The proventriculus is part of the digestive system of birds.^[1] An analogous organ exists in invertebrates and insects.

Birds

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]

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.

Hormones also affect the amount and concentration of these secretions. Some of these include Gastrin, Bombesin, Avian Pancreatic Polypeptide, and cholecystokinin. ^[7] These can prevent or stimulate the release of those secretions.

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]

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]