Chapter 6-Human Digestion

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Chapter 6: Nutrition in Humans

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Learning Objectives(a) describes the functions of main regions of the alimentary canal and the

associated organs: mouth, salivary glands, oesophagus, stomach, duodenum, pancreas, gall-bladder, liver, ileum, colon, rectum, anus in relation to ingestion, digestion, absorption, assimilation and egestion of food as appropriate

(b) Describe peristalsis in terms of rhythmic wave-like contractions of muscles to mix and propel the contents of the alimentary canal

(c) Describe:(i) digestion in the alimentary canal(ii) the functions of a typical amylase, protease and lipase, listing the

substrate and end-products(d) Describe the structure of a villus (including role of capillaries and lacteals) in

absorption(fe State the function of the hepatic portal vein as the route taken by most of the

most absorbed from the small intestine(f) state the role of the liver in:

(i) carbohydrate and fat metabolism(ii) breakdown of red blood cells(iii) metabolism of amino acids and the formation of urea(iv) breakdown of alcohol, including the effects of excessive alcohol

consumption

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6.1 Holozoic Nutrition

What is nutrition?

-Process of taking in food and converting it

into living matter

-The mode of feeding is known as holozoicholozoic

nutrition.nutrition.

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Five 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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DigestionMechanical / 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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6.2 & 6.3 The Mammalian Digestive System

and Digestion in Humans

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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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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

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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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HCl converts inactive pepsinogen and prorennin to their active forms

1.Pepsinogen pepsin

Proteins peptones

2.Prorennin rennin

soluable milk proteins insoluble milk proteins

HCl

HCl

peptones

pepsin

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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.

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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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Duodenum• Starch maltose• Protein polypeptides • Fats fatty acids + glycerol

Ileum• Maltose glucose• Polypeptides amino acids • Fats fatty acids + glycerol • Lactose glucose + galactose• Sucrose glucose + fructose

pancreatic amylase

proteases

lipase

maltase

protease

lipase

lactase

sucrase

Note that the small intestine is the main site of digestion of food and absorption of nutrients.

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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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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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villus

Numerous folds

Epithelial cell

microvilli

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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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• The concentration of simple nutrients (e.g. glucose, amino acids, fatty acids and glycerol) is higher in the lumen of the small intestine then in the blood capillaries that pass through the villi.

• Thus, nutrients diffuse across a region of high concentration (lumen of the small intestine) to the bloodstream, which has a lower concentration. Note that absorption by active transport is also possible.

• The blood capillaries in the small intestine unite to form larger blood vessels, which unite to form the hepatic portal vein, which transports the nutrients to the liver.

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What happens to amino acids and glucose after absorption?

Products released from liver into general blood circulation

Molecules pass into the epithelial cells

Through walls of capillaries in the villus and into bloodstream

The capillaries join up to form veins

Veins unite to form 1 large vein: Hepatic Portal Vein

Hepatic portal vein carries blood to liver

Liver stores or alters products of digestion

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Glucose Amino Acids

• Glucose is used by all cells as a source of energy.

• Excess glucose returned to liver and stored as glycogen. Insulin stimulates liver to convert glucose into glycogen. When the body needs energy, glycogen is converted back to glucose.

• Amino acids which enter the cells are converted into new protoplasm that is used for growth and repair.

• Amino acids used to form enzymes and hormones.

• Excess amino acids deaminated by liver.

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What happens to fatty acids and glycerol after absorption?

Molecules pass into the epithelial cells

Recombine into fats again in the epithelial cells

Fats enter the lacteals

Lymph (fluid in lacteals) + fat = chyle

Lymphatic vessels discharge chyle into bloodstream

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Fats• Blood carries fats to all parts of the body,

especially to the liver.• When there is enough glucose, fats are not

broken down but are used to build protoplam.

• When there is insufficient glucose, fats are broken down to provide energy.

• Excess fats stored in adipose tissues.

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Villi – absorption by diffusion

DiffusionDiffusion

From intestine

To

To liver

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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

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Functions of the Liver1. Regulation of blood glucose concentration

– 70-90mg of glucose / 100cm3 of blood (normal conditions)

2. Production of bile– Liver produces bile which is stored in the gall bladder

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Functions of the liver3. Iron storage

– Red blood cells are destroyed in the spleen and their haemoglobin is sent to the liver to be broken down. The iron released is then stored in the liver. Bile pigments are also formed from the breakdown of haemoglobin.

4. Protein synthesis– Liver synthesizes proteins found in blood plasma,

e.g. albumins, globulins, fibrinogen

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Functions of the liver5. Deamination of amino acids

– Excess amino acids are transported to the liver, where their amino groups are removed and converted to urea.

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Functions of the liver6. Detoxification

– Liver cells contain alcohol dehydrogenase to break down alcohol.

– Excessive alcohol is harmful. Alcohol stimulates acid secretion in the stomach and increases risk of gastric ulcers.

– Prolonged alcohol abuse may lead to liver cirrhosis (destruction of liver cells), which can lead to liver failure and death.

7. Heat production