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Human Human

DigestiveDigestDigestiveDigestive System - ive System - YouTube.flv YouTube.flv

SystemSystem

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5 Digestion Processes (IDAAE)• Ingestion: taking in of food into the body.

• Digestion: breaking down of food into simpler substances

• Absorption: diffusion of food from small intestine into the blood

• Assimilation: using digested nutrients to make new material

• Egestion: removal of undigested waste material

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Recall. . . What are the main organs of the alimentary canal?

Mouth Oesophagus Stomach Small intestine Large intestine Anus

Although not part of the alimentary canal, the liver, gall bladder and pancreas are closely associated with it. They play an important role in digestion by secreting digestive enzymes.

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Digestion– Mechanical / physical digestion physically

breaks down the food in the mouth (chewing). Smaller pieces of food increase surface area for digestion. It also takes place in the stomach (churning of food by the muscular stomach walls)

– Chemical digestion uses enzymes to chemically break down complex food substances into their simplest form. e.g.

Starch maltoseamylase

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Chemical digestion:• Starch (carbohydrate) digestion: in mouth and

small intestine.• Protein digestion: in stomach and small

intestine• Fat digestion: only in small intestine

Why must food be digested???Large molecules of food are unable to pass through cell membranes, thus must be broken down into small molecules so that they can diffuse through cell membranes into the blood stream

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Mouth (Ahhh....)• Mouth ingests food • Teeth masticates food into small pieces to

increase surface area for digestion• Saliva (pH 7) moisten and soften food

• Starch maltose• Tongue mixes food with saliva and rolls

food into a bolus before swallowing• Saliva - water, mucus, salivary amylase

Salivary amylase

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Swallowing https://vimeo.com/76306735

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trachea (windpipe)

glottis

During breathing, the larynx is lowered and the glottis is open.

pharynx

oesophagus

larynx (voice-box)

air

What Happens During Breathing and Swallowing?

Normally, air passes into the trachea (windpipe) while food passes into the oesophagus.

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During swallowing, the larynx is raised and the glottis is covered by the epiglottis. This prevents food particles from entering the trachea.

pharynx

trachea (windpipe)

oesophagus

glottis

epiglottis

food particles

larynx (voice-box)

What Happens During Breathing and Swallowing?

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Occasionally, small particles of food or water may get into the larynx or trachea.

trachea (windpipe)

larynx (voice-box)

food particles

What Happens During Breathing and Swallowing?

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What Happens During Breathing and Swallowing?

This automatically induces violent coughing to force the food particles or water out and to prevent choking.

trachea (windpipe)

larynx (voice-box)

food particles

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Oesophagus• Minimal digestion

• Carries food from mouth to stomach by peristalsis

• Oesophagus has circular and longitudinal muscles which are antagonistic.

• When circular muscles contract, longitudinal muscles relax and vice-versa.

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The two layers of muscles cause rhythmic, wave-like contractions of the gut walls. Such movements are known as peristalsis.

Peristalsis:

• enables food to be mixed with the digestive juices; and

• moves the food along the gut.

Part of the gut wall

Peristalsis

circular muscles

longitudinal muscles

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Wall here constricts. Circular muslces

contract; longitudinal muscles relax

Wall here dilates

Direction of movement of food

Circular muscles relax

Longitudinal muscles contract

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Peristalsis – Move the food down!• When circular muscles

contract, longitudinal muscles relax. Gut wall constricts i.e. gut becomes narrower and longer. Food is squeezed or pushed forward.

• Gravity and slippery mucous lining helps push food down too.

• http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html

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Stomach• Stores food temporarily

• Stomach muscles churns and mixes food (also by peristalsis) with gastric juice to form chyme.

• Gastric juice contains hydrochloric acid (HCl) and enzymes like rennin and pepsin

– HCl is very acidic (pH2), thus it kills bacteria and other microorganisms, as well as stopping the action of salivary amylase

– Provides acidic medium for gastric enzymes to work

• Only protein digestion here

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• The stomach is “guarded” at the entrance and exit points by sphincter muscles which control the amount of food entering and leaving the stomach.

https://vimeo.com/76306734

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Small Intestine

• Subdivided into duodenum, jejunum and ileum

• In the small intestine, chyme stimulates1. Pancreas to secrete pancreatic juice

2. Gall bladder to secrete bile

3. Intestinal glands to secrete intestinal juice

• All three juices secreted are alkaline,

pH 8.5

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bile

intestinal juice

pancreatic duct

pancreatic juicebile duct1

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2•Pancreatic and intestinal juice contain many digestive enzymes.

•Bile does not containenzymes. Bile emulsifies fats, increasing thesurface area for lipase action

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Villi – absorption by diffusion

DiffusionDiffusion

From intestine

To

To liver

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Large Intestine (colon)

• Large inverted U shaped tube.

• No digestion takes place here

• Absorbs water and minerals salts

• Stores the faeces (dead cells, mucus, germs, undigested food)

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Is the colon the main region for water absorption?

No! About 94% of the total amount of water passing through the alimentary canal is absorbed by the small intestine! The large intestine absorbs most of the remaining 6% of water.

Rectum – temporarily stores faecesAnus – egests (= removal of undigested

matter) faeces

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Organs associated with the alimentary canal

These organs do not digest food but aid in digestion

• Gall bladder

• Pancreas

• Liver

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Lacteal – fatty acids and glycerol recombine

in the epithelium toform fat which then enters the lacteal as

fine fat droplets

Blood capillaries – transport sugars and amino acids away from the

small intestine

One cell thick epithelium – for efficient absorption of

food particles

This continual transport of digested food substances maintains the concentration gradient for the absorption

of digested food substances.

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Gall bladder• Temporarily stores bile (smelly green

substance) secreted by liver. • Secretes bile in the presence of chyme.• Bile breaks up large fat droplets into very

small fat droplets to increase surface area for lipase action (Emulsification) Bile emulsifies fats

• *Bile is not an enzyme, so it is not affected by temperature

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Pancreas• Connects to small intestine by pancreatic

duct• Produces pancreatic juice• Secretes hormones like insulin (controls

blood glucose concentration) and glucagon (controls carbohydrate metabolism)

Liver• Produces bile, which is stored in the gall

bladder

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Absorption Adaptations of the small intestine

• Small intestine is very long (~5 m)

• Internal surface of the small intestine has many folds.

• On these folds, there are many finger-like projections called villi

• These 3 adaptations increase surface area for absorption

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Assimilation• After travelling through the blood stream to

the rest of the body, cells can now make use of– glucose as source of energy– amino acids to build new cytoplasm and tissue

cells– fatty acids to build new cell membranes