The Venous Return

If the capillary beds and veins were static tubes with fixed diameters, blood would stream from the arterioles into the capillaries, pour from the capillaries into the veins, and be pushed all the way back to the heart by arterial pressure. But this is not the way the system operates. The capillaries and veins are expandable: they could easily accommodate all the blood in the body. And this can create a serious problem because within certain limits the amount of blood brought to the heart per minute (the ^nous return) regulates the volume of blood pumped by the heart per minute (the cardiac output). Here is what happens: As venous return increases, the additional blood stretches the walls of the ventricles, and when that happens, the stretched muscle fibers in the ventricles automatically pump more strongly, thus increasing cardiac output; as venous return decreases, the ventricles pump less vigorously, thus decreasing cardiac output. Therefore, the mechanisms for moving blood from capillaric venules, and veins back to the heart are critical. If too much blood stap nates in those parts of the system, which can happen for many reason cardiac output decreases and the heart may not receive enough blood t pump to the brain and other vital organs.

When we are in a normal upright posture, the venous return from veil located above the heart is unimpeded, and blood drops like a waterfall I the right atrium. And at heart level (in the middle segment of the arm, fc example) venous pressure is about 15 mm Hg. Since this is more than tl o mm Hg where the blood enters the right atrium, it is still pushed easi' back into the heart. The lower extremities are another story, however, ai 1 to get blood back to the heart from the feet, at least in an upright postu the venous return has to overcome a pressure of about 140 mm Hg, win 1 reflects the height of a static column of venous blood below the heart.

The mechanism for getting blood back to the heart from the low r extremities is beautiful in its simplicity and elegance. The veins below t i heart contain one-way valves, and contraction of the skeletal must s surrounding these valves acts as a "muscle pump" to squeeze bl< 1 through them and back toward the heart. When the muscles relax, t e valves close to prevent backflow, insuring that the flow is unidirectio il blood is pushed through the upper valve blood is pushed through the upper valve

Muscle Pump Venous

figure 8.3. Skeletal muscle pump for venous return. On the left the muscle contracts, and blood can escape this segment of the vein only by being push' I through the upper valve. On the right, the muscle relaxes, and the upper val\ is closed by venous back-pressure. As skeletal muscles become active throug out the body, alternately contracting and then relaxing, blood is pushed baik o the heart mechanically (Dodd).

figure 8.3. Skeletal muscle pump for venous return. On the left the muscle contracts, and blood can escape this segment of the vein only by being push' I through the upper valve. On the right, the muscle relaxes, and the upper val\ is closed by venous back-pressure. As skeletal muscles become active throug out the body, alternately contracting and then relaxing, blood is pushed baik o the heart mechanically (Dodd).

H. llll IIIADSTAND 443

(fig. 8.3). Drill instructors in the military may not he aware of this mechanism, but they know that skeletal muscular activity is needed to get blood back to the heart, and that is why they instruct new recruits who are standing at attention on a hot day to isometrically contract and then relax the postural muscles of their lower extremities, which keeps them from fainting.

There are no valves in the head and neck: our upright posture has rendered them irrelevant. But when you are standing upright, a pool of blood courses slowly through the veins in the lower half of the body, waiting to be pumped back toward the heart by muscular activity. And if you are chronically inactive, fluids move so sluggishly out of this region that the processes of cellular nourishment and elimination are compromised. This gravity-induced congestion can affect any organ or tissue below the heart. Lying down for a night's sleep helps correct the situation, but we still often see the effects of gravity in chronicady swollen ankles, varicose veins, and hemorrhoids. One remedy is vigorous movement in which muscles alternately contract and relax in order to propel blood through the venous valves. And this is one reason nurses try to get people up and about as soon as possible after surgery and why health practitioners constantly preach the benefits of exercise. Yoga teachers do not disagree, but suggest another alternative—inverted postures.

What happens specifically when you turn upside down? In the first moments of the headstand blood pools in the capillary beds and veins in the region of the body superior to the heart—in the head, neck, and shoulders—where it is kept until arterial pressure forces it back around to the heart. And because there are no valves in the veins of this region, skeletal muscle contraction cannot assist its return. This is not very important if you stay in the headstand only 2-5 minutes, but if you want to extend your time in the posture it can become a problem. We'll come back to this issue toward the end of the chapter.

ITechnical note: There are many ways to affect venous return. Without naming the phenomenon, we looked in chapter 2 at the effects of a Valsalva maneuver—holding the breath and straining after an inhalation—in conjunction with hyperventilation. It also sometimes happens that X-ray technicians ask patients to hold their breath after an inhalation in order to get a more elongated and accurate profile of the heart, and if the patient gets overenthusiastic about this after locking the glottis, or if the technician dawdles, the unintentioned Valsalva maneuver impedes the venous return. If you hold the breath in this manner after a deep inhalation, the profile of the heart in a roentgenogram shrinks dramatically, and after 10-15 seconds, depcnd-lnE on how purposely you strain, the venous return is inhibits enough to cause you to pasp out. One would not ordinarily think of trying the Valsalva maneuver in the headstand, and it's plainly inadvisable. It won't result in fainting because the inverted posture sends blood preferentially to the head, but it certainly causes a sharp and immediate rise in blood pressure. This is felt mostly prominently in the foce, where it is disagreeable although probably not harmful—but for the brain and for the retina of the eye, look out: it is assuredly dangerous. I

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The Newbies Guide To Yoga

The Newbies Guide To Yoga

Yoga is extensively know as a form of exercise that stretches and strengthens the body through various poses know as ASANA. For other people yoga is the realization of inner self satisfaction. For other it is a religion that the believe and must follow. Learn more within this guide by downloading today.

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Responses

  • jennifer
    What happens in venous return when you are hot?
    8 years ago

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