. Essentials di biologia ha presentato problemi. Biologia. 374 IL SANGUE E LA SUA circolazione uomo dove le arterie spezzare in capillari, e questi a loro volta formare vene. Struttura di arterie. - Una netta differenza nella struttura , esiste tra le arterie e le vene del corpo umano. Le arterie, a causa della maggiore sollecitazione ricevuta dal sangue che viene pompato dal cuore, hanno più spesse pareti muscolari, e in aggiunta sono molto elastico. / Cause dell'impulso. - L'impulso, che può essere facilmente rilevato da premere- Ang grande arteria di polso o il piccolo uno di fronte e

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. Essentials of biology presented in problems. Biology. 374 THE BLOOD AND ITS CIRCULATION man where the arteries break up into capillaries, and these in turn form veins. Structure of the Arteries. — A distinct difference in structure , exists between the arteries and the veins in the human body. The arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and in addition are very elastic. / Cause of the Pulse. — The pulse, which can easily be detected by press- Ang the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. As the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. At the very end of their course, these arteries are so small as to be almost microscopic in size and are very numerous. There are so many that if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (aorta) which passes blood from the left side of the heart. This fact is of very great impor- tance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. This gushing move- ment is quite lost when the capillaries are reached, first, because there is so much more space for the blood to flU, and secondly, because there is considerable friction caused by the very tiny diameter of the capil- laries. -' Capillaries. — The cap- illaries form a network of minute tubes everywhere in the body, but especially near the surface and in the lungs. It is through their walls that the food and oxygen pass to the tissues, and carbon dioxide is given up to the plasma. They form the connection that ' completes the system of circulation of blood in the body.. Diagram of capillary circulation. Note that the artery breaks up into smaller vessels,

Parole chiave associate all'immagine

• bookcentury1900
• bookdecade1910
• booksubjectbiology
• bookyear1911