Another cause of congestion is the lessening of pressure by the parts surrounding the vessel. Thus in cupping, there is prompt cutaneous congestion, and a similar result occurs in pericardium, pleura, or peritoneum on the withdrawal of the liquid of hydropericardium, hydrothorax or ascites.
Another cause of congestion is found in hypertrophy of the heart and increased force of the blood flow (blood tension). In such cases those organs become congested in which there is some previous debility or disease of the blood vessels.
Symptoms and results. The symptoms are a bright vermillion redness, tension or swelling, heat and tenderness. Pulsation is stronger in the vessels leading into the part, secretions tend to increase but may give place to a serous effusion or hæmorrhage. The bright redness is attributed to the rapid circulation of the red globules which have not time to give up their oxygen to the tissues. It is sharply circumscribed where the affected arterioles have no free anastomosis with those of neighboring parts, diffuse where anastomosis is abundant, and when on the skin it is liable to rise in knots or buttons as in urticaria. When pressed the redness entirely disappears unlike the redness of inflammation.
The swelling may be due to the simple turgescence of the bloodvessels, but also often to transudation of serum as in and around the cow’s udder at parturition. The occasional migration of globules, and their escape through minute lacerations in the vascular walls add alike to color and turgescence.
The elevated temperature, (rising sometimes 3° C.) in the congested area, is attributed to the more active circulation, and Schiff prevented its appearance after section of the cervical sympathetic, by tying the carotid and vertebral arteries on the same side.
The tenderness of the congested parts varies inversely as the looseness of texture and the facility for swelling. It may be scarcely perceptible in the mammary region, and intense under the horn or hoof.
The functions in the congested organ are often seriously interfered with, secretions appearing in excess or entirely altered. When the congestion lasts it may cause hypertrophy, induration or hyperplasia, these are however rather sequels than lesions of the condition. Simple congestion is usually quite transient, and if prolonged, often merges into inflammation.
II. Passive or Venous Congestion. In this there is no excess of blood entering the part, but the regular supply is delayed in the veins by some obstruction, and these vessels and, later, the capillaries are gorged with black blood.
Causes. 1st, Mechanical obstruction to the onward flow of blood, as in the case of disease of the lungs hindering the flow of blood from the right heart; disease of the right heart allowing a reflux of blood into the veins; or pressure by tumors or otherwise on the great or small venous trunks. If in the heart or lungs the whole systemic venous system becomes the seat of passive congestion; if in a single venous trunk then only the parts the venous radicles of which are tributary to this. We find examples of this in phlebitis, in compression by the swellings of strangles, in the result of a bandage or ligature tied round a limb at some distance from its extremity, and in the compression of the iliac veins by a gravid womb.
2d. Diminished force of the blood current in the veins, as from old age or great debility and especially from weakness of the heart’s action. Also from disease of the arterial coats which impairs their tonicity. The force being too weak to force the blood actively through the capillaries and veins, it becomes unduly charged with carbon dioxide and other products of tissue waste, so that nutrition suffers and the walls of the capillaries lose their vital force. This condition is aggravated in the hind limbs by the distance from the heart, and the dependent position, and in decubitus by the compression of the vessels of the limbs. Also by injuries to the vaso-motor nerve supply as œdema appeared in the hind limb after tying of the femoral vein in animals the abdominal sympathetic of which had been cut, but not in animals in which this nerve was left in its normal condition (Ranvier).