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 mucous membrane secrete intestinal juice, a turbid, viscous fluid with an alkaline reaction. Up to 2.5 liters of this secretion are produced daily.
Special attention should be paid to Brunner's (duodenal) glands, localized in the upper part of the 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 slightly breaking down proteins (proteolytic activity), carbohydrates (amylolytic), and fats (lipolytic).
Types of motor activity
Small intestine motility is diverse and serves two main tasks: thoroughly mixing the food bolus (chyme) with digestive juices and gradually moving it toward the large intestine. Several types of contractions are distinguished:
- Rhythmic segmentation. Carried out predominantly by the circular muscle layer. The intestinal contents are divided into parts, and upon the next contraction, a new segment is formed from the halves of the previous one. This leads to efficient mixing of chyme and a local increase in pressure. The frequency of such contractions is 9–12 per minute in the upper sections and decreases to 6–8 per minute in the lower sections.
- Pendular movements. These involve longitudinal muscles supported by circular ones. Chyme moves back and forth, producing a weak forward motion toward the large intestine.
- Peristaltic wave. Represents a coordinated constriction and dilation of the intestine. Usually, the wave moves at a speed of 0.1–0.3 cm/s (faster in the proximal sections), propelling chyme toward the large intestine. Several such waves can occur simultaneously. There is also a rapid (propulsive) wave, the speed of which reaches 7–21 cm/s.
- Antiperistaltic contractions. The wave is directed in the reverse (oral) direction. Not normally encountered, it is characteristic exclusively of the act of vomiting.
- Tonic contractions. Cause narrowing of the intestinal lumen over a considerable distance. They either do not move the contents at all or do so at a minimal speed.
Intestinal pressure dynamics
Muscle contractions directly affect the pressure in the lumen of the small intestine. At rest, the initial (basal) pressure ranges from 5 to 14 cm H2O.
Active motility alters these parameters:
- Monophasic waves can briefly (about 8 seconds) raise pressure 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 of motility regulation
Control of the motor function of the small intestine 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, ensuring phase contractions. The rhythm is set by special "pacemakers". The first is located near the opening of the common bile duct into the duodenum, and the second is in the ileum.
Neural regulation includes several levels:
- The intramural (enteric) system is of primary importance in coordinating motility.
- The extramural system (ANS): parasympathetic influences predominantly enhance contractions, while sympathetic ones inhibit them.
- Central regulation is carried out by structures of the spinal cord and medulla oblongata, the limbic system, and the cortex. A special role is played by the hypothalamus: stimulation of its anterior and middle parts stimulates motility, while the posterior part inhibits the motor activity of the stomach and intestines.
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).
Briefly: answers to topical questions
What enzymes are part of intestinal juice?
Intestinal juice contains over 20 different enzymes. The main enzyme groups include:
- Proteolytic — enterokinase, duodenase, and specific peptidases (aminopeptidase, dipeptidase).
- Lipolytic — lipase, phospholipase.
- Amylolytic and disaccharidases — amylase, maltase, invertase, lactase, sucrase.
- Other enzymes — alkaline phosphatase, nucleases.
A feature of enzyme distribution is the proximodistal gradient: a decrease in their content and activity in the small intestine towards the large intestine.
What is the electrolyte composition of intestinal juice?
The electrolyte composition of intestinal juice is represented by inorganic substances at a concentration of about 10 g/L. These include:
- Chlorides.
- Sodium, potassium, and calcium bicarbonates.
- Sodium, potassium, and calcium phosphates.
Physicochemical properties of intestinal juice: pH is 7.2–7.5, rising up to 8.6 during increased secretion.
To easily remember humoral motility inhibitors, use the acronym SVG (Secretin, VIP, GIP) — these are the ones that inhibit small intestine function.
Class and exam questions
Where are the pacemakers of the small intestine located?
Two main rhythm "pacemakers" are located in the small intestine. The first is in the region where the common bile duct enters the duodenum, and the second is in the wall of the ileum.
What is the difference between pendular movements and rhythmic segmentation?
Rhythmic segmentation works via circular muscles, dividing chyme into segments and increasing pressure to mix it. Pendular movements involve longitudinal muscles, moving contents back and forth with a slight progression toward the large intestine.
How do the act of eating and the properties of chyme affect motility?
The act of eating reflexively causes a brief inhibition of motility, which is then replaced by its enhancement. Rough food and the presence of fats in the chyme also exert a pronounced stimulating effect on intestinal contractions.
What else to study on this topic
- Mechanism of action of the mesenteric nerve plexus
- The role of the autonomic nervous system in extramural regulation
- Interaction of GI hormones: synergism and antagonism
- Features of the generation of propulsive peristaltic waves
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