Bent Elmann Creative Corner Natural Science Section XII (b)

we are distinguished from worms and other invertebrate creatures by having a bony framework that gives rigidity to out several members. The framework is articulated or jointed to enable the body and in attachments to take up different attitudes or configurations. Almost everything we do-eating, running, writing, etc., etc.-involves the movement of limbs or other parts relative to the trunk or body. And in breathing we constantly move our ribs up and down. The motive power for all these movements is provided by the connections between bone and bone that we call muscles. To every joint there is necessarily a pair of muscles. Each forms an elastic attachment not unlike a spring or based of rubber, and whereas one muscle tends to move the joint one way, the other tends to move it the other way. We achieve the movement we want by giving one musacle an ascendancy over its opposite. The muscle is a fleshy band attached to bone at both ends and having its body or central part contractable. Under the requisite nervous impulse the muscle tends to become thicker and shorter,and it exerts a considerable force in so doing-the force that enables us to lift weights, spring into the air or nod our heads. When it is in action a muscle performs work and the energy it needs is drawn from the glucose in the blood. It takes both glucose and oxygen from the blood, returning water and carbon dioxide as waste products. Nearly all the flesh of our bodies consists of muscular tissue, and blood streams through it constantly under the pumping action of the heart. The entry is way of an artery and it passes back to the heart by a vein. The entering artery subdivides again until it forms a network of extremely fine capillaries that can be seen only under a miscoscope. The subdivision of the blood-stream nevertheless leads to the provision of a greater area of flow, so that whereas the velocity of the blood in a large artery may be of the order of 1 ft. per second, in a capillary it will be nearer to 1  in. per minute. The capillary walls are exceedingly thin so that this, in conjunction with the slowness of the blood flow, enables an interchange to take place between the blood and the cellular tissue of the muscle. The blood loses glucose and oxygen while gaining water and carbon dioxide. It the muscle is being overworked, or if the blood flow is too sluggish, carbon dioxide and water will accumulate to excess so that we shall feel fatigue. Normally the heart responds to vigorous muscular exercise by pumping the blood round faster. If the heart is diseased and unable to respond to our normal needs, we may suffer from the accumulation of water in our blood that is known as dropsy. After the blood has served its purpose in the muscle its multitudinous streams recombine to form wider and ever-wider veins. In the veins it recovers its former speed of about 1 ft. per second, though the energy for this acceleration comes only in small part from the heart. Arteries are elestic, and you can feel the pulsations of the heart by pressing on an artery, but almost none of this energy from the heart gets through the capillaries to the veins, so that you cannot feel any pulsation by pressing on a vein. Movement of blood in a vein is promoted mainly by external pressure on the vein itself. If we exercise ourselves vigorously we shall help the blood to travel along the veins by compressing them or squashing them at intervals, their position in the muscles obliging us to do this. Because external pressure tends to drive rthe blood both ways along the vein, non-return valves are provided at intervals along its length; these allow blood to flow towards the heart but not backwards towards the capillaries. We can see now that when the circulation of the blood needs to be most rapid, muscular exercise contributes to the desired end by its mechanical action on the veins just where the influence of the heart is at its feeblest. Blood brings warmth to our muscles, this being one of the results of the oxidation of glucose, so that on a very cold day we can dispel the feeling of numbness by stamping our feet and swinging our arms across our chests. The importance of blood as a vehicle for the fuel needed by our muscles is now self-evident. We have mentioned its circulatory function and said that the heart is a pump that maintains its flow. Where does the blood acquire its oxygen and its glucose? For answer we must refer to the respiratory system and the digestive system. Blood leaving the muscles is impoverished in both respects, so the heart must contrive to pump it to regions in the body where its proper condition is restored. The blood from every muscle in the body goes first of all into a great trunk main leading to what is termed the right heart ( because it is the right-hand portion of that organ) whence it is pumped at once to the lungs to have its oxygen deficiency corrected. The lungs are in the chest cavity that we alternately expand and contract in the act of breathing. Our ribs rise and fall and in rising they spread outwards just as the drooping handle of a bucket spreads out when we first begin to lift it from its lowest position, this enlarges the chest cavity. Separating the chest cavity from the abdomen is a musclar wall called the diaphragm, which we also use in breathing; we depress this when we expand our chests and, by squashing up our abdominal organs, we produce an extra bulge in our bellies. The lungs are a pair of similar organs in the variable chest cavity. Entering each lung is a wide air tube ( the bronchus) which subdivides again and again into smaller and ever-smaller branches. Each of the ultimate branches terminates in a tiny elastic air sac the surface of which is covered with blood capillaries. When we expand our chests we create a partial vacuum in the chest cavity. Air in our lung sacs is this able to expand by blowing out the sac walls like tiny balloons. When the air pressure in a lung sac falls, more air rushes in to make good the deficiency, and therefore we are conscious of inhaling or taking a a breath. The blood in the capillaries lining the lung air sacs has come straight from the heart via a great artery called the pulmonary, artery, that divides and subdivides until it reaches the capillaries. The blood here is deficient in oxygen and in the nutrinebtal principle called glucose, but once it reaches the thin walls of an air sac filled with fresh atmospheric air it can give up its load of carbon dioxide and acquire a fresh charge of oxygen. Having done this, it passes from the lung capillaries into common veins which run together to form at last great pulmonary vein taking oxygen-enriched blood back to the heart ( the left heart this time). The blood entering the lung is a dark colour tending to purple, and even blue, but when its oxygen content is restored to normal it becomes bright scarlet again, so that the pulmonary vein carries scarlet blood to the left heart.