AnS 214: Digestion Part 2
Microbial: most ruminants
Enzymatic: most monogastrics
◼Regulation of the Digestive Tract
❏ Neural Control
❏ Paracrine Secretion
Growth & Repair
◼Passage of Food & Parts that Play a Role in Digestion Diagram
If you want to learn more check out What is the meaning of hydrolysis in biology?
◼Stomach (Temporary “Storage Tank”)
❏ Location of Mechanical and Chemical digestion
Mechanical and chemical digestion breakdown of protein and fat into chyme.
❏ Minimal absorption of nutrients and water occur in the stomach
Mostly proteins, carbs and fats
Some lipidsoluble substances with break down in the stomach
e.g. alcohol and aspirin
❏ Gross Anatomy of the Stomach Diagram
We also discuss several other topics like What are the layers of the dura mater?
❏ Gastric Gland Secretions
2 to 3 L of gastric juice is produced in a day
Components of gastric juice are: H2O, HCl and pepsin
❏ Parietal Cells
Intrinsic factor: Glycoproteins and vitamin B12 absorption
Hydrochloric Acid helps to break down proteins and kill bacteria → will eventually activate pepsin and lingual lipase release
❏ Chief Cells
Pepsinogen (inactive form of enzyme) and lipases
→ pepsin (active form of enzyme)
❏ Enteroendocrine Cells
➤Paracrine, serotonin, histamines
➤Hormones, somatostatin, gastrin
◼Regulation of Gastric Secretions
❏ Occurs in 3 Phases:
1. Phase I: Cephalic Phase (has to do with brain function) We also discuss several other topics like What is a musical comedy called?
Lasts about a few minutes
Phase in which your body begins to prepare itself to receive food
Salivation occurs because of sight, smell, taste or thought of food
Brain stimulates salivary glands and gastric functions
2. Phase 2: Gastric Phase:
Lasts 3 to 4 hours
Occurs after food has been chewed, ingested and swallowed
Food is transported from the pharynx to the esophagus and then to the stomach → stretch receptors are activated
3. Phase 3: Intestinal Phase
A few hours long
The phase in which food begins to digest We also discuss several other topics like Why do cells have semi-permeable membranes?
If you want to learn more check out Can resonance structures have different formal charges?
We also discuss several other topics like What happened to the spanish armada in 1588?
Food starts to pass through the small and large intestine so nutrients can be absorption
◼Regulation of Gastric Motility
❏ Swallowing chewed food signals the stomach to relax so the food ingested can stretch the stomach until the stretch receptors tell the brain to make our bodies stop (the feeling of being full).
❏ Myenteric pacemaker cells→ signal the longitudinal muscle layer to create three peristalsis waves per minute.
Waves of small contractions churns ingested food and mixes it with gastric stomach juices
Stronger contractions sends some food into the duodenum (the first section of the small intestine).
A standard meal is fully digested within 4 hours on average
Borborygmi: stomach rumblings is the result of peristalsis occurring in the stomach and small intestine
When your stomach is empty the rumblings are projected to be louder because there is nothing in the stomach to muffle the rumblings
◼Accessory Organs: Liver, Gallbladder and Pancreas
All release important secretions into the duodenum via ducts to aid in the continuation of digestion of food.
❏ Each organ plays an important role in digestion
➤Produces bile and secretes it
Bile is an alkaline solution containing:
a. Bile Salts: aid in fat emulsification and absorption
Bile salt gets recycled via the enterohepatic circulation
b. Triglycerides and pigments: bile pigmentation is called bilirubin
Bilirubin is formed in the Heme (“Heme” → “Hemoglobin”)
The Sterobilin pigment is what gives healthy feces the
d. Phospholipids and
➤Stores and excretes bile
only then releases when
needed via cystic duct to the
Secretes insulin and
Acini: clusters of secretory cells
Secretes pancreatic juice
Zymogen granules of Acini contain digestive enzymes
Trypsinogen (inactive form) and Trypsin (active form) are secreted
❏ Duodenum: (1) neutralizes acids and breaks down fats
❏ Jejunum: (2) location where most digestion and absorption
❏ Ileum: (3) connects to that large intestine
Peyer’s patches: keeps bacteria where it is supposed to be (in the intestines) ➤ Maximum Absorption → Structural Mods
Increase surface area for nutrient absorption
A. Structural Mod. 1: Circular Folds
Forces chyme to slowly spiral through lumen so maximum absorption can occur
B. Structural Mod. 2: Villi
Motile fingerlike extensions of the mucosa
Villus Epithelium *1
Enterocytes → simple columnar cell → absorption
Goblet Cells → secretory → mucus
C. Structural Mod. 3: Microvilli
Cytoplasmic projections of absorptive cells → brush border *2
Bear brush border enzymes → complete carbohydrates and protein
digestion in the small intestine
1 Villus Epithelium: the epithelium of the structural modification, villi
2 Brush Border: is the microvillicovered surface of simple cuboidal epithelium and simple columnar epithelium cells found in certain locations of the body.
❏ Absorptive epithelial cells
❏ Few goblet cells mucus producers
◼Small Intestinal Motility
❏ Mixes chyme with intestinal juice, bile and pancreatic juice
❏ Churns chyme to increase contact with mucosa for absorption and digestion ❏ Moves residue towards large intestine
➤Segmentation: occurs after you eat something
➤Peristalsis: occurs in between eating something
◼Carbohydrate Digestion in The Small Intestine
❏ Salivary amylase stops working in the stomach because most dietary starch is digested already
❏ Pancreatic amylase completes first steps of digestion in the small intestine ❏ Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose and fructose
❏ Carbohydrate Absorption
SodiumGlucose Transport Proteins that are located in the membrane of the small intestine help absorb glucose and galactose
➤These proteins actively transport across the membrane and into cells
Fructose absorbed by facilitated diffusion then converted to glucose (sugar) inside the cell
◼Protein Digestion and Absorption in The Small Intestine
❏ Pepsin (the active form of pepsinogen) needs an acidic pH to function (pH 1.5 to pH 3.5) ❏ Pepsin becomes inactive when it reaches the duodenum and mixed with the alkaline pancreatic juices (pH 8).
❏ Fat Digestion and Absorption in the Small Intestine
● Micelles: a group of molecules that have both a polar head and nonpolar tail ● Proteins: building blocks of DNA; long chains of amino acids that make up the majority all tissues, organs and etc.
● Chylomicrons: little droplets of fat that are found in the blood after absorption occurs in the small intestine.
● Exocytosis: the release of waste contents of a cell that are collected by the cell’s vacuole organelle.
❏ Fatty acids are mixed with bile salts and form micelles that carry the mixture to the intestinal mucosa
❏ The mixture then leaves the micelle and diffuse into the epithelial cells → combined with proteins and lipids to form Chylomicrons
❏ The chylomicrons are then excreted by the vacuoles of the cells through exocytosis
◼Nucleic Acid Digestion and Absorption in The Small Intestine
❏ The enzymes included in nucleic acid digestion and absorption are:
1. Hydrolyze DNA
2. RNA nucleotides
3. Pancreatic nucleases
❏ The absorption process is carried out by active transport *3
❏ The nucleic acids are transported to the liver by the Hepatic Portal Vein
◼Vitamin Absorption in the Small Intestine
❏ organic compounds that are essential for nutrition and growth of the body
❏ Vitamins are absorbed as they are completely unchanged
❏ Two types of vitamins: fat soluble and water soluble
1. Fat Soluble
a. Vitamins A, D, E and K
2. Water soluble
a. Vitamins B12, B complex and C complex
◼Electrolyte Absorption in The Small Intestine
❏ Mineral or Electrolytes are most actively absorbed by the entire length of the small intestine
◼Water balance in the Intestine
❏ Water is absorbed by osmosis following the ingestion and absorption of salts and organic nutrients
3 Active Transport: the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
❏ when the water is not properly absorbed or balanced by the large intestine
❏ when fecal movements are slow, too much water gets reabsorbed by the large intestine and not enough is left to soften the stool being passed.
❏ Cecum→ Colon→ Rectum
❏ The colon has 3 parts to it:
◼Motility of the Large Intestine
❏ Haustral contractions * occur every 30 minutes
Slow segmenting movements begin
Haustra contracts in response to distension *5
❏ Gastrocolic Reflex
Initiated by the presence of food in the stomach
Activates 3 or 4 slow but powerful peristaltic waves per day → leads to mass movements which sometimes result in defecation
◼Bacterial Flora and Intestinal Gas
❏ Bacterial flora tends to populate in the large intestine
❏ Responsible for producing flatulence or gas
❏ Blind sac extending from the cecum of the large intestine
❏ The lymphoid nodules are the location in which local immune responses occur ❏ Bacterial flora is replenished by the intestinal flora
4 Haustral Contractions: slow segmenting, uncoordinated movements that occur approximately every 25 minutes.
5 Distension: occurs when substances, such as air (gas) or fluid, accumulate in the abdomen causing its outward expansion beyond the normal girth of the stomach and waist.
◼Rectum and Anus
Has three rectal valves that stop feces from being passed with gas (helps the individual to not poop their pants whenever they have to fart)
❏ Anal Canal
The last segment of the large intestine
There are 2 main rectal sphincters:
1. Internal anal sphincters
2. External anal sphincters
❏ Mass movements caused by peristaltic waves forces the feces into the rectum where feces adopts its shape
❏ Distension initiates the spinal defecation reflex
❏ Parasympathetic Signals
Stimulates the contraction of the colon and rectum and then relaxes the internal anal sphincter so feces can pass comfortably
❏ Voluntary control allows the body to relax the external anal sphincter when needed.
◼WHAT YOU SHOULD MEMORIZE AND KNOW: The Checklist