Blood pressure decreases as the distance from the left ventricle increases.

Conversely, decreasing arterial blood pressure initiates the cardioaccelerator reflex, which involves sympathetic impulses to the S-A node. As a result, the heart beats faster. This response increases cardiac output, increasing arterial pressure.

Recall that epinephrine increases heart rate and consequently alters cardiac output and blood pressure. Other factors that increase heart rate and blood pressure include emotional responses, such as fear and anger; physical exercise; and a rise in body temperature.

Changes in arteriole diameters regulate peripheral resistance. Because blood vessels with smaller diameters offer a greater resistance to blood flow, factors that cause arteriole vasoconstriction increase peripheral resistance, and factors causing vasodilation decrease resistance.

The vasomotor center of the medulla oblongata continually sends sympathetic impulses to the smooth mus cles in the arteriole walls, keeping them in a state of tonic contraction, which helps maintain the peripheral resistance associated with normal blood pressure. Because the vasomotor center responds to changes in blood pressure, it can increase peripheral resistance by increasing its outflow of sympathetic impulses, or it can decrease such resistance by decreasing its sympathetic outflow. In the latter case, the vessels undergo vasodila-tion as sympathetic stimulation decreases.

Whenever arterial blood pressure suddenly increases, baroreceptors in the aortic arch and carotid sinuses signal the vasomotor center, and the sympathetic outflow to the arteriole walls falls (fig. 15.39). The resulting vasodilation decreases peripheral resistance, and blood pressure decreases toward the normal level.

Similarly, if blood pressure drops, as following a hemorrhage, the vasomotor center increases sympathetic

Space Medicine

When the rescue team approached the space shuttle Atlantis just after it landed on September 26, 1996, they brought a stretcher, fully expecting to carry off Mission Specialist Shannon Lucid, Ph.D. The fifty-three-year-old biochemist had just spent 188 days aboard the Russian Mir space station, as part of the NASA/Mir Science Program (fig. 15H). Lucid had spent more time in space than any other U.S. astronaut, and it was widely known that about 70% of astronauts cannot stand at all upon reencountering gravity. But she walked, albeit a little wobbly, the 25 feet to the crew transporter, where she received a gift-wrapped big box of M&M chocolate candies from President Clinton. After six months of having to consciously direct her hands to contact objects because of gravity changes, it must have been wonderful to grab the M&M box!

The human body evolved under conditions of constant gravity. So when a body is exposed to micro-gravity (very low gravity) or weightlessness for extended periods, some very obvious changes occur. The field of space medicine examines anatomic and physiologic responses to conditions in space. Shannon Lucid was expected to require the stretcher because of decreased muscle mass, mineral-depleted bones, and low blood volume. The 400 hours that she logged on the Mir's treadmill and stationary bicycle may have helped her stay in terrific physical shape.

From the moment she landed, Lucid was poked and prodded, monitored and tested, as medical researchers attempted to learn how six months in space affects cardiovascular functioning, respiratory capacity, mood, blood chemistry, circadian rhythms, muscular strength, body fluid composition, and many other aspects of anatomy and physiology. Just minutes after greeting her family, Lucid entered a magnetic resonance imaging tube to record the state of her various parts as they adapted to the return to gravity.

The tendency to feel wobbly and be unable to stand upon returning to earth is one of the better-studied physiologic responses to experiencing low-gravity conditions. It is called orthostatic intolerance. Normally, gravity helps blood circulate in the lower limbs. In mi-crogravity or no gravity, blood tends to pool in blood vessels in the center of the body, registering on receptors there. The body interprets this as excess blood, and in response, signals the kidneys to excrete more fluid. But there really isn't an increased blood volume. On return to earth, the body actually has a pint to a quart less blood than it should, up to a 10 to 20% decrease in total blood volume. If blood vessels cannot constrict sufficiently to counter the plummeting blood pressure, orthostatic intolerance results. To minimize the effect, astronauts wear lower-body suction suits, which apply a vacuum force that helps draw blood into the blood vessels of the lower limbs. Maintaining fluid intake helps prevent dehydration.

Shannon Lucid had expected to be in space for 140 days, but problems with the space shuttle and a fierce hurricane extended her stay. Because she spent the longest continuous time in space, her visit is supplying vital information for planning a trip to Mars, which would require that astronauts be in space at least two years, round-trip. ■

Figure 15H

Shannon Lucid's 188-day stay in space revealed to researchers much about the body's responses to microgravity conditions. While aboard the space station Mir, Lucid conducted experiments on quail embryos and growth of protein crystals.

Shier-Butler-Lewis: I IV. Transport I 15. Cardiovascular System I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Music Concept Images


(a) Relaxation of smooth muscle in the arteriole wall produces dilation, whereas (b) contraction of the smooth muscle causes constriction (a and b 1,500x).

Increased blood volume entering heart

Cardiac output increases

Figure 15.37

Cardiac output is related to the volume of blood entering the heart.

Blood pressure returns toward normal

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