Pacemaker

Pacemaker

It is well known to everybody that heart beats automatically. It does not need to be controlled by the brain. Sometimes the heart gets messages that make it change the speed of the beat. It can go faster or slower. These messages can come from special nerve signals. They can also come from harmones carried in the blood. The continuous and regular beatings of the heart is controlled by the pacemaker. The pacemaker lies between the two atria. It is a small group of muscle cells that send out regular electrical signals. These signals make the heart beat regular, that is , about 70 - 80 times a minute in a healthy adult.

Certain heart diseases cause this pacemaker to fail. Under this condition an artificial pacemaker is used to keep the heartbeat regular. An artificial pacemaker is a battery powered electronic device which stimulates the heart and makes it to beat regularly. Some artificial pacemakers are implanted in the chest while others are small enough to be passed along a vein to the heart. The simplest pacemakers keep the heart going at a steady rate of 70 beats per minute. But sometimes it is possible to get the person’s own nervous system to control the pacemaker. In this case the heart can still beat faster or slower according to the need of the body. Battery powered pacemakers can work upto 15 years with just one battery.

Arteries and veins

Arteries and veins are different from one another because they have different parts to play in our blood's circulation. Arteries take blood from the heart to the rest of the body, while veins return it. Blood is pumped from the right side of the heart through the pulmonary artery to the lungs and returns through the pulmonary vein to the left side of the heart. From there it is pumped via the large artery called the aorta to the rest of the body and eventually returns to the right side of the heart again through the veins known as the inferior vena cava, from the legs and abdomen, and superior vena cava, from the head neck and arms.

Blood is forced through the arteries in spurts at high pressure, so their walls are thick and flexible and expand with every pulse of blood. In the veins, blood pressure is low and the blood flow is steadier. The walls of veins are thin and collapsible and valves prevent the blood flowing back in the wrong direction. In general, the arteries lie deep within the body, and the veins nearer the surface, which is why the blood vessels we see on our hands, for instance, appear blue in colour as they contain blood that has lost most of its oxygen and is therefore blue.