Effects of vasopressin and isoprenaline infusions on the distribution of blod flow in the intestine; criteria for the validity of microsphere studies.

1. The distribution of superior mesenteric arterial flow was investigated by radioactive microspheres. The small intestine received 83% of the flow ((85 ml/min)/100 g intestine) and flow was uniform along the length of the small intestine.2. The intestinal wall was separated into 3 layers-muscle, submucosa and mucosa. The muscle received (8 ml/min)/100 g intestine and the combined submucosa and mucosa (77 ml/min)/100 g intestine.3. The distribution of microspheres between the mucosa and submucosa depended on the size of the microspheres; the smaller the spheres, the more were found in the mucosa. It also depended on the state of the vascular bed; if microspheres were given during an infusion of vasopressin, a subsequent infusion of isoprenaline resulted in movement of some of the spheres from the submucosa into the mucosa.4. Histological studies after India ink injection showed few capillary-sized vessels but many large vessels in the submucosa. Capillary-sized vessels arose close to the junction with the mucosa and passed into the mucosa.5. These and other data suggest that the intestine consists of two parallel-coupled sections, one to the muscle and the other through the submucosa to the mucosa. The vessels in the submucosa are in series with those in the mucosa and submucosal shunts do not exist. Redistributions of flow between mucosa and submucosa cannot therefore occur during stimulation of the sympathetic nerves or infusions of drugs such as noradrenaline or adrenaline.6. When microspheres are used in pharmacological investigations on distribution of blood flow in organs, controls to validate the method for the particular areas being studied are essential. If the vessels in the areas studied are in series rather than in parallel, the method is invalid.