Digestive Systems and Nutrition

Digestive Systems and Nutrition

• Digestive systems involve organs, tissues, and cells for ingesting and breaking down food

Digestive System Function

• Digestive systems help maintain homeostasis by providing nutrients needed by cells
• Digestive systems ingest food, break it down into small molecules, absorb nutrients, and eliminate waste

Incomplete vs. Complete Digestive Tracts

• An incomplete digestive tract uses a single opening for both food intake and waste removal
• Planarians use the mouth and muscular pharynx for food entry
• Planarians have a gastrovascular cavity that branches throughout the body
• Planarians excrete wastes through their mouth and muscular pharynx
• Planarians also lack specialized parts
• A complete digestive tract incorporates two openings
• Earthworms ingest food through the mouth
• Earthworms excrete wastes through the anus
• Pharynx, crop, gizzard, and intestine allow for specialization of parts in earthworms

Continuous vs. Discontinuous Feeders

• Clams are continuous filter feeders with water moving into the mantle cavity via the incurrent siphon
• Particles are deposited on gills in clams
• Clams' incurrent siphon size limits the entry of only small particles
• Clams do not need a food storage area
• Squid are discontinuous feeders that move rapidly via jet propulsion
• Squid use tentacles to seize prey
• Squid leverage beak-like jaws to pull pieces into the mouth with the radula
• Squid require a food storage area in the stomach, with cecum, to retain food until their digestion is finished

Adaptations Based on Diet

• Dentition varies depending on an organisms mode of nutrition

Omnivores

• Omnivores have a variety of specializations to accommodate both vegetation and meat diets
• Clams and tube worms are invertebrate omnivores
• Humans, pigs, raccoons, and most bears are omnivores
• Omnivore dentition is adapted for both vegetable and meat diets
• Omnivores also have a better ability to adapt to different food sources

Herbivores

• Herbivores only eat plants
• Herbivores use incisors for clipping
• Herbivores use premolars and molars for grinding
• Land snails and some insects are herbivores
• Koalas are herbivores who eat eucalyptus leaves
• Grazers, like horses, feed on grasses
• Ruminants such as cattle, goats, and sheep, leverage a four-chambered stomach to regurgitate solid material for complete digestion

Carnivores

• Carnivores only eat other animals
• Carnivores have pointed incisors and enlarged canines
• Carnivores also shear off small pieces to swallow
• Spiders and sea stars are carnivores
• Dogs, lions, and dolphins are carnivores
• Lions' pointed canine teeth and sharp incisors assist with killing and scraping

The Human Digestive System

• The human digestive tract is complete, part of a tube-within-a-tube body plan
• The human digestive tract begins with a mouth and ends with an anus
• Digestion in the human digestive system is entirely extracellular, and both mechanical and chemical
• Digestive enzymes are secreted by the wall of the digestive tract and nearby accessory glands

Digestive Organs

Mouth

• Digestion begins in the mouth
• The palate, or roof of mouth, separates oral cavity from nasal cavity
• Uvula is the posterior extension of the soft palate
• There exists three major pairs of salivary glands
• Saliva contains salivary amylase
• Salivary amylase initiates starch digestion
• Starch is present in foods of plant origin
• The tongue is composed of striated muscle and an outer layer of mucous membrane
• The tongue mixes chewed food with saliva and forming a bolus

Pharynx

• Digestive and respiratory passages come together and then separate
• The soft palate closes off nasopharynx during swallowing
• The epiglottis covers the glottis, opening into the trachea, keep food from air passages.

Esophagus

• The esophagus takes food to the stomach
• Sphincters encircle tubes and act as valves

Digestive Tract Wall

• The wall of the digestive tract is composed of the mucosa, submucosa, muscularis, and serosa layers
• Contraction of the muscularis's two smooth muscle layers causes movement of gut contents from esophagus to rectum by peristalsis
• The serosa is part of the internal lining of the abdominal cavity, the peritoneum.

Stomach

• The stomach wall has deep folds, rugae, that disappear as content fills to 1 liter approx
• The epithelial lining contains millions of gastric pits, which lead to gastric glands
• There are three characteristics of the stomach to note

Gastric Glands

• Gastric glands secrete hydrochloric acid (HCl) and pepsin
• Stomach pH is about 2.0, which can kill bacteria in food
• Pepsin is a hydrolytic enzyme that acts on protein to produce peptides.

Stomach Walls

• A layer of mucus protects the stomach wall from enzymatic action
• Gastric acid leaks upward, irritating mucosal linings

Mixing

• Gastrointestinal Reflux Disease (GERD) can cause heartburn
• Food mixing with gastric juices becomes chyme
• The junction between the stomach and small intestine is controlled by a sphincter
• Chyme entering the stomach starts a reflex, causing the sphincter at the stomach base's to contract and close the opening
• The sphincter relaxes, passing small quantities of chyme into the small intestine

Small Intestine

• The first segment of the small intestine is the duodenum
• Chyme from the stomach enters the duodenum and mixes with secretions from the liver and pancreas.

Liver

• Produces bile, which is stored in the gallbladder
• Bile contains bile salts breaking up fat into fat droplets via emulsification

Pancreas

• Releases enzymes and digestive enzymes to the duodenum
• Epithelial cells of the small intestine produce digestive enzymes, completing process of food digestion.
• Villi, ridges on the small intestine surface, contain microvilli
• Villi greatly increase surface area for absorption
• Each villus contains brush-border enzymes, blood capillaries and a lacteal, lymphatic capillary.

Large Intestine

• The large intestine includes the cecum, colon, rectum, and anus
• The large intestine is wider in diameter, shorter in length than the small intestine
• The cecum has a small projection, the appendix
• The large intestine absorbs water, salts, and some vitamins
• It also stores undigestible material until it is eliminated as feces
• A large population of bacteria exists that break down indigestible contents, produce vitamins and also helps prevent dehydration
• Colon joins the rectum, the last 20cm of the large intestine
• Digestive wastes, or feces, leave the body through the anus

Pancreas

• Lies deep in the abdominal cavity
• The pancreas has two functions
• The endocrine function secretes insulin and glucagon to regulate blood glucose levels
• The exocrine function secretes pancreatic juice and digestive enzymes
• Sodium bicarbonate in pancreatic juice neutralizes acid chyme from the stomach
• Pancreatic amylase digests starch
• Trypsin digests protein
• Lipase digest fats

Liver

• The liver is the largest gland in the body
• The liver is located in the upper region, under the diaphragm
• Lobules are its structural and functional units
• The triads consist of a bile duct, a branch of the hepatic artery, and a branch of the hepatic portal vein
• Blood moves from the intestines to the liver via the hepatic portal vein
• Blood moves from the liver to the inferior vena cava via the hepatic veins
• The liver aids in detoxification of the blood
• The liver also helps in storage of iron and some vitamins, production of plasma proteins, and regulation of blood glucose levels/ storage of glucose as glycogen
• The liver also produces urea, stores/removes iron and vitamins, synthesizes bile,
• The liver removes bilirubin and controls regulation of blood cholesterol levels.
• Types of Liver Conditions Include:
• Jaundice occurs due to excess bilirubin, causing yellowing of skin
• Hepatitis is the inflammation of liver, most commonly caused by a virus
• Cirrhosis is the scarring of liver tissue, leading to reduced function
• The liver can regenerate in some cases

Gall Bladder

• The gallbladder is a pear-shaped, muscular sac attached to the liver
• About 1,000 ml of bile is produced by the liver each day; excess is stored in the gallbladder
• Gallstones form when the cholesterol in bile crystallizes

Digestive Enzymes

• Starch + H2O, using salivary amylase, becomes maltose
• Protein + H2O, using pepsin, becomes peptides
• Starch + H2O, using pancreatic amylase, becomes maltose
• Protein + H2O, using trypsin, becomes peptides
• Maltose + H2O, using maltase, becomes glucose + glucose
• Peptides + H2O, using peptidases, becomes amino acids
• Fat utilizes bile salts and become fat droplets
• Fat droplets + H2O, using lipase, becomes glucose + 3 fatty acids

Nutrition and Human Health

• Carbohydrates are present in food in the form of sugars, starch, and fiber
• Fruits, vegetables, milk, and honey are natural sources of sugars.
• Monosaccharides:
  • Glucose.
  • Fructose.
• Disaccharides:
  • Lactose (milk sugar).
  • Sucrose (table sugar).
• After being absorbed from the digestive tract, all sugars are converted to glucose
• Glucose helps in production of ATP by cellular respiration
• Plants store glucose as starch
• Animals store glucose as glycogen
• Starch gets digested to glucose in the digestive tract and excess glucose stores as glycogen.

Fiber

• Includes various undigestible carbohydrates derived from plants
• Food sources rich in fiber include beans, peas, nuts, fruits, and vegetables.
• Fiber is not a nutrient for humans, they cannot be digested
  • Soluble fiber combines with bile acids and cholesterol in the small intestine and prevents absorption.
  • Insoluble fiber adds bulk to feces, which stimulates movement in the large intestine, preventing constipation.
• Fat, oils, and cholesterol are lipids
• Saturated fatty acids (solids at room temperature) usually come from Animals
• Butter, meat, whole milk, and cheeses contain saturated fats
• Unsaturated fatty acids are found in plant oils
• Cholesterol is synthesized by the body and is found in animal foods
• Adequate protein formation requires 20 different types of amino acids
• Eight essential amino acids are required in the diet
• There is variability in vegetable and animal protein sources
• Some foods, such as meat, milk, and eggs, provide all nine complete proteins
• Vegetables supply some essential amino acids, but are usually deficient in at least one, making them incomplete
• Vegetarians and vegans should eat grains, beans, and nuts to meet protein needs

Vitamins

• Vitamins are organic compounds the body is unable to produce, but are required for metabolic purposes
• The absence of a vitamin is associated with a particular disorder.

Minerals

• The body needs about 20 elements for various physiological functions
• Certain individuals do not consume enough iron, calcium, magnesium, or zinc
• High sodium consumption contributes to hypertension

Dietary Guidelines and Disease

• Consuming excess calories contributes to body fat.
• This increases risks of obesity and linked illnesses.
• BMI is a metric of body fat
  • A person’s body mass index, BMI, can be found by dividing weight in kilograms, by height, in meters squared
• Roughly 30% of North Americans are obese

Type 2 Diabetes

• Diabetes mellitus is a term referring to the condition where the hormone insulin isn’t working properly
• Diabetes may occur because of insulin deficiency, or resistance.
  • Excess blood glucose winds up in the urine from it
  • High blood glucose can cause tissue damage, or even death
• Type 1 diabetes can be managed by insulin injections but type 2 is more resistant

Cardiovascular Disease

• Cardiovascular disease is the leading cause of death in the US
• Arteries become blocked with plaque which contains saturated fats and cholesterol
• Low-density lipoproteins, LDL, and high-density lipoproteins, HDL, carry cholesterol in the blood -HDL is 'good'cholesterol, and LDL is considered 'bad'cholesterol
  • American Heart Associations say a positive dietary pattern for circulatory health includes consumption of herring, sardines, tuna, and salmon (which contain polyunsaturated fats.) These nutritional sources can reduce the risk of cardiovascular disease