Chapter 33: The human digestive system Leaving Certificate Biology Higher Level

Chapter 33: The human Chapter 33: The human digestive systemdigestive system

Leaving Certificate BiologyLeaving Certificate Biology

Higher LevelHigher Level

NutritionNutrition

• Autotrophic:– Green plants use light– Some photosynthetic bacteria – use light– Some chemosynthetic bacteria – use

chemicals

• Heterotrophic:– Organisms that have to obtain their energy

from preformed food in the environment (animals; fungi; most bacteria)

Heterotrophic NutritionHeterotrophic Nutrition

• Omnivore:– Eat both plant and animal material – e.g.

human, bears, badgers

• Carnivore:– Eat only animal material – e.g. dogs, cats

• Herbivore:– Eat only plant material – e.g. cattle, sheep

Human NutritionHuman Nutrition

• Digestion is the physical and chemical breakdown of food into its constituent molecules so that they can pass into cells– Need for digestion in heterotrophs: they acquire

preformed food from environment. Preformed food particles, such as carbohydrate, protein, and fats are too big to pass directly into the body’s cells and so must be broken down into smallest possible particles in order to travel across cell membranes

Digestion and Digestive SystemDigestion and Digestive System• Need for digestion and digestive system:

– Digestion: heterotrophs acquire preformed food from environment, such as carbohydrate, protein, and fat particles that are too big to pass directly into the body’s cells and so must be broken down into smallest possible particles in order to travel across cell membranes

– Digestive system: heterotrophs have sophisticated digestive systems that break down food particles and are designed to extract all the nutrients from ingested food and transport them into the bloodstream, while at the same time ensuring that microorganisms are killed and do not infect the body

Human NutritionHuman Nutrition

• Ingestion: taking in of food into the mouth• Digestion: physical and chemical breakdown of

food particles into their constituent molecules• Absorption: constituent molecules of the food

are absorbed into cells that line the digestive tract and transported into the bloodstream for transport to the liver via the hepatic portal vein

• Egestion: removal of undigested and unabsorbed material from the digestive system

Digestive Digestive System System

AnatomyAnatomy

Macrostructure of Digestive SystemMacrostructure of Digestive System• Mouth:

– Physical/mechanical (teeth) digestion of food

– Teeth:• Human dental formula: 2(I2/2; C1/1; PM2/2; M3/3)

• Incisors (8 in total): cutting food• Canines (4 in total): tearing food• Premolars (8in total): grinding/crushing food• Molars (12 in total): grinding/crushing food

– Chemical digestion of food – salivary glands secrete amylase which has an optimal pH of 7

• Starch + Amylase + pH7 → Maltose + Amylase

– Mouths forms a bolus of food that is then swallowed

Dental DecayDental Decay• Dental decay is the destruction of tooth

structure

Dental DecayDental Decay

• Symptoms:– Small holes (caries and/or cavities) appear in

the teeth – especially in areas that are difficult to clean with a tooth brush such as the grooved surfaces of the premolars and molars and between the teeth

– Toothache – only occurs when the cavity has reached the dentine and pulp areas of the tooth where a bacterial infection of the tooth occurs and the tooth has to be removed

Dental DecayDental Decay

• Cause:– occurs when foods containing carbohydrates (sugars

and starches) such as breads, cereals, milk, fizzy drinks, fruits, cakes, sweets are left on the teeth

– Bacteria that live in the mouth digest these foods, turning them into acids

– The bacteria, acid, food debris, and saliva combine to form plaque, which clings to the teeth

– The acids in plaque dissolve the enamel surface of the teeth, creating holes in the teeth called cavities, or caries

Dental DecayDental Decay• Treatment:

– Depends on how bad the tooth decay is– Use of fluoride can prevent further decay within

a cavity– Fillings: cavities can be filled with a substances

such as metals, metal alloys, plastic, porcelain, or a combination of materials (composite fillings)

– Severe tooth decay may need a crown or root canal

– Extreme tooth decay may result in the tooth having to be removed

Dental DecayDental Decay• Prevention:

– Regular brushing (at least 2 times/day) with fluoride toothpaste

– Regular flossing (at least daily)– Healthy diet:

• Avoid snacking on high sugar snacks• Avoid fizzy drinks and alcohol• Include foods rich in calcium and vitamin D• Chew gum after a meal

Macrostructure of Digestive Macrostructure of Digestive SystemSystem

• Oesophagus:– Carries food (bolus) from the mouth to the stomach

by a process called peristalsis– Peristalsis is the rhythmical contractions of smooth

muscle of the alimentary canal that has the effect of moving food along

• Peristalsis is also a form of mechanical digestion

• Fibre stimulates peristalsis thereby preventing constipation

• Good movement of food along the alimentary canal is important in decreasing the chance of contracting colon cancer

Macrostructure of Digestive SystemMacrostructure of Digestive System• Stomach:

– Receives food from mouth via oesophagus and stores food while the cells of the gastric glands secrete:

• HCl (parietal cells) – kills bacteria and denatures protein

• Pepsinogen (chief cells) – is zymogen that is activated by HCl to pepsin (optimal pH=1–2) which then digests proteins to peptides

• Mucous (goblet cells) – is slightly alkaline and protects the stomach lining

– Cardiac sphincter found at entrance to stomach – opens when food enters the stomach

– Pyloric sphincter controls the amount of chyme that leaves stomach

Stomach (Peptic) UlcerStomach (Peptic) Ulcer

• A peptic ulcer is a small lesion in the lining of the stomach wall

• A duodenal ulcer is a small lesion in the duodenal wall

• A perforated ulcer is a complete lesion in the wall of the stomach/duodenum where the contents can leak into the body cavity


• Symptoms of an ulcer:– Persistent or intermittent pain in the upper

abdomen that is relieved by eating or taking antacids

– Vomiting– Fatigue (due to loss of blood due to internal

bleeding)– In the case of a perforated ulcer, internal bleeding

and bacterial infection of body cavity (possibly fatal)

Stomach (Peptic) UlcerStomach (Peptic) Ulcer• Cause:

– Lack of mucous or excess HCl is likely to cause peptic ulcers

– Ulcers can also be caused by the bacterium – Helicobacter pylori – which if untreated can go on to cause stomach cancer

– Ulcers in the digestive system are more likely to occur in people who:

• Take anti-inflammatory drugs such as aspirin, ibuprofen, and naproxen

• Drink alcohol• Smoke• Feel stressed


• Treatment:– If bacterial, then antibiotics are administered– If stress-related then time-out and exercise– If diet-related, then a special diet is made out

by a dietician– If too much acid is being produced, then

antacids are administered– Stop smoking and/or drinking

Stomach (Peptic) UlcerStomach (Peptic) Ulcer• Prevention:

– Prevention of stomach ulcers would involve ensuring good hygiene when preparing food to reduce risk of bacterial infection

– Eating balanced, nutritious meals – several small meals are generally better than 3 big meals

– Exercise as it combats the effects of emotional stress

– Avoid overuse of anti-inflammatory drugs– Do not smoke– Avoid alcohol


• The small intestine:– Consists of three parts:

• Duodenum (25 cm) – digestion• Jejunum – absorption• Ileum – absorption


• Duodenum:– Receives chyme from stomach– Further digestion occurs– Secretions from the pancreas (pancreatic

juice) and gall bladder (bile) are released into duodenum

– Pancreatic amylase, lipase and bile salts are responsible for digestion within the duodenum


• Pancreas:– Secretes pancreatic juice into the duodenum– Pancreatic juice contains:

• Sodium bicarbonate: neutralises stomach acid• Lipase: acts on lipids and converts it into fatty

acids and glycerol (optimal pH of lipase = 7)• Amylase: acts on starch and converts it into

maltose (optimal pH of amylase = 7)– Starch + amylase + pH7 → maltose + amylase


• The liver and its functions in digestion:– Produces bile – which is stored in gall bladder– Bile consists of:

• Bile salts: secreted into duodenum and emulsifies lipids

• Sodium bicarbonate: secreted into duodenum and neutralises acidic chyme

• Bilirubin and biliverdin: breakdown products of haem from the haemoglobin of red blood cells

Macrostructure of Digestive SystemMacrostructure of Digestive System• The liver and its other functions:

– Receives blood from the jejunum and ileum (hepatic portal vein) – detoxifies toxins (e.g. alcohol)

– Breaks down excess amino acids into urea – which is then excreted via the kidneys

– Converts excess glucose to glycogen which it then stores

– Stores fat-soluble vitamin (A, D, E, and K) and trace minerals (iron, copper, zinc)

– Synthesises clotting factors (e.g. fibrinogen)– Synthesises cholesterol – key component of cell

membranes


• Jejunum and ileum:– Absorption of nutrient occurs in this 6 m

section of the small intestine• Monosaccharides (breakdown products of starch):

glucose, fructose, galactose are absorbed directly into bloodstream as they are water-soluble

• Amino acids (breakdown products of protein) are also absorbed into bloodstream as they are water-soluble

• Fatty acids and glycerol (breakdown products of starch) are absorbed into the lacteals and combine with proteins to form lipoproteins which are then transported in the lymph to the blood


• Small intestine:– Small intestine is very well-adapted for

digestion and absorption due to:• Length of small intestine (6–7 m long)• Internal epithelium is structured into villi and

microvilli to increase the surface area for absorption

• The surface of the villi are only one-cell thick in order that absorption into the bloodstream and lacteals is very rapid

• Small intestine has a very good blood supply


• Large intestine:– The large intestine is 1.5 m long and consists

of:• Caecum• Colon• Rectum


• Caecum:– Separated from the ileum by the ileocaecal

valve– It is the beginning of the large intestine– In humans, the caecum's main functions are

to absorb fluids and salts that remain after completion of intestinal digestion and absorption and to mix its contents with mucous


• Appendix:– The appendix is a small branch of the caecum– Its average length is 10 cm– The appendix is rich in infection fighting cell

(white cells) – suggesting that it may have an immune function

– However, the appendix has no function officially

• Appendicitis is painful inflammation and swelling of the appendix

• Symptoms:– Severe and localised pain in the right, lower abdomen

• Cause:– Bacterial growth and build up of toxins and waste

products

• Treatment:– Surgical removal of the entire appendix

• Prevention:– No known preventative measure

Appendicitis


• Colon:– The colon consists of three parts:

• Ascending, transverse, and descending colons

– The function of the colon is to absorb water converting the semi-solid waste that changes into faeces

– The colour of faeces is due to the bile pigments – bilirubin and biliverdin

– Symbiotic bacteria (300 – 1000 different species of bacteria live in the colon!) produce biotin (a B-vitamin) and K which are then absorbed through the colon wall

– Symbiotic bacteria also help to break down unused energy substrates such as small amounts of cellulose


• Importance of dietary fibre for the colon:– Dietary fibre (also called roughage) is necessary to

keep the gut healthy by stimulating peristalsis and ensuring that the undigested material moves along freely without becoming stuck

– Constipation results if there is not enough fibre in the diet

– Persistent constipation increases the risk of colon cancer later in life as the undigested material begins to fester due to bacterial action and toxins can build up


• Rectum:– Stores faeces as it arrives from the colon and

until it is egested from the body via the anus– Egestion from the rectum through the anus is

not excretion as faeces is not a waste product of metabolism – faeces is the undigested food material

• REMEMBER: Excretion is defined as the removal of the waste products of metabolism from the body via the kidneys, skin, and lungs

Vitamin A DeficiencyVitamin A Deficiency• Symptoms:

– Night-blindness; dry, scaly skin; mental retardation; and retarded growth in children

• Cause:– Inadequate intake of food high in vitamin A

• Treatment:– Increase intake of foods rich in vitamin A such as

milk, cheese, cream, liver, cod liver oil, leafy green vegetable, carrots

• Prevention:– Eat adequate amounts of milk, cheese, cream, leafy

green vegetables, carrots

Balanced Human DietBalanced Human Diet• A balanced diet is one in which a

person receives all the essential nutrients in the correct proportions to maintain homeostasis

• A variety of foods should be include in the diet to maintain good health

• No one group of foods should be taken in excess

Balanced Human DietBalanced Human Diet• An individual’s diet will depend on:

– Age: adolescents, adults and pregnant and lactating women will require the largest amount of nutrients

– Sex: generally males require more calories than females (except during pregnancy and lactation)

– Occupation: labourers will require more calories per day than someone with a desk job

– Activity: sports people will require many more calories than a sedentary person

Food PyramidFood Pyramid

Slimming DisordersSlimming Disorders• Anorexia nervosa:

– Psychological condition found mostly in women– They have a perception that they are ‘fat’– They eat very small amounts of food and go on strict

diets or even go long periods of time without eating– Can be fatal

• Bulimia:– Psychological condition similar to anorexia, however,

these people generally eat normally– Difference is that bulimics will induce vomiting after a

meal– Harder to treat as they appear normal because of

their eating patterns

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Chapter 33: The human digestive system Leaving Certificate Biology Higher Level