716
UNIT 4
Maintenance of the Body
19
Check Your Understanding
11.
Suppose you are in a bicycle race. What happens to the
smooth muscle in the arterioles supplying your leg muscles?
What is the key mechanism in this case?
12.
If many arterioles in your body dilated at once, you would
expect MAP to plummet. What prevents MAP from
decreasing during your bicycle race?
For answers, see Appendix H.
Blood Flow Through Capillaries
and Capillary Dynamics
Outline factors involved in capillary dynamics, and explain
the significance of each.
Blood flow through capillary networks is slow and intermittent.
Intermittent flow is due to
vasomotion
, the on/off opening and
closing of precapillary sphincters in response to local autoregu-
latory controls.
Capillary Exchange of Respiratory Gases
and Nutrients
Oxygen, carbon dioxide, most nutrients, and metabolic wastes
pass between the blood and interstitial fluid by diffusion. Re-
call that in
diffusion
, net movement always occurs along a con-
centration gradient—each substance moving from an area of
its higher concentration to an area of its lower concentration.
Hence, oxygen and nutrients pass from the blood, where their
concentration is fairly high, through the interstitial fluid to the
tissue cells. Carbon dioxide and metabolic wastes leave the cells,
where their content is higher, and diffuse into the capillary blood.
Tere are four different routes across capillaries for different
types of molecules, as
Figure 19.16
shows.
1
Lipid-soluble
molecules, such as respiratory gases, diffuse through the lipid
bilayer of the endothelial cell plasma membranes. Small water-
soluble solutes, such as amino acids and sugars, pass through
2
fluid-filled intercellular capillary cleFs or
3
fenestrations.
4
Some larger molecules, such as proteins, are actively trans-
ported in pinocytotic vesicles or caveolae.
As we mentioned earlier, capillaries differ in their “leakiness,”
or permeability. Liver capillaries, for instance, are sinusoids that
allow even proteins to pass freely, whereas brain capillaries are
impermeable to most substances.
Fluid Movements: Bulk Flow
While nutrient and gas exchanges are occurring across the
capillary walls by diffusion, bulk fluid flows are also going on.
±luid is forced out of the capillaries through the cleFs at the
arterial end of the bed, but most of it returns to the bloodstream
at the venous end. Tough relatively unimportant to capillary
exchange of nutrients and wastes, bulk flow is extremely im-
portant in determining the relative fluid volumes in the blood-
stream and the interstitial space. (Approximately 20 L of fluid
filter out of the capillaries each day before being returned to the
blood—almost seven times the total plasma volume!)
As we describe next and as shown in
Focus on Bulk Flow Across
Capillary Walls
(Figure 19.17)
, the
direction and amount
of flow
across capillary walls reflect the balance between two dynamic
and opposing forces—hydrostatic and colloid osmotic pressures.
Hydrostatic Pressures
Hydrostatic pressure (HP)
is the force
exerted by a fluid pressing against a wall. In capillaries, hydro-
static pressure is the same as
capillary blood pressure
—the pres-
sure exerted by blood on capillary walls.
Capillary hydrostatic
pressure (HP
c
)
tends to force fluids through capillary walls (a
process called
filtration
), leaving behind cells and most proteins.
Blood pressure drops as blood flows along a capillary bed, so
HP
c
is higher at the arterial end of the bed (35 mm Hg) than at
the venous end (17 mm Hg).
Basement
membrane
Endothelial
fenestration
(pore)
Intercellular
cleft
Lumen
Pinocytotic
vesicles
Caveolae
Red blood
cell in lumen
Endothelial
cell
Intercellular
cleft
Fenestration
(pore)
Endothelial cell nucleus
Tight
junction
Basement membrane
Pinocytotic
vesicles
4
Transport
via vesicles or
caveolae
(large
substances)
3
Movement
through
fenestrations
(water-soluble
substances)
2
Movement
through
intercellular
clefts (water-
soluble
substances)
1
Diffusion
through
membrane
(lipid-soluble
substances)
Figure 19.16
Capillary transport mechanisms.
The four
possible pathways or routes of transport across the endothelial cell
wall of a fenestrated capillary.
previous page 750 Human Anatomy and Physiology (9th ed ) 2012 read online next page 752 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off