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6.2 The Human Digestive System. In this section, you will: identify the main structures and functions of the digestive system describe the physical and chemical processing of food through the digestive system and into the bloodstream explain the action of enzymes in chemical digestion
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6.2 The Human Digestive System In this section, you will: • identify the main structures and functions of the digestive system • describe the physical and chemical processing of food through the digestive system and into the bloodstream • explain the action of enzymes in chemical digestion • identify and describe, in general terms, how digested molecules enter the bloodstream
Digestive System • The Vertebrate Digestive System • the digestive system functions to break down the macromolecules found in the diet into usable forms for the body • digestion is the process by which large chunks of food are broken down into molecule-sized pieces • products of digestion are used for energy, building blocks, and as enzymes and/or coenzymes
the digestive system is a long tube that extends from the mouth to the anus, and functions to perform four functions: • ingestion (eating) • movement of the food through the tract, which is accomplished by peristalsis (smooth muscle contractions) • digestion – the breakdown of food by both mechanical (chewing and scrunching) and chemical (enzymatic) means • absorption – the nutrients are absorbed from the digestive system into the blood and lymph from the small and large intestines • elimination – defecation – the elimination of indigestible/unabsorbed materials from the body
the digestive system consists of two basic groups – the GI tract (the tube) and the accessory structures • the GI tract is a tube from 6.5 to 9 m long in adults
Mouth • the mouth is where mechanical and chemical digestion begins • the food is moistened by saliva produced by the salivary glands (we produce 1.7 L per day), which moistens the food to make it easier to swallow, and also contains the first digestive enzymes: • amylase which begins the breakdown of starch into maltose (a disaccharide) and dextrin's (short glucose chains)
Swallowing • the tongue is used to push the food to the back of the mouth – this area is called the pharynx, the presence of the food initiates the swallowing reflex • the epiglottis, a small flap of tissue will close over the trachea, preventing food from entering the lungs • the bolus of food is then swallowed, and travels down the 25 cm esophagus to the stomach, helped by peristalsis (you can swallow while standing on your head!) • at the bottom of the esophagus is the lower esophageal sphincter (or cardiac sphincter) – a small ring of smooth muscle that keeps food in the stomach
Stomach • the stomach holds up to 2L (4L) of food, and will hold food for from 3 to 6 hours while it is broken down • There are three important chemicals in the stomach involved in digestion: • HCl(aq) – secreted by the parietal cells of the stomach, lowers the pH of the stomach to 2.0, allowing the activation of pepsin from pepsinogen • Pepsin – secreted as an inactive precursor, pepsinogen (cannot be active or it would digest cellular proteins) by the peptic (chief) cells. It is activated by the lowered pH in the stomach (which changes the shape of the enzyme) • mucous – secreted by the mucous (goblet) cells, protects the stomach endothelium from the acid and enzymes
Two other secretions: • gastrin – a hormone that stimulates gastric secretions (which is stimulated by the presence of proteins in the stomach), and relaxes the pyloric sphincter to allow stomach emptying • rennin – found principally in children, it is involved in the digestion of milk – slowing its emptying from the stomach
physical digestion - the acid along with the peristaltic motions of the stomach contents helps to break up tissues • the pepsin will break up proteins into smaller polypeptides called peptones (protein digestion will be completed in the small intestine) • most materials are not absorbed in the stomach, but some drugs (notably aspirin), some water, electrolytes, alcohol and absorbed in the stomach • the highly acidic stomach contents, called chyme is emptied into the small intestine a bit at a time through the pyloric sphincter
Small Intestine • measures up to 7m in length, but only 2.5 cm in diameter, divided into three sections • duodenum ~25 cm long, where most digestion occurs • jejunum ~2.3 m long • ileum ~ 3.5 m long • the majority of digestion and absorption (90%) takes place here – lipid digestion, continuation of carbohydrate and protein digestion • the accessory organs of digestion, the pancreas, gall bladder and the liver secrete their juices into the duodenum to aid in digestion focusing on absorption
the small intestine, in order to better digest and absorb digested materials is highly folded to increase surface area (by 600x); the folds are called villi, which are covered in small cytoplasmic projections called microvilli • the structure of the small intestine (from the outer surface in) includes: • villi/microvilli – surface (epithelial) cells designed for absorption • lacteal – vessel projection of the lymphatic system, designed to absorb fats • capillaries – vessels of the circulatory system, for absorption of all other nutrients for immediate transport to the liver for processing
as the chyme enters the duodenum, its acidity and contents stimulates a number of hormones to be released, which in turn stimulate the secretion of digestive and protective chemicals from accessory organs and the intestinal epithelium itself • the small intestine produces enzymes to aid in digestions: • carbohydrases: sucrase, maltase and lactase which break down sucrose, maltose and lactose, respectively, into simple sugars • peptidases: peptides smaller peptides and amino acids • nucleosidases:nucleotides bases, sugars, and phosphates
Hormones • Gastrin: stimulated by proteins in food causing the secretion of HCl and pepsinogen • cholecystokinin (CCK): stimulated by partially digested proteins and irritants in the chyme, stimulates pancreatic enzyme release and bile from the gallbladder • Secretin: in response to acidic chyme in the duodenum, inhibits the secretion of gastric juice, stimulates bicarbonate release by the pancreas to neutralize acidic chyme • GIP (gastric inhibitory peptide)
Accessory Organs: Pancreas • a soft tubular gland that lies just behind the stomach, and is connected to the duodenum by two ducts • has both a exocrine (secretory) and endocrine (hormonal) function • its exocrine functions are to secrete digestive enzymes and sodium bicarbonate to neutralize the stomach acid and establish a pH of 7.1-8.2 which will not only neutralize the enzyme pepsin, but activate the pancreatic enzymes
Pancreatic Enzymes: • pancreatic amylase: digest carbohydrates maltose • trypsin: protein digestion: peptides smaller peptides (activated by enterokinase, secreted by the intestinal wall) • chymotrypsin: protein digestion: peptides smaller peptides (activated by trypsin) • peptidase: peptides smaller peptides and amino acids • pancreatic lipase: digests fats into fatty acids and glycerol • ribonuclease: digests RNA to nucleotides • deoxyribonuclease: digests DNA to nucleotides
Liver • the liver has over 500 functions, four being fundamental: • production of bile • storage of glucose in the form of glycogen ( fat if glycogen limits exceeded), conversion of galactose and fructose to glucose • detoxification of the blood (makes enzymes to break down toxins; ex: alcohol, caffeine, nicotine, barbiturates, poisons, excess hormones) • deamination of amino acids – removing nitrogen, producing ammonia and eventually urea (excreted by the kidneys)
it receives two separate blood supplies • via the portal vein – bringing freshly absorbed nutrients from the small intestine • via the hepatic artery – bringing oxygenated blood from the lungs/heart • bile • each day, the liver secretes between 800 – 1000 mL of bile • bile is stored in the gallbladder for release on demand into the small intestine • consists of water, bile salts, cholesterol and bile pigments (made from bilirubin – yellow in colour) • bile acts as an emulsifier, breaking large fat globules into small ones, allowing lipase more surface area for digestion
Gallbladder • is lodged in one of the lobes of the liver • a light muscular bag that stores and releases bile • why store bile? We eat large quantities of fats at a time, so having a store of bile is useful but can lead to problems – bile salts can crystallize in the gallbladder forming gallstones • back to digestion…. • the combination of hormones and the presence of certain foods leads to the secretion of pancreatic enzymes, bile, and intestinal enzymes (maltase, lactase, sucrase and enterokinase), all of which are active in the slightly basic pH of the duodenum
the villi of the small intestine are specialized to absorb the molecules of digestion: • large surface area • transport proteins on the epithelial cells move amino acids, glucose, water soluble vitamins, etc. by facilitated diffusion and active transport into the capillary system, then on to the liver immediately for processing • electrolytes by diffusion, water by osmosis • fatty acids and glycerol are absorbed into the epithelial cells and are repackaged as triglycerides
being hydrophobic, they are packaged into protein packs to enable their transport through the body • the packaged fats are absorbed into the lacteals in the interior of the villi, and are transported through the body’s lymphatic system • fat soluble vitamins will move with fats into lacteals for absorption in the body
Absorption of Nutrients Top: Glucose is actively transported into cells of the intestinal wall to move into the bloodstream. Middle: Amino acids are actively transported into the cells of the intestinal wall to move into the bloodstream. Bottom: Glycerol and fatty acid molecules diffuse into the cells of the intestinal wall where they are resynthesized into fats, coated with proteins, and move into lymph vessels for eventual transport into the bloodstream.
Large Intestine • about 1.5 m in length, a diameter of 6.5 cm. • at the junction of the small and large intestine, exists the appendix, which has no real function given our diet • the large intestine has three main functions: • absorption of water and electrolytes • production of feces – consisting of water, inorganic salts, cells from the GI tract, bacteria, bacterial decomposition and undigested food • housing bacteria, that will use remaining unabsorbed nutrients or undigested carbohydrates to make vitamin K and B vitamins for us to absorb (as well as methane gas)
6.3 Health and the Digestive System In this section, you will: • recognize and appreciate the relationship between health and nutritional decisions • identify conditions that adversely affect the health of the digestive system and the technologies that are available to treat them
Digestive Disorders • Disorders of the digestive system and its accessory organs include: • ulcers, • inflammatory bowel disease, • hepatitis, • cirrhosis, • and gallstones. • All disorders that affect digestion, including eating disorders, can seriously damage overall health by depriving the body cells of the nutrients they need to survive.
Chapter 6 Review • Why are some amino acids classified as essential? • Describe the relationship between the organs in the digestive tract. • What are the benefits of having stomach stapling? What are the risks? • Summarize chemical digestion of macromolecules. • Explain how glucose levels are affected after meals. Include the effects of different foods.
Chapter 6 Summary • The human body takes in matter from the environment in the form of food and water. The human digestive system processes the food and water in order to obtain the macromolecules it needs for survival.
Chapter 6 Summary • Food passes through the digestive tract—the mouth, pharynx, esophagus, stomach, small intestine, and large intestine—during physical digestion. • The accessory organs—the salivary glands, liver, gall bladder, and pancreas—supply chemicals that also contribute to the digestion of food as it passes through the digestive tract. • The stomach supplies chemicals to aid digestion as well as generating physical contractions to physically break down food. • The food is eventually liquefied into soluble units that can pass through cell membranes for transport via the circulatory system to all the cells in the body. • The waste materials from the digestive process leave the body via the large intestine.
Chapter 6 Summary • The nutrients that food supplies include carbohydrates, lipids (fats), protein, and nucleic acids. • Carbohydrates and lipids are broken down to supply energy; lipids also supply material for the cell membranes. • Proteins are more structurally and functionally diverse than carbohydrates and lipids. They assist in transport, immunity, and muscle action and are used to make up most cellular structures. • Nucleic acids direct growth and development. Enzymes speed up chemical reactions, particularly for the production of energy. • Vitamins and minerals are organic and inorganic substances that enable chemical reactions to occur and aid in tissue development and growth and immunity. These substances are needed for a healthy, functional human body.
