FROM FOOD TO FUEL

FROM FOOD TO FUEL

As discussed: Many factors influnce our choice of foods. These include not only what we choose to eat, but also: When we choose to eat Why we choose to eat Where we choose to eat How much we choose to eat Who we choose to eat with

As discussed: Many factors influnce our choice of foods. These include not only what we choose to eat, but also: When we choose to eat Why we choose to eat Where we choose to eat How much we choose to eat Who we choose to eat with

While these are indeed “choices” in the broad sense, they are in fact regulated by the brain to meet the body’s nutritonal needs.

The Beginnings of Our Food Experience

Increased blood flow to digestive organs

Many things trigger appetite and the desire to eat.

The Beginnings of Our Food Experience

Increased blood flow to digestive organs

Digestive System: The big picture:

Food (proteins, carbohydrates, lipids, minerals, vitamins, water, etc.) must be ingested.



Proteins, lipids, & carbohydrates are digested to smaller molecules.

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These smaller molecules as well as water, minerals, and vitamins are absorbed into the blood.

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These smaller molecules as well as water, minerals, and vitamins are absorbed into the blood.

Undigestable material must be eliminated through the anus.

The processes of IngestionMotilitySecretionDigestion (mechanical; chemical)AbsorptionTransport of Nutrients in bloodElimination

define the functions of the digestive sytem

Digestive System:

Digestive System: Two groups of organs

Alimentary Canal or Gut:Hollow tube that begins in the mouth and ends in the anus.

Mouth, pharynx, esophagus, stomach, small intestine, large intestine and rectum.

Sphincters ensure that food moves only in one direction

Digestive System: Two groups of organs

Alimentary Canal or Gut:Hollow tube that begins in the mouth and ends in the anus.

Mouth, pharynx, esophagus, stomach, small intestine, large intestine and rectum.

Sphincters ensure that food moves only in one direction

Accessory Organs:Salivary glands, pancreas, liver, gall bladder

Digestive System:

It all begins in the mouth:

Chewing (“mechanical digestion” begins to break down food.

Digestive System:

It all begins in the mouth:

Chewing (“mechanical digestion” begins to break down food.

Three pairs of salivary glandssecrete saliva, which contains: - water to moisten the food, - mucous to lubricate the food, - enzymes to begin chemical digestion:

amylase to digest starchlipase to digest fats

Digestive System:

Enzymes: By definition – Speed up chemical reactions Either break things apart or put things together

Often (but not always) identifiableby suffix “-ase” after type of

molecule it works on “Lipase” breaks down lipids into fatty acids and glycerol“Protease” breaks down proteins into amino acids“Amylase” breaks down amylose into monosaccharides“Sucrase” breaks down sucrose into glucose and fructose“Glycogen synthase” synthesizes glycogen“Fatty acid synthase” synthesizes fatty acidsetc.

Digestive System:

When you swallow:

The mass, or “bolus” of chewed food and saliva is pushed into the pharynx (throat), then into the esophagus which carries it to the stomach.

Digestive System:

This movement through the esophagus is due to rhythmic contraction of muscle in its wall, called “peristalsis”.

Digestive System:

This movement through the esophagus is due to rhythmic contraction of muscle in its wall, called “peristalsis”.

Peristaltic contractions continue in the stomach,mixing the swallowed food with its secretions

Digestive System:

Secretions of the Stomach: - Water - Mucous to protect the stomach - Hydrochloric acid: digests proteins; kills bacteria - Pepsinogen Pepsin: digests proteins - Gastric lipase: digests fats - Gastrin: hormone which stimulates gastric secretions - Intrinsic factor: will assist absorption of vitamin B12 from the intestine.

Digestive System:

Secretions of the Stomach: - Water - Mucous to protect the stomach - Hydrochloric acid: digests proteins; kills bacteria - Pepsinogen Pepsin: digests proteins - Gastric lipase: digests fats - Gastrin: hormone which stimulates gastric secretions - Intrinsic factor: will assist absorption of vitamin B12 from the intestine.

When the food mixes with these secretions, the fluid is called chyme.

Digestive System:

Chyme is released in small amounts through the pyloric sphincter of the stomach, entering the small intestine.

Thus, food that entered the stomach in a just a few minutes is partially digested and released into the small intestine over many hours, where it will be further digested and absorption will occur.

Digestive System:

Layers of Alimentary Canal:MucosaCircular muscleLongitudinal muscle

It is within that mucosa of the intestine that absorption will occur

Digestive System:The small intestine has 3 parts: Duodenum: Bicarbonate from pancreas neutralizes acid from stomach. Digestive enzymes from pancreas and bile from liver & gall bladder added, so digestion continues.


Jejunum: Some digestion continues, but mostly absorption


Jejunum: Some digestion continues, but mostly absorption

Ilium: Absorption

Total length ~ 3 meters

Digestive System:

As digestion and absorption occur, the contents of the small intestine are pushed along by peristalsis.

Digestive System:


Carbohydrates, proteins, and fats are digested into molecules which are small enough to be absorbed from the intestinal lumen into the capillaries or lymphatic vessels in its wall.

Digestive System:


Carbohydrates, proteins, and fats are digested into molecules which are small enough to be absorbed from the intestinal lumen into the capillaries or lymphatic vessels in its wall.

This is fairly rapid – most of the fluid passes through the small intestine in three to five hours.

Digestive System:

Most of the digestion of proteins, lipids, and carbohydrates occurs in the duodenum:

Proteins Amino acids Lipids Fatty acids & glycerol Carbohydrates Monosaccharides

Those molecules, along with vitamins, and minerals which were ingested , are primarily absorbed in the jejunum, which continues in the ilium.

Digestive System:

The surface area of the small intestine available for absorbtion is tremendously increased in three ways.

1. Its mucosa is thrown into large folds.

2. Millions of finger-like villi project up into the lumen.

3. The membranes of the cells form microvilli.

The net effect is a massive surface area of cellular membranes across which nutrients can be absorbed.

Digestive System:

Some nutrients are absorbed by passive diffusion – they need no assistance to enter the cells lining the intestine.

Digestive System:


Some nutrients need the assistance of a protein carrier to be absorbed into the cells lining the intestine

Digestive System:


Some nutrients need the assistance of a protein carrier to be absorbed into the cells lining the intestine

Some nutrients need to be forced or “pumped” into the cells lining the intestine, requiring the use of energy

Digestive System:After nutrient molecules enter the cells lining the intestine (whether by active transport, facilitated diffusion, or passive diffusion) they are then passed out the other sides of those cells into the connective tissues below, which contain many capillaries and lymphatic vessels.

By the same three mechanisms – active transport, facilitated diffusion, or passive diffusion – those nutrient molecules then enter the capillaries and lymphatics.

Digestive System:

Digestive System:Anything not digested and/or absorbed in the small intestine enters the large intestine, also called the colon.Total length ~ 1.5 to 2 meters

No digestion of nutrients occurs here, although some bacteria digest cellulose (fiber).

The primary function of the large intestine is absorption of water and the minerals (potassium, sodium, chloride) that accompany it.

Digestive System:Anything not digested and/or absorbed in the small intestine enters the large intestine, also called the colon.

This is slow – peristalsis in the large intestine is much slower than it was in the small intestine.

It takes 20 to 40 hours for the material to pass through this organ, during which 80% to 90% of the water is absorbed back into the blood.

The more fiber (undigestable carbohydrate) in your diet, the more water will be retained in the feces.

Digestive System:

Feces consist of approximately 60% solid material (undigestable fiber, dead intestinal cells, bacteria, waste products in bile, etc.) and 40% water.

Digestive System:Let’s back up for a moment to the pancreas, liver, and gall bladder.

1. The liver makes many of the proteins and lipids of the blood and removes many waste products from it.


1. The liver makes many of the proteins and lipids of the blood and removes many waste products from it.

Two functions in nutrition: a. It stores extra glucose after a meal, releasing it back into the blood when needed. b. It produces bile, which helps the digestion and absorption of fats by “emulsifying” them in the water


The liver is constantly making bile, but your intestine only needs it when chymeis present. Thus, extra bile is stored in

2. The gall bladder.

Attached to inferior surface of liver Holds bile until stimulated by hormone cholecystokinin to release it down the common bile duct to the duodenum.


1. The pancreas is located inferior to the liver and stomach.

It produces bicarbonate ions and most of the enzymes which digest nutrients in the small intestine.

These are sent to the duodenum through the pancreatic duct.

Disorders of the Digestive System:

Obviously, things don’t always go right. Many problems can interfere with normal functions of the digestive system.









Next, we’ll move on to discuss the major macronutrients: Carbohydrates, Lipids Proteins

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FROM FOOD TO FUEL