Nutr1010 Chap3 PDF

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3.1 DIGESTION, ABSORPTION, AND METABOLISM -

Remember carbohydrates, vitamins, proteins, and lipids are organic molecules. Because, they contain C-H bonds) Endocrine system → Hormones Integumentary system → Skin and body linings

3.2 OVERVIEW OF DIGESTIVE SYSTEM -

The two major functions of the digestive system are digestion and absorption.

Gastrointestinal tract - AKA GI tract or Alimentary canal. Mouth → Pharynx → Esophagus → Stomach → Small Intestine → Large intestine → Rectum → Anus

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Food is considered to be INSIDE the body once it has been broken down and absorbed (as opposed to being within the lumen of the GI tract). Transit time is the amount of time taken for food to pass the length of the GI tract. GI’s four layers of tissue: 1. Mucosa - It’s a protective layer and absorbs end products of digestion. - Mucosal cells reproduce rapidly and have high nutrient requirements because they have a short life span. → this means they are instantly affected by nutrient deficiencies. 2. Connective tissue - Surround the mucosal layer - Contains nerves and blood vessels - Provide support - Deliver nutrients to mucosa - Provide nerve signals to control secretions and muscle contraction. 3. Smooth muscles layer - No voluntary control of these muscles. - Contractions mix food, break into smaller particles, and propel it through the digestive tract. 4. Connective tissue - Provides support and protection.

Digestive secretions - Mucus and enzymes aid digestion - Mucus is a viscous fluid secreted by glands in the GI and other parts of the body which lubricates, moistens, and protects cells from harsh environments.

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Enzymes speed up reactions and accelerate breakdown of nutrients, without changing themselves.

How is the gastrointestinal function regulated? → Nerves - Nerve signals can cause muscle contractions that churn, mix, and propel food through the gut for optimal nutrient absorption. - Nerve signals can inhibit or stimulate digestive secretions. - E.g. After the food passes through a section of the digestive tract, digestive secretions decrease, and muscular activity slows to conserve energy and resources for other body processes. - Nerves in the GI tract can communicate with the brain so digestive activity can be coordinated with other body needs. → Hormones - Definition: they are chemical messengers produced in one location, released into the bloodstream, and elicit responses at other sites in the body. - They are produced by cells lining the digestive tract and by several accessory organs. - Help prepare different parts of the body for the arrival of food. - Regulate the digestion of nutrients. - Regulate the rate at which food travels through the system.

The Gastrointestinal tract and barrier function What is the barrier function? It is the gastrointestinal cell’s protective role in limiting the absorption of harmful and diseasecausing substances. → Achieved in two ways: 1. The mucosal layer 2. Immune cells below the mucosal layer can detect antigens in any harmful substances or organisms that may breach the layer.

Immune cells - The first immune cells to respond are called phagocytes. They target the invader, engulf it, and break it up to present the antigens at the surface of the phagocyte. - The antigens are detected by lymphocytes, which react by producing and secreting antibodies. - Antibodies bind to invading antigens and help to destroy them. - Each antibody is designed to fight off only one specific antigen. - Once the body makes antibodies for a specific antigen, it remembers and can produce the antibodies rapidly to fight off the antigen anytime it enters the body. - Other types of lymphocytes bind directly to infected cells and destroy them (e.g. eliminate any cells that been infected by a virus) Define antigen A foreign substance (almost always proteins) that, when introduced into the body, stimulates an immune response. Define antibodies They are proteins produced by cells of the immune system. They destroy or inactivate foreign substances in the body. - Immune systems are also responsible for allergic reactions. This occurs when the immune system produces antibodies for allergens. - An allergen is a substance that causes an allergic reaction. It can be found in food or the environment.

3.3 DIGESTION AND ABSORPTION -

The sight, sound, and smell of food cause the nervous system to signal the digestive system to prepare for the meal. This happens when the mouth moistens, and the stomach begins to secrete digestive substances.

Mouth - Mechanical breakdown and chemical digestion occur in the mouth. - Food in the mouth stimulates the flow of saliva from the salivary glands. → Saliva is a watery fluid produced and secreted into the mouth by salivary glands. It contains lubricants, enzymes, and other substances. - Saliva’s functions are: 1. Moisten the food to make swallowing easier 2. Contains salivary amylase, this begins the enzymatic/chemical digestion of starch. 3. Cleanses the mouth and protects from tooth decay due to lysozymes 4. Lubricates the upper GI tract

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Salivary amylase breaks down the long glucose chains of starch into shorter ones. Lysozyme is an enzyme in saliva, tears, and sweat that can destroy certain types of bacteria. Chewing breaks down food mechanically to increase surface area in contact with salivary amylase, and it breaks apart fibres to maximize nutrient absorption.

Pharynx - The pharynx is a funnel-shaped opening that connects the nasal passages and mouth to the respiratory passages and esophagus. It is the passageway for food and air. It is also responsible for swallowing. - The tongue initiates swallowing by moving the bolus (chewed food) back toward the pharynx. - During swallowing, the air passages are blocked by the epiglottis → the epiglottis is a piece of elastic connective tissue at the back of the throat that covers the passageway’s opening to the lungs during swallowing. - The Heimlich manoeuvre is used when the bolus passes into the upper air passageway and blocks it. The sudden pressure to the upper abdomen can blow an object out of the blocked airway. Esophagus - The esophagus is a portion of the GI tract that extends from the pharynx to the stomach. - In the esophagus, the bolus of food is moved along by rhythmic contractions of the smooth muscles called peristalsis. - It only takes about 4-8 seconds for solid food to move from the pharynx down the esophagus to the stomach. Less time is required for liquids. - This process happens throughout the GI tract (from the pharynx to the large intestine). - The food passes through the gastroesophageal sphincter to move from the esophagus and into the stomach. → A sphincter is a muscular valve that helps control the flow of materials in the GI tract. - The valve prevents food from moving back out of the stomach (like heartburn and vomiting).

Stomach - Chyme is a mixture of digested food and stomach secretions. - Some digestion takes place in the stomach except for some water, alcohol, and a few drugs. - Structure: → Have thicker walls and stronger muscles than other segments of the GI tract. → Three layers of muscle: longitudinal, circular, and diagonal. This churns the food efficiently. → The surface of the stomach mucosa is covered with cells that produce large amounts of protective mucus. → This surface has millions of tiny openings called gastric pits. → Gastric pits lead to gastric glands, which secrete substances. → Gastric glands also contain cells that secrete hormones and hormone-like compounds into the body. -

Composition of gastric juice → Consists of a number of substances, including hydrochloric acid. → HCl is produced by parietal cells. → Parietal cells are cells in the stomach that produce HCl and intrinsic factor in response to nervous or hormonal stimulation. → Intrinsic factor is needed for the absorption of vitamin b12 → HCl kills most bacteria present in food by acidifying it. → Gastric juice also contains pepsinogen, produced by chief cells. → Pepsinogen is an inactive form of the protein-digesting enzyme pepsin, which is activated to pepsin by the acid in the stomach. → Pepsin breaks down proteins into polypeptide chains. It is secreted in its inactive form. → Stomach glands also produce enzyme rennin, which curdles milk protein casein.

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Pepsin functions best in the acidic environment of the stomach. This environment inhibits salivary amylase, ceasing digestion of starch in the stomach, although it starts again in the small intestine. But, most digestion of protein occurs in the stomach. If the protective mucus layer is penetrated, pepsin and acid can cause a peptic ulcer. → A peptic ulcer is an open sore in the lining of the stomach, esophagus, or small intestine. Bacteria Helicobactor pylori is acid-resistant and is a leading cause of a peptic ulcer.

Regulation by nerves and hormones - These signals regulate how much your stomach churns and how much gastric juice is released. - These signals come from the brain, stomach, and small intestine. - When food enters the stomach, the local nerves stretch and sends signals to the brain, promoting the secretion of the hormone gastrin. - Gastrin then triggers the release of gastric juice and increases motility. - Basically, gastrin is a hormone secreted by the stomach mucosa that stimulates the secretion of gastric juice. -

When chyme moves out of the stomach and passes through the pyloric sphincter to enter the small intestine, hormonal and nervous signals are triggered. This happens to decrease stomach motility and secretions and slow stomach emptying. This is useful to ensure the amount of chyme entering the small intestine doesn’t exceed the ability of processing. Emotional factors affect this process. → Sadness and fear slow down emptying → Aggression increases gastric motility and speed emptying.

The rate of stomach emptying - How quickly a meal leaves the stomach will depend on the size, nutritional composition, and composition. → What do you mean by nutritional composition? - A high-fat meal will stay in the stomach the longest because fat entering the small intestine causes the release of hormones that slow GI motility, thus slowing stomach emptying. - A high-protein meal will leave more quickly. - A high-carbohydrate meal will leave the quickest. Small intestine - Main site of digestion of food and absorption of nutrients. - A narrow tube, 6m in length and divided into three segments. - The duodenum, jejunum, and the ileum. - Features that increase the area of absorptive surface → The length → Intestinal walls are arranged in large circular folds. → The entire inner surface is covered in villi → Villi are covered in microvilli (aka the brush border) These offer a large surface area for maximum nutrient absorption.

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Each villus contains a blood vessel and a lymph vessel/lacteal. Nutrients must cross the mucosal cell layer to reach the bloodstream or lymphatic system for delivery to the body’s tissues.

Villi (single is villus) are finger-like protrusions of the small intestine lining that participate in the digestion and absorption of nutrients. Microvilli / Brush border tiny, brush-like projections on the mucosal cell membrane. Lacteal is a tubular component of the lymphatic system that carries fluid away from the body tissues. They can transport large particles like the products of fat digestion.

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Chyme is propelled through the small intestine by peristalsis. The chyme is mixed with digestive secretions with the help of rhythmic local constrictions, this speeds absorption by repeatedly moving the food mass over the intestinal wall; this process is called segmentation.

Enzymes and secretions in the small intestine - Cells of the small intestine produce some digestive enzymes that aids in absorption. - Pancreas secretes pancreatic juice which contains both bicarbonate ions and digestive enzymes into the small intestine during digestion. - Bicarbonate ions neutralise the acidic chyme to better suit the small intestine’s neutral environment. - This also allows enzymes from the pancreas and small intestine to function well. -

Pancreatic enzymes include: → Amylase continues breaking down the starch into sugars.

→ Trypsin and chymotrypsin are protein-digesting enzymes. → Lipases are fat-digesting enzymes. -

Small intestine enzymes involved in: → Digesting sugars into single sugar units. → Digesting small polypeptides into amino acids.

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The gallbladder secretes bile which is necessary for fat digestion and absorption. → Bile is produced by the liver and stored in the gallbladder. When bile is secreted into the small intestine it mixes with fats and emulsifies it, or breaks it into smaller droplets, this allows lipases to access fats more efficiently. The tiny droplets of digested fats move up against the mucosal lining, facilitating fat absorption.

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Hormonal control of secretions - Two hormones secreted by the mucosal lining of the duodenum control the release of bile and pancreatic juice into the small intestine. - The hormones are secretin and cholecystokinin (CCK) - Secretin signals the pancreas to secrete pancreatic juice rich in bicarbonate ions and the liver to secrete bile into the gallbladder. - CCK signals the pancreas to secrete digestive enzymes and causes the gallbladder to contract and release bile into the duodenum. How are nutrients absorbed? - The small intestine is the primary site of absorption of water, vitamins, minerals, and the products of carbohydrate, fat, and protein digestion. - They are absorbed when they pass from the GI’s lumen into the mucosal lining, and then into the blood or lymph. -

Simple diffusion occurs. → This process describes the movement of substances from an area of higher concentration to an area of lower concentration (they move down the concentration gradients), no input of energy is required. → The nutrients pass from the lumen of the GI tract across the cell membrane into the mucosal cell, down the concentration gradient. → Vitamin E and fatty acids are absorbed by this process.

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Osmosis → A net movement of water across a semipermeable membrane in a direction that will balance the concentration of dissolved substances on both sides. No energy is required.

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Facilitated diffusion

→ The movement of substances from an area of higher concentration to an area of lower concentration with the aid of carrier molecules, down the concentration gradient. No energy is required. → Sugar fructose is absorbed by this process. -

Active transport → Substances unable to be absorbed by diffusion enter the body by active transport. → The transport of substances across the cell membrane with the aid of a carrier molecule and input of energy. → The substances travel against the concentration gradient. From an area of lower concentration to an area of higher concentration. → Allows nutrients like amino acids and glucose to be absorbed even when they are present in higher concentrations inside the mucosal cells.

Large intestine - The colon is the largest portion of the large intestine. - The rectum is the portion of the large intestine that connects the colon and the anus. - Components of chyme that haven’t been absorbed in the small intestine pass through the ileocecal valve to the large intestine. - Water, vitamins, and minerals are absorbed in the large intestine. - Peristalsis here is slower than in the small intestine, so water, nutrients, and fecal matter may stay in the colon for 24hours. - This slow movement allows the growth of bacteria, referred to as microflora. - The intestinal microflora are microorganisms that are permanently present in the

large intestine. They act on unabsorbed portions of food (like fibre) and produce nutrients that the bacteria can use themselves. - Materials not absorbed in the colon are excreted as waste products in the feces. → Feces is a mixture of undigested, unabsorbed matter, dead cells, secretions from the GI tract. → They have high water content when adequate fibre or fluid is consumed. Feces are stored in the rectum prior to defecation. The rectum is connected to the anus, where feces are expelled. Defecation is regulated by the anal sphincter which can be controlled voluntarily. -

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Probiotics are beneficial live bacteria. Prebiotics are indigestible carbohydrates that pass into the colon and serve as a food supply for bacteria.

3.4 DIGESTION AND HEALTH Common digestive problems

Heartburn - Is one of the most common digestive complaints. - It occurs when the acidic stomach contents leak through the gastroesophageal sphincter and into the esophagus, causing a burning sensation in the chest. - This is also known as gastroesophageal reflux. - If this occurs often it may indicate a chronic condition called gastroesophageal reflux disease (GERD). - When left untreated it can lead to more serious health problems like bleeding, ulcers, and cancer. - Avoiding alcohol, coffee, citrus fruits, fried foods, garlic, onions, spicy foods, fatty and fried foods, and chocolate can reduce symptoms. - Remaining upright after eating helps as well.

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Can be caused by a hiatal hernia

→ The opening in the diaphragm that the esophagus passes through is called the hiatus. → The hernia occurs when the upper part of the stomach bulges through this opening into the chest cavity. → More common in people over the age of 50, women, smokers, overweight. → Most don’t have symptoms. But, larger ones show symptoms like heartburn and chest pain. → Can be treated by heartburn medication; in severe cases, surgery may be needed. Ulcers -

Can arise in both the esophagus and stomach. Occur when the mucosa is eroded away. This exposes the underlying tissues to the gastric juices. If the damage reaches the nerve layer, it causes pain. If the damage reaches the capillaries, this causes gastrointestinal bleeding. Ulcers can result from GERD or the chronic use of drugs (aspirin, ibuprofen) which erode the mucosa. - Can also be caused by bacterial infections in the stomach (Helicobacter pylori) - Mild ulcers can affect nutrient intake by limiting food choices. - Severe ulcers which cause bleeding can be life-threatening. Pancreatic and Gallbladder Problems - If the pancreas is not releasing enzymes, this will reduce the digestion of carbohydrates, fat, and protein. Thus, limiting essential nutrient absorption. -

If the gallbladder is not releasing bile, it can impair fat absorption. Gallstones is a condition that affects the gallbladder. → they are clumps of solid material that form in the gallbladder or bile duct. → Interfere with bile secretion and reduce fat absorption. → Treated by removing them → Bile simply drips into the small intestine, rather than being stored and squeezed out in large amounts.

Diarrhea and constipation - Diarrhea is characterised by frequent watery stools - This happens when material moves through the colon too quickly for sufficient water to be absorbed. - This also happens when water is drawn into the lumen from cells lining the intestinal tract. - This can be a result of the small intestine wall’s lining being inflamed. -

Constipation refers to hard, dry stools that are difficult to pass. Caused by insufficient fluid or fibre, lack of exercise, weakening of the muscles of the large intestine.

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Irritable bowel syndrome can cause either diarrhea or constipation. Its cause is unknown. Symptoms include abdominal pain, cramping, bloating, gas, diarrhea, and constipation.

NOT REQUIRED Alternative feeding methods Enteral/ tube-feeding → A method of feeding by providing a liquid diet directly to the stomach or intestine through a tube placed down the throat or...