Page 778 - Fundamentals of anatomy physiology

The Respiratory Pump. As you inhale, your thoracic cavity Chapter 21 Blood Vessels and Circulation 765
expands, reducing the pressure within the pleural cavities. This
drop in pressure pulls air into your lungs. At the same time, it Filtration
also pulls blood into the inferior vena cava and right atrium
from the smaller veins of your abdominal cavity and lower Filtration is the removal of solutes as a solution flows across a
body. The effect on venous return through the superior vena porous membrane. Solutes too large to pass through the pores
cava is less pronounced, because blood in that vessel is nor- are filtered out of the solution. The driving force for filtration is
mally assisted by gravity. As you exhale, your thoracic cavity hydrostatic pressure. As we saw earlier, it pushes water from an
decreases in size. Internal pressure then rises, forcing air out area of higher pressure to an area of lower pressure.
of your lungs and pushing venous blood into the right atrium.
This mechanism is called the respiratory pump. Such pump- In capillary filtration, water and small solutes are forced
ing action becomes more important during heavy exercise, across a capillary wall, leaving larger solutes and suspended
when respirations are deep and frequent. proteins in the bloodstream (Figure 21–10). The solute mol-
ecules that leave the bloodstream are small enough to pass
Capillary Pressures and Capillary Exchange between adjacent endothelial cells or through the pores in
a fenestrated capillary. Filtration takes place primarily at the
Capillary exchange plays a key role in homeostasis. The most arterial end of a capillary, where capillary hydrostatic pressure
important processes that move materials across typical capillary (CHP) is highest.
walls are diffusion, filtration, and reabsorption.
Reabsorption
Diffusion
Reabsorption occurs as the result of osmosis. Osmosis is a spe-
As we saw in Chapter 3, diffusion is the net movement of ions cial term for the diffusion of water across a selectively per-
or molecules from an area where their concentration is higher meable membrane that separates two solutions of differing
to an area where their concentration is lower. p. 115 The solute concentrations. Water molecules tend to diffuse across
difference between the high and low concentrations represents a membrane toward the solution containing the higher solute
a concentration gradient. Diffusion tends to eliminate that gra- concentration (Figure 3–16, p. 119).
dient. Diffusion occurs most rapidly when (1) the distances
involved are short, (2) the concentration gradient is large, and Figure 21–10 Capillary Filtration. Capillary hydrostatic pressure 21
(3) the ions or molecules involved are small.
(CHP) forces water and solutes through the gaps between adjacent
Different substances diffuse across capillary walls by dif- endothelial cells in continuous capillaries. The sizes of solutes that
ferent routes: move across the capillary wall are determined by the dimensions of
the gaps.
1. Water, ions, and small organic molecules, such as glucose, amino
acids, and urea, can usually enter or leave the bloodstream Capillary Amino acid
by diffusion between adjacent endothelial cells or through hydrostatic Blood protein
the pores of fenestrated capillaries. Glucose
pressure
2. Many ions, including sodium, potassium, calcium, and chloride, (CHP) Ions
can diffuse across endothelial cells by passing through
channels in plasma membranes. Interstitial
fluid
3. Large water-soluble compounds are unable to enter or leave
the bloodstream except at fenestrated capillaries, such as Small solutes
those of the hypothalamus, the kidneys, many endocrine
organs, and the intestinal tract. Hydrogen
bond
4. Lipids, such as fatty acids and steroids, and lipid-soluble materials,
including soluble gases such as oxygen and carbon dioxide, can Water
cross capillary walls by diffusion through the endothelial molecule
plasma membranes.
Endothelial Endothelial
5. Plasma proteins are normally unable to cross the endothelial cell 1 cell 2
lining anywhere except in sinusoids, such as those of the
liver, where plasma proteins enter the bloodstream.