A&P Term 2 - Chapter 5: Digestive System

4 main functions of the digestive system

* Take in food
* Break down food into nutrient molecules
* Absorb nutrient molecules into the bloodstream
* Rid the body of any indigestible remains

Why is food broken down into nutrient molecules?

The body can’t absorb food within larger forms, so they’re broken down (ex. into monomers).

What’s an example of a nutrient which is always discarded as (defecation) waste, and why is this?

Cellulose is always removed from the body as a waste. This is because cellulose cannot be broken down in the human body.

Briefly explain what the GI tract is

A continuous muscular tube. Consists of:

Mouth, pharynx, esophagus, stomach, small and large intestines.

Role of accessory digestive organs

Facilitate mechanical breakdown of food to produce specialized secretions to aid digestion.

Examples of accessory digestive organs

* Teeth
* Tongue
* Salivary glands
* Gallbladder
* Liver
* Pancreas

How are the gallbladder, liver, and pancreas connected?

By a series of ducts

What are the six activities of processing food?

1. Ingestion
2. Secretion (of digestive enzymes)
3. Motility
4. Digestion
5. Absorption
6. Defecation

Ingestion

Takes food into GI via mouth

Motility

* Propulsion (moving of food, ex. swallowing).


* Peristalsis (type of propulsion)
* Mechanical breakdown (increase surface area for digestion, and mixes food with digestive enzyme secretions)
* E.g., mixing with saliva, churning in stomach, and segmentation

Segmentation

Mixing with digestive juices in the intestines

Digestion

Enzymes catabolize food to its chemical components

Absorption

Movement of digested end products from the lumen of the GI tract into blood or lymph

Defecation

Eliminates indigestible substances via the anus

Layers of the digestive tract

* Mucosa
* Submucosa
* Muscularis externa
* Serosa or adventitia

Lumen of the digestive tract

Inner diameter of tract

(includes the mucosa)

Mucosa

Innermost layer of the digestive tract.

Simple or stratified epithelium provides secretions, absorbs digestive end products, and protect against infectious disease.

Submucosa

Layer between the mucosa and muscularis externa of the GI tract.

Connective tissue with blood and lymphatic vessels, and nerves.

Muscularis externa

Layer in between the submucosa and serosa (or adventitia) of the GI tract.

Inner circular and outer longitudinal smooth muscle layers for peristalsis and segmentation.

Serosa and adventitia

Outermost layer of the GI tract

* Serosa: Visceral peritoneum


* Adventitia: Dense network collagen fibers attach the GI tract to adjacent structures (the esophagus)

Visceral peritoneum

Covers external surfaces of most of the digestive organs

Parietal peritoneum

Lines body wall of abdominopelvic cavity

Peritoneal cavity

Space between visceral and parietal peritoneum filled with serous fluid.

* Fluid allows organs to move near each other without friction causing damage to the tissues.

What does the peritoneum ultimately offer protection from?

Movement of organs.

(Intestines move, stomach grows, body moves, etc. Peritoneum protects from all of this).

Mesentar

Double-layer of the peritoneum that extends to the digestive organs from the body wall.

* Allows blood vessels, lymphatics, and nerves to reach digestive organs.
* Holds the organs in place and stores fat.

Explain the roles of the greater and lesser omentum

These are two folds of visceral peritoneum which connect to the stomach. They carry lots of fat (which is one of the reasons that we carry lots of fat in that area).

* Greater omentum: Hangs from the greater curve of the stomach in front of intestines.
* Lesser omentum: Hands from the liver to the lesser curve of the stomach.

Offers physical protection, immunity aid, …

True or False:

Omentums are mesenteries

True

Where are retroperitoneal organs found, and what do they include?

Found posterior to the mesentery, lying against the dorsal abdominal wall.

Includes most of the pancreas, duodenum, and parts of the large intestine.

Retroperitoneal

Sitting behind the mesentery (ex. kidneys, duodenum, …).

They lose some of their visceral peritoneum.

Enteric nervous system (ENS):

* What type of control?
* Sensory receptors
* Myenteric plexus
* Submucosal plexus

* Intrinsic control.
* Sensory receptors: Chemoreceptors and mechanoreceptors in mucosal epithelium.
* Myenteric plexus: Mostly controls motility (strength and rate of contraction).
* Submucosal plexus: Control digestive secretions.

Mechanoreceptors for the regulation of digestive function

Mechanical stimuli (ex. stretch of the GI system).

Mechanoreceptors detect food present, the nature of the food, and decide on the release of digestive enzymes, peristalsis, ….

This means that the brain doesn’t need to play a huge role in this portion of digestion.

Autonomic nervous system (ANS):

* What type of control?
* Parasympathetic
* Sympathetic

* Extrinsic control
* Parasympathetic: Increased enteric nervous system activity → increased motility and digestive canal secretion.
* Sympathetic: Decreased ENS activity → decreased motility and digestive canal section.

Explain endocrine control for the regulation of digestive function

Hormones produced by enteroendocrine cells control secretion or contraction.

Explain how stress and relaxation can affect the digestive tract

* Resting: Parasympathetic activity is more active, allowing for more activity in the digestive tract and secretion of our digestive enzymes.
* Stressed: Sympathetic. Decreases activity of the enteric nervous system, decreasing motility and secretion of enzymes.

When is food considered “chyme” within the digestive tract

Once digested in the stomach

Where in the digestive system is the only location for ingestion (eating)

The mouth (and tongue)

Explain what epithelium makes up the mouth and tongue

Stratified squamous epithelial (mucosa-lined).

What muscles make up the lips and cheeks, and what is the role?

Skeletal muscle (covered by skin), which help to keep food between the teeth when chewing.

Palate:

* Hard
* Soft

Forms the roof of the mouth.

* Bony hard palate: Aids in the manipulation of food.
* Soft palate: Rises during swallowing to close the nasopharynx.

Which type of muscle makes up the tongue?

What is this used for?

Explain papillae

Skeletal muscle.

* Used to reposition food when chewing, mix food with saliva, initiate swallowing, and aid in speech production.

Papillae o the tongue surface house taste buds.

Why is the mouth and tongue made up of stratified epithelium rather than simple squamous?

For protection (of food and friction within the mouth), and because nutrients are not being absorbed within this area of the body.

Which types of salivary glands produce saliva?

Both major and minor

What cell types are saliva and salivary glands made up of?

* Serous cells (produce a watery secretion).
* Mucous cells (produce mucous).

Saliva

Mainly made up of water, but contains:

* Electrolytes (salts)
* Salivary amylase, lingual lipase, mucins, lysozyme
* Antibodies
* Small amount of metabolic waste

Which glands are main glands for saliva secretion?

Parotid, sublingual, submandibular.

Mastication

(Chewing)

Begins mechanical breakdown of food, and mixes food with saliva

Primary dentition

(Deciduous or baby teeth).

20 teeth that are lost to make way for the permanent dentition.

Permanent dentition

32 teeth, including wisdom teeth (third molars)

Roles for the following teeth:

* Incisors
* Canines (cuspids)
* Premolars (bicuspids) and molars (tricuspids)

* Used for cutting
* Tear/pierce
* Used for grinding

What are the three regions of the tooth?

* Enamel-covered crown
* Neck surrounded by gingiva (gums)
* Root embedded in the jawbone

Cement of the tooth

Calcified connective tissue that anchors the root to the periodontal ligaments.

* Holds the tooth in the bony socket of the jaw.

Dentin of the tooth

Bone-like material, underlies the enamel and surrounds the pulp cavity (contains blood and nerve supply)

Explain the most common cause of a toothache

Damage to enamel which can spread down to the dentin. This can extend to the pulp (where there is blood vessels and enrves).

Passage of oropharynx and laryngopharynx

Provide a common passageway for food, fluids, and air.

What does the esophagus provide a passageway for?

Food and fluids from the laryngopharynx to the stomach (where it joins the cardial orifice.

Gastroesophageal (cardiac) sphincter:

Keeps the top (sphincter) of the stomach closed when food is not being swallowed.

Deglutition: what are the two major phases?

(Swallowing)


1. Buccal phase (voluntary)
2. Pharyngeal-esophageal phase (involuntary)

Buccal phase of swallowing (where does it occur, how?)

Occurs in mouth. Bolus of food is forced into oropharynx by tongue.

Pharyngeal-esophageal phase of swallowing (controlled by?)

Controlled by swallowing centre of the medulla and pons.

Explain the steps for turning a bolus into chyme in the stomach

Bolus → churn (mechanical breakdown using the extra oblique layer of smooth muscle) → hydrolysis (protein digestion begins using pepsin) → chyme (storage)

How does fatty food and gastric emptying relate?

The fattier the food, the slower that gastric emptying will be.

What is gastric emptying?

Movement from the stomach to the duodenum

Why is it recommended to eat fatty food when drinking alcohol?

It gives the body a longer time before alcohol is absorbed into the bloodstream (as fatty foods require more digestion).

Rugae

Folds in the stomach wall, allowing the stomach to expand.

* Empty stomach: Volume of around 50 mL.
* Full capacity: Volume of around 4L

Major regions of the stomach

* Cardia
* Fundus
* Body
* Pylorus

What epithelium makes up the surface epithelium of mucosa in the stomach?

* What is the role of surface mucous cells here?

Simple columnar epithelium.

* Surface mucous cells produce a protective coat of alkaline mucous.

Explain gastric cells within the stomach.

* Explain the types

Columns of secretory cells that produce gastric juice.

* Exocrine gland cells
* Enteroendocrine cell (e.g., G cells)

True or False:

Stomach cells are specialized as a protective mechanism from gastric juices.

* Explain

False.

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Thick alkaline mucous layer in the stomach acts as a barrier to protect cells from acidity.

There’s nothing special about cells of the stomach and their ability to withstand acidity.

Briefly explain the locality of enteroendocrine cell hormone production.

These cells produce hormones locally.

Role of surface mucous cells and mucous neck cells (within gastric gland cells)

Secrete mucous.

Helps protect the epithelium from the HCl of the stomach

What do parietal cells (gastric gland cells) secrete?

* HCl
* Intrinsic factor

Role of HCl produced by parietal cells of gastric gland cells

pH 1.5-3.5

Denatures protein, activates pepsin, breaks down plant cell walls, and kills many microbes.

Role of intrinsic factor produced by parietal cells of gastric gland cells.

Glycoprotein is required for absorption of vitamin B12 in the small intestine.

Role of chief cells (within gastric gland cells)

Secretion:

* Pepsinogen
* Gastric lipase

Role of pepsinogen within chief cells of gastric gland cells

Inactive form of pepsin (hydrolyzes proteins).

* Activated by HCl and pepsin (positive feedback mechanism).

Role of gastric lipase within chief cells of gastric gland cells

Hydrolyze a small portion (around 15%) of lipids.

This is because bile salts aren’t yet present.

Explain the role of G cells within gastric gland cells

Enteroendocrine cells that secrete gastrin.

* Stimulates gastric secretions and promotes movement of food along the GI tract.

(Activates things in a downstream way).

Brief role of:

a) Surface mucous cells

b) Mucous neck cells

c) Parietal cells

d) Chief cells

e) G cells

a) Secretes mucus

b) Secretes mucus

c) Secretes hydrochloric acid and intrinsic factor

d) Secretes pepsinogen and gastric lipase

e) Secretes the hormone gastrin

When do gastric acid secretions occur?

When HCl is formed by parietal cells

How are protons (H+) secreted into the stomach lumen?

Proton pumps.

How do chloride ions (Cl_) move into parietal cells?

Via Cl-/HCO3- antiporters.

The ions then diffuse into the stomach lumen through Cl- channels.

* Resulting in an increase in bicarbonate (HCO3-) in the blood leaving the stomach (alkaline tide)

True or False:

Blood becomes more alkaline after you eat.

True

What is HCl secretion stimulated by?

* Ach: Released prom parasympathetic neurons
* Gastrin: Produced by G cells
* Histamine: Paracrine released from mast cells in lamina propria

On the topic of gastric acid secretions, why does histamine act synergistically?

To enhance the effects of Ach and gastrin

Name the three overlapping phases for the regulation of gastric secretion and motility

* Cephalic (reflex) phase
* Gastric phase
* Intestinal phase

Cephalic (reflex) phase for the regulation of gastric secretion and motility

Triggered by aroma, taste, sight, or thought.

* Increased saliva secretion and gastric gland acuity.

Gastric phase for the regulation of gastric secretion and motility

Triggered by stretch and chemical stimuli.

* Neural and hormonal mechanisms increase gastric secretion and motility

Intestinal phase for the regulation of gastric secretion and motility

Distention of the duodenum inhibits gastric motility and emptying

* Neural and hormonal factors slow the exit of chyme and enhance digestion of food in the small intestine.

Role of accessory digestive organs

Facilitate mechanical breakdown of food and produce specialized secretions to aid digestion.

What are the accessory digestive organs?

* Teeth, tongue, and salivary glands
* Gallbladder, liver, and pancreas are connected to the GI tract by a series of ducts

What is our only internal organ that can regenerate?

The liver

What is the liver made up of?

Four lobes of lobules, made of plates (hepatocytes/liver cells).

Role of liver hepatocytes

* Secrete bile
* Process blood-borne nutrients, store fat-soluble vitamins
* Play important roles in detoxification

Hepatocytes surround a central vein which:

Drains hepatic sinusoids (highly permeable capillaries, which are very leaky).

Portal triad of the liver:

* The branch of the hepatic artery: Supplies hepatocytes with oxygen-rich blood.
* Branch of the hepatic portal vein: Carries venous blood rich in nutrients from intestines.
* Bile duct: Attached to bile canaliculi (goes into the gallbladder where it’s stored).

Role of the gallbladder

Storing and concentrating (bile salts, water, etc. to make it stronger) bile that is not needed immediately for digestion

(Bile usually isn’t needed unless you’re eating a meal).

What is bile made up of?

* Water
* Bile salts
* Cholesterol
* Lecithin (phospholipid)
* Pigment
* Ions

Digestive secretion vs. excretory product of bile

Digestive secretion: Emulsification and absorption of lipids.

Excretory product: Bilirubin (derived from heme).

Gallstones

Form when bile salts are insufficient, or excessive cholesterol crystalizes.

What changes in terms of bile secretion when the gallbladder is removed?

Bile is still created, but it cannot necessarily be “dumped” as it used to be able to.

\* Diet needs to be mediated.