Page 779 - Fundamentals of anatomy physiology

766 Unit 4 Fluids and Transport 2. It accelerates the distribution of nutrients, hormones, and
dissolved gases throughout tissues.
The osmotic pressure (OP) of a solution is an indication
of the force of osmotic water movement. In other words, it repre- 3. It assists in the transport of insoluble lipids and tissue
sents the pressure that must be applied to prevent osmotic move- proteins that cannot enter the bloodstream by crossing the
ment across a membrane. The higher the solute concentration capillary walls.
of a solution, the greater is the solution’s osmotic pressure. The
presence of suspended proteins that cannot cross capillary walls 4. It has a flushing action that carries bacterial toxins and
creates an osmotic pressure called blood colloid osmotic pressure other chemical stimuli to lymphatic tissues and organs
(BCOP). Clinicians often use the term oncotic pressure (onkos, a responsible for providing immunity to disease.
swelling) when referring to the colloid osmotic pressure of body
fluids. The two terms are equivalent. Osmotic water movement Capillary blood pressure declines as blood flows from the
continues until either the solute concentrations are equalized or arterial end to the venous end of a capillary. As a result, the
an opposing hydrostatic pressure prevents the movement. rates of filtration and reabsorption gradually change as blood
passes along the length of a capillary. The factors involved are
Now let’s look at the interplay between filtration and reab- diagrammed in Figure 21–11.
sorption along the length of a typical capillary. In this discus-
sion, remember that hydrostatic pressure forces water out of a Net hydrostatic pressure is the difference between the pres-
solution, and osmotic pressure draws water into a solution. sure inside the capillary wall and the hydrostatic pressure outside
the capillary. The net capillary hydrostatic pressure tends to push
The Interplay between Filtration and Reabsorption water and solutes out of capillaries and into the interstitial fluid.
Factors that contribute to the net hydrostatic pressure include
The continuous movement of water out of the capillaries,
through peripheral tissues, and then back to the bloodstream 1. the capillary hydrostatic pressure (CHP), which ranges from
by way of the lymphatic system has four important functions: 35 mm Hg at the arterial end of a capillary to 18 mm Hg at
the venous end, and
1. It ensures that plasma and interstitial fluid, two major
components of extracellular fluid, are in constant commu- 2. the interstitial fluid hydrostatic pressure (IHP). Measurements
nication and mutual exchange. of IHP have yielded very small values that differ from tissue

21 Figure 21–11 Forces Acting across Capillary Walls. At the arterial end of the capillary, capillary hydrostatic

pressure (CHP) is greater than blood colloid osmotic pressure (BCOP), so fluid moves out of the capillary (filtration). Near the
venule, CHP is lower than BCOP, so fluid moves into the capillary (reabsorption). In this model, interstitial fluid colloid osmotic
pressure (ICOP) and interstitial fluid hydrostatic pressure (IHP) are assumed to be 0 mm Hg and so are not shown.

Return to
circulation

3.6 L/day flows
into lymphatic

vessels

Arteriole Venule KEY

Filtration Reabsorption CHP (Capillary
No net fluid hydrostatic pressure)
24 L/day movement 20.4 L/day
BOP (Blood
m3m5 25 25 25 18 25 osmotic pressure)
mm mm mm mm mm
Hg Hg Hg Hg Hg Hg NFP (Net filtration
pressure)

NFP = +10 mm Hg NFP = 0 NFP = –7 mm Hg

CHP > BCOP CHP = BCOP BCOP > CHP
Fluid forced No net Fluid moves
out of capillary movement into capillary
of fluid