Chapter 12: Nutrition in the human body

Three biological important reasons why nutrition is necessary:

• Provision of energy (cellular level as well)
• Availability of material for growth, healing wounds, replacement of damaged/worn tissue
• Regulations of processes in the body

Nutrition involves the following processes: 

• Ingestion: intake of food

Digestion: breakdown of insoluble polymers (macromolecules) into smaller soluble monomers (building blocks)

Absorption: Nutrients is absorbed (used for a variety of processes in the body) -known as the assimilation of nutrition-

Undigested food residues are expelled by egestion

Solid food wastes is called facets -process of defeacation-

If waste products are produced during metabolic reactions; expelled by a specific process called excretion

The digestive consists of the digestive tract & associated organs (glands)

Mouth, Teeth, Tongue and salivary glands: 

Adult person has 32 teeth

Humans are heterodont - our teeth have different structures for different functions

Humans are also difiodont - we have two sets of teen in our life time ( milk teeth and a permanent set)

Animals who do not lose baby teeth are monofiodont

Adaptions of vertebrates to different modes of nutrition: 

Mode of nutrition: 

Herbivores

Adapted structure: 

Jaw bone and teeth

Adaption: 

No incisors in upper jaw

No canines in upper jaw

Upper jaw forms hard bony plate - incisors of lower jaw press against -

Top/ bottom molars fit precisely

Jaws can move side ways

Example of animal: 

Non-ruminant herbivores (sheep)

 Structure/ dental formulas: 

Insert dental formula and picture C 

Adapted Structure: 

• Digestive tract

Adaption: 

Stomach consist of 4 chambers

Large rumen

Small intestine

Long/ thin colon

Large caecum

Examples of animal: 

Ruminant herbivores (cattle)

Picture D insert dental formula 

Adapted structure: 

Digestive tract

Adaption: 

Very large caecum

Short colon

Example of animal:

Rodents (rabbit)

Adapted structure: 

Jawbone

Teeth

Adaption: 

Incisors grow continuously

Kept short by gnawing on food/ hard objects

Carnivores: 

Adapted structure: 

Jaw bone

Teeth

Digestive tract

Adaption: 

Incisors small, poorly developed

Canines developed - long/strong fans (used to tear meat) -

Molars - sharp ridges slide over each other -

Carnassial teeth - sharp ridges/ points (used to break bone) -

Strong jaw muscles (used to tear meat/ crushing bones)

Stomach acid content high; very low pH (digests proteins/bones)

Example of animal: 

Cat family (lions)

Dog family (wolves)

Omnivores: 

Adapted structure: 

Jawbone

Teeth

Digestive tract

Adaption: 

Teeth; grind down meat/ plant material

Have all types of teeth

Canines small (exception: baboon)

Shorter small intestine/ colon, caecum vestigial

Example of animal: 

Human

Other primates

Bears

Types of teeth and their function: 

• 8 incisors (i): Upper jaw + lower jaw (4 each) – used to bite off food (sharp and chisel shaped)
• Two eye teeth, also known as canines (c): Top as well as bottom – used to bite off food
• Premolars (pm): 4 top and bottom (2 each)
• Molars (m): (6 top as well as bottom) – used to grind food –
• Represent the type, position, number of teeth by means of a dental formula
• 2.1.2.3
• ---------                  ß This is the dental formula 
• 2.1.2.3

General structure of a tooth: 

• Tooth consists of dentine – substance similar to bone, it has a higher mineral content
• Dentine is much harder than bone
• Part that protrudes above the gum – crown of the tooth
• The crown of the tooth is covered in enamel
• Enamel is the hardest substance in the body
• The dental cavity is filled with a soft type of connective tissue
• This connective tissue contains blood vessels, lymph vessels, nerves
• The tooth is embedded with cement in the socket – a cavity in the jaw
• Part embedded in the socket: the root
• Nerves, blood vessels, lymph vessels enter the tooth from the jaw – through the root canal

Functions of the tongue: 

Tongue is a muscular organ

• It moves food from one side of the mouth to the other
• Pushes food between the teeth
• Mixes the food with saliva
• Forms the food into a bolus (Bolus: Food mixed with saliva in the form of a ball) 
• Helps with swallowing – pushing food down the throat
• Tongue can also taste five different tastes (sweet, sour, bitter, salt)

• Teeth and tongue are responsible for mechanical breakdown of food – large pieces are broken into smaller pieces; then mixed with saliva to start chemical digestion

Three paired salivary glands: 

• Parotid glands: behind the ears
• Sub-mandibular glands: below the jaw
• Sublingual glands: under the tongue

• Salivary glands are exocrine glands
• Exocrine glands mean that the secretion is transported by tubes
• Endocrine glandes secrete hormones directly into the bloodstream

Saliva consist of: 

• Water: Acts as a 1) Solvent 2) medium for chemical reactions 3) reagent in hydrolysis
• Mucus: Binds chewed food – makes bolus smooth, makes swallowing easier
• Bicarbonate ions: Makes saliva slightly alkaline
• Enzymes salivary amylase: Digests cooked starch into maltose

The Oesophagus (gullet):

• Is a muscular tube – reaching from the pharynx to the stomach
• Leaves the pharynx where the trachea leaves the pharynx
• The trachea is closed by the epiglottis, a valve at the top of the trachea – this helps prevent chocking –
• The longitudinal/ circular muscles work together to move the food down the oesophagus into the stomach (peristalsis)
• The movement of the bolus is made easier by the mucous secreted by the mucosa
• Oesophagus goes through the diaphragm, joins the stomach

The stomach: 

• C shaped organ – valves at both ends –
• Valves are circular muscles – also known as sphincters
• Sphincters that closes the stomach at the oesophagus end – known as cardiac sphincter
• The cardiac sphincter stops gastric acid from going back up
• Pyloric sphincter closes the stomach at the other end – before food can move into the small intestine
• Stomach also goes through peristalsis
• Muscular wall of the stomach has a third layer – oblique muscles
• Three layers enables the stomach wall to contract
• Churning movements mix the food with digestive juices – forming chyme
• Chyme – Bolus that has been mixed with gastric juices
• Lining of the stomach is folded – has finger like outgrowths; this increases the surface that secretes gastric juices
• Gastric juice is acidic
• The stomach lining is well protected by a mucous membrane
• Cells in the mucous – secrete an alkaline substance that neutralises the acid when it comes in contact with the stomach lining
• Stomach wall has gastric glands – secrete gastric juices
• Secretion of gastric juice is stimulated by the secretion of the hormone gastrin
• Gastrin secretion is stimulated by the physical/ chemical stimuli of food entering the stomach – at the cardiac sphincter
• Chyme is forced into the first loop of the small intestine by peristalsis
• Pyloric sphincter controls the movement of chyme to the intestine

Gastric juices consist of:

• Water: Most important
• Mucus 
• Hydrochloric acid: 1) Provides acidic medium of gastric juice; 2) hydrolyses sucrose to glucose/ fructose; 3) antiseptic; 4) activates pepsinogen
• Pepsin:  Protein digesting enzyme – secreted in an inactive form, this is to protect the stomach lining; make sure the enzyme only works on protein in food (Pepsin: Active Pepsinogen: Inactive)
• Pepsinogen is activated by acid in the gastric juices; pepsin digests proteins to peptones
• Lipase: Digests fat to fatty acids/ glycerol – this digestion is not very effective because fat enters the stomach in large chunks
• Rennin: An enzyme in the stomach of small mammals – digests the protein in milk
• Rennin changes the soluble milk protein (caseinogen) into an insoluble form (casein)

Small intestine/ associated glands: 

• Digestion of food takes place mainly in the small intestine
• Soluble nutrients are absorbed by the intestinal wall
• Small intestine is 4-6 meters long; three regions can be identified:
• Duodenum 2) Jejunum 3) Ileum
• Chemical digestion – Duodenum
• Bole from the liver/ enzymes from the pancreas are released into the duodenum – works on chyme
• Enzymes secreted by the intestinal wall complete the chemical digestion process

Liver: 

• Biggest gland in the body
• Most complex organ
• Right side of your body – below the diaphragm
• Single liver cell can perform more than 500 different specialised metabolic activities

Functions of the liver: 

• Secretes bile; plays a role in the physical breakdown of fats
• Maintain homeostasis – releasing nutrients into the blood stream/ withdrawing nutrients
• Stores glycogen
• Converts excess amino acids into fatty acids/ urea through the process of deamination
• Stores iron/ fat-soluble vitamins (Vitamin A, D, K) – water-soluble vitamin B₁₂
• Detoxifies alcohol, drugs, medicines, toxins
• Produces plasma proteins/ blood clotting factors
• Breaks down worn red blood cells

• Bile does not contain any enzymes
• Bile consist of: water, bile salts, bile pigments, cholesterol, salts
• Yellowish-green, bitter liquid

Bile functions: 

• Alkaline liquid, emulsifies fats – breaks it up into small droplets
• Lipase acts more effectively on small droplets
• Neutralises the acidic chyme; contributes to the alkaline medium in the duodenum;
• Enzymes from the pancreas/ intestinal juice prefer to work in the duodenum
• Absorption of fat
• Antiseptic functions
• Makes the chyme less fluid – move more solely through the small intestines
• Ensures maximum digestion/ absorption
• Bile is secreted by the liver – stored in the gall bladder
• Hormone cholecystokinine – secreted by the intestinal wall stimulates the release of bile from the gall bladder

Pancreas: 

• Both an endocrine and exocrine gland
• Tongue shaped
• Greyish-pink gland
• Left of the abdominal cavity – just below the stomach
• Consists of pancreatic cells/ Islets of Langerhans
• Pancreatic duct joins the bile duct from the gall bladder
• Pancreas/ gall bladder release their contents together into the duodenum – common bile/pancreatic duct
• Secretion of pancreatic juice is stimulated by the hormone secretin
• Secretin is secreted by the intestinal wall
• Pancreatic cells secrete pancreatic juice that is transported via small ducts

Pancreatic juice consists of: 

• Water
• Bicarbonate ions: Makes pancreatic juice more alkaline – providing the medium preferred by enzymes (working in the small intestine)
• Trypsinogen (enzyme): Inactive form of trypsin – digests proteins to peptones
• Lipase (enzyme): Digests lipids to fatty acids/ glycerol
• Amylase: Digests starch/ other polysaccharides (except cellulose) to maltose/ other disaccharides
• Ribonuclease/ deoxyribonclease: Digests RNA/ DNA, respectively to their nucleotides
• Trypsinogen is activated by the enzyme enterokinase/ cholecystokinine (to trypsin) -/ In the case of pepsinogen, this protein digesting enzyme is secreted in an inactive form to protect pancreatic tissue

The homeostatic control of blood sugar levels: 

Pancreas is an endocrine gland because the hormones are secreted straight into the blood stream

Glucagon: 

• Converts glycogen back into glucose – when the body needs glucose for cellular respiration/ blood cannot provide sufficient quantities

Insulin: 

• Converts excess glucose into glycogen
• A Polysaccharide stored in the liver/ muscle cells as reserve energy
• Makes cell membrane more permeable – cells absorb glucose more easily for cellular respiration

• When β-cells cannot perform their function, insulin is not secreted
• Β-cells are insulin producing cells
• This leads to the condition known as diabetes mellitus (extremely high blood glucose levels – hyperglycaemia – )
• Type 1 diabetes occurs when a person gets the condition before puberty 
• Type 2 diabetes occurs when the disease starts after the onset of puberty
• Diabetes characteristics: thirst, weight loss, tiredness, excessive urination
• 
  

Structure/ function of the small intestine: 

• Three regions of the small intestine 1) duodenum 2) Jejunum 3) Ileum
• Has an almost velvety appearance – caused by millions of finger-like outgrowths; known as villi
• Villi: Increase surface
• Villi/microvilli enlarge the surface for digestion/absorption
• If the small intestine was smooth on the inside the chyme would move through it too quickly
• Chyme moves through the intestine by means of peristalsis; segmentation/ wave-like movements of the villi (chyme comes into contact with the digestive enzymes
• Intestinal wall has various glands that secrete intestinal juice
• Secretion of intestinal juice is stimulated by the hormone, Secretin
• Secretion of secretin is stimulated when the acidic stomach contents enter the duodenum
• Intestinal juice, secreted by glands in the walls of the duodenum/Jejunum (Glands of Brünner/ crypts of Lieberkühn)

The Glands of Brünner/ Crypts of Lieberkühn consist of: 

• Mucous: Protects the walls of the small intestine against digestive juices
•                  Makes movement of chyme easier
• Maltase: Digests maltose to glucose (Glucose: simple sugar)
• Sucrase: Digests sucrose to glucose/fructose
• Lactase: Digests lactose to glucose/ galactose
• Various peptidases: digests peptones to amino acids  
• Enterokinase: Activates tripsinogen
• Nucleotidases: Digests the nucleotides into their sub-units (building blocks of RNA and DNA)  
• Bicarbonate ions: Alkaline medium in intestinal juice

• Final digestion of food takes place in the small intestine (Into soluble compounds); chyme stays in the small intestine for about 8 hours
• Absorption of nutrients also takes place
• Indigestible waste moves through the ileo-saecal valve  to the large intestine (colon)

Absorption of nutrients in the small intestine: 

End products that need to be absorbed:

• Water soluble amino acids (End product of protein digestion)
• Water soluble monosaccharides (Mainly glucose – end product of carbohydrate digestion)
• Hydrophobic fatty acids (End product of fat digestion)  
• Water soluble (hydrophilic) glycerol (End product of fat digestion)
• Mineral salts 
• Vitamins 
• Water 

Absorption of glucose/amino acids: 

• Absorption of glucose (other monosaccharides)/ amino acids is an active process
• They are transported through the columnar epithelium of the villi by protein carries (Energy is needed for this process)
• Advantage of active transport: Water soluble glucose/amino acids 1) Can move through the hydrophobic fatty layer of the cell membrane 2) Molecules can move against the diffusion gradient 3) Absorption can be faster than normal diffusion   

Absorption of fats: 

Slightly more complex because fats are hydrophobic. Glycerol is water soluble – molecules move into the epithelial cells –

• Fatty acids binds with salts – forms temporary fatty acid-bile salt complexes
• Micelles are water soluble
• Fat soluble vitamins (A,DE,K) – Found in micelles –
• Micelles move through passive diffusion – through epithelium cells –
• When the glycerol/micelle have  moved through the epithelial of the villus, fatty acids break loose from bile salts recombine with glycerol to form fats again
• Fats move into the lymph capillary
• Lymph that contains fats – known as chyle, milky appearance
• Small portions of he absorbed fats move with the lymphatic system (Join the blood at a later stage)
• Small portions of the absorbed fats moves directly into the capillary blood vessels
• All lymph vessels join, forming the large thoracic duct
• Thoracic ducts joins the blood system at the left subclavian vein

Absorption of other substances: 

• Most of the chyme water, 80% of mineral salts, water soluble vitamins are absorbed in the duodenum/ jejunum (together with other nutrients)
• Water soluble nutrients diffuse into blood capillaries in the villi
• Blood capillaries of the small intestine join, forming the hepatic portal vein – transports absorbed nutrients to the liver
• Villi in the duodenum are large, almost flat
• Villi becomes smaller in the jejunum/ ileum
• End of the ileum the contents of the small intestine consist of: Undigested water, water, ions, intestinal bacteria

Large intestine (colon): 

• Ileum of the small intestine joins the colon at the first part, the caecum
• A sphincter, the ileo-caecal valve – separates the two parts from each other
• Base of the caecum a finger-like outgrowth, appendix vermiformis
• Appendix has no obvious function in the human body – plays a role in the cellulose digestion of rodents –
• Colon runs around the whole abdomen area  - ascending, transverse, descending parts –
• Descending part of the colon ends in the rectum (muscular tube) - temporary/ final storage place for solid waste – Faeces –
• Body gets rid of faeces by means of defecation – through the anal opening
• The anus can be closed by an anal sphincter
• Mucous membrane lines the colon
• Contents of the colon is moved along through peristalsis

Role of fibre in the diet: 

• Fibre stimulates the whole of the digestive tract to perform peristalsis
• Fibre prevents constipation in the colon
• Constipation can lead to cancer of the colon – therefore fibre also prevents colon cancer –
• Fibre gives volume to the undigested waste
• Undigested waste moves through the colon/absorbs water that make the faeces soft (making defecation easier – thus preventing piles (haemorrhoids))
• Haemorrhoids: When blood vessels in the colon start to bulge/ bleed under high pressure. This pressure is caused by constipation. Haemorrhoids are usually surgically removed

Absorption in the colon: 

• Final absorption of water/ mineral salts takes place in the colon
• Faeces is the solid waste that remains behind – eventually egested
• Decomposition of the faeces starts in the colon through activity of the intestinal bacteria
• Bacteria live in a mutualistic relationship in the colon
• Bacteria provide vitamin K
• Vitamin K is essential for blood clotting
• Presence of intestinal bacteria is necessary for the normal functioning of the digestive process

Defecation: 

• Characteristics of the colour of faeces is caused by the presence of bile salts
• Darker the faeces the longer it has been in the colon
• Faeces are ready to move into the rectum when all the minerals and water have been absorbed
• When the rectum is full, it contracts, the anal sphincter releases and defecation takes place

End of destination of nutrients: 

• Nutrients are absorbed by the small intestine
• Transported to the liver by the hepatic portal vein
• Liver acts as a type of dispatch depot – fats, glucose, amino acids can be metabolised