Page 123 - Biology Form Two

Biology for Secondary Schools

The aorta, which is the largest artery in tiny pores that allow some components
the body, branches into smaller arteries of blood to filter into the tissues.
that in turn branch into even smaller In the same way, the vena cava, which

vessels called arterioles, as shown is the largest vein, branches into
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in Figure 5.15. Arterioles branch smaller veins called venules as shown
into smallest vessels called arterial in Figure 5.15. These venules branch
capillaries that are in contact with the into smallest vessels called venous

tissues of the body. The capillaries have capillaries that are in contact with the
body tissues.

Artery
Arteriole
Arterial capillaries

Pre-capillaries sphincter

Venous capillaries
Venule

Vein

Figure 5.15: Arterial and venous blood capillaries

At the arterial end of the capillary, These wastes include carbon dioxide,
there is high blood pressure. This minerals, and nitrogenous wastes.
forces fluid out through the tiny pores Figure 5.16 shows the formation of
in the capillaries. The fluid is composed tissue fluid.
of water, oxygen, hormones, and The tissue fluid constitutes an internal
nutrients. This fluid bathes the cells. It environment in which tissue cells bath.
is called tissue fluid or interstitial fluid.
The cells get their oxygen and nutrients
The substances in this fluid diffuse into from the fluid and excrete their metabolic

the cells through the cell membranes. wastes into it.
In addition, the waste products from
the cells diffuse into the tissue fluid.

Student’s Book - Form Two 117

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