 |
| Home | Gallery | Games | Expedition Email Digest | Contact | Postcards | Store | FAQ | Sitemap |
|
|
|
| |||||||||||||||
The heart is located in the chest (thoracic) cavity and is protected by the sternum and the rib cage. The heart is about the size of an adult fist and has a mass of approximately 300 g. The heart is suspended by the large blood vessels that are attached to it.
The heart is basically two pumps each complete in itself. The heart is composed of very special muscle tissue that is capable of contracting without an external stimulus (myogenic). The continuous flow of blood is brought about by the rhythmic contraction and relaxation of heart muscles. The heart is a hollow cavity with muscular walls. A thick partition divides the heart into a right and left side. When the muscular walls relax, both the right and left sides fill with blood. When the heart beats, the muscular walls contract and blood is squeezed into the two large arteries. These arteries are the aorta and the pulmonary artery.
The right side of the heart is responsible for collecting blood from the body and pumping it to the lungs. The blood flows back from the head areas to the right side of the heart by way of a large vein called the superior vena cava. It leads into the upper right-hand chamber, the right atrium of the heart. Blood from the trunk and legs enters this same chamber (right atrium) via another vein called the inferior vena cava. The atria are thin-walled chambers which lie above the ventricles. Their main function is to collect blood and pass it into the main contracting vessels, the ventricles. The right ventricle has thicker, more muscular walls, and is larger than the right atrium. It is connected to the right atrium via the tricuspid valve which prevents blood from flowing back into the atrium when the ventricle contracts. When the right ventricle contracts, it squeezes the blood past the pulmonary semilunar valve into the pulmonary artery. This artery carries oxygen poor blood to the lungs where its load of carbon dioxide wastes is released and a fresh load of oxygen is absorbed by the red blood cells. The oxygenated blood flows back to the heart through the two pulmonary veins. The pulmonary veins return blood to the heart and enter the left atrium. When the left atrium contracts, it squeezes blood past the bicuspid valve into the left ventricle. The left ventricle is the largest and most muscular chamber in the heart. When the left ventricle contracts, the oxygen-rich blood is forced past the aortic semilunar valve into the aorta - the largest artery in the body.
Heartbeat Sounds This sound is caused as the ventricles start to relax and the blood in the major arteries tries to flow backwards into the ventricles. The two valves snap shut to prevent the blood from flowing backward.
A normal heart valve opens when the pressure in front of it is greater than the pressure behind it. For example, the two atria start contracting simultaneously. As the contraction increases, the muscular walls put pressure on the blood contained in these chambers. As soon as this pressure is greater than the pressure in the ventricles, the tricuspid and bicuspid valves are forced open and the blood can flow into the ventricles. As soon as the atria begin to relax and the pressure in them falls below that in the ventricles, the valves snap shut. Similarly, when the ventricles contract and the pressure in them exceeds the pressure in the pulmonary artery and the aorta, the aortic and pulmonary semilunar valves open and the blood is forced into the arteries. As soon as the ventricles start to relax, the valves snap shut and prevent the blood in the arteries from flowing backward into the ventricles. The normal heart sounds are an indication that all these valves are functioning properly. The heart of a resting human adult pumps about five litres of blood every minute (this is approximately the total volume of blood in an adult human). During exercise, both the rate of contraction and the amount of blood pumped increase greatly. This combination may increase the cardiac output four to seven times the resting level. Under such conditions, a single drop of blood may pass through the heart several times in one minute. Cardiac output is the volume of blood pumped by the heart per minute. For an average sized adult (70 kg) at rest, this would be about five litres/min. During severe exercise, cardiac output can increase to over 30 L/min. The cardiac output is determined by the heart rate as well as the volume of blood pumped (stroke volume). At rest, most of your muscles are inactive and your heart rate will be accordingly slow. When standing, muscles in the lower body, as well as the trunk and head, must work and the heart will beat even faster. If you run or exercise, the heart will automatically adjust to speed up in response to the increased activity.
At Altitude: The cardiac output of Everest 2000 team members will increase but the stroke volume will likely decrease. The decrease in stroke volume is likely caused by the a decrease in plasma volume. The lower plasma volume is directly related to an increase in diuresis - the climbers urinate more at higher altitudes. Increased urine output has a tendency to lower the amount of water in the plasma.
Pacemaker Its spontaneous rate can be accelerated and decelerated by nerve impulses from the brain. Each time the S-A node fires, a signal spreads across the two atria causing them to contract. The signal continues to a second mass of tissue called the atrio-ventricular node (A-V node). This mass of tissue sends the nervous impulse throughout the ventricles causing them to contract.
| ||||||||||||||||
