Avian Digestive System
by Sherri Carpenter
Winter 2003
The body is divided into different systems and their organs. Each system has an influence on another system and they all work together to create homeostasis. Our birds’ bodies need fuel to go about their daily activities, and this is where the digestive system plays its role. It converts food into raw materials that build and fuel the body’s cells. It takes in food, breaks down nutrient molecules, absorbs them into the blood stream and rids the body of the indigestible remains.
Esophagus and Crop
As your bird swallows its food, it is mixed with saliva (mucus) to lubricate it down the esophagus (food tube) tube. In humans, the saliva starts the breakdown of starch. The food travels down the esophagus with wave like muscle contractions called peristalsis, and for most birds, it winds up in the crop (ingluvies). Here the food sits and softens before it enters the stomach. Parrots use this softened food to regurgitate to their young, while pigeons and doves make a crop milk to feed, for the first few weeks of their young babies’ lives.
Proventriuculus
Most birds have a two-part stomach, one that is glandular and one that is muscular. The first and glandular one is the proventriculus, which is where digestion first begins. Specialized glands, regulated by hormonal and neural factors, secrete gastric juices. The juices are a mixture (not necessarily equal) of digestive enzymes, hydrochloric acid, and mucus. Hydrochloric acid activates the digestive enzymes (pepsinogen) and the mucus protects the lining from being digested itself. This is the beginning of the chemical digestion of proteins.
Gizzard
The food is then propelled to the second stomach, the gizzard or ventriculus. The ventriculus is a thick-walled, muscular organ, and with some birds may contain small stones that aid in the breaking up the food. The ventriculus grinds up food mixed with gastric juices into small pieces, to make it more easily digested. The food may pass back and forth from the proventriculus to the ventriculus, to better aid in the breaking down of food, if needed.
Small Intestines
From the ventriculus, the food enters the small intestines through a valve called the pylorus. The pylorus only allows a small amount of food to pass at a time, to the small intestines, to allow for digestion to take place.
The small intestines are lined with villi and microvilli. Villi are fingerlike projections and in each villus, there is a rich capillary bed and a modified lymphatic capillary. The microvilli are tiny projections that line the outside of the villi and carry enzymes that break down double sugars into simple sugars and complete protein digestion. As the food is broken down into smaller particles, it is absorbed into the capillary systems and transported back to the liver.
There are three sections in the small intestines, the
- duodenum,
- jejunum, and the
- ileum, which connects to the large intestine.
Liver, Pancreas and Duodenum
The liver and the pancreas secrete their fluids to the duodenum through a common duct. The pancreas produces enzymes that break down all categories of digestible foods. The enzymes will complete the digestion of starch, carry out half of protein digestion and is responsible for all fat digestion. It neutralizes the acid food mixture coming in from the ventriculus, and provides the right environment for the activation of the digestive enzymes. It also produces the hormones insulin and glucagon, which regulate blood sugar.
A bird’s liver has two lobes instead of the four found in humans. In digestion, its primary function is to produce bile. Bile is a watery solution that contains bile salts, bile pigments (mostly bilirubin, a breakdown of hemoglobin), cholesterol, phospholipids, and a variety of electrolytes. In digestion, only the bile salts (derived from cholesterol) and phospholipids aid in the process. Bile salts break down fat globules into smaller ones so that the pancreas’ fat-digestion enzymes have a smaller surface to work with. Certain cells of the liver destroy bacteria that have managed to get through the walls of the digestive tract and into the blood.
Another role of the liver is to detoxify drugs, degrade hormones, make many substances vital to the body as a whole (cholesterol, blood proteins, clotting proteins and lipoproteins), and play a role in metabolism.
The liver processes nearly every class of nutrient that comes in from the blood of the digestive tract. As the blood circulates through the liver, the liver pulls out amino acids, fatty acids and glucose and takes what its cells need and either stores the rest for latter or processes them and returns them to the blood to be used by other cells in the body.
The liver also has a role in maintaining glucose level in the blood. When carbohydrates are broken down to glucose, the liver removes these from the blood and forms large polysaccharide molecules called glycogen and stores them. As the body’s cells use glucose and the level in the blood drops, the liver will than take its polysaccharide molecules and break them down to simple sugars (glucose) and release them bit by bit into the blood to circulate through the body. The liver may also use fats or proteins to make glucose if carbohydrates are not available.
Gallbladder
The gallbladder is the storage house for bile and is connected to the duodenum through the cystic duct. The bile is concentrated by the removal of water and is made available to the duodenum when fatty foods stimulate a hormone response to gallbladder to send out the stored bile. Parrots do not have a gallbladder.
Large Intestine
The large intestines (colon) are relatively short in birds compared to mammals. This helps with quicker elimination to prepare for flight. The large intestine’s main function is to absorb water, dry out indigestible foods and eliminate waste products. The large intestines have bacteria that metabolize remaining nutrients, make the vitamins K and some B, and than absorb them back into the system along with water. Some birds have paired ceca that harbor bacteria, which aid in the breakup of cellulose. Parrots do not have ceca.
Cloaca
The large intestines join the cloaca, which is where the feces meet with urates and urine, from the kidneys, and than pass out through the vent. The urates are usually whitish in color and the feces are usually brown or green depending on what your bird has just eaten.
I’m hoping that by learning more about the digestive system we are better able to understand how important the foods we choose affect our birds bodies. The process of digestion goes on to feed all the cells in the body. As the old saying goes, you are what you eat. Chow.