Digestive System

by Lynn and Ma (Caitlyn and Alma)

Table of Contents


Nutrition is the process by which organisms take in and use food. Herbivores eat plants and algae, carnivores eat animals, and omnivores eat plants, algae, (moreover autotrophs), and animals.

An animal’s diet provides ATP, organic carbon and nitrogen, and essential nutrients, which animals require but cannot make themselves. Eight of the amino acids are essential amino acids, required by most animals and adult humans. Lack of these can slow physical and mental development. “Complete” proteins contain all the essential nutrients, and “incomplete” proteins do not.
Deficiencies of essential fatty acids are rare. Vitamins are essential organic moleculse required in small amounts in the diet. Thirteen that are essential have been identified, like vitamin C for production of connective tissue. Minerals are inorganic nutrients usually needed in very small amounts, like calcium for nerve and muscle function. Undernourishment results from a diet deficient in calories. Malnourishment is the long term absence of one or more of the essential nutrients.
Human dietary requirements are studied by researching genetic defects that affect nutrient uptake and epidemiology, the study of disease among various populations, which have different diets.



Ingestion is the act of consuming food. A large amount of aquatic animals are suspension feeders, which sift small food particles from the water. Substrate feeders live in or on their food, and eat their way through it. Fluid feeders feed on the fluid of a living plant or animal host. Bulk feeders eat large particles and pieces of food.
Fluid Feeder
Bulk feeder
Suspension feeders
Substrate feeder
Digestion breaks up and splits food through enzymatic hydrolysis, which is the addition of water molecules when components’ bonds are broken. In absorption, food is into the animal’s cells, and in elimination, food leaves the digestive system.
Intracellular digestion, common in sponges, is digestion of food molecules in food vacuoles. This follows phagocytosis or pinocytosis, and the food vacuoles then fuse with lysosomes that contain enzymes. Most animals initially break down their food through extracellular digestion in a separate part of the body that connects to the external environment.

A gastrovascular cavity both transports nutrients and digests. Gastrodermal cells take in food particles, and hydrolysis of macromolecules happens in food vacuoles. Undigested materials are eliminated.
Organisms with a gastrovascular cavity
Most animals have
complete digestive tracts, a digestive tract that runs between a mouth and an anus, also known as an alimentary canal. There are also regions in between for processing nutrients. Food ingested through the mouth and pharynx goes through the esophagus and leads to the crop (like in worms), stomach (like in humans), or gizzard (like in owls), which are organs for grinding food. In the intestine, digestive enzymes hydrolise macromolecules, and nutrients are absorbed. Everything not used is eliminated through the anus.

Organs specialized for sequential stages of food processing form the mammalian digestive system


The alimentary canal of a human contains the canal and multiple accessory glands that secrete digestive juices. The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas, the liver, and the gallbladder.
When food is in the alimentary canal, there are alternating waves of contraction and relaxation called peristalsis. Peristalsis moves the food along. At junctions between compartments are ringlike valves called sphincters that regulate the passage of food.

The oral cavity, pharynx, and esophagus
Ingestion starts in the oral cavity. Mechanical digestion occurs as the teeth chew up the food and the salivary glands produce saliva.
      • Amylase, the enzyme in saliva, hydrolyzes starch and glycogen.
      • The lining of the mouth is protected by mucin, a slippery glycoprotein.
      • The tongue checks if the food is okay to be digested, and then it helps shape the food into a ball called a bolus. During swallowing, the tongue pushes the bolus into the esophagus.

The pharynx, or throat region, opens up to 2 passageways: the esophagus and trachea. When you swallow, a flap of cartilage called the epiglottis prevents the food from going down the trachea. The esophagus leads to the stomach.
      • During swallowing, the striated muscle on the top of the esophagus is active.
      • In the rest of the esophagus, the smooth muscle causes peristalsis to occur.

The primary function of the stomach is to store food and continue digestion. A few nutrients are absorbed in the stomach. It’s very elastic and secrets a digestive fluid called gastric juice. Gastric juice mixes with food and produces chyme. There are two components of gastric juice, hydrochloric acid (HCL) and pepsin. HCL has a very low pH of 2 and it denatures proteins, exposing their peptide bonds. Pepsin then attacks those peptide bonds, breaking those proteins into smaller polypeptides. The ingredients of gastric juice are kept inactive until they reach the stomach. The stomach is also covered by a layer of mucus. This way, the stomach doesn’t digest itself. Parietal cells secrete hydrogen and chloride ions, which form HCL in the stomach. Chief cells secret pepsinogen, which is the inactive form of pepsin. HCL converts pepsinogen to pepsin. Pepsin can also activate pepsinogen by clipping it and exposing its active site. The stomach churns its contents about once every 20 seconds. Usually, the somach is closed off at both ends. Sometimes, chyme flows back from the stomach into the esophagus. This is inaccurately called “heartburn.”

The small intestine Most enzymatic hydrolysis of macromolecules from food occurs in the small intestines. It is over 6 meters long. The digestive enzymes can either be secreted into the duodenum or they can be bound to the surface of epithelial cells. The first 25 cm or so of it is the duodenum. In the duodenum, chyme from the stomach mixes with digestive juices from the pancreas, liver, and gallbladder.

      • The pancreas aid digestion by secreting an alkaline solution rich in bicarbonate, which neutralizes the acidity of the chyme. Some pancreatic enzymes include trypsin and chymotrypsin.
      • The liver produces bile, which digests fats and lipids. Bile is stored and concentrated in the gallbladder.
      • The remaining regions of the small intestine is the jejunum and ileum. They absorb nutrients and water.

The small intestine has a very large surface area. On the surface, there are finger-like projections called villi. Each epithelial cell of a villus has its own villi, or microvilli.
Transport across the epithelial cells can be active or passive. Fructose moves by facilitated diffusion wile amino acids, vitamins, and small peptides have to be pumped by active transport. Fats do not leave through the bloodstream. After digestion, they are recombined into triglycerides and then coated with phospholipids, cholesterol, and proteins, forming chylomicrons. Then they are transported to the lacteal, a vessel that is part of the lymphatic system. Then they get transported to the heart. In contrast, the nutrients are transported into the hepatic portal vein, which goes to the liver. The liver removes toxic substances.

The alimentary canal ends with the large intestines, which includes the colon, cecum, and rectum.
      • The colon is one arm of the T shaped intersection that the small intestines forms with the big one. It leads to the rectum and anus. Its main function is to recover lost water.
      • The cecum is a pouch. It is for fermenting ingested materials, especially for animals that eat lots of plants. The appendix is part of the human cecum.
      • The retum is the terminal portion of the large intestines were feces are stored before they are released.

The wastes of the digestive system are called feces. They become increasingly solid as they move along the colon by peristalsis. If less water is absorbed than normal, diarrhea results. If the opposite happens, constipation happens.
Many harmless bacteria live in the colon, including E. coli. These bacteria produce gas as a by-product of their metabolism, which comes out in the form of a fart.

41.4 Evolutionary adaptations of vertebrate digestive systems correlate with diet
Dentition is an animal’s assortment of teeth. The structure of teeth vary among different animals.
      • Carnivores generally have pointed incisors and canines. Their premolars crush and shread food.
      • Herbivores have teeth with broad, ridged surfaces that grind plants.
      • Omnivores have relatively unspecialized dentition.

Animals also vary in stomach and intestinal adaptations. Large, expandable stomachs are common in carnivores. Herbivores and omnivores have longer alimentary canals than carnivores because vegetation is harder to digest than meat because of the cell walls.
Some digestive adaptations involve mutualistic symbiosis. These relationships between host and microoganisms vary. For example:
      • A herbivorous bird in South America called the hoatzin has microorganims living in its crop. The microorganisms break down glucose.
      • Horses and other herbivorous mammals have microorganisms in their cecums.
      • In rabbits and other rodents, the mutualistic bacteria live in the large intestines and cecum. However, most nutrients are absorbed in the small intestines, the nourishing by-products of the bacteria are lost in the feces. As a result, the rabbits and rodents eat their feces.
      • In the koala, the mutualistic bacteria ferment shredded eucalyptus leaves in the cecum.
      • Ruminants are herbivores with the most elaborate adaptations for diet. They include deer, sheep, and cattle.
      • 41.20.png

Some animals, such as the giant tubeworms, have no mouth or digestive system and rely entirely on mutualistic bacteria to generate energy and nutrients.

41.5 Homeostatic mechanisms contribute to an animal’s energy balance

Animals can use carbohydrates, proteins, or fats as fuel for energy. However, most animals use proteins after they exhaust their supply of carbs and fats. When animals take in more energy rich molecules than they can absorb, the molecules get converted to glycogen and they are stored primarily in the liver and muscle cells. When fewer calories are taken in than used, the glycogen is oxidized. The hormones insulin and glucagon maintain glucose homeostasis by regulating glycogen breakdown (see Chapter 45 or Endocrine system wiki by the Zebra, Fat Cat, and Lulu). Animals also store energy at a secondary site, which is the adipose (fat) cells. When glycogen deposits are full, the excess is stored as fat. Humans draw energy from liver glycogen first, then muscle glycogen and fat. When a person eats more calories than the body needs for normal metabolism, overnourishment occurs. This leads to obesity. This is a problem. Obesity can lead to diabetes, colon and breast cancer, cardiovascular diseases, or strokes.

Insulin injection

Several hormones regulate body weight.

      • Leptin is produced by adipose tissue. It suppresses appetite as fat levels increase.
      • PYY is secreted by the small intestine after meals and suppresses appetite.
      • Ghrelin is secreted by the stomach wall, and it triggers the feelings of hunger as mealtimes approach.
      • Insulin is secreted by the pancreas after a rise in blood sugar. It suppresses appetite, along with other functions explored elsewhere.

Why do we love fats? Because our ancestors survived mainly on seeds and plants. Rich, fatty foods were very rare so they gorged on them when they were available. That’s why we love those fats.
Petrels are birds that are fat as babies because their parents have to fly far away to get food and bring back mostly lipids. Then, they fast for several days to get rid of all the fat.

Web sources: