The proventriculus is a standard part of avian anatomy, and is a rod shaped organ, located between the esophagus and the gizzard of most birds.[2] It is generally a glandular part of the stomach that may store and/or commence digestion of food before it progresses to the gizzard.[3] The primary function of the proventriculus is to secrete hydrochloric acid (HCl) and pepsinogen into the digestive compartments that will churn the ingested material through muscular mechanisms.[4] The Encarta (2007) holds that the proventriculus is:
The first part of a bird's stomach, where digestive enzymes are mixed with food before it goes to the gizzard. It is analogous to the gizzard in insects and crustaceans.[1]
Thomas Cecere (College of Veterinary Medicine of VirginiaTech)[5] discusses the proventriculus of the avian stomach and opines that:
The proventriculus is the glandular portion of the avian compound stomach, and a rather peculiar organ it is. There's nothing like it in mammals.[6]
Anatomy of the avian digestive system. The proventriculus is located early in the digestive tract, and is associated with the gizzard.
Secretions
The secretory glands that line the proventriculus gives it the nickname of the "true stomach," as it secretes the same components as a mammalian stomach. [7] It contains glands that secrete HCL and pepsinogen. The gastric glands of birds only have one type of cell that produces both HCL and pepsinogen, unlike mammals which have different cell types for each of those productions. In mammals, HCl is secreted into the lumen of the proventriculus using parietal cells, while chief cells secrete the pepsinogen into the lumen. [7] In birds, the gastric glands of the proventriculus secrete both the HCl and pepsinogen.[8] Since pepsinogen is a zymogen, it is then activated to pepsin, using the HCl. Once activated, pepsin can break the peptide bonds found in peptides and proteins.[9] Since the digesta in birds has not been chewed, the secretions are important to break the particles down.
The roles of these secretions are to reduce the pH of the digesta and begin protein digestions, however the distribution of the secretions differ depending on the avian species. These secretions cause the stomach to be very acidic, but the exact value will differ based on the species, as seen in the table below. [7]
pH of proventriculus
Species
pH value
Chicken
4.8
Turkey
4.7
Pigeon
4.8
Duck
3.4
In petrels, the proventriculus is much larger and the mucus secretions are arranged into longitudinal ridges, which creates more surface area and more concentrated cells.The secretion of the oil is quite common to birds, and are usually undigested fatty acids, but can vary depending on the diet. [10] Petrels also have a unique mechanism where they can shoot stomach oil from their beak when alarmed. [10]
Motility
The muscle contractions of the gizzard push material back into the proventriculus, which then contracts to mix materials between the stomach compartments. This transfer of digested material can occur up to 4 times per minute, and the compartments can hold the stomach contents for thirty minutes to an hour.[4] The contractions are regular and rhythmical, and are more frequent in intact males than in females because there is more concentration of the hormone Androgen. This also causes castrated males to have a decrease in contractions and their amplitude. [8] Smaller and less frequent contractions lead to less effective digestion of food.
Culinary use
Chicken proventriculus is eaten as street food in the Philippines. It is dipped in flour and deep fried until golden brown. It is served best with spiced vinegar and is often sold in a small kiosk. This dish is called proben.
Insects
Insects also have a proventriculus structure in their digestive track, although the function and structure differ from avian proventriculi. They still contain secretions, but its main role is to help the passage of food and connect the crop to the stomach. [11] The area within the proventriculus called the proventriculus bulb contains hairs or teeth like structures that help filter food and make it easier to digest. [11] The proventriculus is organized in a central framework with muscles surrounding it on the outside. Different species have different combinations of longitudinal and circular muscles around it. In general, Hymenoptera contain sphincter muscles that add pressure on the digestive components and help pass the food into the midgut. [12] Ants have more longitudinal muscles rather than circular muscles around the proventriculus, and its purpose is to act as a barrier to increase storage capacity of the crop. The amount of time that it can hold food depends on the strength of the muscles around the proventriculus bulb. [11]
In bees, it not only controls the movement of food, but also helps separate nectar that will later be converted to honey, from pollen that will be digested. Since the proventriculus controls the passage of food into the digestive track, it is important that it is functional. Male bees are commonly affected by an abnormal proventriculus conditions that prevent the passage of food [13] In most cases, the proventriculus is swollen and shaped differently. This can later affect how their body absorbs the nutrients since the proventriculus cannot effectively push food into the gut. The exact reason for this abnormality is still unknown, as there are a variety of possibilities. Since it seems to be a sex specific condition, some theories are: changes in the gut biome due to diet or sex-specific pathogens, variability compared to females, or differences in behavior that could affect their exposure to biological threats and toxins [13]
↑ Caceci, Thomas (undated). Proventriculus. Source: "Example: Proventriculus". Archived from the original on 2007-12-10. Retrieved 2007-12-18. (accessed: December 18, 2007)
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is involved in the gastric phase of digestion, following chewing. It performs a chemical breakdown by means of enzymes and hydrochloric acid.
Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.
Pepsin is an endopeptidase that breaks down proteins into smaller peptides and amino acids. It is produced in the gastric chief cells of the stomach lining and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pepsin is an aspartic protease, using a catalytic aspartate in its active site.
Saliva is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells, enzymes, antimicrobial agents.
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs, earthworms, some gastropods, some fish, and some crustaceans. This specialized stomach constructed of thick muscular walls is used for grinding up food, often aided by particles of stone or grit. In certain insects and molluscs, the gizzard features chitinous plates or teeth.
The pylorus or pyloric part connects the stomach to the duodenum. The pylorus is considered as having two parts, the pyloric antrum and the pyloric canal. The pyloric canal ends as the pyloric orifice, which marks the junction between the stomach and the duodenum. The orifice is surrounded by a sphincter, a band of muscle, called the pyloric sphincter. The word pylorus comes from Greek πυλωρός, via Latin. The word pylorus in Greek means "gatekeeper", related to "gate" and is thus linguistically related to the word "pylon".
Gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids of proteins. Gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated pH. These cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. The pancreas further produces large amounts of bicarbonate and secretes bicarbonate through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract.
Parietal cells (also known as oxyntic cells) are epithelial cells in the stomach that secrete hydrochloric acid (HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and body regions of the stomach. They contain an extensive secretory network of canaliculi from which the HCl is secreted by active transport into the stomach. The enzyme hydrogen potassium ATPase (H+/K+ ATPase) is unique to the parietal cells and transports the H+ against a concentration gradient of about 3 million to 1, which is the steepest ion gradient formed in the human body. Parietal cells are primarily regulated via histamine, acetylcholine and gastrin signalling from both central and local modulators.
Achlorhydria and hypochlorhydria refer to states where the production of hydrochloric acid in gastric secretions of the stomach and other digestive organs is absent or low, respectively. It is associated with various other medical problems.
Brunner's glands are compound tubuloalveolar submucosal glands found in that portion of the duodenum proximal to the hepatopancreatic sphincter.
Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of animals and in the tracts of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes of diverse specificities are found in the saliva secreted by the salivary glands, in the secretions of cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the secretions of cells lining the small and large intestines.
In human anatomy, there are three types of chief cells, the gastric chief cell, the parathyroid chief cell, and the type 1 chief cells found in the carotid body.
The gastric glands are glands in the lining of the stomach that play an essential role in the process of digestion. All of the glands have mucus-secreting foveolar cells. Mucus lines the entire stomach, and protects the stomach lining from the effects of hydrochloric acid released from other cells in the glands.
A gastric chief cell is a type of gastric gland cell that releases pepsinogen and gastric lipase. It is the cell responsible for secretion of chymosin in ruminant animals and humans. The cell stains basophilic upon H&E staining due to the large proportion of rough endoplasmic reticulum in its cytoplasm. Gastric chief cells are generally located deep in the mucosal layer of the stomach lining, in the fundus and body of the stomach.
Gastric pits are indentations in the stomach which denote entrances to 3-5 tubular shaped gastric glands. They are deeper in the pylorus than they are in the other parts of the stomach. The human stomach has several million of these pits which dot the surface of the lining epithelium. Surface mucous cells line the pits themselves but give way to a series of other types of cells which then line the glands themselves.
The gastric mucosa is the mucous membrane layer of the stomach, which contains the glands and the gastric pits. In humans, it is about 1 mm thick, and its surface is smooth, soft, and velvety. It consists of simple columnar epithelium, lamina propria, and the muscularis mucosae.
Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.
Foveolar cells or surface mucouscells are mucus-producing cells which cover the inside of the stomach, protecting it from the corrosive nature of gastric acid. These cells line the gastric mucosa. The mucus-secreting cells of the stomach can be distinguished histologically from the intestinal goblet cells, another type of mucus-secreting cell.
The nervous system, and endocrine system collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the gastrointestinal tract, including peristalsis, and segmentation contractions.
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.
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