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Functions of the digestive system
• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Excretion
The Digestive process can be broken down into 4 parts:
1. Ingestion – taking in of nutrients
2. Digestion – breakdown of complex molecules into smaller components by
enzymes
3. Absorption- transport of digested nutrients to the tissue of the body
4. Excretion – removal of waste food material from the body
Food Facts:
 Takes up to 3 days for food to fully travel through the digestive system
 Adults digestive system is 9m long
 Each person eats about 2kg of food a day → 50 000kg over a lifetime (10
elephants)
Ingestion: eating
 Physical digestion begins in the mouth, where food is chewed and formed
into a bolus (greek word for ball) by the tongue.
In the mouth: physical and chemical digestion starts!
• Teeth: physical breakdown of food into smaller pieces.
• Saliva: chemical breakdown of starch, lubricates food so it can be swallowed
and makes food taste…good or bad
Saliva:
 Contains the enzyme AMYLASE that breaks down starch into simple sugars
 Dissolves food particles
 Activates the taste buds
 the saliva is produced in the salivary glands
 your body can produce between 250 mL to 1500 mL of saliva per day
 the functions of saliva are:
1. thin watery saliva, wets the food
2. a thicker mucus secretion acts as a lubricant and causes food
particles to stick together to form a food mass called bolus
3. softens food
4. saliva also contains a digestive enzymes which breaks down starch
into a sugar – ex. salt crackers (sweet)
Tongue : versatile organ, attached at the floor of the mouth
 it can be twisted, rolled, turned and extended
 it moves food onto the teeth
 it mixes with saliva
 tongue rolls it into a ball called bolus
 taste buds, detect dissolved chemicals
Teeth
• Adult humans have 4 different kinds of teeth
1. Incisors
2. Canines
3. Premolars
4. Molars
• Children are born with primary teeth (20) which get replaced ( also called
deciduous teeth)
Teeth uses
• Incisors (8) and canines (4) are used for biting, tearing and cutting through
food
• Premolars (8) and molars (12) are at the back of the mouth and are used for
grinding the food down
Parts of the tooth:
 Crown – the visible part that is above the gum
 Root – the part below the gum line that is what holds the tooth in
 Enamel – protective coating - hardest substance in the body, can not be
replaced if chipped
 Dentine – hard bone that gives the tooth shape and strength. It is sensitive
to temperature, sugar, touch, acids, etc.
 Pulp Chamber – receive and transmits messages of any sensation in the
dentine
 Gum Line – Help hold and protect teeth, very important to keep healthy
 Root canal – carry blood and nerve endings
 Cementum – connect tooth to the jaw bone
What causes tooth decay?
• -Bacteria, called Plaque, live in your mouth and eat any food that stays on
your teeth. This bacteria excretes acid which eats away at the enamel on
your teeth causing pits called cavities.
• Sugar does not cause tooth decay, it feeds the bacteria that do
Keep your teeth healthy
• brushing and flossing remove the plaque from your teeth and help the gums
stay healthy
• protect your teeth from injury by wearing mouth guard in sports
• Don’t put hard objects in your mouth to chew on them, or try to open bottles
with your teeth
• Regular dental check up- clean away tartar build up and can catch problems
before they worsen.
• Calcium, phosphorus, vitamins A, C and D all help maintain health of teeth
and gums
Dental Problems
• Cavities remove protective enamel leaving dentine open to the environment
causing sensitivity and aches
• If bacteria get into the root and get infected you get a very painful condition
called an Abscess
• Periodontal disease affects the tissue around the teeth including retreating
gum line, sore gums or bleeding. Usually caused by poor diet or hygiene.
• Halitosis – bad breath caused by smoking, infection, tooth decay, sinus
infections.
Swallowing :
 The tongue humps itself up and closes off the front part of your mouth – the
soft palate moves upward to partially seal off the nasal passage
 A flap of tissue closes off the nasal passage – so food doesn’t enter your
nose
 A flap of tissue called the epiglottis closes off the trachea so food can’t
enter your windpipe, therefore it enters your esophagus
Review:
• Food has been:
mechanically chewed by the teeth
moistened and partially digested by saliva
• Saliva contains the enzyme AMYLASE that breaks down starch into simple
sugars
• The bolus is swallowed and then goes through the pharynx, closes the
epiglottis (so food doesn’t enter the trachea)
• and enters the esophagus…
Esophagus
• Food and fluids move thru the esophagus NOT because
of gravity but because peristalsis.
• Peristalsis = rhythmic, wavelike muscle contractions
• Peristalsis is why we can drink while standing on our
head!
Sphincters
• Is a ring-shaped muscle that relaxes or
tightens to open or close a passage or
opening in the body.
• Examples are the anal sphincter (around the
opening of the anus) and the pyloric
sphincter (at the lower opening of the
stomach).
• There is a sphincter at the top and the
bottom of the esophagus.
Hiccups?
• The diaphragm is used in the respiratory system but also plays a role in the
digestive system.
• When a person gets the hiccups, it is due to uncontrolled spasms of the
diaphragm.
• It is not entirely known what causes hiccups as it may be different for each
person.
– One hypothesis is that it is due to stomach acid moving up through the
lower oesophageal sphincter
Functions of the stomach
 Bulk storage (capacity of approx
1.5L) of undigested food
 Mechanical breakdown of food
(muscular contractions of the
stomach)
 Disruption of chemical bonds via
acids and enzymes
Structure of stomach
• Millions of cells line the inner wall of the stomach
• These cells secrete gastric fluids
Gastric Juices:
1. Mucus: protects stomach against digestion and corrosion
2. HCl: hydrochloric acid kills harmful substances ingested with food
3. Pepsinogen: is activated to pepsin in the presence of HCl, the enzyme pepsin
digests protein
Problems: ulcers and heartburn
Chemical digestion
Part
Enzyme
Digests
Example
Mouth
Amylase
Carbs
Bread
Stomach
Pepsin
Proteins
Meat
HCl (hydrochloric acid) allows pepsin to do its digestion and kills pathogens that
come into the stomach.
Mechanical digestion
Physically breaking food into smaller pieces so there is more surface area for
chemical digestion.
Mouth
Teeth
Stomach
Muscular contractions
From the Stomach to the Intestines
 Pyloric Sphincter – Controls the flow of partially digested food out of the
stomach into the duodenum.
 The food is called chyme at this point.
Small Intestine
 a long, coiled and looped tube
 Most digestion takes place here
 2.5 cm across and 7m long
 Fills most of the abdomen and is held in
place by a thin membrane called
mesentery. The mesentery is also
responsible for carrying blood vessels to
the intestine – providing oxygen to the
tissue and transporting nutrients away to be used by
the rest of the body
 Secretions and buffers provided by pancreas, liver,
gall bladder
 Three subdivisions:
 Duodenum (most digestion occurs here)
 Jejunum
 Ileum
Small Intestine
Small Intestine Cont.
 The chyme coming from the stomach is very acidic….. the small intestine
does not have a way to protect itself like the stomach (mucus)
 Many digestive juices are added at the top of the small intestine through the
common bile duct
 The Pancreas releases biocarbonate (HCO3-) into the top of the small
intestine that neutralizes the acid (HCl)
 Continues to digest food - Enzymes continue the chemical reactions on the
food.
 Food stays in your small intestine for 4 to 8 hours
 The nutrients are broken down small enough to pass through the lining of the
small intestine, and into the blood (diffusion).
 Nutrients are carried away to the liver and other body parts to be processed,
stored and distributed.
 Most nutrient absorption occurs in the small intestine
 The walls have long finger like tubes called villi which increase surface area
for absorption
4 functions of the intestine:
1) keep the food moving along by peristalsis
2) secretes enzymes which continue the digestive process
 Erepsin/Trypsin – completes protein digestion
 Maltase/sucrase/lactase – complete the breakdown of complex
sugars into simple sugars (carbs)
 Lipase – completes the digestion of fats
 Enterokinase – helps pancreas enzymes work
3) Is the site where digestion by chemicals from the pancreas and liver takes
place.
4) Nutrients are absorbed into the blood stream – lined with villi to increase
surface area to increase absorption
The Liver:
 largest gland in the body
 blood that is rich in nutrients and other material is carried from the intestine,
through the liver where toxins are filtered out and many vitamins are stored.
 Produces Bile – it is stored in the gall bladder and is used to emulsify fats (it
helps fats be able to dissolve in water so it can be more easily digested and
absorbed).
 Main Functions of the liver
 Synthesis: produce bile salts
 Breakdown: removes nitrogen
(toxic) from digested food to
be excreted in urine
 Storage: glucose to glycogen
(storage molecule) and back to
maintain blood sugar levels
 Detoxification: convert
harmful compounds (i.e.
alcohol) to less harmful
substances to be excreted
The Pancreas
 produces pancreatic juices which contain
about 28 digestive enzymes and sodium
bicarbonate
 sodium bicarbonate neutralizes the acidic
chime as it comes out of the stomach so no
damage is done to the intestinal wall
 Amylase – completes carbohydrate digestion
 Lipase – completes fat breakdown
 Trypsin and peptidase – complete protein digestion
 Secretes insuline to keep blood glucose levels in equilibrium (homeostasis)
Large Intestine
 1.5m long and 7.6cm wide
 By the time food reaches the large intestine,
digestion is complete.
 Main function is to reabsorb water to
prevent dehydration (makes feces solid
rather than liquid).
 Vitamins B12 and K are produced by
bacteria that live in the colon
 These vitamins and salt are absorbed across the large intestine wall into the
blood stream
 No digestion of nutrients occurs here
 As the waste material is moved through the large intestine, it is packed
together, water is removed (absorbed into blood stream) and it fills the anus
where it is called feces.
 When the rectum is full you get a feeling of discomfort, relaxation of the anal
sphincter allows the feces to pass out of the body
 If the blood vessels surrounding the anus get strained (caused by hard feces,
lack of fibre, etc) the swell – called haemorrhoids.
Troubles with your digestive system
 Many medications may irritate your system
 Most say to take on a full stomach
 Follow these directions!
 Many people over use antacids or laxatives which can lead to a weaker
stomach lining and decreases your own ability to deal with the situation.
 good diet, lots of water and fibre and regular exercise should control
bowels
 avoiding foods that cause stomach irritation is better for you
 over use of antibiotics kills the bacteria that live in your digestive tract,
making digestion harder
 eat yogurt if you are on a antibiotics – it replenishes the bacteria