BLOOD-PRESSURE IN THE VEINS. 175 



ing tissues. The resistance to the blood-stream is greatest in the capillary area, 

 and it is evident that the blood in a long capillary must exert more pressure at the 

 commencement than at the end of the capillary; in the middle of the capillary area 

 the blood-pressure is just about one-half of the pressure within the large arteries 

 (Bonders). The capillary pressure must also vary in different regions of the body. 

 Thus, the pressure within the intestinal capillaries, in those constituting the 

 glomeruli of the kidney, and in those of lower limbs when the person is in the erect 

 posture, must be greater than in other regions, depending in the former cases partly 

 upon the double resistance caused by two sets of capillaries, and in the latter case 

 partly on purely hydrostatic causes. 



87. Blood-Pressure in the Veins. 



In the large venous trunks near the heart (innominate, sub-clavian, 

 jugular) a mean negative pressure of about - O'l mm. Hg. prevails (H. 

 Jacobson). Hence, the lymph-stream can flow unhindered. As the 

 distance of the veins from the heart increases, there is a gradual increase 

 of the lateral pressure ; in the external facial vein (sheep) = + 3 mm. ; 

 brachial, 4*1 mm., and in its branches 9 mm.; crural, 11*4 mm. 

 (Jacobson). [The pressure is said to be negative when it is less than 

 that of the atmosphere.] 



Conditions Influencing the Venous Pressure. (1.) All conditions 

 which diminish the difference of pressure between the arterial and 

 venous systems increase the venous pressure and vice versa. 



(2.) General plethora of blood increases it ; anaemia diminishes it. 



(3). Respiration, or the aspiration of the thorax, affects specially the 

 pressure in the veins near the heart ; during inspiration, owing to the 

 diminished tension, blood flows towards the chest, while during expira- 

 tion it is retarded. The effects are greater the deeper the respiratory 

 movements, and these may be very great when the respiratory passages 

 are closed ( 60). 



[When a vein is exposed at the root of the neck, it collapses during inspiration, 

 and fills during expiration a fact which was known to Valsalva. The respiratory 

 movements do not affect the venous stream in the peripheral veins. The veins of 

 the neck and face become distended with blood during crying, and on making 

 violent expiratory efforts, as in blowing upon a wind-instrument; while every 

 surgeon is well acquainted with the fact that air is particularly apt to be sucked 

 into the veins, especially in operations near the root of the neck. This is due to 

 the negative intra-thoracic pressure occurring during inspiration.] 



(4.) Aspiration of the Heart. Blood is sucked or aspirated into 

 the auricles when they dilate, so that there is a double aspiration 

 one synchronous with inspiration, and the other, which is but 

 slight, synchronous with the heart-beat. There is a corresponding 

 retardation of the blood-stream in the venae cavse, caused by the 

 contraction of the auricle (see p. 77, a). The respiratory and cardiac 



