Rt. ft.irlolc Kt. Vfiitriulp luipure Blood (veiioiu) k tree iiiiiil the aricncs are sc* hinall that they are hair-iike in size, wlien they are called capillaries. The pure blood passes from the heart through the arteries to the capillaries. There the blood parts -with its oxygen and nourishing material to the tissues, and takes up carbonic acid, when it loses its red colour and becomes purple, or " venous." The capillary arteries o{)en into the little capillary veins, with which they form a communicating network, and these veins gradu- ally get larger and larger as they join other veins from different parts of the body, and pass up the bodv back to the heart. When this venous blood reaches the heart, it must be purified and made fresh, red, arterial blood again before the heart can send it on through the arteries to nourish the body. To this end it must be purified by passing through the lungs to get a fresh supply of oxygen, and give up its poisonous carbonic acid. Imagine the heart as a hollow organ divided into two by a median partition. The right side of the heart contains impure blood com- ing up from all parts of the body through the veins. The veins open into a cham- ber of the heart called the right auricle. The blood passes from the right auricle through an opening into a second cham- ber, the right ventricle, which pumps it on through a large blood-vessel to the lungs. There it receives oxygen and gives up carbonic acid, when it at once loses its purple colour, becomes red, or arterial, blood, and passes back to the left side of the heart. The " left heart " also consists of two chambers — an auricle and a ventricle. The pure blood reaches the left auricle and ])asses on to the left ventricle. The left ventricle pumps it into arteries, on to the capillaries, whence the blood •flows back by the veins to the right side of the heart. So that the complete circulation of blood is from the left side of the heart, through the arteries, capillaries, veins, to the right side of the heart, from the right side of heart to the lungs, and back once more to the left side of the heart. The blood always flows in one direction,because the arteries have the power of contracting like the heart, and thus the blood is sent forward with every beat of the heart and every pulsation of the arteries. This pulse wave passes from the heart to the smaller arteries, and can be felt when the arteries come near the surface — for example, at the wrist, or in front of the ankle, or at the temples. The veins do not pulsate, but the blood is kept going in one direction in the veins by valves, which prevent the backward flow of the blood. The heart beats from sixty to eighty times a minute. First the two auricles contract and then the two ventricles, and the blood flows from the right ventricle to the lungs, and irom the left ventricle to the aorta, and on to the arteries. Between each auricle and each ventricle are Diagram showing the circulation of the blood MKDICAL valves which prevent any risk of the blood flowing back to the auricles, because, when the ventricles contract, the valves close, and the blood has to flow forwards to the arteries. These valves do not stretch across the vessel as they should do when a vein becomes dilated as in varicose veins. The result is .sometimes fatal haemorrhage if the vein gets punctured or " wears through." The walls of veins are not contractible like arteries, and haemorrhage cannot stop spontaneously. In a varicose vein haemorrhage is often very alarming for this reason. The vein bleeds at both ends. Blood is being pumped into the vein from the arteries, and that in itself will cause severe haemorrhage. At the same time blood is welling back from the larger veins, because the diseased valves cannot hold the column of blood up. The patient may die in a few minutes from loss of blbod unless simple " first aid " measures are taken to stop the haemor- rhage. Blood itself consists of a fluid or plasma, in which large numbers of cells, or corpuscles, are held in sus- pension. There are two kinds .uiig circulation ipuliiioiiMry; Left Aitrlde U'ft Ventricle . , Pure BUmhI (art^Tial) The diagram shows the heart as a four^chambered organ, containing two auricles and two ventricles. The pure blood is pumped by the left ventricle through of COrpuSCles — red and white, the arteries to the capillaries, where it loses its oxygen The red give the colour tO and becomes venous. The venous blood returns by the ^.l ui„of1 hprausp +hpv rnn- veins to the right auricle, then on to the right ventricle. ^^^ DlOOCl, Decause they COH- The right ventricle pumps the venous blood through the tam a COlourmg matter, Or lungs, where ii is purified (gets a new supply of pigment, called haemoglobin. oxygen). From the lungs it returns to the left auricle, Ji^^y ^arry the OXygen. The and thence to the left ventricle, which pumps it again , ./ -^ i •^** .. . throughout the body. white corpuscles are called phagocytes, or scavengers, because they devour bacteria or germs, cat up dead tissue cells and other waste products. If it were not for these active phagocytes we should not have a chance in our daily warfare against disease. They form the standing army of the body, and die in protecting us. War is going on all the time between microbes and phagocytes. Here is an example of what takes place when we " catch " a disease — for example, influenza. The microbes first settle in our respiratory tract; the vitality of the tissue cells is not strong enough to resist the infection, to kill the attacking microbes, so they get a hold. Now the phagocytes attack them, the circulation slows in the blood-vessels of that part — that is, there is congestion — then the white blood corpuscles, or phagocytes, become very active. They push their way through the walls of the blood-vessels all round, they fall upon the microbes. A certain number of them die in the attack, and these form *' pus," or " matter," which is coughed up as expectoration in the case of a lung affection, such as pneumonia or bronchitis. All the time, although the phagocytes are dying, they are killing or destroying a certain number of microbes, and, unless these are very powerful, the phagocytes are at last victorious, the inflammation subsides and the patient recovers. The white blood corpuscles behave in the same way when the body is injured. In the next aiticle we shall consider ihj physiology of respiration.
