Materials needed for a reusable class set:
1 standard garden hose
1 soaker hose
1 discharge hose (find in pool supply stores)
This is an easy way to have students carefully consider the characteristics of blood vessels. To set it up, you will need three hoses (or lengths of hose) of three different types: a simple garden hose with fairly thick walls (red?), a soaker hose with mesh walls, and a discharge hose (blue?) such as is used to drain the water from swimming pools. The latter can be ordered from a pool supply company. It will flatten when not full of liquid.
Directions: Simply cut the hoses in lengths of 12 to 18 inches. Each student or small group is then given one of each type of hose. They are asked to decide which best represents a capillary, vein, or artery. There is not one right answer, although some choices may seem more appropriate than others. Students must consider the properties of each type of blood vessel and make reasoned decisions regarding their choices, citing characteristics to back up their reasoning. It might be helpful to allow them to take the hoses to the sink and run water through them.
Arteries are thick-walled vessels that carry oxygenated blood on its journey away from the heart. (Exception: the pulmonary arteries also carry blood away from the heart, but it is deoxygenated blood on its way to the lungs where oxygen is added.) It is helpful to remember that arteries always carry blood away from the heart, Arteries Always Away. Arterial walls are composed of three tissue layers called the tunica interna, the innermost layer which includes the epithelial lining, tunica media in the middle, and tunica externa, the outer layer. The arterial walls contain more smooth muscle and elastic fibers than do the walls of the veins. This makes arterial walls thicker and more elastic, so they retain their circular shape in cross-section, even when emptied of blood on the commercially prepared slides your students may view. The walls are thicker than the walls of veins and more contractile. Blood pressure in the arteries is high and pulses as the heart beats. The greater elasticity of the walls allows them to spring back from pressure. Their greater contractility allows arteries to constrict or dilate to control blood flow to areas of the body. No valves are present.
Veins are vessels that carry deoxygenated blood on its return trip to the heart. (Exception: the pulmonary veins also carry blood back to the heart, but it is oxygenated blood returning to the heart from the lungs.) Veins are composed of three layers as are the arteries, but with less smooth muscle and elastic fibers. In a tissue sample such as those on commercially prepared slides, the emptied veins may collapse and flatten. Reduced connective tissue content in the venous walls allows the veins to lose their round shape readily when emptied. The blood in the veins is under lower pressure since it is farther removed from the pumping action of the heart. Since the blood pressure is lower and the journey through the veins is mostly against the pull of gravity, extra help comes in the form of valves present inside the veins. Contraction of skeletal muscles surrounding the veins also helps to push the blood back to the heart.
Capillaries are very thin-walled vessels with a very small diameter. Some are only 4 microns across. (Compare this to the diameter of the red blood cell: 8 microns!) They are the site of gas exchange and chemical exchange between the blood and the body’s cells. The capillary walls are only one cell thick, being composed simply of epithelium, and oxygen and carbon dioxide can pass through them readily. They make up for their small diameter by being extremely plentiful, existing in capillary beds in all tissues of the body except epithelial tissue.
Blood vessel illustration by National Cancer Institute, National Institutes of Health – http://training.seer.cancer.gov/anatomy/cardiovascular/blood/classification.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=45154160