Untitled Quiz

Lecture 12: Digestive Physiology

• BIO108 - General Physiology course
• Professor Thucydides L. Salunga, Ph.D.
• Lecture notes focus on digestive physiology in animals.

Digestive System Interactions

• Diagram illustrates the interconnectedness of the digestive, respiratory, circulatory, and urinary systems.
• Food and water intake fuels the digestive system.
• Oxygen intake fuels the respiratory system.
• Nutrients, water, oxygen, and salts from digestion move to the circulatory system for rapid transport.
• The circulatory system delivers these to all living cells in the body.
• Excess water, salts, and wastes are processed through the urinary system for elimination.

Digestion and Nutrition

• Nutrition encompasses all processes where an animal consumes, digests, absorbs, stores, and utilizes nutrients to meet metabolic needs.
• Digestion is the chemical and/or mechanical breakdown of food to be absorbed by individual cells.
• An animal's diet must provide chemical energy for cellular processes, building blocks for macromolecules, and essential nutrients for biochemical processes.

Processes Involved in Digestion

• Ingestion is the act of eating.
• Digestion is the process of breaking down food into absorbable components.
• Absorption is the uptake of nutrients by body cells.
• Elimination is the route of undigested material from the digestive system.

Dietary Categories

• Herbivores consume plant life for energy and building blocks.
• Carnivores feed on herbivores and other carnivores.
• Omnivores consume both plants and animals.
• Saprophagous animals feed on decaying organic matter.
• Insectivores/insectivorous animals feed primarily on arthropods.

Feeding Adaptations

• Suspension feeders sift small food particles from water, like filter feeding baleen whales.
• Substrate feeders live on or in their food source (like caterpillars).
• Fluid feeders extract nutrient-rich fluids from a living host.
• Bulk feeders consume relatively large food pieces.
• Deposit feeders consume partially decayed organic matter in soil or sediment.

Intracellular Digestion

• Protists and sponges utilize intracellular digestion, where cells take in and break down food using enzymes within the cell.
• Food particles are engulfed through phagocytosis.
• Food vacuoles fuse with lysosomes containing hydrolytic enzymes for breakdown.

Extracellular Digestion

• Enzymes break down food outside cells within a cavity or organ.
• Large animals possess specialized organs for digestion.
• Extracellular compartments are continuous with the external body environments.

Digestive Systems

• Incomplete digestive systems have one opening in the body surface for nutrient intake and waste elimination (e.g. planarians).

• Complete digestive systems have two openings (e.g. mouth and anus); includes mouth, pharynx, esophagus, stomach, small/large intestines, and anus.

• Mammalian digestive systems have accessory glands: salivary glands, liver, and gallbladder; crucial for digestion and nutrient absorption.

• One-way digestive systems carry food, from mouth, pharynx, esophagus, stomach, intestines, rectum, and anus.

Diversity in Digestive Structures

• Evolutionary and structural adaptations of digestive systems in various animals correlate with eating habits, metabolic rate, and body size.
• Specific tongues and bill structures adapt to various diets among diverse vertebrate animals.

Dietary Adaptations

• Diverse vertebrate animals exhibit varied tongue and bill structures for feeding mechanisms, such as projecting tongues for capturing insects (frogs, salamanders).
• Birds exhibit diverse bill shapes adapted to specific diets (e.g., nut cracking, mud sifting).
• Mammals adapt teeth structure to their diets (carnivores, herbivores, omnivores).

Feeding Adaptations in Snakes and Other Animals

• Distensible jaws in snakes and fishes allow consumption of larger prey or diverse foods.

Adaptations in Birds

• Crops and gizzards in birds assist in breaking down plant material; crops are storage organs and gizzards have high levels of muscularity.
• Multiple stomach chambers (rumen, reticulum, omasum, abomasum) are present in ruminant mammals (e.g., cows) for efficient digestion of plant material. Ruminants have multiple stomachs adapted to digesting cellulose through microbial fermentation.

The Human Digestive System

• Major organs include mouth, pharynx, esophagus, stomach, small intestine, and large intestine; these organs interact in a one-way digestive system
• Accessory organs- salivary glands, liver, and gallbladder- aid in the digestive process by secreting enzymes and other substances. These organs contribute to the breaking down of macromolecules into absorbable components

Three Categories of Digestive Systems

• Batch reactors - blind tubes or cavities where one batch of food is processed and eliminated before the next is processed.
• Continuous-flow stirred tank reactors - GI tract extends through the animal with continual digestion and processing of food.
• Continuous-flow plug-flow reactors - food progresses through a long tubular digestive system, with composition varying along its length.

High Quality Food and Low Quality Food

• High-quality food maximizes the energy extraction with a shorter time spent in the digestive system.
• Low-quality food requires a longer time for energy extraction.
• Transit time through the GI tract varies with anatomic design, body mass, and body temperature.

Digestive System of Invertebrates

• Digestive systems in invertebrates range from simple to complex; these systems have headguts, midguts, and hindguts.
• General trends for invertebrates are simple to highly complex in their digestive systems.

Digestive System of Vertebrates

• Head, mid, and hindgut parts make up the vertebrate body's digestive system.
• Digestive systems in vertebrates share similar basic elements; overall length of the digestive tract correlates with diet.

Processes of Digestion

• Food is partially broken down in the mouth (mechanical), forced into the pharynx and esophagus (peristalsis), and then to the stomach (acidity).
• The stomach produces chyme (food mixes with acids and enzymes); nutrients are absorbed in the small intestine, and water/ions are absorbed and waste is compacted/excreted in the large intestine.
• Processes of the human digestive system are regulated by different mechanisms, such as hormones and nerves; hormones affect the digestive processes; nerves help regulate digestive functions.

Accessory Digestive Organs

• Accessory organs include salivary glands, pancreas, liver, gallbladder.
• These organs secrete enzymes and substances essential in the digestion process, which break down food into smaller molecules for absorption.

Liver and Pancreas

• Lipids are digested in the small intestine via bile (liver produced, stored in the gallbladder).
• Bile acts as an emulsifier that breaks down lipids into smaller droplets. The enzymes break down fatty acids into fatty acids and monoglycerides for nutrient absorption.

Liver Function

• The liver detoxifies dangerous compounds (alcohol, ammonia), and stores fat-soluble vitamins and glucose.
• The liver supplies glucose to the brain during periods with low food intake and stored glycogen to provide energy.

Water and Electrolyte Balance

• Water and electrolyte absorption occurs mostly near the tips of the villi (small intestines).
• Excessive uptake is prevented by several hormones (e.g., gastrin, CCK, cholic acid).
• Fluid fluxes are necessary for several digestive processes.
• Several processes regulated by different factors influence fluid balance within the gut.

Regulation of Digestion

• Each step of the digestive system is initiated as needed in response to the need.
• The enteric nervous system and endocrine system regulate digestive processes through hormones and neurotransmitters.
• The digestive system processes are regulated by various factors. These factors include several mechanisms.

Regulation of Energy Storage

• Energy-rich molecules are stored for future use instead of being immediately used by the body.
• Glycogen is stored in the liver and muscles; excess energy is stored as fat.
• The body prioritizes the use of stored glycogen, followed by muscle glycogen, and finally fat when more calories are taken in than expended.

Glucose Homeostasis

• Glucose is a crucial fuel for respiration, contributing to the human body's energy needs and biosynthesis process.
• Insulin and glucagon regulate the breakdown of glycogen into glucose.
• The liver plays a critical role in glucose homeostasis.
• During digestion, carbohydrate-rich foods provoke elevated insulin levels for glycogen synthesis in the liver and various organs.
• Low blood sugar triggers glucagon release, promoting glycogen breakdown and glucose release for maintaining blood sugar levels.
• Several mechanisms help manage glucose levels in the human body.

Regulation of Appetite and Consumption

• Excessive food intake results in obesity, contributing to various health issues.
• Ghrelin, leptin, PYY, and insulin regulate appetite and consumption.
• Ghrelin stimulates hunger feelings, whereas leptin suppresses appetite. PYY and insulin aid in regulating appetite and overall consumption.

Nutritional Requirements

• A balanced diet meets three core requirements: fuel for bodily functions, organic raw materials for biosynthesis, and essential nutrients for fundamental processes.
• Essential nutrients cannot be produced by the body.

Essential Nutrients: Amino Acids

• Twenty amino acids are crucial for protein synthesis.
• Eight are essential for adults to be obtained from food.
• Additional amino acids are essential for children's development.
• Consuming a balanced mix of foods, including proteins from both plant and animal sources, addresses the requirement for essential amino acids; an optimal diet would come from a proper combination of various food sources, such as grains and beans.

Essential Nutrients: Essential Fatty Acids

• Unsaturated fatty acids found in various plant foods serve as structural components of fat molecules.

Essential Nutrients: Vitamins

• Thirteen essential vitamins are necessary for optimal body function.
• Water-soluble vitamins aid enzyme function.

Essential Nutrients: Minerals

• Calcium and phosphorus are critical for bone formation.
• Iron is a component of cytochromes and hemoglobin and is crucial for the transport of oxygen through the body.
• Sodium, potassium, and chloride control osmotic balance between cells and interstitial fluids, with excess salt intake posing harm.
• Various minerals are vital for optimum bodily function.

Hormones and Hunger

• Leptin and ghrelin are essential hormones in regulating fat storage and use. These hormones aid in maintaining a balanced distribution of fats in the body; hormonal mechanisms influence appetite and hunger.

MSG-Mouse Model for Non-Alcoholic Steatohepatitis (NAFLD)

• MSG-induced animal model is effective for evaluating NAFLD/NASH-related health impacts.
• MSG-treated mice exhibit characteristics of NAFLD. This model is used to evaluate the effectiveness of new treatments and to understand the disease's pathology.

NAFLD, NASH, and Related Diseases

• NAFLD, the most common chronic liver disease (25% prevalence rate), progresses to NASH and cirrhosis and is associated with metabolic syndrome.
• NAFLD and metabolic syndrome can be studied using an animal model (MSG-mouse) to evaluate treatment outcomes, understand disease severity and prevalence among various groups, and evaluate risk of neoplastic transformation through a histological analysis.

Clinical Impact of NAFLD

• NAFLD has a notable prevalence (14-21%) in Japan that progresses into NASH (15%) and cirrhosis/death (3% after 10-20 years).
• NAFLD shares significant lifestyle-related risk factors linked to obesity and type 2 diabetes.
• This condition influences the body's overall metabolism.

Two-Hit Theory in NAFLD

• The progression of NAFLD to NASH involves two sequential "hits" —initial fat accumulation and then chronic oxidative stress.
• Several organ-specific factors, including genetic mutations, deficiencies, and drug exposure, contribute to the disease development.
• The two-hit theory helps explain the progression of NAFLD to NASH.

Histopathological Characteristics of NAFLD/NASH

• Typical NASH features may include micro/macro-vesicular fatty change, neutrophil aggregation, ballooning and Mallory body formation, which are important in diagnosing and studying the disease mechanism.

Monosodium Glutamate (MSG)

• Glutamate is a natural component of foods.
• MSG is sodium salt of glutamic acid/glutamate.
• MSG is a common food additive that enhances flavors, and its average daily consumption is around 10 grams.
• MSG can cause various toxic effects in adults, such as symptoms complex.

Materials

• This section describes materials used to conduct animal studies; various materials include needles, boxes, and tools.

MSG-Mouse Model (6-month-old)

• Control and MSG groups are used for comparisons- MSG-group demonstrates obesity, diabetes mellitus and central obesity, particularly in the males of the group.

MSG-Mouse Model, Body Weight

• Body weight increases more in the MSG-mouse model compared to the control group, impacting the overall health of the group and leading to obesity at 29 weeks.

MSG-Mouse Model and Blood Chemistry

• Blood concentrations of various substances, like glucose, insulin, LDL, HDL, cholesterol, triglycerides, AST, and ALT, are measured and presented.
• Blood chemistry is significantly altered in the MSG-mouse model.
• Several measures are affected, like cholesterol, glucose, insulin and others.

MSG-Mouse Model and Organ Weights

• Organ weights are taken; weights in the MSG group were significantly higher than the controls.
• A notable variation in organ weights highlights the impact of the treatment.

Summary of MSG-Mouse Model

• The findings support the suitability of the model to establish a comprehensive link between MSG consumption and NAFLD/NASH development.
• The model shows higher BMI, glycosuria, and glucose tolerance decrease in male mice compared to the control group.

Histological Analysis of MSG-Mouse Liver

• The control and MSG groups are compared via histology using specific staining techniques to evaluate the impact and effects, such as liver nodule formation and inflammation.

Conclusions and Recommendations

• The collected data suggests the MSG-treated mouse is a well-suited model for various studies relating to NAFLD/NASH progression.
• Future investigations should involve further observations from multiple groups, examining the natural history progression in the animal model, analyzing the impact of hormones and other pathways in fatty liver development, and confirming possible links to HCC in various groups.