Human Digestion and Anatomy

• The digestive system and anatomy of the human digestive system will be covered in these notes
• MD Ketevan Dundua made this presentation for BAU

General Terminologies

• Nutrition is the process of taking in and using food
• Nutrients are substances in food that are used as energy sources to run the body's systems
• The digestive system is the organs and glands in the body responsible for digestion
• Digestion is the breakdown of food to smaller molecules

Types of Digestive Systems

• Incomplete digestive systems are common among invertebrates, there is only one opening for food ingestion, and they lack an anus
• Complete digestive systems are common among nematodes, annelids, mollusks, echinoderms, and vertebrates, and they have digestive tubes with two openings: a mouth and an anus

The Human Digestive Tract

• The human digestive tract is a long, coiled, muscular tube stretching from the mouth to the anus
• From the mouth to the anus, the human food tube or digestive tract is about nine meters long

Digestion Regions

• Food processing starts in the mouth
• The bolus food is then swallowed and moved through the pharynx into the esophagus
• Food is mechanically and enzymatically digested in the stomach
• Enzymatic digestion takes place mostly in the small intestine
• The large intestine then eliminates waste leading to the anus, for waste elimination

Digestive System Assisting Organs

• Food processing starts in the mouth with assisting organs secreting saliva, such as the salivary glands
• Bolus is swallowed and enters the pharynx then into the esophagus
• Food is mechanically and enzymatically digested in the stomach
• The liver secretes bile stored in the gallbladder, emulsifying fats with a detergent-like action assisting in the absorption of fats
• The pancreas secretes digestive enzymes
• Enzymatic digestion occurs in the small intestine
• Nutrients are digested as they move along the digestive tract
• Nerves and hormones regulate digestion
• Absorption occurs mainly through the villi of the small intestine
• The large intestine eliminates wastes through the anus

Digestion Phases

• Mechanical phase is breaking food into small pieces, pushing food down the food tube, and mixing it with digestive juices
• Chemical phase further breaks down larger food molecules into smaller molecules due to digestive enzymes

Mechanical Digestion in the Mouth

• During mastication in the mouth, four types of teeth tear the food into pieces:
  • Incisors: Thin-edged for cutting food
  • Canines: Pointed and used for tearing
  • Molars and premolars: Specialized for crushing and grinding
• Humans have a temporary (deciduous) dental set
  • Incisors: 2 in the upper jaw and 2 in the lower jaw, totaling 8
  • Canines: 1 in the upper jaw and 1 in the lower jaw, totaling 4
  • Premolars: 0 in both upper and lower jaws, totaling 0
  • Molars: 2 in the upper jaw and 2 in the lower jaw, totaling 8
  • Total teeth: 20
• Humans have a permanent dental set
  • Incisors: 2 in the upper jaw and 2 in the lower jaw, totaling 8
  • Canines: 1 in the upper jaw and 1 in the lower jaw, totaling 4
  • Premolars: 2 in the upper jaw and 2 in the lower jaw, totaling 8
  • Molars: 3 in the upper jaw and 3 in the lower jaw, totaling 12
  • Total teeth: 32

Chemical Digestion in the Mouth

• Salivary glands secrete saliva that contains the enzyme salivary amylase
• Salivary amylase and the enzyme maltase (catalyzes maltose into glucose) enable chemical digestion in the mouth
• Starch is broken down via Salivary Amylase / Ptyalin into Maltose
• Maltose is broken down via Maltase into Glucose
• The tongue aids in mechanical digestion, pushing and mixing food, while its back secretes mucus easing swallowing

The Tongue, Pharynx, & Esophagus

• The pharynx and esophagus conduct food to the stomach
• After chewing, food is swallowed through the pharynx, extending to the esophagus

Pharynx and Esophagus

• The pharynx and esophagus are used for both the digestive and respiratory system
• The esophagus is a canal connecting the throat to the stomach
• The epiglottis tissue closes the airway during swallowing
• Peristalsis propels chewed food (bolus) and mixes it with digestive juices in the stomach and small intestine
• Anti-peristalsis results in vomiting, protecting the body

Food's Digestion in the Stomach

• The stomach's entrance is closed by a ring muscle at the end of the esophagus
• When empty, the stomach is collapsed and shaped like a hot dog
• Once food enters, the rugae folds of the stomach wall smooth out and expand to over a liter

Enzymatic Digestion in the Stomach

• Parietal cells in the gastric glands secrete hydrochloric acid and intrinsic factor, crucial for vitamin B12 absorption
• Chief cells secrete pepsinogen (an inactive enzyme precursor); when pepsinogen contacts acidic gastric juice, it becomes pepsin

Food's Digestion in the Small Intestine

• Digestion is completed, and nutrients are absorbed through the wall of the small intestine
• The small intestine is about 5 to 6 meters (about 17ft.) long
• It has three regions:
  • Duodenum
  • Jejunum
  • Ileum
• Chemical digestion mostly occurs in the duodenum, the first layer of the small intestine

Liver Secretion

• The liver is the largest internal organ and one of the most complex, lying in the upper right abdomen under the diaphragm
• The liver secretes bile, a fluid that emulsifies fats

Liver Functions

• Secretes bile
• Helps maintain homeostasis by removing or adding nutrients to the blood
• Converts excess glucose to glycogen and stores it
• Converts excess amino acids to fatty acids and urea
• Stores iron and certain vitamins
• Detoxifies alcohol and other drugs and poisons

Pancreas Secretion

• The pancreas secretes both digestive enzymes and hormones
• These help regulate the level of glucose in the blood
• Enzymes secreted by the pancreas are called trypsin and chymotrypsin
• Trypsin and chymotrypsin digest polypeptides to dipeptides
• Pancreatic Lipase degrades fats
• Pancreatic Amylase breaks down almost all types of carbohydrates including cellulose disaccharides
• Ribonuclease and Deoxyribonuclease split RNA and DNA to free nucleotides

How Chemical Digestion Done in Small Intestine

• Bile from the liver and digestive enzymes from the pancreas are released into the duodenum
• These act on the chyme (a semi-fluid mass of digested food expelled by the stomach into the duodenum)
• Enzymes produced by the epithelial cells lining the duodenum catalyze the digestion of the major nutrient types

Digested Nutrients

• Chyme moves via peristalsis, contractions, and motions of the villi for nutrient digestion
  • Carbohydrates are digested to monosaccharides or simple sugars
  • Proteins are digested to amino acids
  • Fats are digested to fatty acids and monoacylglycerols

Chemical Digestion of Complex Molecules

• Proteins are broken down into Maltose by proteases
• Fats broken down into fatty acids & glycerol by lipase
• Carbohydrates are broken down into simple sugars by carbohydrases

Nerves and Hormones

• Digestion is regulated by nerves and hormones including the enteric nervous system regulates the digestive system
• Several hormones including gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP) regulate the digestive system

Regulatory Hormones of the Digestive System

• Gastrin: Stomachs mucosal cells creates Gastric glands to secrete pepsinogen
• Secretin: Duodenum creates in the Pancreas & Liver that Signals secretion of sodium bicarbonate, and stimulates bile secretion
• Cholecystokinin (CCK): Duodenum creating in the Pancreas & Gallbladder that stimulates release of digestive enzymes & Stimulates emptying of bile
• Gastric Inhibitory Peptide (GIP): Duodenum creating in the Stomach that decreases stomach churning, thus slowing emptying

Absorption

• Absorption is the process in which substances are taken in by cells of the food tube
• It takes place mainly through the villi of the small intestine.
• Digested food forms in amino acids, simple sugars, fatty acids, and glycerol, and diffuses into the cells of the villi

Absorption Process

• Digested food diffuses into capillaries and reaches the blood, while fatty acids and glycerol diffuse into the lacteals which lead to lymph
• Digested food reaches the blood & lymph through diffusion and undergoes circulation
• Circulating fluids distribute the digested food to the body's cells

Large Intestine Elimination

• Undigested food passes through the large intestine that leads to waste
• The large intestine is shorter than the small intestine; however, its diameter is greater at 1.3m (about 4ft.)

Elimination vs Excretion

• Elimination gets rid of digestive wastes – materials that have not been absorbed from the digestive tract
• Excretion is getting rid of metabolic wastes, occurring mainly via the kidneys and lungs; the large intestine excretes bile pigments

Illnesses Involving Elimination

• Diarrhea is when chyme passes too rapidly caused by anxiety, foods, or disease lining irritation
• Constipation is a result of chyme passing through the intestine too slowly
• Colorectal cancer, cancer of the colon and rectum is also possible

Functions of Food

• Food is comprised of organic nutrients, water, minerals, and vitamins

• Body tissues are 67% water, 15% proteins, and 13% fats

• Mineral solids (calcium, phosphorus, iron, etc.) are also used as building materials

• Lipids provide energy and make biological molecules

• Proteins serve as enzymes and cell structures

• Vitamins are organic compounds for metabolism

• Minerals are inorganic nutrients

• Antioxidants protect against oxidants

• Phytochemicals maintain health

Imbalance

• Undernutrition is a serious problem, resulting from not enough to eat, or from lack of a balanced diet

  • Malnourished individuals are weak, fatigued, and susceptible to infections
  • Severe protein malnutrition results in kwashiorkor in young children, where a first child is displaced from breastfeeding due to the arrival of a sibling

• Obesity accumulates body fats

  • The World Health Organization (WHO) ranks obesity among the top 19 global health issues
  • Obesity is a major risk factor for heart disease, diabetes mellitus, osteoarthritis, and certain cancers

Tips for a Healthy Body

• Watch the amount of food consumed
• Chew food well and eat slowly to facilitate digestion
• Drink plenty of water
• Balance food intake with activities

Balanced Diet

• A balanced diet with vegetables and fruits
• It should be a balance of milk and milk products
• Meat including fish
• Along with cereals such as rice
• As well as other healthy foods
• Always remember, the greatest wealth is your health