Chapter 21 - Digestion

Digestion in the small intestine

1) Carbohydrates -Pancreatic Amylase - polysaccharide to disaccharide -Maltase. lactase, sucrase enzyme - splits it into monosaccharide glucose. 2) Proteins -Trypsin, chymotrypsin - polypeptides to smaller ones -Peptidases enzyme to amino acids 3) Nucleic Acids -Nuclease enzyme to nucleotides -Other enzymes to nitrogenous bases, sugar, phosphates 4) Fats -Bile salts - Fat globules to droplets -Lipase enzyme to Fatty acids and glycerol.

Gastric Glands - 3 types of cells

1) Mucus cells: -Secrete mucus -lubricates + protects cells lining the stomach 2) Parietal cells: -Secrete H + CL ions -Combine with lumen (cavity) of stomach - forms Hydrochloric acid (HCl). 3) Chief cells: -secrete pepsinogen - inactive form of enzyme pepsin.

Bolus

A ball-like mixture of food and saliva that forms in the mouth during the process of chewing. Under normal circumstances, the bolus is swallowed, and travels down the esophagus to the stomach for digestion. Once the bolus reaches the stomach, it mixes with gastric juices and becomes chyme, which then travels through the intestines for further digestion and absorption, and eventual discharge as feces.

Gastric juice

A digestive fluid formed in the stomach and is composed of protein-digesting enzyme, mucus and strong acid. pH 2 Acid functions - breaks apart cells in food, denature proteins. One important component of gastric juice is pepsin. Pepsin is the chief digestive enzyme in the stomach that breaks down proteins. Hydrochloric acid is the primary digestive acid and has several roles in digestion. The acid prevents harmful bacteria from entering your system by sterilizing it. Another function of hydrochloric acid is to trigger the release of pepsin. You need hydrochloric acid to absorb minerals like magnesium, sodium and iron. Gastric acid is so strong that it could digest the stomach if not for a mucus lining protecting the organ.

Peristalsis

A series of wave-like muscle contractions and relaxation of the smooth muscles lining the canal that moves food to different processing stations in the digestive tract. The process of peristalsis begins in the esophagus when a bolus of food is swallowed. The involuntary constriction and relaxation of the muscles of the intestine or another canal, creating wavelike movements that push the contents of the canal forward.

Digestion

Breakdown of food substances into simpler forms that can be absorbed and used

small intestine

Digestive organ where most chemical digestion and absorption of food takes place. Functions: The small intestine is the part of the intestines where 90% of the digestion and absorption of food occurs, the other 10% taking place in the stomach and large intestine. The main function of the small intestine is absorption of nutrients and minerals from food.

Epiglottis

Door like flap of cartilage, over the opening to the trachea. Function: -Directs the closing of the trachea; ensure food goes down esophagus

salivary glands

Glands of the mouth that produce saliva, a digestive secretion. Composition: mucus -H2O, salts, slippery glycoprotein comes Glycoprotein: -Protects soft lining of mouth -Lubricate food for easier swallowing Buffers: -Neutralize food acids, prevent tooth decay Anti-bacterial agents (Lysozyme): -Kill bacteria that enter mouth with food. Salivary amylase: -start chemically digestion of starch

Gallbladder

In vertebrates, the gallbladder is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although the structure and position of the gallbladder can vary significantly among animal species. It receives and stores bile, produced by the liver, via the common hepatic duct, and releases it via the common bile duct into the duodenum, where the bile helps in the digestion of fats.

Ingestion

Intake of food

Villi

Millions of tiny finger-like structures called villi project inwards from the lining of the small intestine. The large surface area they present allows for rapid absorption of digestion products. Villi are specialised for absorption in the small intestine as they have a thin wall, one cell thick, which enables a shorter diffusion path. They have a large surface area so there will be more efficient absorption of fatty acids and glycerol into the blood stream.

large intestine

The large intestine, also known as the large bowel or colon, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation.

Pancreas

The pancreas is a glandular organ in the digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity behind the stomach. It is an endocrine gland producing several important hormones, including: -insulin -glucagon, -pancreatic polypeptide all of which circulate in the blood. The pancreas is also a digestive organ, secreting pancreatic juice containing bicarbonate to neutralize acidity of chyme moving in from the stomach, as well as digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme. The pancreas is known as a mixed gland.

Microvilli

They are tiny projections that exist on, in, and around cells. Microvilli occur in various structures and sites in an organism. One such place is the small intestine where the microvilli and villi work to increase the surface area of the intestine, allowing for more absorption of vital nutrients for the organism.

pancreatic juice

a fluid secreted into the duodenum by the pancreas The pancreas makes pancreatic juices and hormones. The pancreatic juices contain enzymes that help digest food in the small intestine. Bicarbonate neutralizes acids in food. Insulin is among the hormones made by the pancreas. It controls the amount of sugar in the blood. Both enzymes and hormones are needed to keep the body working right.

digestion mechanically in mouth

bite and chew mechanically moistens food and breaks it down into pieces small enough to swallow; increases surface area available for chemical processes.

Digestion chemical

catabolic breakdown of food in presence of enzyme; macromolecules broken down in to building blocks (i.e. fat to glycerol, fatty acids, protein to amino acids).

oral cavity

cavity that receives food for digestion, the mouth Function: A key role in the first step of digestion: ingestion. This is where the teeth and tongue work with salivary glands to break down food into small masses that can be swallowed, preparing them for the journey through the alimentary canal.

alimentary canal

digestive tube that extends from the mouth to the anus

Stomach

large muscular sac that continues the mechanical and chemical digestion of food The main function of the stomach is to break down and digest food in order to extract necessary nutrients from what you have eaten. Main Functions: -Store food temporarily -Break it down with acid and enzymes

Sphincter

muscular ring-like valves that regulate the passage of food into and out of the stomach.

pyloric sphincter

ring of muscle that guards the opening between the stomach and the duodenum Function: It plays an important role in digestion, where it acts as a valve to controls the flow of partially digested food (chyme) from the stomach to the small intestine.

Rectum

the final section of the large intestine, terminating at the anus. Function: The rectum acts as a temporary storage site for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate.

Cardio-esophageal sphincter

the lower esophageal sphincter (or cardiac sphincter), which resides at the top of the stomach, keeps stomach acids and other stomach contents from pushing up and into the esophagus.

How is active pepsin formed?

1) Pepsinogen and HCl are secreted into the lumen of the stomach. 2) HCl converts pepsinogen to pepsin. 3) Pepsin helps activate more pepsinogen, starting chain reaction. Example of positive feedback - end product of a process promotes the formation of more end product.

functions of large intestine

1. The large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. 2. The colon absorbs vitamins that are created by the colonic bacteria, such as vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation), vitamin B12, thiamine and riboflavin. 3. It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation. 4. The large intestine also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. 5. Reducing the acidity + protecting from infections. The mucosa of the large intestine also secretes bicarbonates to neutralise the increased acidity resulting from the formation of these fatty acids and other digestive components at earlier parts of the intestines. The mucosal layer of the large intestine also acts as a mucosal barrier and protects from microbial infections and invasions. 6. Producing antibodies The large intestine, particularly the appendix, is a confluence of several lymphoid tissues. These play an important role in immunity. The lymphoid tissues of the large intestine also help in the production of antibodies and cross reactive antibodies. These antibodies are produced by the immune system against the normal commensal bacteria but may also be active against related harmful bacteria and thus prevent infections.

Hepatic portal vein

A large vessel that transports blood to the liver, giving the liver first access to nutrients absorbed in the intestine. This blood is rich in nutrients that have been extracted from food, and the liver processes these nutrients; Functions: -processes blood received from small intestine -filters toxins that may have been ingested with the food -removes excess glucose from blood, converts it to glycogen, stored in liver cells; -regulates metabolism -converts nutrients received into new substances. 75% of total liver blood flow is through the portal vein. The blood leaves the liver to the heart in the hepatic veins. The portal vein is not a true vein, because it conducts blood to capillary beds in the liver and not directly to the heart.

Chyme

Acidic, nutrient broth (partly digested food) coming from the stomach to the upper portion of the small intestine. The semi-fluid mass of partly digested food that is expelled by the stomach, through the pyloric valve, into the duodenum (the beginning of the small intestine).

Bile

Bile, also called gall, greenish yellow secretion that is produced in the liver and passed to the gallbladder for concentration, storage, or transport into the first region of the small intestine, the duodenum. Its function is to aid in the digestion of fats/lipids in the duodenum/small interesting. In humans, bile is produced continuously by the liver (liver bile), and stored and concentrated in the gallbladder. Bile contains bile salts - act as emulsifiers that break up fats into small droplets, making them more susceptible to attach by digestive enzymes.

Gastrin

Hormone, circulates in the bloodstream, returning to the stomach. Function: Stimulates secretion of gastric juice. Gastric juice regulated by combination of nerve signals and hormones. When see/smell/taste food, signal from brain stimulates gastric glands. Once food in, trigger release of hormone gastrin.

Circular folds of the small intestine

The circular folds slow the passage of the food along the intestines, and afford an increased surface for absorption. They are covered with small fingerlike projections called villi (singular, villus). Each villus, in turn, is covered with microvilli. The microvilli absorb fats and nutrients from the chyme.

Esophagus

The esophagus is a tube that connects the throat (pharynx) and the stomach. It is about 8 inches (20 centimeters) long. Function: the conduit for food and liquids that have been swallowed into the pharynx to reach the stomach. The esophagus is made of muscles that contract to move food to the stomach. This process is called peristalsis. At the top of the esophagus is a band of muscle called the "upper esophageal sphincter", which are under voluntary control. Another band of muscle, the "lower esophageal sphincter" is at the bottom of the tube, slightly above the stomach, which are under involuntary control (muscles contract involuntarily). When a person swallows, these sphincters relax so food can pass into the stomach. When not in use, they contract so food and stomach acid do not flow back up the esophagus.

Liver

The liver, an accessory digestive gland, produces bile, an alkaline compound which helps breakdown fat. Bile aids in digestion via the emulsification of lipids. The gallbladder, a small pouch that sits just under the liver, stores bile produced by the liver. The liver's highly specialized tissue consisting of mostly hepatocytes regulates a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions. The liver is a metabolically active organ responsible for many vital life functions. The primary functions of the liver are: Functions: - Bile production and excretion -Excretion of bilirubin, cholesterol, hormones, and drugs -Metabolism of fats, proteins, and carbohydrates -Enzyme activation -Storage of glycogen, vitamins, and minerals -Synthesis of plasma proteins, such as albumin, and clotting factors -Blood detoxification and purification

Absorption

The process by which nutrient molecules pass through the wall of the digestive system into the blood; Products travel to bloodstream to cells.

Digestion - Accessory glands

The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs; secrete digestive chemicals.

Elimination

Undigested material passes out of the digestive tract.

duodenum

first portion of the small intestine Function: The duodenum is the first and shortest segment of the small intestine. It receives partially digested food (known as chyme) from the stomach and plays a vital role in the chemical digestion of chyme in preparation for absorption in the small intestine. Chyme squirted from the stomach, mixes with digestive juices from pancreas (pancreatic juice), liver (bile), gallbladder (bile), gland cells from intestinal wall. Hormones from duodenum stimulate release of bile + digestive juice in pancreas.

Pharynx

the membrane-lined cavity behind the nose and mouth, connecting them to the esophagus. It is commonly referred to as the throat. The pharynx is part of both the digestive and respiratory systems. Function: For the digestive system, its muscular walls function in the process of swallowing, and it serves as a pathway for the movement of food from the mouth to the esophagus.