Monary Circulation

The Arteries.

In the great arterial trunks such as the aorta, the pulmonary, the carotid and iliac arteries, the tunica media is divided by elastic fibres and membranes into a large number of concentric layers containing (especially in the aorta) only a few muscle fibres. The adventitia of the large arteries is also composed of fibro-elastic tissue, but its structure is looser and the fibrous tissue is more abundant. The medium-sized arteries differ in structure from the larger ones in that the elastic elements of the intima and media are replaced to a considerable extent by non- , striated muscular fibres (fig. 13).

To this type belong the majority of the arterial vessels.

The Capillaries.

These (fig. 14) consist solely of a single layer of endothelial cells, which present little resistance to the passage of substances dissolved in the blood, such as oxygen, carbon dioxide, sugar and salts. According to some observers, the cells forming the capillary wall are contractile; according to others, the actual contractile elements are special Stellate cells which encircle the capillaries. But whatever the mechanism, the contractility of the capillaries is unquestionable, and must play a considerable part in regulating the blood flow through an organ. The average length of capillaries is between 0.4 and 0.7 m.m.; in most organs they freely anastomose with each other, forming a more or less loose network.

The Vein.

The smallest veins result from the fusion of a variable number of capillaries. They assume a connective tissue coat, the adventitia (fig. is), and with increase in size muscle-fibres appear in the form of a badly defined media. Some of the larger veins, such as the brachial and subcutaneous veins, have a better developed muscular layer, while in others (the jugular, the subclavian, the veins of the dura and pia mater) the media is entirely lacking. Generally speak ing, the muscular and elastic ele ments are much less prominent in the veins than in the arteries, and they contain a preponderance of inelastic connective tissue fibres. In many veins the adventitia shows an inner longitudinal muscular layer. All subcutaneous veins and some internal veins are supplied with valves which restrict any possible back flow.

The valves of the veins are reduplications of the intima, and the greater part of the valvular structure consists of fibrous connective tissue and elastic fibres.

The Blood Pressure.

It has long been known and can easily be demonstrated that the blood is under different pressures in the various parts of the vascular system. When an artery is cut, blood

flows out with great force in a series of jerks which are synchron ous with the heart beat. When a large vein is cut, the blood also flows out rapidly, but the stream has very little force.

The first measurement of arterial pressure was made by the Rev. Dr. Stephen Hales. ("Statical Essays, containing Haema staticks" ) Since Hales' work, the chief improvements in the method have been the application of the mercury manometer by Poiseuille, the invention of the recording manometer and the kymograph by Lud wig (figure i6), and the intro duction of the more accurate membrane manometer by Hiir thle and Frank.

The manometer of Ludwig con sists of a U-tube which is half filled with mercury. On the sur face of the mercury of one limb is a float from which a stiff light rod projects, bearing on its upper end a writing point which is made to write on the smoked surface of a revolving drum. The other limb of the manometer is connected by means of a glass or metal can nula to the artery. The tube be tween the artery and the manom eter is filled with a solution of some anticoagulant salt (sodium citrate, magnesium sulphate).

The mercury manometer allows a direct reading of pressure, but on account of its inertia it does not accurately record rapid changes in the pressure. The membrane manometer, which is merely a tube filled with fluid and sealed at one end with a stretched rubber membrane, records rapid changes in pressure more accurately, but each membrane requires special calibration if absolute values are desired. The venous pressure is recorded by a similar method, but with a water manometer or a manometer with a lightly stretched membrane.

The highest pressure, which occurs while the blood is passing from the heart into the aorta, is called the systolic arterial pres sure, and the pressure at the end of diastole is the diastolic pres sure; the range between these two extremes is known as the pulse pressure. In the dog, with a mean arterial pressure of about 120 mm. of mercury, the systolic pressure may be as high as 16o mm., and the diastolic pressure as low as 65 mm. ; here the pulse pres sure would be 95 mm. of mercury. By taking the pressure at dif ferent parts of the vascular system, we obtain a result which is diagrammatically represented in figure 18.