Blood and the Circulatory System Help (page 2)
Introduction to Blood and the Circulatory System
This section discusses the circulatory ( SIR -kyuh-luh- tor -ee) or cardiovascular ( car -dee-oh- VAS -kyuh-lar) system. The circulatory (cardiovascular) system is an organ system including the “heart” (cardi) and “little vessels” (vascul) that carries the blood in a “little circle” that both begins and ends with the heart. Along the way, the blood passes by the main body tissues. In so doing, the blood delivers hormones, glucose, and various other nutrients to the main body tissues. And, as it leaves, the blood picks up a number of waste products from the main body tissues.
How Does the Circulatory Sytem Work?
Glance at Figure 16.1. You will see that if the heart represents a pump for the blood, then the blood vessels represent the pipes carrying it! The arteries ( AR -ter- ees ) are large-diameter vessels that always carry blood away from the heart. Thus, the arteries immediately receive the blood being pumped from the ventricles ( VEN -trih- kls ) – the “little belly”-like lower chambers on either side of the heart. Specifically, the right ventricle (abbreviated as RV) pumps blood out into the common pulmonary ( PULL -mun- air -ee) artery (abbreviated as CPA), which in turn sends blood out towards both “lungs” (pulmon). And the left ventricle (abbreviated as LV) pumps blood out into the aortic (ay- OR -tik) arch. The aortic arch sends the blood from the LV out towards the tissues of the major body systems (other than the lungs) (See Figure 16.2, A).
The Arteries - Arterioles and Cappilaries
As the major arteries (such as the common pulmonary artery and aortic arch) travel farther from the heart, they branch into smaller arteries (Figure 16.2, B). And these smaller arteries, in turn, branch into even smaller arterioles (ar- TEER -ee-ohls) or “little arteries.” As the arterioles approach the cells of the body tissues, they branch into the smallest blood vessels of all, the capillaries ( CAP -ih- lair -ees). Each capillary is very narrow (much like a strand of hair). This characteristic is reflected in the translation of capillary, which “pertains to a hair.”
Since the wall of each capillary is only a single cell thick, nutrients and waste products diffuse across the wall. Nutrients (such as oxygen, O 2 , and glucose) diffuse out of the bloodstream, and into the tissue cells. And waste products (such as carbon dioxide, CO 2 ) diffuse out of the tissue cells, and into the bloodstream.
Venules - Little Veins
After the capillaries run past the tissue cells, several of them merge together to form the venules ( VEN -yewls). The venules are the “little veins,” in the sense that they connect the capillaries to the much larger veins. The veins are all wide-diameter vessels that return blood towards the heart.
Several of the largest veins return blood back to the atria ( AY -tree-ah) – the small “entrance rooms” (atri) or chambers located at the top of the heart. Among the biggest sets of veins are the superior and inferior vena ( VEE -nah) cavae ( KAY -veye), or “upper and lower cave veins.” The superior vena cava (SVC) drains blood down into the right atrium ( AY -tree-um), or RA , from the area above the heart, while the inferior vena cava (IVC) returns blood up into the right atrium from the entire region below the heart.
The four pulmonary veins, as their name suggests, return blood from the lungs and empty it into the left atrium, LA.
The Systemic And Pulmonary Circulations
For ease of study, the major vessels and heart chambers are often grouped together into two connected circulations. The pulmonary or right-heart circulation involves the circulation of blood to, through, and from the lungs. The systemic (sis- TEM -ik) or left-heart circulation, in contrast, represents the circulation of blood to, through, and from the tissues of all the major body organ systems (except for the lungs).
- Which specific chamber of the heart begins the pulmonary circulation?
- Which specific blood vessels end the pulmonary circulation?
- What particular heart chamber begins the systemic circulation?
- What particular blood vessels end the systemic circulation?
Comparison To Circulations In Other Vertebrates
Endotherms - Mammals
The four-chambered human heart (two upper atria, two lower ventricles) plus a double circulation (pulmonary circulation plus systemic circulation) is pretty typical of the heart and circulation found in both birds and other mammals. Being endotherms (or homeotherms) that regulate their own internal body temperature (Chapter 12), birds and mammals have a great need for energy and oxygen to keep their body heat always at a fairly high level. A double circulation plus powerful four-chambered heart provide both the required force to pump a large volume of heated blood and a separate pulmonary circulation that adds plenty of oxygen to the blood.
The pulmonary (right-heart) circulation in mammals, therefore, is specialized for adding oxygen to the bloodstream. But the systemic (left-heart) circulation specializes in extracting oxygen from the bloodstream and delivering it to the tissues of the major organ systems. In the process, carbon dioxide is picked up from the tissues.
Poikilotherms - Fish, Amphibians, and Reptiles
The circulation in vertebrates that are heterotherms (poikilotherms), such as fish, amphibians, and reptiles, is quite another matter. Fish have a two-chambered heart with only a single circulation. Amphibians rely upon a three-chambered heart (two atria, one ventricle) beating within their chests. Reptiles, likewise, possess a three-chambered heart, with two atria and one ventricle that is partially subdivided. Both amphibians and reptiles have double circulations, but there is much less specialization of function of the two circulations.
Practice problems for these concepts can be found at: Blood And The Circulatory System Test
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