Chyme Definition
Chyme is a semi-fluid pulp formed in the stomach made of partly digested food and the secretions of the gastrointestinal tract. It is initially acidic in pH and also contains salivary enzymes and gastric enzymes. Chyme is created from the ingested bolus through muscular contractions of the stomach, which mix food with the secretions of the stomach. Chyme passes from the stomach to the small intestine in short spurts and influences pancreatic secretions and the release of bile from the gall bladder and liver.
Composition of Chyme
Chyme contains food, water, salivary secretions, gastric secretions and partially digested carbohydrates and proteins in the stomach. It also contains cells that were sloughed off from the mouth and esophagus in the process of chewing and swallowing. Gastric secretions include hydrochloric acid secreted by the parietal cells of the stomach and this makes chyme have an extremely low pH. In addition to destroying most pathogens found in food, the pH of chyme is optimal for the action of pepsin. Pepsin is secreted by special cells in the stomach called chief cells. This enzyme is often the starting point for the digestion of proteins, and prefers to catalyze the hydrolysis of peptide bonds between hydrophobic and aromatic amino acids. Therefore, when chyme enters the duodenum, it contains many short peptides with either a hydrophobic or an aromatic residue at each end.
The overall water and electrolyte content of chyme remains nearly constant when it exits the stomach, irrespective of the nature of food ingested. However, the time spent in the stomach, and the relative amounts of different nutrients can vary depending on the individual and the meal. For instance, a meal rich in fat and protein and low in carbohydrates will result in the chyme being ‘oily’ and ‘frothy’ with some of the partially digested peptides acting as emulsifiers for the fats. Alternatively, a meal that was improperly masticated, and heavy in carbohydrates could result in chyme containing chunks of unprocessed food. There is also some evidence that the presence of whole unprocessed grains delays gastric emptying and results in the chyme staying within the stomach for longer periods of time. In fact, the nature of chyme is often used in forensic analysis, to estimate the time of death. Additionally, conditions like peptic ulcers, chronic stress, hormonal imbalances, or alcohol and tobacco consumption could alter gastric secretions and the composition of chyme.
Chyme is occasionally mentioned as being distinct from chyle, which is formed when the fats within the food also start getting digested in the small intestine. Chyle therefore contains emulsified fatty acids, in addition to carbohydrates and proteins in various stages of digestion. However, this distinction is often overlooked and the term ‘chyme’ is used to refer to food as it travels from the stomach through the intestine, until most of the nutritive material has been absorbed and only fecal matter remains.
Generation of Chyme
Chyme is generated from the bolus of food that enters the stomach through the esophagus. Digestion of food in humans begins in the mouth where food is hydrated and mechanically broken down into smaller pieces. Salivary enzymes also begin to work on carbohydrates, which is why grains such as rice and wheat begin to taste sweet when they are chewed for an extended period of time. This mixture of masticated food containing salivary secretions is called a bolus, which reaches the stomach through the esophagus.
Within the stomach, two events begin to occur in concert with each other. First, the three layers of smooth muscle in the stomach arranged in circular, diagonal and longitudinal rows contract to squeeze food mechanically. This churning allows the bolus to be mixed with water, ions, hydrochloric acid, and mucin, enhancing the catalytic activity of gastric enzymes. Gastrin, a hormone secreted by the stomach and small intestine, influences these secretions and the peristaltic movements of the gastrointestinal tract.
Image shows the diagonal (oblique) and circular muscle fibers of the stomach.
The presence of hydrochloric acid not only allows pepsin to function optimally, but also influences the hydrolysis of many bonds within biological polymers and helps in creating a relatively uniform consistency for chyme. The lining of the stomach contains a thick coating of mucus to prevent these powerful forces from digesting the cells of the organ itself. Parietal cells contain an important proton pump that uses the energy from ATP hydrolysis to pump hydrogen ions (H+) into the lumen of the stomach, in exchange for potassium ions (K+) being imported into the cell. These proteins are usually sequestered within vesicles inside parietal cells. However, the presence of food in the stomach (or other stimuli such as the smell or sight of food) can translocate these proteins to the apical membrane of the cells. Since acid secretion is an important part of chyme generation and digestion, it is controlled by a number of different molecules, including acetylcholine, histamine, gastrin and somatostatin. Some of these act like paracrine signaling molecules, being secreted by the cells of the stomach itself. Others behave like hormones. The enteric nervous system also plays an important role in regulating these secretions.
While food is being digested in the stomach, the pyloric sphincter separating the stomach from the duodenum remains closed. As chyme is slowly moved towards the small intestine through regulated opening of the sphincter, more enzymes get added, and the muscles of the intestinal walls then continue to mix the secretions with chyme. After digestion is complete, nutrients like monosaccharides, fatty acids, glycerol, and amino acids are absorbed through the walls of the intestine. In the large intestine, some bacteria in the colon continue the process of digestion. As chyme travels further along the digestive tract, water is absorbed, making it more concentrated. When nutrient absorption is complete, the remaining waste material is expelled from the body as feces.
Functions of Chyme
There are two major functions of chyme – the first is to increase the surface area of food to allow digestive enzymes to complete their work, and the second is to stimulate various digestive glands to release their secretions.
The action of enzymes requires direct contact with the molecules of the substrate. When food is first ingested, it is in the form of large chunks. Such particles have a very low surface area for their volume, and therefore, enzymes will only have access to a small proportion of the molecules in the substrate. Mastication of food, and the subsequent churning through the muscles of the stomach and small intestine repeatedly break down food through mechanical processes. The importance of this mixing and roiling can be illustrated by a simple example. When a cubical object of 10 ml volume is split into eight pieces of 1.25 ml each, the surface area doubles while the volume remains constant. When food is broken down through the contraction of stomach muscles, there is an even greater increase in surface area since it leads to the formation of many irregular surfaces and niches. This allows an enzyme to access the interior of the substrate, brings it into contact with a number of new surfaces and vastly increases the rate of the reaction. Additionally, continued mixing during a reaction can also prevent an enzyme from catalyzing the reverse reaction, since the products of catalysis are quickly removed from the enzymatic active site. Digestion can therefore proceed efficiently and be completed in time to provide energy to the organism.
The second function of chyme is to stimulate different organs of the digestive and endocrine system. When chyme enters the duodenum from the stomach, it influences the secretion of bicarbonates from the pancreas and the release of alkaline bile from the gall bladder and liver. Its acidity also determines whether parietal cells of the stomach are stimulated to produce more hydrochloric acid or inhibited.
Related Biology Terms
- Aminopeptidases – Enzymes that hydrolyze the last peptide bond of the amino terminus (N-terminus) of proteins or peptides.
- Carboxypeptidases – Enzymes that hydrolyze the last peptide bond of the carboxy terminus (C-terminus) of proteins or peptides.
- Jejunum – The middle section of the small intestine, occurring after the duodenum and preceding the ileum, consisting of nearly 40% of the length of the small intestine. Involved in digestion and absorption.
- Pylorus – Part of the stomach that connects it to the first section of the small intestine – the duodenum.
Quiz
1. Which of these molecules is associated with the chief cells of the stomach?
A. Hydrochloric acid
B. Somatostatin
C. Gastrin
D. Pepsin
2. How is a bolus different from chyme?
A. Bolus is only seen in the stomach and small intestine while chyme is present through the entire digestive system
B. Bolus is derived from masticated food in the mouth, while chyme represents a pulpy mass that is first seen in the stomach
C. Bolus is hydrated food, while chyme is digested food
D. Bolus contains intestinal enzymes, while chyme only has gastric secretions
3. What are the functions of chyme?
A. Increase in surface area and inducing release of digestive secretions
B. Activation of salivary enzymes
C. Protection of the stomach from the action of digestive enzymes
D. All of the above