Small Intestine and Digestion Notes

Small Intestine

  • Small in diameter, long in length (3-4 meters)
  • 90% of nutrient absorption occurs here
  • Waste is transported to the large intestine
  • Peristalsis and segmentation move everything through the digestive system
  • Longitudinal and circular muscles contract
    • Move chyme
    • Maximize digestion and absorption
  • Segmentation = churning motion, reverse pushing

Duodenum

  • First 20 cm of the small intestine
  • Secretions from the liver and pancreas enter here
  • Detects chyme from the stomach
  • Special cells release hormones
    • Cause sphincters to open
    • Allow fluids to enter (common bile duct & pancreatic duct)
    • Neutralize chyme

Ligaments and Ducts

  • Falciform ligament: above belly button to diaphragm (not essential to remember)
  • Common hepatic duct: drains bile from the liver to the gallbladder
  • Cystic duct: drains bile from the gallbladder to the hepatic duct
  • Common bile duct: bile from both liver and gallbladder enter the duodenum
    • After gallbladder removal, bile comes directly from the liver
  • Pancreatic duct: drains enzymes from the pancreas into the duodenum

Digestive Fluids in Small Intestine

  • Bile from the gallbladder (made by the liver)
  • Pancreatic enzymes and buffer
    • Buffer: Bicarbonate ion from the pancreas.
  • Intestinal enzymes from the wall of the small intestine
  • Mechanical and chemical digestion occur here
    • Mechanical: Muscles break down size.
    • Chemical: Polymers broken down to monomers.

Goal of Digestive System

  • Break food particles into monomers
  • Absorb them into the bloodstream
  • Mechanical digestion enhanced by peristalsis and segmentation
  • Large fat droplets are broken down mechanically into small fat droplets by bile salts from the liver and gallbladder
    • Bile salts are an emulsifying agent (like soap on fat)

Chemical Digestion

  • Salivary amylase: in the mouth; carbohydrates to maltose
  • Pepsinogen: in the stomach; large polypeptides to smaller polypeptides
  • Small intestine enzymes:
    • 4 from the pancreas
    • 2 from the cells lining the small intestine

Pancreatic Enzymes

  • Starch to maltose
  • Large polypeptides into smaller polypeptides
  • Triglycerides (neutral fats) into fatty acids and glycerol
  • Nucleic acids (DNA and RNA) into nucleotides

Enzyme Optimal Conditions

  • Temperature: 37∘C37^\circ C
  • pH: 8-8.5

Pancreatic Enzyme Breakdown

  • Starch: Pancreatic amylase.
  • Large polypeptides: Trypsin.
  • Triglycerides/lipids: Lipase.
  • Nucleic acids: Nuclease.
    • Connections to remember:
      • nucleic -> nuclease
      • lipids -> lipase
      • pancreas -> pancreatic amylase

Small Intestine Enzymes from Crypts

  • Breaks down:
    • Maltose to glucose
    • Small polypeptides to amino acids
  • Maltose breakdown:
    • Maltase
      • maltose -> maltase -> glucose
  • Small polypeptides:
    • Peptidases
    • small polypeptides -> amino acids
      Completes chemical digestion

Eight Enzymes Total

  • Mouth: Salivary amylase (starch to maltose, pH 7)
  • Stomach: Pepsin (large polypeptides to smaller polypeptides, pH 2)
  • Pancreas:
    • Pancreatic amylase (starch to maltose, pH 8)
    • Trypsin (large polypeptides to smaller polypeptides, pH 8)
    • Lipase (triglycerides/lipids to fatty acids and glycerol, pH 8)
    • Nuclease (DNA/RNA to nucleotides, pH 8)
  • Small intestine:
    • Maltase maltose to glucose, pH 8
    • Peptidases (small polypeptides to amino acids, pH 8)

Final Products for Digestion (Monomers)

  • Produced by:
    • Lipase (fatty acids and glycerol)
    • Nuclease (nucleotides)
    • Maltase (glucose)
    • Peptidases (amino acids)

Duodenum - Busy Place

  • Secretions from liver and pancreas enter
  • Pancreas secretes 4 digestive enzymes and bicarbonate
  • Liver produces bile, stored in the gallbladder
  • Everything enters the duodenum after the stomach releases its contents

Nutrient Absorption

Four modifications to increase surface area:

  • Villi & Microvilli
  • Tennis court area
  • Water, salts, sugars, amino acids, nucleotides are absorbed by blood vessels
  • Fats are absorbed into lacteals within the villi (part of lymphatic system)
  • Lymphatic system empties into the bloodstream

Structure of Small Intestinal Walls

Enhance surface area:

  • Length: 3-4 meters (15-20 feet) long.
  • Inner folds (circular pleats): ability to stretch, add surface area.
  • Villi: Finger-like projections. Contain capillaries and lacteals.
  • Microvilli: Submicroscopic projections on single cells. Add surface area for materials to come in through the villi membrane.
  • Crypts: Produce intestinal juice, the production of two enzymes, and cell regeneration happens from here.

Villi

  • Villi are covered by millions of microvilli.
  • Each villus has a capillary bed and lacteal.
  • They are very large.

Absorption

  • As chyme is digested, it contains many nutrients ready for absorption:
    • Simple sugars
    • Amino acids
    • Fatty acids and glycerol
    • Nucleotides
    • Water
    • Electrolytes
    • Vitamins
    • Toxins
  • Alcohol starts absorption in the mouth.

Lacteal and Hepatic Portal Vein

  • Glucose, amino acids, nucleotides, fatty acids, glycerol, water, electrolytes, and toxins are absorbed.
  • Lymph vessels drain into the blood at the subclavian blade.
  • Hepatic portal vein:
    • Blood from the villi capillaries is collected.
    • Taken to the liver to be conditioned.
    • Nutrient-rich.
  • Materials that get absorbed into our blood in our digestive system goes directly to our liver to pretty much be filtered and cleaned before it goes to the rest of our body.

Simple Sugars, Amino Acids, and Nucleotide Absorption

  • Digested into monomers by enzymes
  • Actively pumped out of the lumen by cells lining the small intestine (ATP required)
  • Passively diffuse into the capillaries of the villi
  • Hepatic portal vein takes them to the liver

Lipid Absorption

  • Bile salts physically break fat into smaller pieces
  • Enzyme lipase breaks triglycerides into monomers
  • Monomers are actively pumped out of the lumen into the villi
  • Lacteals gather the fat and eventually dump them into the blood at the subclavian vein

Villi

  • Cross section of the villi. Lacteal absorbs fats.
  • Arterial and venule side of the capillary.
  • Goblet cells keep mucus layers on to protect the villi.

Lymphatic System

  • Thoracic ducts bring fat and put it back in right under our clavicles
  • Subclavian veins

Hepatic Portal System

  • Small intestine villi capillaries absorb products of digestion
  • Nutrient molecules travel via the hepatic portal vein to the liver.
  • Liver monitors the blood by removing toxins and excess glucose
  • Conditioned blood enters the hepatic vein for general circulation

Basic Vessels

  • Hepatic portal vein: From the intestines and the stomach to the liver (capillary bed to capillary bed).
  • Three capillary beds from the stomach, intestines, and pancreas go directly to the liver to get conditioned.

Large Intestine (Colon)

  • Not as long as the small intestine: 1.5 meters (5 feet)
  • Begins with the cecum and the appendix
  • Absorbs water
  • Concentrates the waste
  • Absorbs vitamins and ions (produced by bacteria)
  • Stores waste for elimination (feces)

Parts of the Colon

  • Ileocecal valve: Drains the small intestine into the cecum
  • Cecum: Base of ascending colon
  • Appendix: Base of the cecum
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon: Storage area
  • Rectum: Where poop sits before elimination
  • Anus: Sphincters (internal and external)

Sphincters in the Bum Hole

  • Internal anal sphincter
  • External anal sphincter
  • Anus

Large Intestine

  • Produces alkaline (basic) mucus to neutralize acids produced by bacterial metabolism
  • Water, salts, and vitamins are absorbed
  • Remaining contents form feces:
    • Cellulose
    • Bacteria
    • Bilirubin (gives feces the yellowy-brown color)
  • E. Coli produces vitamins (including vitamin K) that are absorbed

Regions of the Large Intestine

  • Cecum: Pocket at the proximal end with the appendix
  • Colon: Ascending, transverse, and descending.
  • Sigmoid: S-bend near the terminal end
  • Rectum: Terminal end (anal canal ending at the anus)
    • Internal involuntary sphincters
    • External voluntary sphincters

Diverticulitis

  • Small bulges in the colon called diverticula
  • Occurs in 50% of the population over 60
  • Diverticula become inflamed or infected (10-25% of people with diverticula develop the condition)
  • Probable cause: Too little fiber in the diet

Accessory Organs

  • Food does not pass through these organs, but they assist in digestion
    • Salivary glands
    • Liver
    • Gallbladder
    • Pancreas

Salivary Glands

  • Produce watery fluid (saliva)
    • pH around 7
    • Salivary amylase (starts carbohydrate digestion)
  • Softens and moistens food
  • Three pairs: Submandibular, sublingual, and parotid
  • Sublingual: below the tongue
  • Sublandibular:
  • Parotid: in front of ears here, and it comes out just by your molars

Gallbladder

  • Pear-shaped muscular sac attached to the surface of the liver
  • Stores excess bile
    • Liver produces about 1 liter of bile per day (1000 mL)
    • Water is reabsorbed to becomes a consistency of thick mucus
  • Bile duct empties into the duodenum
  • Bile composition:
    • Bile salts (emulsify fats)
    • Bile pigments (waste from red blood cell removal)

Gallstones

  • Crystallized bile
    • 80% cholesterol
    • 20% bilirubin
  • Treatment:
    • Surgical removal (colextectomy) - 500,000 surgeries per year (potentially outdated number)
    • Oral dissolution (early stages)
    • Ultrasound waves (break stones into passable pieces)

Pancreas

  • Exocrine function (ducts used):
    • Pancreatic juice:
      • Sodium bicarbonate (neutralizes stomach acid)
      • Four enzymes: pancreatic amylase (starch), trypsin (proteins), lipase (fats), nucleases (DNA/RNA)
  • Endocrine function (releases directly into the blood):
    • Hormones: insulin/glucagon (control blood sugar levels)

Glycogen, Glucose, Insulin, and Glucagon

  • Low glucose levels in the blood:
    • Glucagon stimulates the breakdown of glycogen into glucose
  • Insulin (helps store glucose and glycogen in the liver and muscles)
  • When no glucose or glycogen is available:
    • Amino acids are converted into glucose in the liver
    • Deamination of amino acids to produce glucose (removal of the amino group)

Plasma Concentrations

  • Glucagon is released when sugar levels are low
  • Insulin levels go up when glucose levels go up
  • Low glucose and insulin levels when it's food time (especially in the morning)

Liver

  • Largest organ in the body (skin is the largest overall organ)
  • Digestive functions:
    • Stores iron, vitamins A, D, E, and K
    • Stores glycogen
    • Produces bile
    • Detoxifies poison (hydrogen peroxide - broken down with catalase)
  • Nondigestive functions:
    • Synthesis of blood proteins (maintain osmotic conditions)
    • Destruction of old red blood cells (recycles iron and globin/amino acids)
      • Heme group is the waste (becomes bilirubin)
      • Responsible for: color of feces and urine
    • Deamination of amino acids (produces toxic waste that is converted into less toxic urea)

Exocrine and Endocrine Function

Exocrine

  • Glands that secrete using a duct or vessel
  • Pancreatic amylase, trypsin, lipase, and nucleases are the four enzymes
  • Bicarbonate ion (buffer) neutralizes the stomach acid

Endocrine

  • Glands that make hormones that diffuse into the bloodstream
  • Insulin causes the liver and muscle cells to store glucose as glycogen to drop blood sugar levels
  • Glucagon has the opposite effect to bring back up blood sugar levels

Liver Functions

  • Hepatic portal system (blood from digestive tract is moved to the liver to be conditioned before it is placed in general circulation)
    • Blood from the heart passes through the digestive system, picking everything up, taking it to the liver to get cleaned and conditioned
      *Capillary bed -> capillary bed (hepatic portal vein)
Basic Liver functions

*Hepatic artery (from heart to liver)
*Hepatic vein is blood leaving the liver.
*Hepatic portal vein from the digestive system bringing blood in.

Liver Primary functions

  • Making bile. Bile salts (emulsifying agent for physical digestion of fats/mechanical digestion of fats)
  • Nutrient metabolism is the second process, including:
    • glucose to glycogen and backwards, glycogen back to glucose
    • deamination of amino acids
  • storage is the third function including:
    • glycogen, minerals, and vitamins
      • vitamina, b 12, d, e, k
      • minerals including iron and copper
  • detoxification is another primary fucntion, where liver cleans up toxins and drugs. Alcohol is removed as well, if there is a lot of alcohol, this leads to liver damage.
    *Blood protein funciton where blood proteins are created that maintain the blood tenacity.
    *Liver removes old and damaged red blood cells, with cells lasting approximately 105-135 days per cell.
    *Excreation of the waste bile pigments, from recycling machines to produce hemoglobin in the form of bilirubin.