Recent research has revealed that the large intestine1 and its resident bacterial population2 have key roles to play in determining our health and wellbeing. It is much more than just a waste storage facility.
Structural features
In an average adult, the large intestine is about 1.5 m long and 5 cm wide. It consists of the caecum3, appendix4, colon5 and rectum6.
The large intestine
The large intestine consists of the caecum, colon, rectum and anal canal. It is about 1.5 metres long and has an average diameter of about 6 cm.
The 4 major functions of the large intestine are recovery of water and electrolytes, formation and storage of faeces and fermentation of some of the indigestible food matter by bacteria.
The ileocaecal valve7 controls the entry of material from the last part of the small intestine8 called the ileum9.
The human appendix has no known function and is thought to be a remnant10 from a previous time in human evolution11.
What does the large intestine do?
The 4 major functions of the large intestine are:
- reabsorption of water and mineral12 ions13 such as sodium and chloride
- formation and temporary storage of faeces14
- maintaining a resident population of over 500 species15 of bacteria16
- bacterial fermentation17 of indigestible materials.
By the time partially digested foodstuffs reach the end of the small intestine (ileum), about 80% of the water content has been absorbed. The colon absorbs most of the remaining water.
As the remnant food material moves through the colon, it is mixed with bacteria and mucus, and formed into faeces for temporary storage before being eliminated.
It has been estimated that there are about 500 species of different bacteria found inhabiting the adult colon. Most of these bacteria can only survive in oxygen-free environments and are referred to as anaerobes. These bacteria ferment some of the undigested food components, converting them into short-chain fatty acids18 (SCFAs19) and releasing gases20 like carbon dioxide21, hydrogen22 and methane23. SCFAs such as acetic, propanoic and butyric acids24 then serve as an energy source for the bacteria as well as the cells lining the colon.
| CH3COOH | CH3CH2COOH | CH3CH2CH2COOH |
Gut bacteria and health
The resident bacterial species in the large intestine form complex inter-relationships with themselves as well as their human host26. It is now thought that, rather than being a non-harmful coexistence, it is in fact a symbiotic27 relationship where each gains benefit from the other.
Recent research has revealed that gut bacteria perform a host of useful functions apart from fermenting undigested macronutrient28 material. These include interacting with the immune system29, producing vitamins such as vitamin30 K, stimulating the release of hormones31 involved in storage of fats and influencing mood and our feeling of wellbeing.
This high level of activity, which impacts on our health and wellbeing, has led some researchers to regard the gut bacteria as a body organ in its own right rather than a population of bacteria that just happen to live in the gut.
In addition to the importance of the ‘bacterial organ’, researchers now believe that the network of interconnected nerve cells lining the large intestine has a key role to play in food intake and its digestion32. This enteric nervous system33 is now often referred to as the ‘second brain’. It is capable of directing messages to the brain as well as controlling the release of hormones that influence the movement of food down the gut, feelings of wellbeing and the sensations of being hungry or of being full.
This new field of scientific research – known as neurogastroenterology34 – is helping to explain how the ‘second brain’ influences the body’s immune response35. This will lead to a greater understanding of how diseases36 such as inflammatory bowel37 disease38 develop and how they can be prevented.
Related content
It has been estimated that there are more bacteria in the large intestine than there are cells in the body. Find out more about Healthy gut bacteria and the impartance of maintaining a healthy bacterial population in the large intestine plays a key role our sense of wellbeing.
Discover more about bacteria and Bacteria – good, bad and ugly. In Microorganisms – friend or foe? learn how we have trillions of microorganisms39 living in and on our bodies and, though we need most of them to survive, a small number are harmful (pathogenic40).
- large intestine: The end part of the gastrointestinal tract that includes the caecum, colon and rectum.
- population: In biology, a population is a group of organisms of a species that live in the same place at a same time and that can interbreed.
- caecum: A pouch connecting the last part of the small intestine, called the ileum, with the first part of the colon, known as the ascending colon.
- appendix: A blind-ended tube, about 10 cm long, that runs out from the caecum just below the ileocaecal valve. Its functions are not yet fully understood.
- colon: That part of the large intestine between the caecum and rectum. It consists of four sections: the ascending colon, the transverse colon, the descending colon and the sigmoid colon.
- rectum: The end portion of the large intestine, about 12 cm long in humans, that temporarily stores faeces prior to egestion.
- ileocaecal valve: A muscular valve that controls the entry of undigested material from the last part of the small intestine (the ileum) into the first part of the large intestine (caecum).
- small intestine: That part of the gastrointestinal tract that connects the stomach to the large intestine. It consists of three parts: the duodenum, the jejunum and the ileum.
- ileum: The last part of the small intestine (about 3 m long in humans). It links to the large intestine.
- remnant: The remaining part of something that was originally bigger.
- evolution: In biology, the change in the genetic material and/or the behaviour of a population of organisms over time.
- mineral: 1. (Geology) A naturally occurring solid formed through geological processes. Any given mineral has a characteristic chemical composition and a specific set of physical properties. 2. (Dietary) An inorganic compound needed for proper body function and maintenance of health, for example, iron in the form of haeme present in red meat.
- ion: An atom or group of atoms that has gained or lost electrons and has an electrical charge.
- faeces: The excreted waste product of digestion in animals – poo.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
- fermentation: The chemical breakdown of molecules like glucose in the absence of oxygen. Fermentation is used to create food products such as beer, wine, kombucha, bread and yoghurt. In ruminants, microbial populations ferment feed and water into volatile fatty acids and gases – methane and carbon dioxide.
- short-chain fatty acids (SCFAs): In digestion, the products of fermentation of undigested carbohydrate by bacteria in the large intestine.
- short-chain fatty acids (SCFAs): In digestion, the products of fermentation of undigested carbohydrate by bacteria in the large intestine.
- gases: The state of matter distinguished from the solid and liquid states. Gases have the ability to diffuse readily and to become distributed uniformly throughout any container.
- carbon dioxide: CO2 is a colourless, odourless, incombustible gas. It is a product of cellular respiration and combustion and is an essential component in photosynthesis.
- hydrogen: First element on the periodic table – symbol H, with the atomic number of 1, meaning that it has a single proton in its nucleus.
- methane: CH4, a gas that contributes to the greenhouse effect.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- host: An organism that contains a parasite or other dependent organism. Hosts usually provide the dependent organism with food and shelter.
- symbiotic: A relationship between two species that benefits both species. For example, lichen results from a mutually beneficial partnership between fungi and algae.
- macronutrient: 1. In human nutrition, a nutrient that is required in large amounts and provides the energy needed to maintain body functions and carry out the activities of daily life. There are three macronutrients – carbohydrates, proteins and fats. 2. In agriculture, a macronutrient is any of the chemical elements required by plants in relatively large amounts: carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, magnesium, sulfur and calcium.
- immune system: The natural defence system found in living things. In humans, a network of organs, cells and proteins that helps to protect us against anything it recognises as being an ‘invader’ or ‘foreign’, for example, bacteria, viruses, cancer cells, parasites and transplanted organs and tissues. In plants, a network of disease resistance genes enable plants to detect and resist things like fungi, bacteria, viruses, nematodes and insects.
- vitamin: An organic compound required as a nutrient in tiny amounts by an organism.
- hormone: A chemical substance secreted by an endocrine gland into the bloodstream. It acts on specific target cells to produce a given response to control and regulate the activity of certain cells or organs.
- digestion: The mechanical and chemical breaking down of food into smaller components that can be absorbed into a bloodstream.
- enteric nervous system: The digestive system’s own local nervous system that can and does function autonomously but normally works in association with the central nervous system.
- neurogastroenterology: A research area in medical science that is concerned with the interactions of the brain and the gut.
- immune response: How your body recognises and defends itself against bacteria, viruses and substances that appear foreign and harmful.
- diseases: 1. An abnormal condition of an organism that impairs bodily functions. 2. In plants, an abnormal condition that interferes with vital physiological processes.
- bowel: A term to describe the intestines. The term ‘large bowel’ is sometimes used to describe the colon and rectum.
- diseases: 1. An abnormal condition of an organism that impairs bodily functions. 2. In plants, an abnormal condition that interferes with vital physiological processes.
- microorganism: A living organism which is too small to be seen with the naked eye and can only be observed using a microscope. Includes bacteria and most protists.
- pathogenic: An organism that causes disease. Certain types of bacteria are pathogenic to humans – they cause diseases such as food poisoning. Some fungi are pathogenic to plants – they cause diseases such as potato blight.
