. 17.28 ). As discussed previously, when a person stands, hydrostatic pressure increases in the legs and decreases in the head.
Tissue pressure, or, more specifically, interstitial fluid pressure (Pi) outside the capillaries, opposes capillary filtration. The difference between Pc and Pi constitutes the driving force for filtration. Normally, Pi is close to zero, and so Pc essentially represents the hydrostatic driving force.
Osmotic Forces. The key factor that restrains fluid loss from capillaries is the osmotic pressure of plasma proteins (such as albumin). This osmotic pressure is called colloid osmotic pressure or oncotic pressure (πp). The total osmotic pressure of plasma is approximately 6000 mm Hg (reflecting the presence of electrolytes and other small molecules, as well as plasma proteins), whereas oncotic pressure is only approximately 25 mm Hg. This low level of oncotic pressure is an important factor in fluid exchange across the capillary because plasma proteins are essentially confined to the intravascular space, whereas electrolytes are virtually equal in concentration on both sides of the capillary endothelium. The relative permeability of solute by water influences the actual magnitude of osmotic pressure. The reflection coefficient (σ) is the relative impediment to the passage of a substance through the capillary membrane. The reflection coefficient of water is 0, and that of albumin (to which the endothelium is essentially impermeable) is 1. Filterable solutes have reflection coefficients between 0 and 1. In addition, different tissues have different reflection coefficients for the same molecule. Hence, movement of a given solute across the endothelial wall varies with the tissue. The actual oncotic pressure of the plasma (πp) is defined by the following equation (see also
Equation 17.19 1 µm 0.5 µm 0.1 µm0.1 µm GJTJ CT Nu PC V BM * V A B C D •Fig. 17.26 A, Electronmicrographofacross-sectionofacapillaryinamouseventricle.Theluminaldiameterisapproximately4µm.Inthissection,thecapillarywallisformedbyasingleendothelialcell(Nu,endothelialnucleus).Thethinpericapillaryspaceisoccupiedbyapericyte(PC)andaconnectivetissue(CT)cell(“fibroblast”),whichformsafunctionalcomplex(arrow)withitself.V,plasmalemmalvesicles.B, DetailoftheendothelialcellinA showingplasmalemmalvesicles(V)attachedtotheendothelialcellsurface.Thesevesiclesareespeciallyprominentinvascularendotheliumandareinvolvedintransportofsubstancesacrossthebloodvesselwall.Notethecomplexalveolarvesicle(asterisk). BM,basementmembrane.C, Junctionalcomplexinacapillaryofamouseheart.“Tight”junctions(TJ)typicallyforminthesesmallbloodvesselsandappeartoconsistoffusionsbetweenapposedendothelialcellsurfacemembranes.D, Interendothelialjunctioninamusculararteryofapapillarymuscle.Althoughtightjunctionssimilartothoseofcapillariesarefoundintheselargebloodvessels,extensivejunctionsthatresemblegapjunctionsintheintercalateddisksbetweenmyocardialcellsoftenappearinarterialendothelium(exampleshownatGJ).
CHAPTER 17  Properties of the Vasculature 365
Junction of two Discontinuousendothelial cells endothelium • Fig. 17.27 Illustration of an electron micrograph of a capillary in cross-section.