9 Gastrointestinal physiology – the normal stomach and small intestines
The stomach lies to the left of the median plane of the body. When empty, it is within the costal arch and a normal empty stomach cannot be palpated during physical examination. Even when full, the examiner may need to hook his or her fingers underneath the costal arch to feel a normal stomach.
The stomach is divided anatomically into five regions: cardia, fundus, body, antrum and pylorus (Fig 9.1). Physiologically, the stomach has a proximal part which stores food temporarily and a distal part which regulates the release of hydrochloric acid, grinds food particles and controls the emptying of the stomach. The fundus of the stomach dilates in response to the entry of food in receptive relaxation which results in a decrease in fundic motor activity and pressure. As dogs tend to eat large meals as opposed to cats, which tend to eat frequent small meals, the storage capacity of the stomach is likely to be of greater importance to dogs.
The stomach contributes to the initial stages of digestion by secreting hydrochloric acid and pepsinogen. Muscles of the antrum grind food particles and peristaltic waves move from the body of the stomach to the antrum towards a usually partially closed pylorus. A strong retrograde wave then moves the food back into the proximal antrum resulting in grinding into particles small enough to be allowed through the pylorus.
The pylorus and the antrum function as a unit to regulate the emptying of solid food. In dogs the particles of food are usually less than 2 mm in size before they move through the pylorus. Large indigestible particles of food do not leave the stomach until the interdigestive period (after digestion is complete). In fasted dogs an interdigestive motor complex (migrating motor complex) moves through the stomach and intestines to clear these larger particles (and sometimes also foreign bodies) into the intestines. This is also called a ‘housekeeping wave’. The electrical impulse in cats differs from that of dogs; the wave is stimulated by a migrating spike complex, which likely serves the same function in the cat.
Most of the enzymatic digestion of food occurs within the small intestine. The small intestine is divided into the duodenum, jejunum and ileum, although no anatomic distinction divides one section from another. The small intestine is 1.80 to 4.80 m long in the dog and about 1.3 m long in the cat (Fig 9.2). The pancreas lies near the duodenal flexure (Fig 9.3).