- Ingestion: taking in.
- Digestion: breaking down.
- Absorption: food entering body.
- Egestion: removing undigested food, bacteria dead cells.
Think of the digestive system as a long tube that goes through the body, starting at the mouth and ending at the anus. This is called the alimentary canal. (Alimentary means “relating to food”)
- There are four types of teeth here used to help with mechanical breakdown.
- Front to back: Incisors, canines, premolars and molars.
- Chemical breakdown also occurs as enzyme amylase digests starch to maltose.
- Alkaline environment.
- Saliva contains lysozyme that kills microogranisms.
- This is a muscular pipe.
- Moves food down the body by peristalsis.
- Peristalsis is an involuntary wave of muscular contraction.
- Peristalsis happens in the intestines too.
- Fibre stimulates peristalsis.
- There is a sphincter between the oesophagus and the stomach. A sphincter is a collection of muscle fibres that encircle a tube to stop movement (think of it like a hair bobbin). This is known as the cardiac sphincter (nothing to do with the heart as such. The name simply means that it is located closer to the heart than sphincter at the far side of the stomach, known as the pyloric sphincter).
- The stomach stores and digests food.
- It is a muscular bag, which churns food to make chyme.
- The stomach lining makes:
the activated enzyme breaks down protein to peptides
- hydrochloric acid (HCl)
it softens food
denatures amylase (that comes in from the salivary glands of the mouth and functions best in an alkaline environment)
- Peptic ulcers (“stomach ulcers”) develop when the protective mechanisms fail or, more commonly, due to a bacterial infection with Helicobacter pylori. This presents with symptoms of pain, heartburn and nausea. Treatments: antibiotics. If the ulcer isn’t treated, it can eat through the entire stomach wall, or “perforate”. This is a life-threatening emergency treated surgically.
- Makes sodium hydrogen carbonate to neutralise acid from the stomach.
- Enzymes such as amylase (starch to maltose) and lipase (digests fats to fatty acids and glycerol) and proteases (proteins to amino acids)
- Has an important endocrine function (insulin, glucagon)
- Makes bile which is alkaline and emulsifies fat to aid digestion. Bilirubin and biliverdin (breakdown products of old red blood cells) and excess cholesterol and excreted with bile. Bile is stored in the gall bladder.
- Stores many fat-soluble vitamins (A, D, E, K), glycogen and minerals
- Important in maintaining the temperature of the blood passing through it
- Receives blood from the intestines via the portal system. The intestines are different to all other organs: the veinous blood from the intestines doesn’t drain straight to the heart. It drains to the liver first. Why? The nutrients/chemicals/toxins/drugs that we ingest end up being absorbed into our blood. The liver is a metabolic sorting station that deals with all of this input, detoxifies and maintains homeostasis. It literally serves a filter between the intestines and the heart by filtering the blood that arrives through the portal vein.
- Forms cholesterol (used in sex hormones)
- Deamination (breakdown of amino acids with the production of urea)
- Makes fibrinogen (a clotting protein)
- Main location for digestion.
- Contains a range of digestive enzymes. These are delivered from the pancreas.
- Receives bile from the gall bladder
- Alkaline environment
- Absorbs food (by this point the enzymes and the food have had a chance to mix, so the food has been broken down into the basic building blocks that can be absorbed into the blood). The walls of the small intestine are one cell thick to aid this.
- Lined with many villi (finger-like outgrowths, villi = plural, villus = single) to increase surface area. Each villus is covered in microvilli.
- It has a generous blood supply with extensive capillary networks (this allows quicker absorption). Each villus has a “lacteal” inside it, which drains into the lymphatic system and carries away the digested fats.
- Glucose and amino acid are absorbed into the bloodstream and taken into the liver.
- Fatty acids and glycerol enter the lacteals and are transported in lymph and returned to the bloodstream for distribution around the body.
The appendix and caecum are vestigial meaning they have lost their former use.
- Reabsorbs water from waste, forming faeces. If not given enough time to reabsorb, this results it diarrhoea. The opposite is constipation and it results in hard stools.
- There are symbiotic bacteria in the colon.
- Some make vitamin K and digest cellulose.
- Some prevent harmful microorganisms from growing.
- The caecum is a part of the colon that has the appendix. It is located in the right lower part of the abdomen. The appendix is a worm-like projection that doesn’t serve any significant function today (a “vestigial” organ). If a piece of food or faeces gets lodged in the appendix, it becomes inflamed. This is called appendicitis. This presents with pain, nausea, vomiting, fever. Appendicitis is treated with antibiotics and surgical removal of the appendix.
The rectum stores faeces.
Components of a balanced diet:
- Fibre (stimulates peristalsis (prevents constipation), soaks up toxic chemicals and cholesterol as it travels through the gut unabsorbed, it may prevent obesity
- The amount of food a person requires depends on age, activity, gender and health.
BMI = body weight in kg/ (height in m)2
Being overweight (BMI above 25) or obese (BMI over 30) is associated with heart disease, stroke, diabetes and arthritis.
is an eating disorder.
[You don’t need all this detail for the exam, but people always seem to like to know this.] Symptoms include persistent restriction of energy intake leading to significantly low body weight, an intense fear of gaining weight or of becoming fat, or disturbance in the way one's body weight or shape is experienced, undue influence of body shape and weight on self-evaluation, or persistent lack of recognition of the seriousness of the current low body weight. It can cause lack of periods, cardiac problems and life-threatening electrolyte imbalances. It is treated with psychiatric input: counselling and medication, as well as medical input for physical problems (e.g. osteoporosis, infertility, etc).
is also an eating disorder.
It involves recurrent episodes of binge eating characterised by eating for an amount of time (within a 2 hour period )large amounts of food and a sense of lack of control over eating during an episode. It is followed by inappropriate compensatory behaviour in order to prevent weight gain (purging).
Four food groups:
- Cereals, breads, potatoes (6+ servings per day)
- Fruit and vegetables (4+)
- Milk, cheese and yoghurt (4)
- Meat, fish and poultry (2)
HL Paper 2008 Q12:
(i) Distinguish between mechanical and chemical digestion.
Mechanical digestion is carried out by the action of the teeth. It involves the physical break down of food.
Chemical digestion involves the use of enzymes, such as amylase, which breaks down cooked starch into maltose. This is salivary amylase and can be found in the mouth.
(ii) Name a structure in the human digestive system, other than teeth, which is involved in mechanical digestion.
In the small intestine, via peristalsis.
(b) The diagram shows the human digestive system.
(i) Name the parts A, B, C, D, E and F.
A = oesophagus.
B = stomach.
C = small intestine.
D = rectum.
E = appendix.
F = colon.
(ii) Describe two functions of bile in relation to digestion.
Bile emulsifies lipids. This means it breaks down large fats and oils into tiny droplets. This increases the surface area for enzyme digestion.
It contains sodium hydrogen carbonate, which helps to neutralise chyme from the stomach.
(iii) Answer the following in relation to a lipase:
1. Where is it secreted?
2. Where does it act?
3. What is the approximate pH at its site of action?
(c) (i) What are symbiotic bacteria?
These are bacteria that live closely together with another species, in this case humans, with at least one species receiving a benefit.
(ii) Give two activities of symbiotic bacteria in the human digestive system.
Bacteria in the colon feed on the waste and produce some B group vitamins and vitamin K, which we absorb.
The presence of beneficial bacteria prevent the growth of pathogenic bacteria.
(iii) Name the part(s) of the digestive system in which the following are absorbed into the blood.
1. the products of digestion,
The villi in the small intestine.
In the colon.
(iv) Name a process involved in the passage of the products of digestion into the blood.
(v) Explain how the structure that you have named in (iii) 1. is adapted for the absorption of the products of digestion.
Their walls are only one cell thick.
There are numerous infoldings to increase surface area.
There is a rich blood supply just inside each villus.
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