Structures and functions of the human digestive system
In an average person, the stomach is about the size of their two fists placed next to each other. Organ systems Digestive system Metabolism. The serous fluid is produced by serous glands in these salivary glands which also produce lingual lipase. The movement of the tongue against the hard palate and the cheeks helps to form a rounded mass, or bolus , of food. Here it is mixed with gastric acid until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas. Some people have bowel movements three times a day; others, only one or two times a week. The vomitus in a dog or cat with a bleeding lesion gastric ulcer or neoplasm may contain frank blood or have the appearance of coffee grounds.
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It goes into the small intestine to await disposal. Solid waste leaves the body through the rectum then the anus. Liquid waste leaves the body after passing through the Digestion takes place in a long tube-like canal called the alimentary canal, or the digestive tract. Food travels through these organs in the following order: The liver is located in the abdomen and performs many functions. Which of the following is NOT a function of the liver? Healing itself when it is damaged. Digestive System What is the digestive system?
The body's breathing system The body's system of nerves The body's food-processing system The body's blood-transporting system Digestion begins in the mouth. The tongue keeps the food in place in the mouth while the food is being chewed. The digestive juices can react more easily with the food when chewed.
The food is completely digested and is absorbed by tiny blood vessels in the walls of the stomach. It passes from the small intestine into the large intestine, then into the blood. Mouth, gullet, stomach, small intestine, large intestine and rectum. Mouth, oesophagus, stomach, large intestine, small intestine and rectum.
The efficiency of the absorption is influenced by the surface area available for the nutrients to move through i. They also provide a means of concentrating the nutrients collection ability once they have moved through the intestine wall. After the duodenum the small intestine forms a coil and is suspended from the dorsal wall of the abdominal wall by a thin membrane called the mesentery. This membrane carries the blood vessels associated with the intestine. The duodenum starts at the gizzard and forms an elongated loop that is approximately 20 centimetres long.
The pancreas lies between the arms of the loop and is attached to, and actually holds together, each arm of the duodenum. Lymphoid tissue in the duodenum is very plentiful and is usually located in the corium. The lymphoid tissue collects the lymph and the lymph vessels transport fluid, other than blood, that is found in the spaces between cells and tissues until it passes into the blood system.
Bile ducts from the gall bladder that are attached to the liver and two to three pancreatic ducts enter the small intestine by a common papilla at the caudal end closest to the rear of the duodenum. The jejunum and the ileum, together about cm long, commence at the caudal end of the duodenum where the bile and the pancreatic duct papilla are located and terminates at the ileo-caecal-colic junction.
This junction is where the small intestine, the two caeca and the colon all meet. This portion of the small intestine is similar in structure to the duodenum except that:. This projection is where the yolk sac was attached during the development of the embryo. The large intestine is very short and does not differ to any extent from the calibre of the small intestine.
It runs in nearly a straight line below the vertebrae and ends at the cloaca. Sometimes this section is referred to as the colon and the rectum the rectum being the terminal section.
The bursa of fabricius is located immediately above the cloaca of young birds but disappears when the birds have reached approximately one year old. The two caeca or blind pouches are about centimetres long in the adult. They extend along the line of the small intestine towards the liver and are closely attached to the small intestine along their length by the mesentery. Each caecum has three main parts:. The large intestine terminates in the front part of the cloaca.
The cloaca is a tubular cavity opening to the exterior of the body and is common to the digestive and urogenital tract. The structure of the cloaca is very similar to that of the intestine except that the muscularis mucosa disappears near the vent. It divides into three chambers, each separated by a constriction not readily defined:. The liver is a bi-lobed organ that lies ventrally below and posterior in rear of to the heart and is closely associated with the proventriculus and the spleen.
The right side lobe is larger. The liver is dark brown or chocolate in colour except for the first days when it may be quite pale due to the absorption of lipids fats from the yolk as an embryo. It weighs approximately 50 grams in adult birds. The capsule, or glissosis, is the membrane that covers the liver and is thinner than that of mammals. The gall bladder lies on the right lobe beneath the spleen. Two bile ducts emerge from the right lobe and one of these originates from the gall bladder and the second provides a direct connection from the liver to the small intestine.
A system of ducts connects the right and left lobes. The liver cells have a high rate of destruction and a good regenerative capacity re-growth ability. Notwithstanding this, in the normal animal, much of the organ is in reserve and can be removed or destroyed without causing undue stress. There are two blood supply systems. One originates from the coelic artery for normal maintenance of the liver as an organ and the second, called the hepatic portal system, transports the nutrients from the small intestine after absorption to the liver.
This latter system enters the liver via two veins one for each lobe. The two blood supply systems join together inside the organ. The liver is drained via the hepatic veins into the posterior vena cava hepatic — relating to the liver; vena cava — one of the main veins that enters the heart.
The liver has a network of sinusoids empty holes in the tissues as in a sponge. The hepatic portal system, the capillaries of the arterial blood supply and the hepatic veins are in close association with each other in these sinusoids. The liver consists of a series of tissue sheets that are two cells thick, with a sinusoid on either side of the sheet.
Bile is made by the cells. The blood vessels, when they enter these sinusoids, become closely associated with them to provide for the easy transfer of material from one system to another. Minute canals called canaliculi that have the task of collecting and transporting the bile are associated with the cells in the tissue sheets.
These canals eventually join together to form the bile ducts with one going directly to the intestine and one to the gall bladder before it connects to the small intestine. This organ has three lobes that occupy the space between the two arms of the duodenal loop.
Two or three ducts pass the secretions of this organ into the distal end of the duodenum via papillae common with the ducts from the gall bladder and the liver.
These are mainly associated with the production of hormones. In poultry the cells of the islets of Langerhans are less defined than those in mammals.
The functions of the pancreas are:. The pattern of food intake and its passage through the digestive system are the main factors that influence secretory and hence digestive activity. Probably because of the high metabolic rate of the fowl, a more or less continuous supply of food is required by the digestive system. This is provided for by the crop that acts as a reservoir for the storage of food prior to its digestion and consequently permits the fowl to eat its food as periodic meals.
There is quite wide variability between birds in relation to eating behaviour, even between those in the same flock. Some eat small amounts at short intervals while others eat larger amounts at wider intervals. The food is delivered into the crop for storage after the first few boli have passed into the proventriculus. The crop is quite distensible and will hold a large amount of undigested food that is then moved on as required by the proventriculus.
This function of the crop is less important when there is a plentiful supply of food available. There is no relationship between the length of time of food deprivation and the amount of food consumed. While there is a wide variation between the eating habits of different birds in the flock, fowls do tend to eat meals on about minute intervals through the daylight hours and, to some extent, during darkness. They tend to eat larger portions at first light and in the late evening.
Similar factors affect the rate of movement of the food through the digestive system with a meal of normal food taking approximately 4 hours to pass through in the case of young stock, 8 hours in the case of laying hens and 12 hours for broody hens.
Intact, hard grains take longer to digest than the cracked grain and, quite often some whole grain will pass through unchanged. After ingestion, the food is mixed with saliva and mucous from the mouth and oesophagus and these secretions thoroughly moisten the food.
The enzyme amylase, which is produced by the salivary and oesophageal glands and found in the saliva and mucous, can now commence to breakdown the complex carbohydrates. However, the amount of enzyme action at this stage is minimal and the first major enzyme activity takes place in the proventriculus and in the gizzard.
The secretions of the proventriculus, or glandular stomach as it is often called, include hydrochloric acid to lower the pH of the system and the food mixture, the enzyme pepsin that acts on protein, and the hormone gastrin that stimulates the production and release of gastric juice in the proventriculus and pancreatic juice from the pancreas.
The gizzard is a very powerful organ which physically breaks the food particles into smaller sizes to make the work of the enzymes easier. At the same time, the enzymes previously released into the food with the saliva and by the proventriculus are thoroughly mixed into the food which improves their opportunity to carry out their work.
This breaking and mixing function of the gizzard is enhanced by the presence of insoluble grit such as stones. The food material enters the duodenum from the gizzard. Enzyme activity in this region is, in the main, a continuation of the breakdown of proteins started in the gizzard. Pancreatic juice and bile from the liver enters via ducts located at the distal end of the duodenum at about the junction of the duodenum and the jejunum if it were differentiated.
However, because of back flow of pancreatic juice and bile towards the gizzard, the actions of these secretions start earlier in the digestive process than would be expected by their entry point to the small intestine. One effect is an increase in the pH of the intestinal contents of the latter half of the duodenum from strongly to weakly acid.
The small intestine also produces enzymes that plays part in the digestive process of reducing the complex food compounds eaten to the simple compounds or building blocks that can be absorbed across the intestinal wall for transport to the organ or location where either they will be further processed, stored or used. Food materials that escape enzyme action along this tract are subjected to bacterial breakdown in the caeca which provides a system of at least partial recovery of some nutrients.
The remainder of the material consists of waste and undigested food and are mixed with the urine in the cloaca and eliminated from the body as faeces. The appearance of the faeces varies considerably, but typically is a rounded, brown to grey mass topped with a cap of white uric acid from the kidneys.