In vertebrates there are three kinds of blood vessels, arteries, veins and capillaries. These vessels differ from each other in the structure of their walls, which is related to their functions.
Arteries
Arteries are the vessels which have to carry blood from the heart to all tissues of the body. The structure of the walls of the arteries is related to their function. They have the strongest and the thickest walls. The arterial walls are made up of three layers. The innermost layer, forming the lining of the inner surface of the arterial wall, is called endothelium. The middle layer consists of smooth muscles and elastic tissue. The outermost layer is also elastic and consists of connective tissue. Because of their elastic walls, the arteries are stretched when the blood enters them and then recoil slowly.
Thus the blood in the arteries is ‘pulsing’and the pressure in them fluctuates with the heart beat. You can feel the pulse if you place your fingers on an artery in the neck or the wrist. Because of their strong and elastic walls, the arteries are able to withstand the high pressure of blood in them. The arteries, on reaching closer to the tissues, divide into arterioles. The arterioles, on entering the tissues, further divide into very fine branches, the capillaries.
Capillaries
The function of the capillaries is to deliver the blood as close to all the body cells as possible. The walls of capillaries are extremely thin, consisting of only a single layer of cells, the endothelium. The lumen of the capillaries is so narrow that the erythrocytes can move through them only in a single file. As the blood moves through the capillaries, the gases (oxygen and carbon dioxide), hormones, and other materials are exchanged with the surrounding tissues by diffusion.
Veins at the venous ends of the capillary beds, the blood flows back in slightly thicker vessels, called the venules. The venules join together to form larger vessels, the veins, which ultimately transport the blood from the tissues back to the heart. The walls of the veins, like those of the arteries, are also three-layered but they are less elastic and more pliable. The walls of the veins are thin and almost inelastic, that an empty vein collapses, whereas empty artery does not collapse (fig. 12.14).
Practical work; study the blood circulation in a fish tail or a frog’s web or fish tail
1. Take the live specimen (frog or fish). If frog is selected pith it and then spread its web in the hind limb. Hold it in two slides and keep it at a place with a proper clip or thread. For fish keep wet cotton on gills for respiration and spread the tail as described in frog’s web study.
2. Place it under the microscope and study the blood circulation.
3. How blood moves in capillaries? Draw a label.
Tissue Fluid
We have already learnt that capillaries reach closest to the tissues of the body and carry the blood to all tissues of the body. Here in the tissues, some fluid oozes out of the capillaries and fills the intercellular spaces. This colourless fluid thus accumulated in the intercellular spaces is called tissue fluid.
The escape of tissue fluid from the capillaries occurs through small gaps present in the walls of the capillaries. The number of gaps in the capillary walls varies greatly in different tissues of the body. These gaps are quite big so as to allow the passage of fairly large molecules but are not large enough to allow the escape of erythrocytes.
The tissue fluid is the medium through which the exchange of materials occurs, by diffusion, between the cells and the blood in the capillaries.
The tissue fluid also maintains a constant aquatic environment around the cells of the body.