Chapter 5: Nutrition in Humans

• The buccal cavity, also known as the mouth or oral cavity, is the space at the beginning of the alimentary canal, spanning from the lips to the back of the throat.
• The function of the buccal cavity is to receive food by ingestion, break it into small particles by mastication (physical digestion), and mix it with saliva (chemical digestion).
• The parts of the buccal cavity include: teeth to break large pieces of food into smaller pieces and increase surface area of the food so enzymes can work more efficiently, salivary glands that secrete saliva into the mouth, and the tongue that mixes food with saliva and moves food to the back of the mouth during swallowing.
• Pharynx Definition: Muscular tube in the neck, the soft part at the top of the throat that connects the mouth and nose to the oesophagus

  Function: serves both the respiratory and digestive systems by receiving air from the nasal cavity and air, food and water from the oral cavity behind the mouth and in front of the oesophagus

  
• Oesophagus: Definition: Narrow, muscular tube; passes through the thorax and connects mouth to the stomach

  
  

  Peristalsis in the Oesophagus: Definitions: Peristalsis is the rhythmic, wave-like muscular contractions in the wall of the alimentary canal.

  enables food to be mixed with digestive juices

  propel food along the gut

  
• Peristalsis movement involves the contraction of longitudinal muscles, which are located on the outer side of the gut wall, and the relaxation of circular muscles, which are located on the inner side of the gut wall.
• Longitudinal and circular muscles are antagonistic muscles, meaning when one set of muscle contracts, the other set relaxes.
• In the process of peristalsis, circular muscles contract, constricting the lumen and pushing food forward.
• Longitudinal muscles relax, allowing the gut to dilate and widen the lumen for food to enter.
• Muscles behind the bolus involve the contraction of the circular muscle, which propels the food forward as the lumen is narrowed. Vice versa for the muscles in front of the bolus.
• Enzymes:

  definition: Biological catalysts, made of proteins and speed up the rate of chemical reactions.

  Enzymes in the mouth: 

  Salivary amylase secreted by salivary glands in the mouth :

  • chemically digests starch into maltose
  • optimum pH level for salivary amylase is at pH 7
• The stomach

  • Definition
  • Distensible muscular bag with thick and well-developed muscular walls
  • Peristalsis occurs
  • Only protein is digested into polypeptides
  • Location
  • Lies to the left side of the abdominal cavity
  • Partly covered by the liver

  Physical digestion

  • Peristalsis occurs, and the food is mixed with the protease enzymes and gastric juice
  • Churning action 

  Chemical digestion 

  • The protease digests the complex protein molecules into polypeptides
• Features

  • Stomach wall
  • Numerous pits
  • Lined with gastric glands
  • Gastric glands
  • Secrete gastric juice (important in digestion)
  • Contains water and HCl
  • Provides an acidic environment for the protease to work well, pH 2
  • Kills bacteria in the food
• The structure of the small intestine includes the duodenum, jejunum and the ileum.

  
• Duodenum

  • definition
  • the first part of the small intestine extending from the opening from the stomach
  • location
  • it is located between the stomach and the jejunum (middle part of the small intestine)
  • structure
  • a u-shaped segment
  • Approximately 25-30cm long
  • Consists of 4 segments
  • Made up of 4 layers of tissue that are identical to other layers of the gastrointestinal (GI) tract
  • function
  • to mix food in the small intestine with bile from the gallbladder as well as enzymes such as pancreatic juices from the pancreas, to digest the food

  
• Jejunum

  • middle part of the small intestine
  • absorbs sugars, amino acids, lipids and fatty acids
• Ileum 

  • coiled part at the end of small intestine
  • absorption takes place 
  • absorbs remaining nutrients that were not absorbed into bloodstream
• Adaptations of the small intestine 

  1. Villi

  • The inner surface of the ileum has numerous folds and finger-like projections called villi to increase surface area
  • To increase the exposed surface area for absorption
• Adaptations of the small intestine 

  2. One-cell-thick epithelium (wall)

  • The wall of the villus is one cell thick and is really thin
  • Provides a short diffusion distance for nutrients to pass through
• Adaptations of the small intestine

  3. MicrovilliLarge amount of blood capillaries in each villus

  • Allow blood to transport the absorbed glucose and amino acids
  • Maintains a steep diffusion gradient, increasing the rate of diffusion

  1. Lacteal

  • Each villus contains a lacteal to transport the fatty acids away
  • Maintain a steep diffusion gradient, increasing the rate of diffusion

  1. High amounts of mitochondria in epithelial cells

  • Provide energy for active transport of nutrients into the villi
• Absorption taking place in the small intestine 

  • Glucose + amino acids
  • Absorbed by diffusion into the blood capillaries of the villi 
  • also absorbed by active transport into the blood capillaries 
  • lower concentration of digested food substances in the lumen of the small intestine than in the blood capillaries 
  • Glycerol + fatty acids
  • Diffuse into epithelium (one-cell-thick)
  • Combine to form minute fat globules which enter the lacteals
• Active transport: movement of substances from a region of lower concentration to higher concentration (going against the concentration gradient)