chapter 8 Flashcards
(44 cards)
Define ‘cardiovascular system’
Collectively known as the cardiovascular system (from the Greek kardia, heart, and the Latin vasculum, small vessel), the heart and blood vessels function as a centrally controlled blood distribution network. The heart provides the power to move the blood, and the vascular system represents the network of branching conduit vessels through which the blood flows. Central control (oversight and management, if you will) is provided by the nervous system. By controlling the rate at which the heart pumps and the resistance to flow through blood vessels, the nervous system apportions blood flow to the various tissues and organs according to need.
Know that vessels form a circuit from: Left Side of heart –> Arteries –> arterioles –> capillaries –> venules –> veins –> Right side of heart
know it
Do arteries carry blood toward or away from the heart?
away
Is blood pressure high or low in arteries as compared with veins?
high
How does the middle smooth muscle layer of arteries differ from the same layer in veins?
Veins are less rigid and tough as they are under lower pressure and can expand more
Describe the function of precapillary sphincters in arterioles
Right where an arteriole joins a capillary is a band of smooth muscle called the precapillary sphincter (Figure 8.2). The precapillary sphincters serve as gates that control blood flow into individual capillaries.
A narrowing of blood vessel diameter caused by contraction of the smooth muscle in the vessel wall.
vasoconstriction
A widening of blood vessel diameter caused by relaxation of the smooth muscle in the vessel wall.
vasodilation
Describe the function of capillaries.
capillaries (Figure 8.3a). Capillaries are thin-walled vessels that average only about one-hundredth of a millimeter in diameter—not much wider than the red blood cells that travel through them. In fact, they are so narrow that red blood cells (RBCs) often have to pass through them in single file or even bend to squeeze through (Figure 8.3b)
What is the diameter of a capillary in relation to a RBC?
Barely bigger
Where are capillary beds located? Why is this essential?
Extensive networks of capillaries, called capillary beds, can be found in all areas of the body, which is why you are likely to bleed no matter where you cut yourself. The branching design of capillaries and their thin, porous walls allow blood to exchange oxygen, carbon dioxide, nutrients, and waste products with tissue cells. Capillary walls consist of a single layer of squamous epithelial cells (Figure 8.3c). Microscopic pores pierce this layer, and the cells are separated by narrow slits. These openings are large enough to allow the exchange of fluid and other materials between blood and the interstitial fluid (the fluid that surrounds every living cell), yet small enough to retain RBCs and most plasma proteins in the capillary. Some white blood cells (WBCs) can also squeeze between the cells in capillary walls and enter the tissue spaces. In effect, capillaries function as biological strainers that permit selective exchange of substances with the interstitial fluid. In fact, capillaries are the only blood vessels that can exchange materials with the interstitial fluid.
Why is it important that capillary walls be thin and porous?
To allow exchange
More fluid gets filtered out of blood capillaries on the arteriole side than get reabsorbed on the venule side. What system is responsible for collecting and returning this excess fluid to circulation?
lymphatic system
Summarize the function of veins.
From the capillaries, blood flows back to the heart through venules (small veins) and veins (see Figures 8.1 and 8.4). Like the walls of arteries, the walls of veins consist of three layers of tissue. However, the outer two layers of the walls of veins are much thinner than those of arteries. Veins also have a larger diameter lumen than arteries. The anatomical differences between arteries and veins reflect their functional differences. As blood moves through the cardiovascular system, blood pressure becomes lower and lower. The pressure in veins is only a small fraction of the pressure in arteries, so veins do not need nearly as much wall strength as arteries. The larger diameter and high distensibility of veins allows them to stretch like thin balloons to accommodate large volumes of blood at low pressures. In addition to their transport function, then, veins serve as a blood volume reservoir for the entire cardiovascular system. Nearly two-thirds of all the blood in your body is in your veins. Thanks to their blood reservoir function, even if you become dehydrated or lose a little blood, your heart will still be able to pump enough blood to keep your blood pressure fairly constant.
Skeletal muscles squeeze veins
Skeletal muscles squeeze veins On their path back to the heart, veins pass between many skeletal muscles. As we move and these muscles contract and relax, they press against veins and collapse them, pushing blood toward the heart. You may have noticed that you tire more easily when you stand still than when you walk around. This is because walking improves the return of blood to your heart and prevents fluid accumulation in your legs. It also increases blood flow and the supply of energy to your leg muscles.
One-way valves
One-way valves permit only one-way blood flow Most veins contain valves consisting of small folds of the inner layer that protrude into the lumen. The structure of these valves allows blood to flow in one direction only: toward the heart. They open passively to permit blood to move toward the heart and then close whenever blood begins to flow backward. what is called the “skeletal muscle pump” (Figure 8.5). Once blood has been pushed toward the heart by skeletal muscles or drained in that direction by gravity, it cannot drain back again because of these one-way valves. The opening and closing of venous valves is strictly dependent on differences in blood pressure on either side.
Pressures from breathing
Pressures associated with breathing push blood toward the heart The third mechanism that assists blood flow involves pressure changes in the thoracic (chest) and abdominal cavities during breathing. When we inhale, abdominal pressure increases and squeezes abdominal veins. At the same time, pressure within the thoracic cavity decreases, dilating thoracic veins. The result is to push blood from the abdomen into the chest and toward the heart. This effect is sometimes called the “respiratory pump.”
What is the myocardium?
In cross section, we see that the walls of the heart consist of three layers: the epicardium, myocardium, and endocardium (Figure 8.7). The outermost layer, the epicardium, is a thin layer of epithelial and connective tissue. The middle layer is the myocardium. This is a thick layer consisting mainly of cardiac muscle that forms the bulk of the heart. The myocardium is the layer that contracts every time the heart beats. The structure of cardiac muscle cells allows electrical signals to flow directly from cell to cell. An electrical signal in one cardiac muscle cell can spread to adjacent cells, enabling large numbers of cells to contract as a coordinated unit. Every time the myocardium contracts, it squeezes the chambers inside the heart, pushing blood outward into the arteries. The innermost layer of the heart, the endocardium, is a thin endothelial layer resting on a layer of connective tissue. The endocardium is continuous with the endothelium that lines the blood vessels.
Differentiate between atria and ventricles.
Taking a closer look at the details of the structure of the heart, we see that it consists of four separate chambers (see Figure 8.7). The two chambers on the top are the atria (singular: atrium), and the two more-muscular bottom chambers are the ventricles. A muscular partition called the septum separates the right and left sides of the heart. Blood returning to the heart from the body’s tissues enters the heart at the right atrium. From the right atrium, the blood passes through a valve into the right ventricle. The right ventricle is more muscular than the right atrium because it pumps blood at considerable pressure through a second valve and into the artery leading to the lungs. Blood returning from the lungs to the heart enters the left atrium and then passes through a third valve into the left ventricle. The very muscular left ventricle pumps blood through a fourth valve into the body’s largest artery, the aorta. From the aorta, blood travels through the arteries and arterioles to the systemic capillaries, venules, and veins and then back to the right atrium again. The left ventricle is the most muscular of the heart’s four chambers because it must do more work than any other chamber. The left ventricle must generate pressures higher than aortic blood pressure in order to pump blood into the aorta. (We’ll see how high aortic pressure is in a minute.) The right ventricle has a thinner wall and does less work because the blood pressure in the arteries leading to the lungs is only about one-sixth that of the aorta.
Be able to describe the flow of blood through the heart:
From systemic circulation (Vena Cava) –> Rt. Atrium –> Rt. Ventricle –> Lungs (Pulmonary circulation) –> Lt. Atrium –> Lt. Ventricle –> Aorta (out to systemic circulation)
What is the function of the AV valves?
Four heart valves enforce the heart’s one-way flow pattern and prevent blood from flowing backward. The valves open and shut passively in response to changes in the pressure of blood on each side of the valve. The right and left atrioventricular (AV) valves located between the atria and their corresponding ventricle prevent blood from flowing back into the atria when the ventricles contract. The AV valves consist of thin connective tissue flaps (cusps) that project into the ventricles. The right AV valve is called the tricuspid valve because it has three flexible flaps. The left AV valve has two flaps, so it is referred to as the bicuspid or mitral valve. These valves are supported by strands of connective tissue called chordae tendineae that connect to muscular extensions of the ventricle walls called papillary muscles. Together, the chordae tendineae and papillary muscles prevent the valves from everting (opening backward) into the atria when the ventricles contract.
Differentiate between the pulmonary and systemic circuits.
The heart pumps blood through two circuits simultaneously; the pulmonary circuit (lungs), where blood picks up oxygen and gets rid of CO2, and the systemic circuit (the rest of the body) where oxygen is used and CO2 waste is produced. The pattern of blood flow within the cardiovascular system is shown in Figure 8.8. Let’s follow the flow of blood through the system, starting with the return of blood to the heart from the systemic circuit.
1 Deoxygenated venous blood returns to the heart and
2 enters the right atrium.
3 From there it passes through the right atrioventricular valve into the right ventricle. The right ventricle pumps blood through the pulmonary semilunar valve into
4 the pulmonary trunk (the main pulmonary artery) leading to the lungs.
5 The pulmonary trunk divides into the right and left pulmonary arteries, which supply the right and left lungs, respectively.
6 Blood entering the lungs passes through the pulmonary capillaries. This is where gas exchange occurs; blood gives up CO2 and receives a fresh supply of O2 from the air we inhale.
7 The freshly oxygenated blood flows into the pulmonary veins leading back to the heart. On returning to the heart after its trip through the lungs,
8 the now-oxygenated blood flows into the left atrium and then
9 passes through the left atrioventricular valve to enter the left ventricle.
10 The left ventricle pumps the blood into the aorta.
11 Some of the blood travels up the main arteries to the head and upper body, and the rest passes down the aorta to the torso and lower limbs.
12 Upon arrival at the capillaries in the systemic circuit, blood delivers O2 to the tissues and picks up waste CO2. Then it returns to the venous system, eventually making its way back to the great veins that enter the heart. Note that for every one trip around the body the blood passes through the heart twice; once as deoxygenated blood destined for the lungs, and once as oxygenated blood to be delivered to the systemic circuit. Deoxygenated blood passing through the right side of the heart never mixes with oxygenated blood passing through the left.
Describe the function of the coronary arteries.
Thus, the heart has its own set of blood vessels called the coronary arteries that supply the heart muscle (Figure 8.10). The coronary arteries branch from the aorta just above the aortic semilunar valve and encircle the heart’s surface (the word coronary comes from the Latin corona, meaning “encircling like a crown”). From the surface, they send branches inward to supply the myocardium. Cardiac veins collect the blood from the capillaries in the heart muscle and channel it back to the right atrium.
Describe the cardiac cycle:
Atrial systole –> Ventricular systole –> Diastole
1 Atrial systole As contraction starts, the heart is already nearly filled with blood that entered the ventricles and atria passively during the previous diastole. Contraction of the heart begins with the atria. During atrial systole, both atria contract, raising blood pressure in the atria and giving the final “kick” that fills the two ventricles to capacity. Atrial systole also momentarily stops further inflow from the veins. Both atrioventricular valves are still open, and both semilunar valves are still closed.
2 Ventricular systole The contraction that began in the atria spreads to the ventricles, and both ventricles contract simultaneously. The rapidly rising ventricular pressure produced by contraction of the ventricles causes the two AV valves to close, preventing blood from flowing backward into the atria and veins. At this time, the atria relax and begin filling again. The pressure within the ventricles continues to rise until it is greater than the pressure in the arteries, at which point, the pulmonary and aortic semilunar valves open and blood is ejected into the pulmonary trunk and the aorta. With each ventricular systole, about 60% of the blood in each ventricle is forcibly ejected
3 Diastole Both atria and both ventricles are relaxed throughout diastole. At this point pressure within the ventricles begins to fall. As soon as ventricular pressures fall below arterial pressures during early diastole, the pulmonary and aortic semilunar valves close, preventing backflow of arterial blood. Once ventricular pressure falls below blood pressure in the veins, the AV valves open and blood begins to flow passively into the heart. A complete cardiac cycle occurs every 0.8 second or so. These cycles repeat, from birth to death, without ever stopping. Atrial systole lasts about 0.1 second; ventricular systole about 0.3 second. During the remaining 0.4 second, the heart relaxes in diastole.
