No abstract

No abstract

Bayliss showed in 1902 that a sudden rise of arterial pressure causes the volume of the hindlimb of dogs, cats and rabbits to decrease below the original after a preliminary increase. The response was unaltered by acute denervation. He also found that arrest of the circulation to a curarized hindlimb for as little as 2 sec was followed by a large increase in the volume of the limb. These effects he ascribed to a direct reaction on the part of the muscular walls of the arteries, an increase in pressure causing constriction and a decrease dilatation. This explanation has been criticized by many workers, but has recently received support. Folkow (1949) has measured directly the arterial perfusion pressure in, and the rate of venous outflow from, the hindlimbs of anaesthetized cats. Very short (5 sec) periods of interrupted arterial inflow were followed by a pronounced vasodilatation which was substantially the same in the normal, acutely and chronically sympathectomized and chronically de-afferented limb respectively. In the acutely sympathectomized limb, a partial arterial occlusion, sufficient to lower the perfusion pressure, but not sufficient to lower the rate of blood flow below that occurring before sympathectomy, was also followed by vasodilatation. The dilatation following partial arterial occlusion was unaltered by large variations in the oxygen and carbon dioxide percentages in the inspired air. Sudden steps up or down in the arterial perfusion pressure of the acutely denervated hindlimb of the cat (Folkow, 1953) caused sudden rises and falls respectively in the rate of venous outflow; in all cases, however, the rate of venous outflow gradually returned over the next minute towards, but not to, its original level. These observations support the view that the resistance vessels respond by dilatation to a fall and by constriction to a rise of perfusing pressure, although it is not clear whether the mean or the pulsatile pressure constitutes the predominant stimulus. Hilton's (1953) observation that the outflow from the cat's hindlimb is increased after arterial occlusion but not after venous occlusion gives further support.

1. Exposure of the body from iliac crests to feet of a horizontal subject to a pressure 70 mm Hg below atmospheric causes a displacement of about 10 g of blood/kg total body weight from the upper to the lower part of the body. Much of this blood is returned very rapidly at the end of suction. 2. During suction, the changes in the circulation resemble those during a foot‐down tilt. After suction, the changes resemble to some extent those following the Valsalva manoeuvre. 3. The overshoot of forearm blood flow following suction is caused by variations in the activity of adrenergic vasoconstrictor nerves. The receptors for this reflex have not been identified, but their stimulation depends upon a rapid and large return of blood to the central circulation.

Many investigators have injected acetylcholine and histamine into man both intra-arterially and intravenously. The effects of intra-arterial injection of acetylcholine on the circulation in the legs were studied by Carmichael & Fraser (1933) who observed skin colour, and by Ellis & Weiss (1932) who measured skin temperature and arterio-venous oxygen differences. We have been unable, however, to find detailed quantitative measurements of their effects on the vessels. Since such observations were required for another purpose we have made them, and this paper describes the results. METHODSObservations have been made on twenty-six men and three women between the ages of 19 and 34. All were in good health, and none suffered from peripheral vascular disease. They arrived at the laboratory at least 1 hr before observations started and were recumbent during, and for half an hour before, the observations. They wore normal indoor clothing, and the trunk and legs and as much as possible of the arms were wrapped in blankets. The laboratory temperature was 20-23' C.Blood flow The blood flow through the hands and forearms was measured every 15 sec, or more frequently, by venous occlusion plethysmography. In most experiments the plethysmographs were filled with stirred water at 32°C for the hand and at 340 C for the forearm. They were fitted with loose gloves for the hands or loose sleeves for the forearms. In some experiments, in which it was desired to measure simultaneously the hand and forearm flow on the same side, a light celluloid air-filled plethysmograph was used on the forearm.The venous occlusion cuffs, at the wrist for the hand, and above the elbow (proximal to the needle in the brachial artery) for the forearm, were, unless otherwise stated, inflated to 70 mm Hg during the periods of collection. A wrist cuff was inflated to 200 mm Hg for 1 min before, and during, observations of forearm blood flow. When, however, the hand and forearm blood flows