blood vessels Flashcards
What is the Vascular System and the History of Blood Circulation?
Blood circulates inside the blood vessels, which form a closed transport system called the vascular system. The idea that blood circulates, or “makes rounds,” through the body is only about 300 years old. The ancient Greeks believed that blood moved through the body like an ocean tide, first moving out from the heart and then ebbing back to it in the same vessels to get rid of its impurities in the lungs. It was not until the seventeenth century that William Harvey, an English physician, proved that blood did, in fact, move in circles.
What are the Components and Functions of the Vascular System?
Like a system of roads, the vascular system has its freeways, secondary roads, and alleys. As the heart beats, it propels blood into the large arteries leaving the heart. As the large arteries branch, blood moves into successively smaller and smaller arteries and then into the arterioles (ar-ter′e-ōlz), which feed the capillaries (kap′ĭ-lar″ez) in the tissues. Clusters of capillaries, called capillary beds, are drained by venules (ven′ulz), which in turn empty into veins that merge and finally empty into the great veins (venae cavae) entering the heart. Thus arteries, which carry blood away from the heart, and veins, which drain the tissues and return the blood to the heart, are simply conducting vessels—the freeways and secondary roads. Only the tiny hairlike capillaries, which extend and branch through the tissues and connect the smallest arteries (arterioles) to the smallest veins (venules), directly serve the needs of the body cells. The capillaries are the side streets or alleys that intimately intertwine among the body cells and provide access to individual “homes.” It is only through their walls that exchanges between tissue cells and the blood can occur.
Why are Arteries Depicted in Red and Veins in Blue? Are There Any Exceptions?
Notice that we routinely depict arteries in red and veins in blue. By convention, red indicates oxygen-rich blood, the normal status of blood in most of the body’s arteries, and blue indicates relatively oxygen-depleted, carbon dioxide–rich blood, the normal status of blood in most of the veins. However, there are exceptions. For instance, we have seen that oxygen-poor blood is carried in the pulmonary trunk, an artery, while oxygen-rich blood is transported back to the heart in pulmonary veins. An easy way to remember this difference is the following: Arteries are red and veins are blue, but for the lungs there’s an exception of two.
what is the vascular system and what is the history of it
Blood circulates inside the blood vessels, which form a closed transport system called the vascular system. The idea that blood circulates, or “makes rounds,” through the body is only about 300 years old. The ancient Greeks believed that blood moved through the body like an ocean tide, first moving out from the heart and then ebbing back to it in the same vessels to get rid of its impurities in the lungs. It was not until the seventeenth century that William Harvey, an English physician, proved that blood did, in fact, move in circles.
Step One:
Front:
Where does blood go after the heart pumps?
Big Arteries:
Blood is pumped from the heart into the large arteries (such as the aorta), which carry it away from the heart to the rest of the body.
Step Two:
Front:
What happens after the blood moves through the large arteries?
Back:
Arteries to Arterioles:
The large arteries branch into smaller arteries, and the blood flows into even smaller vessels called arterioles.
Step Three:
Front:
What happens after blood enters the arterioles?
Back:
Capillaries:
Arterioles feed blood into capillaries, tiny, hairlike vessels that form capillary beds. These are where exchanges of oxygen, nutrients, and waste occur between blood and body cells.
Step Four:
Front:
What happens after blood passes through the capillaries?
Back:
Venules:
After the exchange in the capillaries, blood drains into venules, small vessels that collect blood from the capillaries.
Step Five:
Front:
Where does the blood go after the venules?
Back:
Veins:
Venules merge into veins, which carry the blood back toward the heart.
Step Six:
Front:
What is the final step in the blood’s journey
Back:
Great Veins:
The veins eventually merge into the great veins (like the vena cavae) which return the oxygen-poor blood to the heart.
How are arteries and veins typically depicted in terms of color?
Basic Concept:
Arteries are usually depicted in red because they generally carry oxygen-rich blood away from the heart to the body.
Veins are typically depicted in blue because they carry oxygen-poor blood back to the heart.
This is the general rule for most arteries and veins in the body.
Exceptions:
However, there are two exceptions to this rule, specifically when it comes to the lungs:
Pulmonary Artery:
Even though it’s an artery, it carries oxygen-poor blood from the heart to the lungs.
This is an exception because, normally, arteries carry oxygen-rich blood, but the pulmonary artery is carrying oxygen-poor blood.
Pulmonary Vein:
Even though it’s a vein, it carries oxygen-rich blood from the lungs back to the heart.
Normally, veins carry oxygen-poor blood, but the pulmonary veins are carrying oxygen-rich blood.
Memory Trick:
To help you remember this, you can use this saying:
“Arteries are red and veins are blue, but for the lungs, there’s an exception of two.”
This means that in the lungs, the pulmonary artery carries oxygen-poor blood (even though it’s an artery), and the pulmonary vein carries oxygen-rich blood (even though it’s a vein).
Tunics
Except for the microscopic capillaries (which have only one layer), the walls of blood vessels have three layers, or tunics (Figure 11.10). The tunica intima (tu′nĭ-kah in-tim′ah), which lines the lumen, or interior, of the vessels, is a thin layer of endothelium (squamous epithelial cells) resting on a basement membrane. Its cells fit closely together and form a slick surface that decreases friction as blood flows through the vessel lumen.
The tunica media (me′de-ah)
is the bulky middle layer, made up mostly of smooth muscle and elastic fibers. Some of the larger arteries have elastic laminae, sheets of elastic tissue, in addition to the scattered elastic fibers. The smooth muscle, which is controlled by the sympathetic nervous system, is active in changing the diameter of the vessels. As the vessels constrict or dilate, blood pressure increases or decreases, respectively.
The tunica externa (eks′tern-ah
) is the outermost tunic. This layer is composed largely of fibrous connective tissue, and its function is to support and protect the vessels.
Structural Differences in Arteries, Veins, and Capillaries
The walls of arteries are usually much thicker than those of veins. The arterial tunica media, in particular, tends to be much heavier. This structural difference is related to a difference in function of these two types of vessels. Arteries, which carry blood away from the heart, must be able to expand (stretch) as blood is forced into them and then recoil passively as the blood flows off into the circulation during diastole. Their walls must be strong and stretchy enough to take these continuous changes in pressure without stretching out permanently (see Figure 11.19).
Veins, in contrast, carry blood back toward the heart, and the pressure in them tends to be low all the time. Thus veins have thinner walls. However, because the blood pressure in veins is usually too low to force the blood back to the heart, and because blood returning to the heart often flows against gravity (up the legs, for example), veins are modified to ensure that the amount of blood returning to the heart (venous return) equals the amount being pumped out of the heart (cardiac output) at any time. The lumens of veins tend to be much larger than those of corresponding arteries, and they tend to have a thinner tunica media but a thicker tunica externa. The larger veins have valves that prevent backflow of blood like those in the heart (see Figure 11.10).
muscular pump, also enhances venous return.
Skeletal muscle activity, known as the muscular pump, also enhances venous return. As the muscles surrounding the veins contract and relax, the blood is squeezed, or “milked,” through the veins toward the heart (Figure 11.11). Finally, the drop in pressure that occurs in the thorax just before we inhale causes the large veins near the heart to fill. Thus, the respiratory pump also helps return blood to the heart (see Figure 11.9).
Why are capillary walls so thin, and how does this affect exchange?
Capillary Wall Structure:
One cell layer thick (tunica intima).
Benefit: This thinness allows easy exchange of substances (oxygen, nutrients, wastes) between the blood and tissue cells.
What are capillary beds and how do they function?
Capillary Beds:
Interweaving networks of capillaries.
Form between an arteriole (small artery) and a venule (small vein).
Allow for microcirculation, the flow of blood from an arteriole to a venule through the capillary bed.
What are precapillary sphincters and their role in capillary beds?
Special capillary beds are found in the mesenteries (serous membranes holding the intestines in place). These capillary beds have precapillary sphincters, which regulate blood flow in the capillary bed, and a vascular shunt (Figure 11.12c). A vascular shunt is a vessel that directly connects the arteriole and venule at opposite ends of the bed. When the precapillary sphincters are relaxed (open), blood flows through the capillaries and takes part in exchanges with cells. When the sphincters are contracted (closed), blood flows through the shunt and bypasses the cells in that region.
How is blood flow through capillary beds regulated?
Blood Flow Regulation:
Controlled by the constriction or dilation of the terminal arteriole and other arterioles feeding into the capillary bed.
Dilated terminal arteriole: Blood flows into the capillary bed, allowing for exchange.
Constriction of terminal arteriole: No blood flows through the capillary bed.
Varicose veins and thrombophlebitis (throm″bo-flĕ-bi′tis),
Varicose veins are common in people who stand for long periods of time (for example, cashiers and hairdressers) and in obese (or pregnant) individuals. The common factors are the pooling of blood in the feet and legs and inefficient venous return resulting from inactivity or pressure on the veins. In any case, the overworked valves give way, and the veins become twisted and dilated. A serious complication of varicose veins is thrombophlebitis (throm″bo-flĕ-bi′tis), inflammation of a vein that results when a clot forms in a vessel with poor circulation. Because all venous blood must pass through the pulmonary circulation before traveling through the body tissues again, a common consequence of thrombophlebitis is clot detachment and pulmonary embolism, which is a life-threatening condition in which the embolism lodges in a vessel in the lung.
