Digestive System Ch. 25

Food for Energy and Growth Carbohydrates are obtained primarily from cereals, grains, and breads  on the average, carbohydrates contain 4.1 calories per gram  the body uses carbohydrates for energy Dietary fats are obtained from oils, margarine, and butter and are abundant in fried foods, meats, and processed snack foods  fats contain 9.3 calories per gram  the body uses fats to construct cell membranes, to insulate nervous tissue, and to provide energy

Food for Energy and Growth Proteins can be obtained from many foods, including poultry, fish, meat, and grains  proteins have 4.1 calories per gram  proteins are used for energy and as building materials for cell structures, enzymes, hemoglobin, hormones, and muscle and bone tissue

Food for Energy and Growth In wealthy countries, being significantly overweight is common  this is due to habitual overeating and high-fat diets, in which fats constitute over 35% of the total caloric intake  the standard measure of appropriate body weight is the body mass index (BMI), estimated as your body weight in kg, divided by your height in meters squared

Are you overweight? 66% of Americans are overweight with a BMI of 25 or more

Types of Digestive Systems Heterotrophs are divided into three groups on the basis of their food sources  herbivores eat plants exclusively  carnivores are meat eaters  omnivores eat both plants and animals Single-celled organisms, as well as sponges, digest their food intracellularly All other animals digest their food extracellularly, within a digestive cavity

Types of Digestive Systems A gastrovascular cavity is found in cnidarians and flatworms  this cavity has only a single opening that serves as both a mouth and an anus  there is no specialization within this type of digestive system because every cell is exposed to all stages of digestion

Types of Digestive Systems The alimentary canal is a digestive tract with a separate mouth and anus  this permits specialization and the transport of food is one way physical forces, such as chewing and grinding, break the ingested food into smaller fragments chemical digestion involves hydrolysis reactions that liberate the subunits of food the products of digestion are absorbed into the blood any molecules in the food that are not absorbed by the animal are excreted through the anus

One-way digestive tracts

Vertebrate Digestive Systems In humans and other vertebrates, the digestive system consists of a tubular gastrointestinal tract and accessory organs  in general, carnivores have shorter intestines for their size than herbivores  Herbivores have long, convoluted small intestines because they ingest a large amount of plant cellulose, which resists digestion  the tubular gastrointestinal tract has a layered structure

The human digestive system

The layers of the gastrointestinal tract

The Mouth and Teeth Many vertebrates have teeth, and chewing (mastication) breaks up food into small particles and mixes it with fluid secretions

The Mouth and Teeth Inside the mouth, the tongue mixes food with a mucous solution called saliva  saliva moistens and lubricates food so that it is easier to swallow  saliva also contains a hydrolytic enzyme called amylase this enzyme initiates the breakdown of starch into the disaccharide maltose

The Mouth and Teeth Food is moved by the tongue to the back of the mouth for swallowing The soft palate is raised, closing off the nasal cavity, and muscles push the food past the larynx  food is prevented from going into the respiratory tract by the epiglottis Figure The human pharynx, palate, and larynx.

The Esophagus and Stomach The esophagus is a muscular tube that connects the pharynx to the stomach  the upper third is enveloped in skeletal muscle for voluntary control of swallowing  the lower two-thirds is surrounded by involuntary smooth muscle  rhythmic waves of contractions, called peristalsis, propel food towards the stomach

Peristalsis

The Esophagus and Stomach The movement of food from the esophagus into the stomach is controlled by a ring of circular smooth muscle, called a sphincter  contraction of the sphincter prevents food in the stomach from moving back into the esophagus  in humans, stomach contents can be brought back out during vomiting  the relaxing of the sphincter may lead to acid reflux, which is when stomach acid moves into the esophagus this produces a burning sensation known as heartburn

The Esophagus and Stomach The stomach is a saclike portion of the digestive tract  the stomach contains an extra layer of smooth muscle for churning food  gastric juice is released by gastric glands in the lining of the stomach parietal cells secrete hydrochloric acid (HCl) chief cells secrete pepsinogen –pepsinogen requires a low pH to be activated into pepsin, a protease that begins the digestion of proteins

The stomach and gastric glands

The Esophagus and Stomach Gastric juice has a pH of 2, much more acidic than the 7.4 pH of blood  the low pH helps to denature protein, keep pepsin active, and kill most bacteria  active pepsin hydrolyzes food proteins into short chains of polypeptides that are not fully digested until the mixture enters the small intestine  chyme is the name for the mixture of partially digested food and gastric juice

The Esophagus and Stomach Overproduction of gastric acid can occasionally eat a hole through the wall of the stomach, called a gastric ulcer  normally the stomach epithelial cells are protected by alkaline mucus  susceptibility to ulcers is increased by an infection of the bacterium Helicobacter pylori

The Small and Large Intestines The small intestine is the true digestive vat of the body  only relatively small portions of chyme are introduced into the small intestine at one time this allows time for acid to be neutralized and enzymes to act  in the small intestine, carbohydrates, protein, and lipids are broken down and absorbed into the bloodstream

The Small and Large Intestines While some enzymes necessary for digestion are secreted by the cells of the intestinal wall, most are made in the pancreas  the pancreas is an exocrine gland, meaning it secretes through ducts  the pancreas sends it products via a duct that empties into the first part of the small intestine, the duodenum

The Small and Large Intestines Much of the food energy the vertebrate body harvests is obtained from fats  fat digestion involves bile salts that are secreted into the duodenum by the liver  bile salts act like detergents and combine with drops of fat to form microscopic droplets this process is known as emulsification this increases the surface area for the enzyme lipase to work on in order to breakdown the fat

The Small and Large Intestines The small intestine also includes  jejunum where digestion continues  ileum where water and digested products are absorbed The lining of the small intestine is folded into ridges, which are covered with fine projections called villi (singular, villus)  each of the cells covering the villus is covered by a field of projections called microvilli

The Small and Large Intestines The large intestine has a wider diameter than the small intestine  no digestion takes place here  only about 6% to 7% of fluid absorption occurs here some water, sodium, and vitamin K  the main function of the large intestine is to compact and store undigested material as feces

Accessory Digestive Organs The pancreas secretes fluid through the pancreatic duct into the duodenum  the fluid contains a host of enzymes trypsin and chymotrypsin digest proteins pancreatic amylase digests starch lipase digests fats  the pancreas also secretes bicarbonate, which neutralizes the HCl from the stomach

Accessory Digestive Organs In addition to being an exocrine gland, the pancreas is also an endocrine gland  it produces hormones in the islets of Langerhans the two most important pancreatic hormones are insulin and glucagon

Accessory Digestive Organs The liver is the largest internal organ of the body  the liver produces bile and stores it in the gallbladder where it is concentrated if the bile duct becomes blocked, a gallstone forms  the arrival of fatty food in the duodenum triggers a neural and endocrine reflex that stimulates the gallbladder to contract

Accessory Digestive The liver removes toxins, pesticides, carcinogens, and other poisons by converting them into less toxic forms  excess amino acids that may be present in the blood are converted to glucose an amino group (-NH 2 ) is removed from the amino acid to become ammonia (NH 3 ) NH 3 then combines with CO 2 to form urea, which then goes to the liver