Digestive System: Mouth to Microbiome — Comprehensive Notes

Mouth and Oral Digestion

• The mouth is the first place digestion starts; saliva contains amylase that begins breaking down carbohydrates from the very beginning.
• Chewing and swallowing challenges can increase with aging (e.g., dentures fitting, difficulty chewing food).
• Bolus formation: mechanical breakdown plus saliva forms a bolus to help swallow.
• Peristalsis: the contracting and squeezing motion that moves content through the GI tract; occurs in the esophagus, stomach, and intestines.
• Swallowing moves contents down the esophagus; the feeling of something going down your throat is the bolus traveling toward the stomach.
• The esophagus is connected to the trachea; a flap (epiglottis) closes the trachea to prevent food from entering the airway, then opens for breathing.
• If airway protection fails, choking can occur; this is a common risk when swallowing is impaired.
• Neurological issues (e.g., stroke) can cause dysphagia (difficulty swallowing), and cancer can also affect swallowing.
• Puree diets or baby-food textures are used for people with dysphagia or swallowing difficulties in clinical settings (e.g., oncology clinics).
• The mouth and oral cavity lead into the esophagus, which leads to the stomach; digestion begins in the mouth and continues through the GI tract.

Gastric Phase and Enzymatic Digestion

• The stomach mixes and churns contents via peristaltic contractions and forms chyme (partially digested food).
• Digestive juices in the stomach include proteases and other enzymes (the speaker mentions protease and lipase and lactase; end with -ase are enzymes, and their job is to break down macronutrients).
• Enzymes floating in saliva continue to act on food, contributing to digestion before the stomach.
• Carbohydrates: amylase begins carbohydrate digestion in the mouth; in the stomach, chemical digestion continues to break down components in preparation for absorption.
• Bolus becomes chyme as it mixes with gastric juices; chyme passes from the stomach into the small intestine for further digestion and absorption.
• The term for the digestive fluids and their mixture in the stomach is sometimes referred to as chyme (note the speaker said “clime”; the standard term is chyme).
• Digestive secretions include acids and enzymes; the liver, gallbladder, and pancreas contribute bile and pancreatic enzymes later in digestion.

Pancreas, Liver, and Gallbladder Roles

• Pancreatic juices provide enzymes to break down carbohydrates, fats, and proteins in the small intestine.
• The three main pancreatic enzymes referenced for macronutrient digestion are amylase (carbohydrates), protease (protein), and lipase (fat).
• The liver produces bile, which is essential for fat digestion; bile is stored in the gallbladder and released as needed.
• Gallbladder issues (e.g., gallstones) can be impacted by diet and activity; a very low-fat diet can lessen symptoms during gallbladder attacks.
• If symptoms like heartburn or reflux are present, it’s important to seek medical evaluation; early symptoms can indicate underlying issues.
• The pancreas and biliary system play a critical role in preparing nutrients for absorption in the small intestine.
• A cautionary note: some statements in the session link pancreatic juice absence to severe GI issues; in clinical practice, such a direct link to esophageal cancer is not typical. Always rely on medical diagnosis.

Small Intestine: Absorption and Structure

• The small intestine is the primary site of nutrient absorption.
• The inner lining has villi and microvilli (finger-like projections) that dramatically increase surface area for absorption.
• Microvilli increase absorptive surface area; the surface is lined by enterocytes that transport nutrients into the bloodstream or lymphatics.
• Underneath the microvilli are transport systems; nutrients enter either:
  • blood capillaries (for water-soluble nutrients) and the hepatic portal vein to the liver,
  • or lymphatic vessels (lacteals) for fats transported as chylomicrons.
• The jejunum (the middle portion of the small intestine) is described in the session as being the lower portion of the small intestine in their diagram.
• Enzymes and secretions from the pancreas and liver/biliary system act in the small intestine to continue digestion.
• Absorption in the small intestine includes carbohydrates, proteins, fats, vitamins, minerals, and water from chyme.
• Fat absorption requires fat to aid the absorption of fat-soluble vitamins (e.g., vitamins A and D) and other lipids; fat acts as the transport medium for these nutrients.
• Carbohydrates digested into glucose: ext{Carbohydrate}
  ightarrow ext{glucose} via ext{amylase} activity; glucose is absorbed into the bloodstream.
• Fat digestion and absorption: ext{Triglyceride}
  ightarrow ext{monoglyceride} + 2~ ext{fatty acids} via ext{lipase}; fatty acids and monoglycerides form mixed micelles, are absorbed by enterocytes, packaged into chylomicrons, and transported via the lymphatic system.
• Protein digestion: ext{Protein}
  ightarrow ext{peptides}
  ightarrow ext{amino acids} via ext{protease} activity.
• The small intestine also relies on hormonal and neural signals to regulate digestion and transit.
• The feeling of peristalsis and gut motility continues to move chyme through the small intestine toward absorption and into the large intestine.

Large Intestine: Fiber, Stool, and Water Absorption

• The large intestine functions as a highway and storage reservoir for waste, and is where water reabsorption occurs.
• Fiber adds bulk to stool and helps move contents through; insufficient fiber can lead to diarrhea or stool that may be poorly formed and can contribute to inflammation if contents stagnate.
• Stool consistency discussion:
  • Diarrhea: watery stool with little formed shape.
  • Normal stool: softer-serve ice cream-like consistency or slightly more formed.
• There are pockets in the large intestine where material can become stuck; fiber helps prevent this by promoting bulk and transit.

The Microbiome: Gut Flora and Health

• The gut microbiome consists of the gut bacteria that keep the system functioning; it includes both beneficial and potentially harmful bacteria.
• The microbiome supports the immune system and contributes to overall health; it can influence inflammation in the body.
• Some discussions suggest the microbiome may influence body weight; evidence comes from experiments including transferring gut microbiota between individuals with different body types (and animal studies).
• Fermented and pickled foods (e.g., sauerkraut, pickled carrots) introduce probiotics and can influence gut bacteria composition.
• Social-media discussions around the microbiome emphasize critical thinking and encourage further independent research; the instructor frames this as an opportunity to learn beyond class.
• The microbiome is a system-wide component affecting digestion, immunity, and metabolic processes; it interacts with dietary inputs and other organ systems.

Clinical Context and Dietary Implications

• Dysphagia and swallowing disorders may require texture-modified diets (e.g., puree) to reduce choking risk and assist with safe feeding in oncology and other clinical settings.
• Reflux and heartburn are common complaints; persistent symptoms should be evaluated to rule out underlying pathology.
• Fat in the diet is essential for absorbing fat-soluble vitamins and overall energy; extremely low-fat diets can impact brain function and nutrient absorption over time; dietary balance is important.
• The digestive system is interconnected with nervous and endocrine signals; changes in one part can influence digestion and absorption elsewhere in the tract.

Digestive Transit Timelines

• Time from mouth to stomach: t_{ ext{mouth→stomach}} < ext{minutes}. (Note: the transcript states this generally; exact times vary among individuals.)
• Time in the stomach (gastric dwell time): t_{ ext{stomach}} \,\in\, [1,2]\text{ hours}.
• Overall transit time varies widely between individuals and depends on factors like diet, motility, health status, and age.

Key Terms and Concepts

• Bolus: the ball of chewed food mixed with saliva ready to swallow.

• Bolus → chyme: after mixing with gastric juices; chyme is the semi-liquid substance that leaves the stomach.

• Peristalsis: rhythmic, wave-like muscle contractions that propel contents through the GI tract.

• Amylase: enzyme that digests carbohydrates (carbohydrates → simple sugars such as glucose).

• Protease: enzyme that digests proteins (proteins → peptides → amino acids).

• Lipase: enzyme that digests fats (triglycerides → monoglycerides + fatty acids).

• Lactase: enzyme involved in lactose digestion (present in the small intestine).

• Villi and microvilli: finger-like projections that increase the absorptive surface area of the small intestine.

• Enterocytes: absorptive cells lining the villi with microvilli on their surface.

• Lacteals: lymphatic vessels in the villi that transport absorbed fats into the lymphatic system.

• Hepatic portal vein: vascular connection transporting water-soluble nutrients from the intestine to the liver.

• Chyle: lipid-rich lymph formed after fat absorption that travels via lymphatics.

• Chyme: semi-fluid mass of partly digested food that moves from stomach to small intestine.

• Jejunum: the middle portion of the small intestine (here described as the lower half of the small intestine in the lecture visuals).

• Bile: digestive fluid produced by the liver and stored in the gallbladder; emulsifies fats to aid digestion.

• Dysphagia: difficulty swallowing, often associated with neurological disorders or structural issues.

• Reflux/Heartburn: backflow of stomach contents into the esophagus; can be a symptom of GERD or other conditions.

• Dysphagia/dysphasia: ensure terminology is used correctly; dysphagia = swallowing difficulty; dysphasia is a language impairment related term.

• Puree diet: a texture-modified diet used for swallowing difficulties, common in oncology settings and geriatrics.

• Microbiome: community of microorganisms living in the GI tract; influences digestion, immunity, and metabolism.

• Fiber: dietary component that adds bulk to stool and supports regular bowel movements.

• Equations and transformations (highlights):

  • ext{Carbohydrate} \rightarrow \text{glucose} \text{(via amylase)}
  • ext{Protein} \rightarrow \text{peptides} \rightarrow \text{amino acids} \text{(via proteases)}
  • ext{Triglyceride} \rightarrow \text{monoglyceride} + 2 \, ext{fatty acids} \text{(via lipase)}
  • Fat absorption pathway: ext{Fatty acids + monoglycerides} \rightarrow \text{chylomicrons} \rightarrow \text{lacteals} \rightarrow \text{lymphatic system}
  • Transit timing: t{ ext{mouth→stomach}} < ext{minutes}, \ t{ ext{stomach}} \in [1,2] \text{hours}