The Stomach

  • An organ connected to the small intestine via the pylorus, a valve that is usually closed.

  • Cardia:the valve at the entrance of the stomach which is always ope

  • Food stays in the stomach for 3-4 hours, mixing with gastric juice, making chyme.

  • Gastric juice contains pepsin, an enzyme that begins the digestion of proteins.

  • It also contains hydrochloric acid, which activates pepsin and facilitates its action, and destroys bacteria. 

  • The inside of the stomach contains mucus which protects the stomach wall and prevents it from being attacked by hydrochloric acid and enzymes.

  • When chyme becomes very acidic, the pylorus opens and it moves into the small intestine.

  • Gastric acid is produced through involuntary processes

The Small Intestine

  • A tube around 6 or 7 meters long

  • Located between the pylorus and ileocaecal valve, which separates it from the large intestine

  • Here, chyme comes into contact with digestive juices secreted by the liver and pancrea.

    Divided into 3 parts:

  1. Duodenum: 1st section, U-shaped, around 25 cm long

  2. Jejunum: 2nd & longest section

  3. Ileum: final section, leads to large intestine

The Liver

  • Large organ in the top-right of the abdomen

  • Reddish brown color because it contains blood

  • Produces bile which enters the duodenum

  • Also carries out non-digestive functions, such as destroying toxins            


  • Is stored in the gallbladder & only released when food enters the intestine

  • Does not contain digestive enzymes

  • Contains bile salts, which help to digest fats by emulsifying them.

The Pancreas

  • Long, greyish-white organ behind and underneath the stomach


  1. Produces hormones (insulin & glucagon) that regulate the amount of glucose in the bloodstream

  2. Secretes pancreatic juice which contains digestive enzymes & sodium bicarbonate              

How digestion works

  • Most digestive processes take place in the small intestine

    Digestion produces nutrients that cells need:

  • Monosaccharides (simple sugars): obtained from complex carbohydrates

  • Glycerol & fatty acids: obtained from fats

  • Amino acids: obtained from proteins 

Nutrient Absorption

  • Nutrients pass from the small intestine into the bloodstream through absorption and then are delivered to cells by the blood.

  • The inside wall of the small intestine is covered in folds that are lined with thousands of finger-like structures called villi.

  • These villi are full of capillaries.

  • The plasma membrane of villi cells is covered in microvilli which increase the surface area of the intestine.

The Large Intestine

  • Undigested food moves to the large intestine

  • Wider & shorter than the small intestine & doesn’t have microvilli

  • Almost all absorption of water & minerals takes place here

  • Waste products are then expelled through defecation

The Large Intestine

 Made up of 3 parts:

  1. Caecum: 1st portion, connected to the appendix

  2. Colon: has 3 parts and contain symbiotic bacteria that produce certain vitamins, like vitamin B₁₂ and K

  3. Rectum: final section, leads to the anus


What is it?

  • Breaks nutrients into parts small enough for your body to absorb and use for energy, growth, and cell repair.

  • It is made up of the gastrointestinal (GI) tract, and the liver, pancreas and gallbladder.

  • Digestive functions: ingestion (entry of food into the digestive tract) and digestion (transformation of food into simple nutrients)

Types of digestion 

  1. Mechanical digestion: physical transformations such as chewing, squeezing, mixing, etc. to reduce the size of the food and make it easier for chemical digestion.

  2.  Chemical digestion: transformation of complex nutrients from food into simple nutrients through chemical reactions. Carried out by enzymes in digestive juices such as stomach acid and bile

  • Absorption: the passing of nutrients from the digestive tract to the blood or lymph, done by the small intestine

  • Defecation: the expulsion of non-digested or non-usable parts of food 

GI/Digestive tract

  • A series of hollow organs joined in a long, twisting tube from the mouth to the anus where food moves through.

  • The hollow organs that make up the GI tract are the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

     Made up of 3 layers of tissue:

  1. Mucosa– internal layer, lined with glands

  2. Muscularis- middle layer, two layers of smooth muscle

  3. Serosa- connective tissue layer, joins digestive tract to other organs


  • Help carry out chemical digestion by secreting digestive juices into the digestive tract

  • Includes salivary glands, gastric glands, intestinal glands, the liver and pancreas

  • Organs involved in chemical digestion are the mouth (saliva), stomach (stomach acid & digestive enzymes), pancreas, liver (bile), gallbladder (stores bile), small intestine (digestive juice)


  • Food moves through the digestive tract through a process called peristalsis.

  • The hollow organs of the GI tract have a layer of muscle which allows them to move, pushing and mixing the food forward through the GI tract

  • Mouth ⇢ esophagus ⇢ stomach ⇢ small intestine ⇢ large intestine ⇢ rectum



  • Our cells need energy for things such as contracting muscles, cell division, producing heat and making protein molecules.

  • We get this energy from the food we eat.

  • Food is broken down into smaller molecules (digested), which are absorbed into the blood and then transported to our cells.


  • The main nutrient used to provide energy to cells is called glucose.

  • Glucose contains a lot of chemical energy.

  • Respiration is a series of metabolic reactions that break down glucose molecules and release energy from them.

  • This is done by our cells.

  • It involves the action of enzymes.


  • Most of the time our cells release energy from glucose by combining it with oxygen.

  • This is known as aerobic respiration.

  • It happens in a series of small steps, most of which take place in the mitochondria


  • Releasing energy from glucose without using oxygen (“an” means “without”).

  • It is not as efficient as aerobic and not much energy is released per glucose molecule.

  • Yeast (single-celled fungus), plants, and our muscle cells can respire anaerobically for short amounts of time.

Yeast & Plants

                    Glucose        ——> Alcohol & CO2 

                                  Muscle cells

          Glucose     ——> Lactic Acid


  • Oxygen is obtained directly from our surroundings.

  • Carbon dioxide is a waste product that must be removed after aerobic respiration.

  • There are special areas in organisms where oxygen enters and carbon dioxide leaves.

  • This is called gas exchange.


  • Surfaces used for gas exchange have to be permeable.

  • Other characteristics of exchange surfaces:

  1. Thin to allow gases to diffuse quickly

  2. Close to an efficient transport system to take gases to and from the surface

  3. Large surface area to allow a lot of gas to diffuse at once

  4. Good supply of oxygen (caused by breathing movements)


  • Most important structures are the 2 lungs.

  • Each lung is filled with tiny air spaces called air sacs or alveoli.

  • Here, oxygen diffuses into the blood

  • The lungs are supplied with air through the trachea.


  • Air can enter the body through either the nose or the mouth.

  • The nose and mouth are separated by the palate, so you can breathe through your nose when you’re eating.

  • It is better to breathe through your nose because the structure of the nose warms, moistens, and filters air before it enters the lungs.

  • Goblet cells make a liquid with water and mucus which moistens the air in our nose

  • Other cells have cilia, hairlike projections that trap and push dust and bacteria in mucus up to the back of the throat   


  • After the nose or mouth, air passes to the trachea

  • At the top of the trachea is a piece of cartilage called the epiglottis

  • This closes the trachea and stops food from going down it

  • This happens automatically (refex)

  • Below is the larynx or voice box which contains the vocal cords


  • The trachea goes down through your neck and into the thorax.

  • Here, the trachea divides into two, and the branches are called the right and left bronchi.

  • Each bronchus goes to a lung and then branches out into smaller tubes called bronchioles.

  • On the end of each bronchiole are many tiny air sacs or alveoli, where gas exchange takes place


  • The walls of the alveoli are the gas exchange surface.

  • Tiny capillaries are closely wrapped around the outside of the alveoli.

  • Oxygen diffuses across the walls of the alveoli into the blood.

  • Carbon dioxide diffuses the other way.


  1. Thin: they are only one cell thick. 

  2. Have an excellent transport system: blood is constantly pumped to the lungs along the pulmonary artery, which branches into thousands of capillaries which takes blood to all parts of the lungs.

  3. Large surface area: total surface area is over 100 m².

  4. Good supply of oxygen: your breathing movements supply your lungs with oxygen.


  • In order to breathe, you change the volume of your thorax.

  • First, you make it large so air is sucked in.

  • Then, you make it smaller so air is squeezed out.

  • This is called breathing.


  1. Intercostal muscles: in between the ribs. 

  2. Other muscles are in the diaphragm.

  • The diaphragm is a large sheet of muscle and elastic tissue underneath the lungs and heart.


  1. The muscles of the diaphragm contract.

  2. This pulls the diaphragm downwards, increasing the volume of the thorax.

  3. The external intercostal muscles contract, raising the ribcage.

  4. The volume of the thorax increases, so air rushes into the lungs.


  1. The muscles in the diaphragm relax.

  2. The diaphragm springs back up into its domed shape.

  3. External intercostal muscles relax.

  4. Rib cage lowers into its normal position.

  5. Volume of the thorax decreases, so air is forced out of the lungs

  • Expiration can be natural or forced (ex: coughing)


  • Respiration: a series of chemical reactions which happen in all living cells, in which food is broken down to release energy, usually by combining it with oxygen.

  • Gas exchange: the exchange of gases across a respiratory surface; (ex: oxygen is taken into the body and carbon dioxide is removed); also takes place during photosynthesis and plant respiration.

  • Breathing: muscular movements which keep the respiratory surface supplied with oxygen