Gastrointestinal tract

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Some nutrients are complex molecules for example vitamin B 12 which would be destroyed if they were broken down into their functional groups. Field metabolism, water requirements, and foraging behavior of wild Ostriches in the Namib. Carnivores have canine teeth which are used to kill and tear meat. In these sections of the gut there is clear boundary between the gut and the surrounding tissue. Cardiovascular system peripheral Artery Vein Lymphatic vessel Heart. They blend into the surrounding tissue and are fixed in position. The mucusal tissue of the small intestine is alkaline so that pH of the semi-digested food moving through the small intestine gradually increases to around 8.

The mouth of the squid is equipped with a sharp horny beak mainly made of cross-linked proteins. It is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species.

The tongue is skeletal muscle on the floor of the mouth that manipulates food for chewing mastication and swallowing deglutition. It is sensitive and kept moist by saliva. The underside of the tongue is covered with a smooth mucous membrane. The tongue also has a touch sense for locating and positioning food particles that require further chewing. The tongue is utilized to roll food particles into a bolus before being transported down the esophagus through peristalsis.

The sublingual region underneath the front of the tongue is a location where the oral mucosa is very thin, and underlain by a plexus of veins. This is an ideal location for introducing certain medications to the body. The sublingual route takes advantage of the highly vascular quality of the oral cavity, and allows for the speedy application of medication into the cardiovascular system, bypassing the gastrointestinal tract. Teeth singular tooth are small whitish structures found in the jaws or mouths of many vertebrates that are used to tear, scrape, milk and chew food.

Teeth are not made of bone, but rather of tissues of varying density and hardness, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if we were to bite into something too hard for our teeth, such as a chipped plate mixed in food, our teeth send a message to our brain and we realise that it cannot be chewed, so we stop trying.

The shapes, sizes and numbers of types of animals' teeth are related to their diets. For example, herbivores have a number of molars which are used to grind plant matter, which is difficult to digest. Carnivores have canine teeth which are used to kill and tear meat. A crop , or croup, is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion.

In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds. Certain insects may have a crop or enlarged esophagus. Herbivores have evolved cecums or an abomasum in the case of ruminants.

Ruminants have a fore-stomach with four chambers. These are the rumen , reticulum , omasum , and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud or bolus. The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.

Fibre, especially cellulose and hemi-cellulose , is primarily broken down into the volatile fatty acids , acetic acid , propionic acid and butyric acid in these chambers the reticulo-rumen by microbes: In the omasum, water and many of the inorganic mineral elements are absorbed into the blood stream. The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach e.

It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs.

Microbes produced in the reticulo-rumen are also digested in the small intestine. Regurgitation has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by regurgitation. Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents perhaps developed as a way to reduce exposure to toxins. Other animals, such as rabbits and rodents , practise coprophagia behaviours — eating specialised faeces in order to re-digest food, especially in the case of roughage.

Capybara, rabbits, hamsters and other related species do not have a complex digestive system as do, for example, ruminants. Instead they extract more nutrition from grass by giving their food a second pass through the gut. Soft faecal pellets of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten. Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation.

When they are born, their intestines do not contain these bacteria they are completely sterile. Without them, they would be unable to get any nutritional value from many plant components. An earthworm 's digestive system consists of a mouth , pharynx , esophagus , crop , gizzard , and intestine. The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew.

The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt.

When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter.

By peristalsis , the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body. In most vertebrates , digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms.

Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:. Underlying the process is muscle movement throughout the system through swallowing and peristalsis.

Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.

The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digests before excretion.

In mammals , preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed , and mixed with saliva to begin enzymatic processing of starches.

The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract , and the process finishes with defecation. The human gastrointestinal tract is around 9 meters long. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours.

Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection.

In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile , pancreatic juice and intestinal juice. The intestinal walls are lined with villi , and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.

In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place. Here water is absorbed and waste material stored as feces to be removed by defecation via the anal canal and anus.

Different phases of digestion take place including: The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal secretes acid and G cell secretes gastrin activity via D cell secretion of somatostatin.

The gastric phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine , which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach.

Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin , which in turn stimulates parietal cells to secrete gastric acid.

Gastric acid is about 0. Acid release is also triggered by acetylcholine and histamine. The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens.

For example, trypsin is secreted by pancreas in the form of trypsinogen , which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides. Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. However fats are mainly digested in the small intestine.

In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase , resulting in progressively smaller chains of glucose. This results in simple sugars glucose and maltose 2 glucose molecules that can be absorbed by the small intestine. Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose.

Glucose and galactose can be absorbed by the small intestine. Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat unfermented milk-based foods.

This is commonly known as lactose intolerance. Lactose intolerance varies widely by ethnic heritage; more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent.

When amino acids are used for energy or converted to fats or carbohydrates, the amine NH 2 group must be removed. These amine groups are toxic and must be eliminated. Some organisms excrete these nitrogenous wastes as ammonia e. Birds and reptiles excrete these wastes primarily as uric acid. Although excreting nitrogenous wastes as uric acid has its advantages e.

Molecular structure of a typical amino acid, ammonia, urea, and uric acid. The homeostasis of fluid and ions in birds involves several organ systems Figure 10 and is a more complex phenomenon than in other vertebrates. In birds, the kidneys and lower gastrointestinal tract cloaca, rectum, and ceca are involved in the regulation of extracellular fluid composition.

Many birds also have functional salt glands see below. Osmoregulatory organs of birds Hughes As noted above, the avian kidney has a limited capacity for the conservation of body water and electrolytes via elimination of hyperosmotic urine. This low capacity to concentrate urine is not a liability because urine formed by the kidneys travels along the ureters into the cloaca Figure From there, it may move by retrograde peristalsis into the lower intestine colon and cecae.

Fluid from the upper gastrointestinal tract also enters the cloaca. Therefore, the cloaca receives an influx of water from the kidneys and the small intestine. The influx of water into the cloaca can be reabsorbed through the epithelium of the lower intestinal tract to maintain hydration.

In the lower intestine and cecae, water and sodium chloride are reclaimed by the process of sodium-linked water reabsorption Figure In other words, positively-charged sodium ions are actively transported out of the intestine and negatively-charged chloride ions follow. Water then follows by osmosis. Uric acid is, as a result, concentrated and excreted as a relatively dry mixture with feces Hildebrandt Diagram of the cloaca and lower intestine of a domestic chicken Gallus gallus.

Initially, the solutes in the urine cause water to move by osmosis out of the surrounding tissues and into the coprodeum the section of the cloaca adjacent to the colon or large intestine. After being transported by peristalsis into the colon, however, NaCl is transported out of the colon and water follows the concentration gradient osmosis and is reabsorbed Laverty and Skadhauge The predominate form in which nitrogen is excreted by birds uric acid requires little water for excretion because it isn't very soluble in water.

However, it does require a significant amount of protein to maintain it in a colloidal suspension in the urine i. The source of some of this protein is the plasma, as significant amounts pass through the glomerular filtration barrier.

This protein is not lost because it is broken down when the urine enters the lower colon Goldstein et al. In the colon, the composition of the urine is altered in several ways. The urine spheres are broken down, as is the protein that aided the formation of those spheres.

Much of this degradation is accomplished by bacteria. The amino acids or peptides that result are either used by the bacteria or absorbed by the epithelium of the colon Braun Casotti and Braun In birds, urine is conveyed to the cloaca, and moved by reverse peristalsis into the colon and digestive ceca.

Digestive ceca have been well studied for non-passerine birds and have been shown to absorb substrates and water. The ceca of passerine birds have been suggested to be non-functional because of their small size. Three-dimensional reconstruction of the ceca of House Sparrows Passer domesticus from serially-sectioned tissue showed that the ceca have a central channel with a large number of side channels.

Electron micrographs indicated that all of the channels are lined by epithelial cells with a very dense microvillus brush border as well as a region densely packed with mitochondria just below the brush border. It is possible that the row of mitochondria below the brush border is present to provide ATP to power substrate transport. Although the importance of small ceca for fluid homeostasis remains to be determined, these data suggest that the small ceca of passerine birds may function in fluid and electrolyte e.

Light micrograph and electron micrographs of a House Sparrow cecum. A A light micrograph of a cecum cut in sagittal section. The image shows one central channel with a proximal opening to the colon upper right of section.

Side channels arrows can be seen branching from the central channel. B Higher power light micrograph of a cecum. The image illustrates the columnar cells that line the channels bracket with their well developed brush border arrow. C Electron micrograph of a House Sparrow cecum.

The image shows a portion of the epithelial cells that line channels of the ceca. Evident is a dense microvillus brush border on the apical surface of the cells and tight junctions TJ between the cells. Just below the brush border is a very dense layer of mitochondria bracket. Reyes and Braun As described above, the colon and ceca of birds can substantially modify the urine that initially enters the cloaca coprodeum. As a result, the composition of bird droppings can differ dramatically from the composition of the urine and feces that initially entered the colon or large intestine Figure Depending on diet and access to water, droppings may have considerably less water, solutes sodium and potassium , and uric acid or urate than the original urine and feces Figure Of course, if a bird consumes more of these substances than needed, the composition of droppings will differ and more water, solutes, and urates can, as needed, be excreted in the droppings.

The coordinated action of the kidneys, lower GI tract, and the salt glands see below in the regulation of fluid and ion balance is a classic example of the integration of organs required to maintain homeostatic balance. No single organ appears to have an outstanding capacity to conserve ions and water, but instead they all function in concert to maintain total body fluid homeostasis to allow birds to inhabit a wide range of environments Braun Modification of urine and feces in the lower GI tract colon and ceca of birds.

Can birds be ammonotelic?? Uric acid is a relatively non-toxic nitrogen end product. It is relatively insoluble and hence excreted with little water. Uricotely, however, is costly. More energy is needed to excrete a unit of waste nitrogen as uric acid than as urea or ammonia.

In contrast to uric acid, ammonia is highly soluble, cheap to synthesize, but fairly toxic. It can only be used as a nitrogenous waste by animals with high rates of water turnover that permit almost continuous elimination, such as in aquatic animals Tsahar et al. Preest and Beuchat suggested that it might be advantageous for birds that ingest large amounts of dilute, protein-poor nectar to shift from uricotely to ammonotely.

Thus, ammonia can be voided rapidly, and the costs of synthesizing urates can be reduced. Thus, Preest and Beuchat proposed that high energy demands and high water fluxes favor ammonotely. Subsequently, Roxburgh and Pinshow found that the Palestine Sunbirds also switched from uric acid to ammonia excretion under some conditions.

However, Roxburgh and Pinshow noted that in sunbirds with high water intake, the concentration of urate was higher in the ureters the tubes that carry urine from the kidneys to the cloaca than in excreta.

They argued that ammonotely in Palestine Sunbirds was only 'apparent' because it was not a result of excessive excretion of ammonia, but rather the result of a reduction in excreted urate resulting from post-renal modification of urine.

Recently, Tsahar et al. These authors, like Roxburgh and Pinshow found that protein concentration was lower in excreta than in ureteral urine, and hypothesized that some of the protein associated with urate spheres was digested in the lower intestine and recovered.

Why would it be advantageous for birds to recover a nitrogenous metabolic waste? Although uric acid is considered primarily a nitrogenous waste, it also has a major function as a powerful antioxidant in both birds and mammals Tsahar et al. So, can birds be ammonotelic? In some cases, as with Anna's Hummingbirds, ammonotely may be a response to the ingestion of lots of water facultative ammonotely.

In other cases, ammonotely may simply be 'apparent', with urine produced by the kidneys being urotelic but the actual excreta being ammonotelic because of reabsorption of uric acid in the lower intestine or colon prior to defecation. Because the kidneys of birds cannot produce a hypertonic urine with lots of ions like sodium , the excretion of excess salt is a potential problem.

Even quicker than humans, birds would be severely dehydrated after drinking saltwater and ingesting salty food. However, many species of birds, especially marine birds and shorebirds, can drink seawater as their only source of water. This is possible because these birds have another way other than the kidneys to eliminate excess salt - salt glands.

Salt glands of birds likely evolved from nasal glands of reptiles, probably in the late Paleozoic. They lie immediately under the skin in supraorbital depressions of the frontal bone in the skull of Charadriiform birds, but in other groups they may be located above the palate or within the orbit of the eye. Skulls of fossil birds, Ichthyornis and Hesperornis, have similar depressions Figure 14 , suggestin g these birds lived in marine habitats.

The salt glands of marine birds and some falconiform and desert birds secrete excess NaCl via the salt glands using less water than is consumed, which generates free water Hughes Hesperornis skull note depression above eye socket where salt gland would be located.

Salt glands have been reported in several avian orders Spheniciformes , Procellariformes , Charadriiformes, Pelecaniformes, Anseriformes, and Phoenicopteriformes. Even though most studies of osmoregulation in birds have been conducted with marine taxa, nasal secretions are not to be restricted to these species. The presence of functional salt glands has been documented in several terrestrial orders. For example, Roadrunners Geococcyx californianus and Savanah Hawks Heterospizias meridionalis , have active salt glands and can produce hypertonic secretions in response to their protein-rich diets.

Although these species are not stressed by high saline load, the active secretions of salt gland allows them to minimize water losses. Other desert birds, such as the Sand Partridge Ammoperdix heyi and the Ostrich Struthio camelus , have functional salt glands that are stimulated in response to high temperature. Thus, salt glands are not restricted to birds that live in saline or maritime habitats, but are also present in some terrestrial forms that consume little water Sabat Salt glands have a system of countercurrent blood flow to remove and concentrate salt ions from the blood Figures 15 and The paired, crescent-shaped glands each contain several longitudinal lobes approximately 1 mm in diameter and each lobe contains a central duct from which radiate thousands of tubules enmeshed in blood capillaries.

These tiny capillaries carry blood along the tubules of the gland, which have walls just one cell thick and form a simple barrier between the salty fluid within the tubules and the bloodstream. It is here that salt excretion occurs. When a bird drinks seawater, sodium enters the blood plasma from the intestine and the solute concentration of the blood plasma increases. This causes water to move out of cells osmosis , increasing the extracellular fluid volume ECFV. The increases in blood plasma solute concentration and ECFV stimulate salt gland secretion Hughes Glomerular and medullary architecture in the kidney of Anna's Hummingbird.

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Journal of Experimental Zoology Hypertonic fluids are secreted by medial and lateral segments in duck Anas platyrhynchos nasal salt glands.

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