BIO Vascular wall tension and aortic blowout The walls of blood vessels contain vary...

BIO Vascular wall tension and aortic blowout The walls of blood vessels contain varying amounts of elastic fibers that allow the vessels to expand and contract as the pressure and amount of fluid inside vary (these fibers are more prevalent in the aorta and large arteries than in the small arterioles and capillaries). These fibers in the cylindrical walls produce a wall tension T, defined as

where L is the length of an imaginary cut parallel to the axis of the vessel and F is the magnitude of the force that each side of the cut must exert on the other side to hold the two sides together. Three forces are exerted on a short section of wall fiber— the system. (1) The fluid inside pushes outward, due to fluid pressure from inside P_{inside fluid pushing out} = P_out; (2) fluid outside the vessel pushes inward, due to fluid pressure from outside P_{outside fluid pushing i}n = Pin; and (3) the wall next to the system exerts wall tension T on each side of that wall. The pressure difference across the wall ΔP = P_out - Pin, the wall tension T, and the radius R of the cylindrical vessel are related by Laplace’s law:

The inward gauge pressure Pin of tissue surrounding the vessels is approximately zero. Thus, the pressure difference ΔP = P_out - P_in = P_vessel - 0 = P_vessel is the gauge pressure in the blood vessel. We can now estimate the wall tension for different

types of vessels. The tension in the aorta is approximately

Using similar reasoning, the wall tension in the low-pressure, very small radius capillaries is about 0.016 N/m—about 0.0003 times the tension needed to tear a facial tissue. Because such little tension is needed to hold a capillary together, its wall can be very thin, allowing easy diffusion of various molecules across the wall. The walls of a healthy aorta can easily provide the tension needed to support the increased blood pressure when it fills with blood from the heart during each heartbeat. However, aging and various medical conditions may weaken the aortic wall in a short section, and increased blood pressure can cause it to stretch. The weakened wall bulges outward; this is called an aortic aneurism. The increased radius causes increased tension, which can increase bulging. This cycle can result in a rupture to the aorta: an aortic blowout. a windowed cylindrical portable hyperbaric chamber constructed of nonpermeable nylon that requires constant pressurization with a foot pump attached to the bag. The climber enters the bag, and a person outside pumps air into the bag so that the air pressure inside is somewhat higher than the outside pressure. The bag and pump have a 6.76-kg mass. The volume of the inflated bag is 0.476 m3. The maximal bag pressure is 0.14 X 10⁵ N/m² above the air pressure at the site where it is used. In the 1997 climb, with the temperature at -20 °C, the bag was filled in about 2 min with 10–20 pumps per minute. This raised the pressure in the bag to 0.58 * 105 N>m2 (equivalent to an elevation of 4400 m) instead of the actual outside pressure of 0.43 X 105 N/m² at the 6450-m elevation. The treatment lasted for 2 h, with the person inhaling about 15 times/min at about 0.5 L/inhalation, and was successful—the pulmonary edema disappeared.

Aortic blowout occurs when part of the wall of the aorta becomes weakened. What does this cause?

(a) A bulge and increased radius of the aorta when the blood pressure inside increases

(b) An increased radius of the aorta, which causes increased tension in the wall

(c) An increased tension in the aorta, which causes the radius to increase

(d) a and b

(e) a, b, and c