Essentials of Anatomy and Physiology

liver and pituitary gland, which produce and secrete
proteins into the blood.

EXCHANGES IN CAPILLARIES

Capillaries are the sites of exchanges of materials
between the blood and the tissue fluid surrounding
cells. Some of these substances move from the blood
to tissue fluid, and others move from tissue fluid to the
blood. The processes by which these substances are
exchanged are illustrated in Fig. 13–2.
Gases move by diffusion, that is, from their area of
greater concentration to their area of lesser concen-
tration. Oxygen, therefore, diffuses from the blood in
systemic capillaries to the tissue fluid, and carbon
dioxide diffuses from tissue fluid to the blood to be
brought to the lungs and exhaled.
Let us now look at the blood pressure as blood
enters capillaries from the arterioles. Blood pressure

here is about 30 to 35 mmHg, and the pressure of the

surrounding tissue fluid is much lower, about 2

mmHg. Because the capillary blood pressure is higher,

the process of filtrationoccurs, which forces plasma

and dissolved nutrients out of the capillaries and into

tissue fluid. This is how nutrients such as glucose,

amino acids, and vitamins are brought to cells.

Blood pressure decreases as blood reaches the

venous end of capillaries, but notice that proteins

such as albumin have remained in the blood. Albumin

contributes to the colloid osmotic pressure(COP)

of blood; this is an “attracting” pressure, a “pulling”

rather than a “pushing” pressure. At the venous end

of capillaries, the presence of albumin in the blood

pulls tissue fluid into the capillaries, which also brings

into the blood the waste products produced by

cells. The tissue fluid that returns to the blood

also helps maintain normal blood volume and blood

pressure.

The Vascular System 295

Precapillary sphincter Tissue fluid Cells Capillary

Tissue fluid -

Hydrostatic pressure

2 mmHg

COP 4 mmHg

CO 2

CO 2

CO 2

O 2

O 2

O 2

Plasma, glucose,

amino acids, vitamins

B. P. 33 mmHg

Albumin - COP 25 mmHg

B. P. 15 mmHg

Albumin - COP 25 mmHg

Tissue fluid,

waste products

VENOUS END

Outward forces 19

Inward forces 27

ARTERIAL END

Outward forces 37

Inward forces 27

Figure 13–2. Exchanges between blood in a systemic capillary and the surrounding tis-

sue fluid. Arrows depict the direction of movement. Filtration takes place at the arterial end

of the capillary. Osmosis takes place at the venous end. Gases are exchanged by diffusion.

QUESTION:Of all the pressures shown here, which one is the highest, and what process

does it bring about?