A capillary bed can consist of two types of vessels: true capillaries, which branch mainly from arterioles and provide exchange between cells and the circulation, and vascular shunts, short vessels that directly connect arterioles and venules at opposite ends of the bed, allowing for bypass. Capillary beds may control blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure.
This is achieved by myogenic response and by tubuloglomerular feedback in the kidney. When blood pressure increases, the arterioles that lead to the capillary bed are stretched and subsequently constrict to counteract the increased tendency for high pressure to increase blood flow.
In the lungs, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When heart rate increases and more blood must flow through the lungs, capillaries are recruited and are distended to make room for increased blood flow while resistance decreases.
Privacy Policy. Skip to main content. Cardiovascular System: Blood Vessels. Search for:. Artery Function Arteries are high-pressure blood vessels that carry oxygenated blood away from the heart to all other tissues and organs. Learning Objectives Distinguish the function of the arterial system from that of venous system. Key Takeaways Key Points Arteries are blood vessels that carry blood away from the heart.
This blood is normally oxygenated, with the exception of blood in the pulmonary artery. Arteries typically have a thicker tunica media than veins, containing more smooth muscle cells and elastic tissue. This allows for modulation of vessel caliber and thus control of blood pressure. The arterial system is the higher-pressure portion of the circulatory system, with pressure varying between the peak pressure during heart contraction systolic pressure and the minimum diastolic pressure between contractions when the heart expands and refills.
The increase in arterial pressure during systole, or ventricular contraction, results in the pulse pressure, an indicator of cardiac function. Key Terms systolic pressure : The peak arterial pressure during heart contraction. Elastic Arteries An elastic or conducting artery has a large number of collagen and elastin filaments in the tunica media. Learning Objectives Distinguish the elastic artery from the muscular artery. Key Takeaways Key Points Elastic arteries include the largest arteries in the body, those closest to the heart.
They give rise to medium-sized vessels known as muscular, or distributing, arteries. Elastic arteries differ from muscular arteries both in size and in the relative amount of elastic tissue contained within the tunica media. Arterial elasticity gives rise to the Windkessel effect, which helps to maintain a relatively constant pressure in the arteries despite the pulsating nature of blood flow.
Key Terms elastic arteries : An artery with a large number of collagen and elastin filaments, giving it the ability to stretch in response to each pulse. The Aorta Due to position as the first part of the systemic circulatory system closest to the heart and the resultant high pressures it will experience, the aorta is perhaps the most elastic artery, featuring an incredibly thick tunica media rich in elastic filaments. Muscular Arteries Distributing arteries are medium-sized arteries that draw blood from an elastic artery and branch into resistance vessels.
Learning Objectives Distinguish muscular arteries from elastic arteries. Key Terms muscular arteries : Medium-sized arteries that draw blood from an elastic artery and branch into resistance vessels, including small arteries and arterioles. It is readily visualized with light microscropy in sections of muscular arteries. Anastomoses A circulatory anastomosis is a connection or looped interaction between two blood vessels. Learning Objectives Explain the function of arterial anastomoses.
Key Takeaways Key Points Anastomoses occur normally in the body in the circulatory system, serving as backup routes for blood flow if one link is blocked or otherwise compromised.
Anastomoses between arteries and between veins result in a multitude of arteries and veins, respectively, serving the same volume of tissue. Pathological anastomoses result from trauma or disease and are referred to as fistulae. Veins return blood back toward the heart. Capillaries surround body cells and tissues to deliver and absorb oxygen, nutrients, and other substances. The capillaries also connect the branches of arteries and to the branches of veins.
The walls of most blood vessels have three distinct layers: the tunica externa, the tunica media, and the tunica intima. These layers surround the lumen, the hollow interior through which blood flows. The left ventricle of the heart pumps oxygenated blood into the aorta. From there, blood passes through major arteries, which branch into muscular arteries and then microscopic arterioles.
The arterioles branch into the capillary networks that supply tissues with oxygen and nutrients. The walls of arteries are thicker than the walls of veins, with more smooth muscle and elastic tissue. This structure allows arteries to dilate as blood pumps through them.
After the capillaries release oxygen and other substances from blood into body tissues, they feed the blood back toward the veins. First the blood enters microscopic vein branches called venules. The venules conduct the blood into the veins, which transport it back to the heart through the venae cavae. Vein walls are thinner and less elastic than artery walls. The pressure pushing blood through them is not as great. Veins have a thick outer layer made of collagen and below this are thin bands of smooth muscle and elastic tissue, with the innermost layer being made up of endothelium cells.
Veins are a lot further from the pumping action of the heart than arteries and as a result have much thinner walls because they operate under much less pressure. The skeletal muscles surrounding veins expand and contract which presses on veins causing a pumping effect. The surge of pressure in adjacent arteries also provides a similar effect. The adjacent diagram also illustrates valves that are located at regular intervals in large veins. These valves ensure that blood only flows in one direction towards the heart, as they are automatically closed by the backflow of blood, similar to the valves in the heart.
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