单选题
Blood
To do its job properly, blood must travel continually around the body. The pumping heart forces blood around the circulatory system to constantly change the fluid surrounding the living cells so that fresh supplies of oxygen and food are brought in as fast as they are used up and poisonous end-products are discarded.
The arteries carry blood to every organ of the body, such as the liver, the stomach, the kidneys and the intestines. The liver and kidneys play a vital role in the blood system. Many of the chemical activities carried out within the body result in toxic byproducts. These substances diffuse into the lymphatic system and are carried off into the plasma. When they eventually reach the kidneys, a large proportion of this noxious waste is excreted through the outer organs. The blood passes regularly through the liver and kidneys, picking up nutrients from the former and recycling the waste through the latter.
While the kidneys acts as a flushing device for food residues, the liver is mainly a storage system. Only a select amount of nutrients is transferred from the liver to the blood, depending on the body"s immediate need. The rest is stored in the liver until it becomes necessary. The arteries get smaller and smaller as they pass through the various organs and tissues. The smallest vessels are the capillaries. It is through the thin walls of these capillaries that the body cells get their food and oxygen and release their waste. The capillaries join up again to form veins. Blood flows through the veins back to the heart. The arteries have much thicker wails than the veins to withstand the pressure of blood coining directly from the heart. There is little blood pressure in the veins. The action of the muscles of the body helps to massage blood through the veins, and the valves in the vein walls prevent blood in the veins from flowing the wrong way.
When blood returns from the veins, the heart pumps it to the lungs. There it gives up carbon dioxide and takes in oxygen. After being refreshed, the blood returns to the heart and is again pumped out to all parts of the body through the arteries. This double flow of fresh blood from the heart to various parts of the body, followed by the transfer of used blood first to the lungs then back to the heart, is called double circulation.
The continuous double circulation of the blood system can be measured my means of external devices. Heart monitors are used to record a person"s systolic or diastolic pressure, as the two main components of the double circulation system are termed. There is a healthy range for each blood pressure measurement, which depends on an individual"s age, diet and activity. Strenuous activities are known to temporarily increase blood pressure above the usual top limit.
Another sign of increased cardiovascular activity is the bulging of the vein walls during exercise. Contrary to popular belief, this slight enlargement of the veins is not caused by higher blood pressure but rather by enzymatic activity. When more blood is pumped through the system, capillaries dilate and release enzymes to facilitate better arterial flow. The enzymes get into the muscles, and this hardening of the muscles pushes the veins towards the skin giving them a bulging appearance.
Glossary
circulatory:
relating to circulatory system or to circulation of the blood
plasma:
the fluid part especially of blood lymph, or milk that is distinguished from suspended material
capillary:
one of the minute blood vessels that connect arterioles and venules
artery:
a blood vessel that carries blood from the heart to the body