Chyme: Definition, Composition, and Function

What is Chyme?

Chyme, a thick, semi-liquid mass of partially digested food and digestive secretions that form in the stomach and intestines during digestion. In the stomach, digestive juices are produced by the gastric glands; These secretions include the enzyme pepsin, which breaks down proteins, and hydrochloric acid.

Chyme is a semi-liquid pulp that is formed in the stomach from partially digested food and secretions from the gastrointestinal tract. It is initially acidic in pH and also contains salivary and gastric enzymes.

Chyme is created from the ingested bolus by muscle contractions of the stomach that mix food with gastric secretions. Chyme makes its way from the stomach to the small intestine in short bursts, influencing pancreatic secretion and the release of bile from the gallbladder and liver.

Composition of Chyme

Chyme results from the mechanical and chemical breakdown of a bolus and consists of partially digested food, water, hydrochloric acid, and various digestive enzymes. Chyme slowly passes through the pyloric sphincter and into the duodenum, where the extraction of nutrients begins.

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, which is secreted by the parietal cells of the stomach. As a result, chyme has an extremely low pH value. In addition to destroying most of the pathogens found in food, the pH of chyme is optimal for pepsin to work. 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 preferentially catalyzes the hydrolysis of peptide bonds between hydrophobic and aromatic amino acids. Therefore, when chyme gets into the duodenum it will contain many short peptides with either a hydrophobic or an aromatic residue on each end.

The total water and electrolyte content of chyme remains almost constant when it leaves the stomach, regardless of the type of food consumed. However, the time spent in the stomach and the relative amounts of various nutrients can vary by person and by meal.

For example, a meal that is high in fat, protein, and low in carbohydrates causes the chyme to be “oily” and “frothy” and some of the partially digested peptides to act as emulsifiers for the fats. Alternatively, a meal that hasn’t been properly chewed and is high in carbohydrates could result in chyme, which contains chunks of unprocessed foods.

There is also evidence that the presence of whole unprocessed grains delays gastric emptying and causes the chyme to stay in the stomach for longer. In fact, the nature of chyme is widely used in forensic analysis to estimate the time of death.

In addition, diseases such as stomach ulcers, chronic stress, hormonal imbalances, or the consumption of alcohol and tobacco can change gastric secretion and the composition of the 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 the only fecal matter remains.

Generation of Chyme

Chyme is generated from the bolus of food that enters the stomach through the esophagus. 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 mechanically compress food. This agitation allows the bolus to be mixed with water, ions, hydrochloric acid, and mucin, which increases the catalytic activity of gastric enzymes. Gastrin, a hormone secreted by the stomach and small intestine, affects these secretions and the peristaltic movements of the gastrointestinal tract.

The 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 the 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 the 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 systems. 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.

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