1.2 Mouth-Stomach

Lumen

Space inside the gastrointestinal tract

Where does the lumen start and where does it end?

Goes from the mouth to the anus 

Is the lumen located inside or outside the body?

Outside of the body  

Where does digestion occur in the lumen?

Digestion occurs in the lumen  

Where does absorption occur in the lumen?

Absorption occurs at the surface of the lumen  

What is the function of the sphincters?

They work to slow down the flow of digesta. In order to digest something, you have to slow it down long enough so the body can absorb it. Another way to think of this is that it will increase retention.  

How do you slow down the flow of digesta and increase retention?

• Block the flow --> Sphincters  

• Increasing the length of the tract  

• Make the nutrients smaller for absorption 

How do you block the flow of digesta?

Sphincters

Kp

Rate of passage; how fast the digesta move 

How does slow Kp affect digestion?

Slow Kp = Increases retention time = Increases digestion  

How do you make nutrients smaller for digestion?

• Mechanical breakdown  

  • Chewing  

• Enzymatic breakdown 

• !Mechanical and enzymatic breakdown influence Kd!

Kd

Rate of digestion; how fast nutrients are digested  

Do Kp and Kd work together?

No, Kp and Kd are in competition! Food can be absorbed, or it can be excreted.  

% of a nutrient absorption

extent of digestion= Kd/ (Kd+Kp) 

How are Kd and Kp related?

When we have an increase of Kd, the result is an increase in the extent of digestion. When you have a faster rate of digestion, more will be digested. In contrast, if we increase Kp,  less will be digested. If something moves through faster, the extent of digestion will decrease.  

How do we think of Kd in an animal?

In an animal, we think of Kd as a function of the feedstuff the animal is eating.  

Oral Cavity

Mouth; mechanical breakdown; mastication

Mastication

The process of chewing food

How does an increase in surface area increase digestion?

Decreases particle size, which increases digestion because of increased surface area  

• More surface area results in increased enzymatic access  

Functions of the oral cavity

• Decreases particle size, which increases digestion because of increased surface area  

• Mixing of saliva and food  

  • Mouth is used to bring food into the body, whereas humans can use their hands  

Why is saliva important?

Important for lubrication when swallowing food  

What does saliva contain?

• Saliva has enzymes, which allow digestion to start  

• Saliva contains buffers & minerals 

Why is saliva important?

Salvia is important for oral health  

• Lack of saliva promotes tooth decay  

Esophagus

Tube that transports food from the mouth to the stomach  

Functions of the stomach:

• Storage: provides a place to store food  

• Controls the flow of digesta (food) and secretions to the small intestine 

• Mixing of the digesta (aka physical processing)  

• Secretions: enzymes, acid, buffer, and mucus  

• Digestion: Chemical and Enzymatic  

• Absorption

What does the stomach secrete and what are their functions?

• Enzymes: Aid in digesting food  

• Acid: Secreted HCl, which lowers the pH (makes the stomach more acidic). The low pH denatures proteins and kills microbes.   

• Buffer: HCO3-, which raises the pH (makes the stomach more basic). Close to the surface. Helps to maintain a neutral pH, which helps protect the surface of the stomach (prevents ulcers!) 

• Mucus: Protects the surface of the stomach from acid & enzymes.  

  • Problems protecting the stomach can result in a gastric ulcer!! 

What is a gastric ulcer, and which mammals are we concerned about getting them?

A gastric ulcer is damage to the surface of the stomach (an erosion or hole). The animals we are concerned about getting gastric ulcers are the horse, pig, and human.

What causes gastric ulcers?

Gastric ulcers can be caused by stress and the use of NSAIDs (Nonsteroidal anti-inflammatory drugs). NSAIDs erode the mucus and make it harder to protect it from enzymes.  

Types of digestion:

Chemical and Enzymatic

Chemical Digestion

Acid; low pH

Enzymatic digestion

Activating zymogens

What do we absorb from the stomach?

• Water 

• Alcohol  

• Copper  

• Florine  

• Molybdenum  

• Aspirin  

What is digestion preparing?

Digestion is preparing ingested food for absorption.  

Mechanical absorption

Reducing particle size

Enzymatic absorption

Hydrolytic reaction (hydrolysis) --> inserting water

Hydrolysis turns NCHOs (Nonstructural carbohydrates) into what?

Monosaccharides to be absorbed

Hydrolysis turns proteins into what?

Amino acids, dipeptides, & tripeptides to be absorbed

Hydrolysis turns lipids (fats) into what?

Monoglyceride and 2 fatty acids to be absorbed

Absorption

When a molecule moves from the lumen (outside the body) into the body (typically it crosses a cell membrane)