398 BIOLOGY FOR BEGINNERS 



these automatically change the rate of heart beat. When we run 

 upstairs (a bad habit, by the way) we use more energy, hence oxi- 

 dize more tissue, hence need more oxygen to be brought by the 

 blood, and produce more waste, which must be carried off, and the 

 heart has to work harder to meet this demand. 



Blood Vessels. Arteries. All the vessels that carry blood away 

 from the heart are arteries regardless of whether they carry red 

 (oxygenated) or dark (de-oxygenated) blood. Arteries have elastic 

 muscular walls, and very smooth linings. Their function is to assist 

 and to regulate blood flow. Since they are elastic they expand 

 when blood is forced into them, and as the valves prevent it from 

 returning to the heart, their elastic contraction forces it to flow 

 on through the arteries and exerts pressure clear to the capillaries. 



If it were not for this elasticity, which is greatest in the large 

 arteries, the circulation would be slow and unsteady and the ar- 

 teries themselves in danger of bursting under the sudden strain, 

 when the ventricles contract. In " hardening of the arteries " 

 this elasticity is lost and produces serious and usually fatal 

 results. 



In general the arteries are protected by location beneath thick 

 muscles, but at the wrist and neck some large ones come near the 

 surface and this elastic wave of expansion can be felt, and is known 

 as the pulse. 



The muscles in the artery walls perform the very important 

 function of regulating the amount of blood that reaches a given 

 organ. By a very complicated system of nerve control, these 

 muscles expand when more blood is required and contract when the 

 supply is not needed. 



Capillaries. As the arteries leave the heart they divide again 

 and again, becoming smaller and thinner walled till they develop 

 into microscopic tubes with a wall of only one layer of cells. These 

 tiny blood vessels are the capillaries (" hair like ") and are so 

 numerous that they reach every living tissue of the body. Their 

 large area and thin walls permit osmosis to go on readily and it is 

 by way of osmosis from the capillaries that food actually reaches 

 the body cells. Absorption of food in the digestive tract and ex- 



