Secretory Function of the Small Intestine
Secretion in the small intestine provides an optimal environment for the enzymatic processing of chyme. Glands located throughout the mucosa secrete succus entericus (intestinal juice) — a turbid, viscous fluid with an alkaline reaction. Up to 2.5 liters of this secretion are produced daily.
Of particular note are Brunner's glands (duodenal glands), located in the proximal duodenum. Their secretion is a thick, colorless fluid with a weakly alkaline reaction. This juice has relatively low enzymatic activity: it contains enzymes capable of minimal breakdown of proteins (proteolytic activity), carbohydrates (amylolytic), and fats (lipolytic).
Types of Motor Activity
Small intestinal motility is diverse and serves two main functions: thorough mixing of the food bolus (chyme) with digestive juices and its gradual propulsion toward the large intestine. Several types of contractions are distinguished:
- Rhythmic segmentation. Carried out primarily by the circular muscle layer. Intestinal contents are divided into segments, and subsequent contractions form new segments from halves of the previous ones. This leads to effective mixing of chyme and local pressure elevation. The frequency of these contractions is 9–12 per minute in the upper segments, decreasing to 6–8 per minute in the lower segments.
- Pendular movements. These involve longitudinal muscles supported by circular muscles. Chyme moves back and forth, producing weak forward propulsion toward the large intestine.
- Peristaltic waves. Represent a coordinated ring of contraction followed by relaxation. Typically, a wave moves at a speed of 0.1–0.3 cm/s (faster in the proximal segments), propelling chyme toward the large intestine. Several such waves may occur simultaneously. There is also a rapid (propulsive) wave reaching speeds of 7–21 cm/s.
- Antiperistaltic contractions. Waves directed in the reverse (oral) direction. Not normally present, characteristic exclusively of the vomiting reflex.
- Tonic contractions. Cause narrowing of the intestinal lumen over a significant length. They either fail to propel contents or do so at minimal speed.
Intraluminal Pressure Dynamics
Muscle contractions directly affect intraluminal pressure in the small intestine. At rest, the baseline (basal) pressure ranges from 5 to 14 cm H₂O.
Active motility alters these parameters:
- Monophasic waves can transiently (for about 8 seconds) raise pressure up to an impressive 30–90 cm H₂O.
- The slow component of contractions increases pressure only slightly, but its duration ranges from one to several minutes.
Mechanisms Regulating Motility
Control of small intestinal motor function is provided by three interrelated mechanisms: myogenic, neural, and humoral.
The myogenic mechanism is based on the background rhythmic activity of neurons in the myenteric plexus, which drive phasic contractions. The rhythm is set by specialized pacemakers. The first is located near the ampulla of Vater in the duodenum, and the second is in the ileum.
Neural regulation includes several levels:
- The intramural (enteric) nervous system plays a leading role in coordinating motility.
- The extramural system (ANS): parasympathetic influences generally enhance contractions, whereas sympathetic influences inhibit them.
- Central regulation is carried out by structures of the spinal cord and medulla oblongata, the limbic system, and the cerebral cortex. The hypothalamus plays a special role: stimulation of its anterior and middle regions triggers motility, whereas the posterior region inhibits gastric and intestinal motor activity.
Humoral regulation is carried out by hormones and biologically active substances:
- Enhance motility: serotonin, histamine, gastrin, motilin, cholecystokinin-pancreozymin (CCK-PZ), vasopressin, oxytocin, bradykinin, substance P, as well as acids, alkalis, and salts.
- Inhibit motility: secretin, vasoactive intestinal peptide (VIP), and gastric inhibitory peptide (GIP).