digestive system and nutrition

• Function of digestion

  To break down large insoluble molecules into small soluble ones that can be absorbed
• Human gut - anatomy

  • Not all sections are the same!
  • Changes across the gut
• Epithelium in the mouth

  It also lines the oesophagus
• Epithelium in the small intestine

  Columnar epithelium, with a microvillus brush border increases surface area for absorption
• types of digestion in the mouth

  1. Mechanical

  2. Chemical
• Enzyme produced in the mouth

  Salivary amylase (also known as ptyalin)
• Where is salivary amylase released?

  Parotid gland
• Contents and function of saliva

  • Water
  • Mucus
  • Enzymes - parotid only
• Function of salivary amylase

  Hydrolyses starch to maltose
• Types of adult human teeth

  • Incisors
  • Canines
  • Premolars
  • Molars
• Tooth structure

  • Closed roots, with a pulp cavity containing nerves and blood vessels
  • Crown made up of enamel, dentine and cementum
• Movement of food through the oesophagus

  Circular muscle layer contracts to move the bolus of food towards the stomach
• the oesophagus

  • layers of muscle - longitudinal and circular
  • epithelium = Stratified squamous epithelium
• Function of the stomach

  To further digest and store food
• Stomach muscle layers

  • Circular
  • Longitudinal
  • Oblique
• Sphincters in the stomach

  • to stop the contents from leaking out as the stomach contracts:
  • Cardiac sphincter
  • Pyloric sphincter
• Gastric glands and pits

  • Situated in the mucosa of the stomach
  • Contain three types of cells: oxyntic, chief, and enteroendocrine
• Small intestine

  • 6.5 m long in the average person
  • 3 regions:
  • duodenum - bile duct and pancreas open into here via the ampulla of Vater
  • jejunum - absorption of sugars amino acids and lipids
  • ileum - absorption of sugars amino acids and lipids plus vitamin B12
• Structure of small intestine wall

  • Thrown into folds (plicae)
  • wall, Villi and microvilli increase surface area
  • Two layers of muscle
  • Brunner’s gland and mucus glands
  • The Crypts of Lieberkuhn
  • Two nerve plexuses:

  1. Auerbach's - controls the contraction of the gut
  2. Meissner's - controls glandular secretion and blood flow
• Villi

  • Finger-like projections into the gut
  • Enterocytes responsible for absorption
  • Goblet cells produce mucus
  • Paneth cells produce antimicrobial peptides and proteins
  • Endocrine cells produce hormones
  • Stem cells in the Crypts of Lieberkuhn
  • capillary and lacteal for absorption of products of digestion
• Digestive enzymes - proteases

  • Produced in the stomach, pancreas and small intestine
  • Hydrolyse proteins to amino acids for absorption
  • two types:

  1. Endopeptidases
  2. Exopeptidases

  • Plus dipeptidases on the intestinal mucosa which hydrolyse dipeptides to amino acids for absorption
• Pepsin

  • Produced in the stomach in gastric glands by chief cells as pepsinogen (an inactive form)
  • activated by HCl, produced by oxyntic cells
  • Pepsin is an endopeptidase
  • HCl also hydrolyses proteins
• Pancreatic proteases

  • Produced in inactive form
  • Trypsin and chymotrypsin are endopeptidases, activated in the small intestine mucosa
  • Carboxypeptidases
• Dipeptidases and aminopeptidases

  • Produced by the intestinal mucosa
  • Break down dipeptides into amino acids
• Amino acid absorption

  By co-transporter proteins
• Pancreatic lipase

  Hydrolyses most lipids
• Lipid digestion and absorption

  1. Lipids are hydrophobic in a hydrophilic environment

  2. Fats need to be emulsified by bile produced by the gall bladder

  3. Bile salts surround triglycerides to form micelles

  4. Pancreatic lipase enters the micelles and hydrolyses triglycerides to monoglycerides and free fatty acids

  5. Small and medium fatty acids and glycerol move into blood capillaries of the villus

  6. Long chain fatty acids and monoglycerides reform as triglycerides and are formed intochylomicrons and are transported in the lacteals
• Salivary amylase (ptyalin)

  Not very effective at starch hydrolysis
• Pancreatic amylase

  Hydrolyses starch to maltose
• Disaccharidases on intestinal mucosa

  • Maltase to glucose (maltase)
  • Sucrase to glucose and fructose (sucrase)
  • Lactase to glucose and galactose (lactase)
• Absorption of carbohydrates

  • Glucose and galactose are absorbed by Na+ cotransporter proteins
  • Fructose is absorbed by facilitated diffusion
  • Once in the epithelial cells all monosaccharides enter the capillaries of the villi and then go into the hepatic portal vein
• Absorption of glucose

  1. Na+/K+ pump in basal membrane pumps Na+ out and K+ into the cell

  2. This lowers Na+ concentration in cell

  3. Na+ comes in down its diffusion gradient through the Na+ symport in apical membrane, bringing glucose with it, raising the concentration of glucose in the cell

  4. Glucose moves out of cell by facilitated diffusion through glucose channels in basal membrane

  5. Glucose diffuses across tissue fluid into capillaries of villi and is carried away in the blood
• Balanced diet

  Includes proteins, carbohydrates, lipids, vitamins, minerals, water, and non-digestible carbohydrates
• Malnutrition

  Includes undernutrition, inadequate vitamins or minerals, overweight, obesity, and resulting diet-related noncommunicable diseases
• Types of malnutrition

  • Undernutrition (wasting, stunting, underweight)
  • Micronutrient-related malnutrition (deficiencies or excess)
  • Overweight, obesity and diet-related noncommunicable diseases
• Body mass index (BMI)

  Over 25 is considered overweight, over 30 is obese
• BMI does not distinguish between excess fat, muscle, or bone mass, nor indicate distribution of fat
• Causes of malnutrition

  • Physical illness
  • Mental illness
  • Poor diet (poverty, homelessness, social isolation)
  • Alcohol related
• Kwashiorkor

  Protein and energy undernutrition, with weight loss, stunted growth, dry skin and eyes, brittle hair, diarrhoea, lower immunity, stomach infection and lactose intolerance, respiratory infections
• Marasmus

  Severe protein deficiency, with loss of muscle mass, enlarged tummy ("pot belly"), regular infections, red inflamed skin patches, dry brittle hair that falls out, tiredness or irritability