The Digestive Tract: Digestion and Absorption

By Rosh Health Center

June 27, 2022

The digestive system is a fascinating network that is involved in motility, digestion, absorption, and secretion. Food moves through the digestive system in an incredible fashion, from the top to the bottom.

Motility (Peristalsis)

Peristalsis is a series of wave-like muscle contractions that move food through the digestive tract
Involuntary constriction and relaxation of muscles push content forward
Involved in breakdown and mixing of ingested nutrients all the way through the elimination of undigested waste from the body

Secretion

This is the release of enzymes, hormones, and other substances that help the body digest the food that is eaten
Hormones tell the body when to produce digestive juices and signal the brain when you are hungry or full

Digestion

Ingested nutrients are reduced into molecules small enough to pass through the lining of the gut and so they can enter the bloodstream
Digestive system breaks down food
- Proteins break down to amino acids
- Fats break down to fatty acids and glycerol
- Carbohydrates break down to simple sugars
Nutrients are absorbed for energy, growth, and cell repair

Mouth

The mouth is the beginning of the digestive tract
Food enters the digestive system through the mouth
The process of digestion starts before the first bite of food
Seeing, smelling, or tasting food activates salivary glands
Salivary glands are located under the tongue and near the lower jaw
Begin producing saliva

Chewing

Physically break down food
Reduce the size of ingested food
Smaller pieces are easily digested
Activates the functions of stomach and intestines
Chewing thoroughly and eating slowly prevents overeating
Chewing slowly gives the stomach enough time to signal the brain for satiety

Bolus formation 

Small pieces of food get mixed with saliva by chewing is called a bolus
Mixing food with saliva moistens the food
- Prepares it for swallowing
Saliva serves the role of lubrication
- Promotes a smoother journey to the esophagus and eventually into the stomach
Saliva has enzymes for digestion
- Salivary amylase initiates the digestion of starches (complex carbohydrates)
- Lingual lipase initiates the digestion of dietary fat
When swallowing the bolus, tongue passes the food into pharynx and into the esophagus

Esophagus

The esophagus is a hollow, muscular tube that connects the throat (pharynx) with the stomach
Lies behind the trachea (windpipe) and in front of the spine
Is an extension between the pharynx and the stomach
Receives swallowed food from the mouth

Epiglottis

Cartilaginous leaf-shaped flap
Located in the throat behind the tongue and in front of the larynx
Usually upright at rest, allows air to pass into the larynx and lungs
Folds over the larynx (windpipe) when swallowing
When swallowing
Epiglottis folds backward to cover the entrance of the larynx
Prevents food and liquid from entering the windpipe and lungs
Prevents choking (when food goes into the larynx (windpipe))
After swallowing epiglottis returns to its original upright position

Esophagus: function

Receives swallowed food from the mouth
Used for transportation of food and water that travels to the stomach
A series of muscular contractions within the esophagus called peristalsis delivers food to the stomach
The contractions of the muscle to move the food through the system is involuntary, so we have no control over it
Esophageal sphincter is a ring like muscle that relaxes to let the food enter the stomach

Lower esophageal sphincter 

Bundle of muscles at the low end of the esophagus called the lower esophageal sphincter must relax to allow the passage of food
Once food enters the stomach
Sphincter contracts and prevents acid and stomach contents from traveling backwards from the stomach
When sphincter fails to contract
Stomach contents flow back into the esophagus
Results in acid reflux or heartburn
It is best to sit up after eating

Esophagus disorders: Signs and Symptoms 

Heartburn, chest pain, difficulty swallowing, acid regurgitation, sensation of a lump in the throat, nighttime acid reflux, chronic cough, hoarse voice, new or worsening asthma, disrupted sleep, difficult swallowing, painful swallowing, decreased appetite, nausea, vomiting, sore throat, etc.

Few examples of Esophagus disorders

GERD, Achalasia, Barrett’s esophagus, Eosinophilic esophagitis, Esophageal diverticulum, Esophageal spasms, Esophageal strictures, etc.

Stomach

A muscular hollow organ (pear-shaped pouch) that lies across the abdominal cavity to the left
Holds food while it is being mixed with stomach enzymes
Stomach can alter size and shape
Empty stomach is about 12 inches long and widest point is about 6 inches across
An adult stomach has the capacity of about 2.5 ounces when empty and relaxed
The stomach can expand to hold about 1 quart of food
Receives food from the esophagus, mixes it, breaks it down, and then passes it on to the small intestine in small portions
Cells in the lining of stomach secrete a strong acid and enzymes that are responsible for the breakdown process
The pH level in the stomach ranges from 1.5 to 3

Gastrin

Hormone secreted by the stomach wall in response to the presence of food
Stimulates secretion of gastric juice
Gastric juice comprises water, mucus, hydrochloric acid, pepsin, and intrinsic factor
About 3 to 4 liters of gastric juice are produced per day
Gastric juice kills bacteria

Hydrochloric acid

Important acids in the stomach
Aids digestion of proteins
Catalyzes the action of pepsin
Digestive enzymes split up the protein

Stomach wall

The mucus will act to protect and lubricate the stomach wall to offer a buffer for the secretion of acids
The mucus covers the stomach wall with a protective coating
Together with the bicarbonate, this ensures that the stomach wall is not damaged by hydrochloric acid

Stomach disorders: Signs and Symptoms 

Bloating, constipation, diarrhea, heartburn, nausea, vomiting, weight gain or loss, abdominal pain, acid reflux, lack of appetite, acid reflux, brittle nails, hair loss, undigested food in stool, nutrient deficiency, belching, bloody stool, cramping, diarrhea, flatulence, indigestion, etc.

Few examples of stomach disorders

Crohn’s disease, Peptic Ulcer Disease (PUD), Gastritis, Hypochlorhydria, Pernicious anemia, Gastroparesis, Gastroenteritis, Hiatal hernia, etc.

Small Intestine

Made up of three segments
- Duodenum
- Jejunum
- Ileum
Small intestine is a 22-foot-long muscular tube that breaks down food
It is called small due to lumen diameter being smaller than that of the large intestine
Longer in length but not in width than the large intestine
Most nutrients from food are absorbed in the small intestine
Contents of the small intestine start out semi-solid and end in a liquid form after passing through the organ
- Water, bile, enzymes and mucus contribute to the change in consistency
Water is absorbed by osmosis throughout the small intestine
Once nutrients have been absorbed
- Leftover-food residue pass to the large intestine, or colon

Duodenum

Duodenum is the first segment of the small intestine
Largely responsible for the continuous breaking-down process
Widest part of small intestine and the shortest
- About 10 inches long
When food moves into the duodenum, it mixes with digestive enzymes secreted by the pancreas
These enzymes break down the largest molecules of food, such as proteins and starches
Duodenum neutralizes stomach acid
- Bile is a substance that breaks down the fats
- Empties into the duodenum by the common bile duct
- Vitamins and minerals absorbed
- Vitamin A, D, E, and K
- Calcium, iron, and folate

Jejunum

The middle part of your small intestine
Responsible for absorption of nutrients into the bloodstream
Absorbs most of the nutrients
- Carbohydrates
- Proteins
- Fats
- Minerals
- Vitamins

Ilium

Lowest part of small intestine
Final parts of absorption take place in the ilium
The ileum absorbs
- Bile acids
- Fluid
- Vitamin B-12
Absorbs any remaining nutrients that did not get absorbed by the duodenum or jejunum
- Particularly vitamin B12

Disorders of the Small Intestine: Signs and Symptoms 

Cramping, abdominal pain, bloating, flatulence, constipation, diarrhea, burning stomach pain, feeling of fullness, belching, intolerance to fatty foods, loss of appetite, diarrhea, unintentional weight loss, nutrient deficiencies and their associated symptoms, constipation, depression, anxiety, nausea, heartburn, etc.

Few examples of Small Intestine Disorders 

Celiac disease, Crohn’s disease, Infection, Intestinal obstruction, Irritable bowel syndrome, Peptic ulcer, SIBO, Dyspepsia, etc.

Pancreas

Found along the small intestine close to the stomach
An accessory organ for the gut
Produce important secretions in the absorption and digestion process
Works primarily on the small intestine

Some Example of pancreatic enzymes include

Lipase
Amylase
Trypsin
Nuclease

Pancreas makes insulin, releases it directly into the bloodstream

Insulin is the chief hormone for metabolizing sugar
Insulin allows the cells in the muscles, fat, and liver to absorb glucose that is in the blood
Glucose serves as energy to these cells, or it can be converted into fat when needed

Pancreas secretes digestive enzymes into the duodenum that breaks down

Protein
Fats
Carbohydrates

Pancreas disorders: Signs and Symptoms

Nutritional deficiencies, weight loss, upper abdominal pain, abdominal pain that radiates to the back, tenderness when touching the abdomen, fever, rapid pulse, nausea, vomiting, abdominal pain that feels worse after eating, oily bowel movements, etc.

Few examples of Pancreas disorders

Acute Pancreatitis, Chronic Pancreatitis, Hereditary Pancreatitis, Exocrine pancreatic insufficiency, Cystic fibrosis, etc.

Liver

The liver has many functions
Main function within the digestive system is to process nutrients absorbed from the small intestine
Bile from the liver secreted into the small intestine also plays an important role in digesting fat and fat-soluble vitamins
Liver is the body’s chemical “factory”
Takes raw materials absorbed by the intestine and makes all the various chemicals your body needs to function
Detoxifies potentially harmful chemicals

Production of bile

Carry away waste product
During digestion, breaks down fats in small intestine

Production of certain proteins for blood plasma

Production of cholesterol and proteins

Help carry fats through the body

Conversion of excess glucose into glycogen for storage 

Glycogen can later be converted back to glucose for energy

Regulation of blood levels of amino acids

Which form the building blocks of proteins

Processing of hemoglobin 

For use of its iron content
- Liver stores iron

Removing waste products

After blood leaves the digestive tract and flows into the liver
Liver gears up to process a variety of dangerous chemicals in the bloodstream
Toxics, alcohol, medicine, ammonia
- Alters and removes it from the body

Conversion of poisonous ammonia to urea 

Urea is end-product of protein metabolism
Excreted in urine

Regulating blood clotting

Controls immune responses

Produces immune factors
Removes bacteria from the bloodstream
- Bacteria, viruses, and other harmful organisms that have entered the body
- Liver can find and destroy them
- This is done by specialized cells in liver

Clearance of bilirubin

Helps body get rid of bilirubin
breakdown of your red blood cells
Too much bilirubin in your body can cause jaundice
- Yellowing of skin and eyes

Liver disorders: Signs and Symptoms

Fatigue, sudden nausea and vomiting, abdominal pain or discomfort, especially on the upper right side beneath the lower ribs (by the liver) Clay-colored bowel movements, loss of appetite, low-grade fever, dark urine, joint pain, loss of appetite, muscle aches and pains, nausea and vomiting swollen belly (ascites), enlarged blood vessels underneath the skin, yellowing of the skin and the whites of the eyes (jaundice), intense itching, etc.

Few examples of Liver disorders:

Infection (Hepatitis A, B, C), Immune system abnormality (Autoimmune hepatitis, Primary biliary cholangitis, Primary sclerosing cholangitis), Genetics (hemochromatosis, Wilson’s disease, Alpha-1 antitrypsin deficiency), Nonalcoholic fatty liver disease (NAFLD), Non-Alcoholic Steatohepatitis (NASH), Budd-Chiari Syndrome, Cirrhosis, Gilbert Syndrome, Reye Syndrome, etc.

Gallbladder

Gallbladder is pear-shaped, hollow structure, a small pouch located under the liver and on the right side of the abdomen
Gallbladder is part of the biliary system
- Made up of liver, gallbladder, and associated ducts
- This system is needed for the production, storage, and secretion of bile
Bile is continuously produced by the liver
During a meal, bile moves from the liver directly to the small intestine
When you are not eating, bile is stored in the gallbladder
Gallbladder function is to store and concentrate bile until it is needed for digestion
Consumption of fatty food causes the gallbladder to contract
- Gallbladder releases stored bile into the duodenum in the small intestine to help absorb and digest fats

Bile

Bile stored in the gallbladder and released into the proximal small intestine when a meal is eaten
Aid in the digestion of fats in the duodenum
Carries away waste products

- - Bilirubin, a waste product of hemoglobin that carries oxygen in the blood
  - Drugs
  - Toxins

When digesting fats, bile acts as an emulsifier to break the large fat globules into smaller emulsion droplets
Emulsified fats provide a larger area for the fat-digesting enzymes (lipase) to act, making the process quicker
Bile is essential for the absorption of fat, iron, calcium, and vitamins
Bile salts help in the absorption of lipid-soluble vitamins A, D, E and K and the provitamin, beta carotene

Gallbladder disorders: Signs and Symptoms

Feeling sick, sweating, loss of appetite, skin and eyes that appear yellowish (jaundice), abdominal pain that gets worse when taking a deep breath, abdominal pain and cramping after eating (especially fatty food), bloating, nausea, vomiting, abdominal pain (typically localized to the right upper quadrant of the abdomen), abdominal swelling, distension or bloating, abdominal tenderness, pale or clay-colored stools, fever and chills, loss of appetite, pain that radiates from the abdomen to the right shoulder or back

Couple examples of Gallbladder disorders

Cholelithiasis, Cholecystitis

Large Intestine

Wraps around the border of the abdominal body cavity from the right side of the body, across the top of the abdomen, and finally down the left side
Muscular tube that connects the small intestine to the rectum
About 5-foot long and 3 inches in diameter
Named Large
- Due to lumen (diameter) larger than the small intestine
- Length is not greater than the small intestine

The large intestine is made up of 

Cecum
Ascending (right) colon
Transverse (across) colon
Descending (left) colon
Sigmoid colon, which connects to the rectum

3 primary functions

Absorbing water and electrolytes
Producing and absorbing vitamins
Forming and propelling feces toward the rectum for elimination

Most of the microbes in the intestine are found the cecum

Referred to as the gut microbiome
Several useful functions
Synthesize various vitamins
Vitamin K e.g.
Process waste products and food particles
Protect against harmful bacteria
Digest fiber
Produce short-chain fatty acids
Important for gut health

Proper stimulation of colon causes emptying of content into the rectum

This triggers defecation
High fiber intake
Softer feces
Smooth elimination

Rectum

The rectum is a straight 8-inch chamber
Connects the colon to the anus
Receive stool from the colon
Let you know that there is stool to be evacuated
Hold the stool until evacuation

Anus

The anus is the last part of the digestive tract
2-inch-long canal
Consists of the pelvic floor muscles and the two anal sphincters (internal and external)
The lining of the upper anus can detect rectal contents
It lets you know whether the contents are liquid, gas or solid

Large Intestine disorders: Signs and Symptoms

Right side abdominal pain, nausea, vomiting, fever, chills, abdominal tenderness, constipation, diarrhea, bloating, feeling of fullness, rectal bleeding, weight loss, flatulence, mild abdominal cramps, loss of appetite, fatigue, anal fissures, anal fistulas

Few examples of Large Intestine disorders

Diverticulitis, diverticulosis, Crohn’s disease, ulcerative colitis, Irritable bowel syndrome, Intestinal obstructions, C difficile infection

Absorption

Digested nutrients pass from the gut into the blood so the circulatory system or lymph system can pass them on to the rest of the body to use or store
The lymph system absorbs fatty acids and vitamins
The blood carries simple sugars, amino acids, glycerol, and some vitamins and salts to the liver
The liver stores, processes, and delivers these nutrients to the body when needed

Factors that can lead to digestive dysfunction: Food sensitivities

Self
- Elimination diet
Healthcare practitioner
- Food sensitivity testing

Factors that can lead to digestive dysfunction: Stress

Self
- Follow simple methods to lower stress, walk, exercise, journal, herbal teas, meditation
Healthcare practitioner
- If emotions such as anger, grief, and resentment are not resolved can lead to chronic stress (Seek counseling)

Factors that can lead to digestive dysfunction: Nutritional deficiencies