Maintenance of the Body
Check Your Understanding
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?
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
, the on/oﬀ opening and
closing of precapillary sphincters in response to local autoregu-
Capillary Exchange of Respiratory Gases
Oxygen, carbon dioxide, most nutrients, and metabolic wastes
pass between the blood and interstitial ﬂuid by diﬀusion. Re-
call that in
, 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
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
ﬂuid-ﬁlled intercellular capillary cleFs or
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
direction and amount
across capillary walls reﬂect the balance between two dynamic
and opposing forces—hydrostatic and colloid osmotic 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
sure exerted by blood on capillary walls.
tends to force ﬂuids through capillary walls (a
), leaving behind cells and most proteins.
Blood pressure drops as blood ﬂows along a capillary bed, so
is higher at the arterial end of the bed (35 mm Hg) than at
the venous end (17 mm Hg).
cell in lumen
Endothelial cell nucleus
via vesicles or
Capillary transport mechanisms.
possible pathways or routes of transport across the endothelial cell
wall of a fenestrated capillary.