Download DIGESTION

DIGESTIVE SYSTEM.
Function: To provide the body with water and other nutrients that
are essential for life.
To function effectively the digestive system carries out
five basic activities:
 Ingestion (taking food into the mouth – eating).
 Movement (passage of food along gastrointestinal tract (GI)).
 Digestion(breakdown of food by two types of process:
- Mechanical.
- Chemical.
 Absorption (passage of digested food from GI tract
cardiovascular (circulatory) system to be used by cells.
 Elimination (food substances that cannot be digested and
absorbed are excreted as faeces.
There are four stages of digestion:
 Mouth.
- Ingestion.
- Chewing.
- Swallowing.
- Start of starch digestion.
 Stomach.
- Mixing.
- Protein digestion.
 Small Intestine.
- Carbohydrate digestion.
- Fat digestion.
- Absorption.
 Large Intestine.
- Water, vitamins, minerals absorbed. Waste and excretion.
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The GI Tract (or Alimentary Canal).
 The GI tract is a continuous tube that extends from the mouth
to the anus.
 A GI tract taken from a cadaver (dead body) is about 9m (30ft).
It is shorter in a living person because the muscles are toned.
 GI Tract consists of:
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Mouth.
Pharynx.
Oesophagus.
Stomach.
Small Intestine.
Large Intestine.
Accessory Organs of GI Tract.
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Salivary Glands.
Liver.
Gall Bladder.
Pancreas.
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The GI tract has four basic layers.
 Mucosa. Secretes mucus for lubrication and protection.
 Submucosa (connective tissue that binds mucosa to third
layer), which is:
 Muscularis.
- In the mouth and pharynx the muscularis consists of
skeletal (voluntary) muscle for swallowing.
- The anus is also made of skeletal muscle as we have
voluntary control over defaecation.
- Rest of GI tract consists of smooth (involuntary) muscle.
- Muscularis is usually in two sheets;
o Inner circular fibres.
o Outer longitudinal fibres.
- Involuntary contractions of smooth muscle, peristalsis,
help to break food down mechanically, mix it with
digestive enzymes and propel it along the tract.
- Muscularis also contains major nerve supply to GI tract.
 Serosa.
- Outermost layer of most portions of the GI tract.
- It is a serous (watery) membrane composed of connective
tissue and epithelium.
- Abdominal serosa is called the peritoneum.
- Peritoneum is the largest serous membrane in the body.
(Other main serous membranes are the pericardium of
the heart and pleura of the lungs).
- Mesentery is part of peritoneum that attaches small
intestine to posterior abdominal wall.
- Mesocolon attaches large intestine to posterior abdominal
wall.
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Stage 1: Mouth (Oral Cavity).
 Cheeks.
- Muscles that form walls of mouth, terminating in superior
and inferior lips.
- Keep food in place whilst masticated (chewed) by teeth.
 Hard palate (bony).
 Soft palate (muscle).
 Tongue.
- Skeletal muscle covered with mucous membrane.
- Manoeuvres food for mastication.
- Shapes food into bolus.
- Forces bolus into back of mouth to be swallowed.
- Contains taste buds.
 Teeth.
- Adult x 32.
- Primary x 20.
- Incisors x 8.
- Canine x 4.
- Premolar x 8.
- Molar x 8.
- Wisdom x 4.
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Pharynx.
 No voluntary control from here.
 Bolus passes through into the;
Oesophagus.
 Muscular collapsible tube lying behind the trachea (10in long
approximately).
 Transports food to stomach.
 No digestive enzymes produced.
 No absorption takes place.
 Peristalsis (involuntary wave like movement of muscularis
controlled by medulla oblongata – see diagram.).
 Mucus helps passage of bolus.
 Solid food takes 4-8 seconds to reach stomach.
 Fluid takes approximately one second to reach stomach.
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Stage 2. Stomach.
 Stomach is a ‘J’ shaped organ of the GI tract.
 Stomach is situated under diaphragm and lower ribs on left
side of abdomen.
 Cardiac sphincter at top end of stomach (ring like muscle)
prevents stomach contents going back into the oesophagus.
 Pyloric sphincter at bottom end of stomach prevents
stomach contents entering small intestine too soon.
 Empty stomach is approximately the size of a large sausage,
however it stretches to contain a large amount of food.
 When empty the mucosa of the stomach lies in large folds
called rugae. As the stomach fills up with food these folds
become ‘ironed out’.
 Stomach is a temporary storage area for food.
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Fatty foods – 6hours.
Protein – 4hours.
Carbohydrate – 2hours.
Water – 20minutes.
 In stomach food is mixed by peristalsis.
 Gastric juices mixed into the food.
 Absorption: Water, glucose, alcohol and some drugs.
 Food is churned until it becomes a fluid called chyme.
 Chyme is pushed into the small intestine via the pyloric
sphincter by peristaltic action.
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Stomach Secretion: Gastric Juice.
Gastric juice starts the chemical breakdown of proteins.
 Water: further liquefies the swallowed food.
 Mucus: for lubrication and protection of stomach.
 Hydrochloric acid:
- acidifies the food and stops the action of salivary
amylase/ptyalin.
- converts pepsinogen into active enzyme pepsin for
breakdown of proteins).
- kills harmful micro-organisms.
 Pepsinogen: Changed into active enzyme pepsin by
hydrochloric acid. Pepsin starts the digestion of proteins.
Pepsin works best in pH range 1.6-3.2.
 Intrinsic factor: A protein necessary for absorption of
Vitamin B12. Vitamin B12 is essential for the production of
erythrocytes.
Stage 3: Small Intestines.
 Duodenum.
 Jejunum.
 Ileum.
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Functions of the small intestines.
 Each segment allows chemical digestion to continue and
absorption of nutrients to take place.
 90% of nutrients are absorbed in the small intestines.
 Onward movement of nutrients (peristalis).
 Secretion of intestinal juices.
 Digestion of carbohydrates, fats and proteins completed.
 Many lymph nodes throughout protect the body from microorganisms that survive gastric hydrochloric acid.
 Secretion of hormone cholecystokinin (causes gall bladder to
contract thus sending bile into duodenum when fat and acid
present, also stimulates pancreas to secrete enzymes.
 Mucosa and submucosa of small intestines are adapted for
absorption of food by having fingerlike tufts called villi.
 Superior mesenteric artery and vein responsible for blood
flow to and from GI tract.
 Nerve supply is from the sympathetic and parasympathetic
nerves (autonomic nervous system). This nerve supply
forms an important role in homeostasis.
- Sympathetic nerves:
 Contracts sphincter muscles.
 Decreases activity.
 Inhibits secretary glands.
- Parasympathetic nerves:
 Relaxes sphincter muscles.
 Increases activity.
 Stimulates secretary glands.
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Duodenum.
 The duodenum is approximately 25cm (10in) long.
 Ducts from the pancreas and gall bladder enter via a
common opening in the middle.
 Chyme leaves the stomach in small amounts to enter the
duodenum.
Jejunum.
 The jejunum is approximately 2.6m (8ft) long.
 ‘Jejunum’ means ‘empty’ because at death it is always found
to be empty.
 Absorption continues until remaining nutrients enter final
part of small intestines.
Ileum.
 The ileum is approximately 3.6m (12 ft) long.
 Ileum means ‘twisted’.
 Nutrients are absorbed at different sections of the ileum by:
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Diffusion.
Osmosis.
Facilitated osmosis.
Active transport.
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Stage 4: Large Intestine.
Large intestines are 1.5m (5ft) long and 6.5cm (2.5in) diameter.
 Caecum – 6cm (2.5in).
 Ascending colon.
 Transverse colon.
 Descending colon.
 Sigmoid colon.
 Rectum – 20cm (8in).
 Anus – opening to exterior.
Function of the Large Intestine.
 Absorption of water, salt, minerals, vitamins and drugs.
 Storage of food residues, cells, and bacteria until eliminated
by defaecation.
Rectum.
 Immediately in front of sacrum.
 Forms the last 20cm (8in) of large intestine.
 Rectum acts as reservoir for faeces.
 When rectum is full of faeces the sensation of pressure
signals relaxation of sphincter muscles around anus to allow
faeces to be expelled (defaecation).
Anus.
Sphincter muscle that opens to allow faeces to be expelled.
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Accessory Organs That Do Not Come Into Direct Contact
With Food.
Salivary Glands.
- Secrete saliva continuously into the mouth, usually just
enough to keep mouth and pharynx moist.
- When food enters the mouth saliva secretion increases to
lubricate, dissolve and begin chemical breakdown of food.
- Three main pairs of salivary glands;
o Parotid (in front of ear).
o Sublingual (under tongue).
o Submandibular (bottom of jaw).
- Saliva is;
o Water (99.5%).
o Other solutes (0.5%) Including;
o Lysosome (bacterial enzyme).
o Amylase/Ptyalin (starts breakdown of starch).
Pancreas.
 The pancreas is a soft pink gland that lies across the back of
the abdomen behind the stomach.
 Its tip extends to the spleen and the rounded head fits into
the curve of the duodenum.
 The pancreas is both an endocrine gland and an exocrine
gland. It excretes pancreatic juice and insulin.
 Endocrine glands produce hormones that are secreted
directly into the blood steam instead of through duct/tubes.
 The Islets of Langerhans are endocrine cells.
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Hormones produced by the Islets of Langerhans in the
pancreas
 Glucagon: To raise blood sugar.
 Insulin: To lower blood sugar.
 Somatostatin: To maintain homeostasis by inhibiting
production of insulin and glucagons to ensure that blood
levels of glucose remain within the normal parameters
range of 4-9mm (millimoles).
Exocrine gland secretions reach their destination via
ducts.
Pancreatic Juice.
 Up to two litres produced daily.
 Pancreatic juices enter the duodenum via the pancreatic
duct.
Pancreatic Juice contains.
 Water.
 Mineral salts.
 Enzymes.
- Trypsinogen – turns proteins into amino acids when
converted into active trypsin by enterokinase, which is
found in the duodenum.
- Pancreatic amylase – turns starch into maltose (malt
sugar)
- Lipase – splits fat into fatty acids and glycerol following
action of bile.
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Liver.
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Liver is the heaviest gland in the body around (1.4kg (3lb).
It is the second largest organ in the body (after the skin).
Located under the diaphragm on right side of abdomen.
It is estimated that it carries out over 500 functions.
Functions of the liver.
 Produces plasma proteins including albumen and blood
clotting factors.
 Metabolises heparin (anti-coagulant).
 Forms cholesterol from fat that is used for cell membranes.
 Neutralises toxins produced by the body and from food and
drugs.
 Neutralises and excretes bilirubin (produced when old
erythrocytes are broken down.
 Regulates levels of nutrients in the blood (eg glucose).
 Stores iron, vitamins A, D, E and K.
 To maintain homeostasis liver can produce erythrocytes, but
only in an emergency when there is a severe deficiency of
red blood cells.
 Liver has a high metabolic rate, which causes it to be
warmer than other organs.
 Liver produces and excretes bile.
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Bile.
 Bile is produced by the liver and stored in the gall bladder.
 The gall bladder is a pear shaped sack that is attached to the
liver by connective tissue.
 Approximately 2 pints of bile is produced daily.
 Bile is expelled into the duodenum via the bile duct.
Bile consists of:
 Water.
 Bile salts (emulsifier).
 Cholesterol.
 Bile pigments (colour).
 Salts/ions.
 Lecithin (emulsifying agent).
Functions of Bile:
 Main role is to emulsify fatty lipids into smaller droplets.
This gives a much larger surface area thus allowing the
pancreatic juices and lipase to digest them very quickly.
 Bile salts assist the absorption of digested fats, cholesterol
and fat soluable vitamins (A,D, E, K).
 90% of bile salts are reabsorbed in the lower small intestine
and returned to the liver.
 Bilirubin is the bile pigment that gives colour to and
deodorises faeces.
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