Digestive System: An Overview

Digestive System Introduction

• Humans are heterotrophs, meaning they depend on other organisms for food
• Humans are also omnivores, consuming both plants and animals
• Humans rely on external sources for raw materials to produce energy for bodily functions
• The major components of food include carbohydrates, fats, proteins, minerals, vitamins, and water
• The digestive system converts complex, insoluble food into simpler, absorbable forms
• This breakdown is called digestion, facilitated by digestive enzymes in the alimentary canal

Types of Digestion

• Digestion breaks down food into smaller components for easier absorption
• There are three types of digestion: mechanical, chemical/enzymatic, and microbial
• Mechanical digestion occurs mainly in the oral cavity, physically breaking down food by chewing
• Chemical/enzymatic digestion breaks down food with chemicals like HCl and enzymes such as amylase, lipase, and pepsin
• Microbial digestion occurs in the large intestine, fermenting indigestible matter using gut bacteria and reabsorbing water

Steps of Digestion

• Digestion is a multistage process
• Ingestion is the intake of food into the mouth or digestive system
• Digestion involves mastication and mixing with water, acids, bile, and enzymes to break down complex molecules
• Absorption is the entry of simple molecules (nutrients) into circulatory and lymphatic capillaries via active transport and diffusion
• Egestion is the removal of undigested materials from the digestive tract through defecation

Gastrointestinal Tract (GIT)

• The GIT facilitates the breakdown and absorption of food and liquids
• The GIT begins at the mouth, continues to the esophagus, stomach, and small intestine (duodenum, jejunum, ileum), and ends with the large intestine (cecum, colon, rectum) terminating at the anus

Histological Structure of GIT

• The GI tract consists of four layers, also known as Tunics
• From the inside out these are called mucosa, sub-mucosa, muscularis and serosa
• The mucosa is the innermost layer and aids in absorption and mucus secretion
• Mucosa consists of simple columnar epithelium with specialized goblet cells
• Mucosa is folded to form Villi and Micro-Villi
• The submucosa is thick, highly vascular, and serves the mucosa
• Elements absorbed by the mucosa are picked up by submucosa blood vessels
• The submucosa also contains glands and nerves
• The Muscularis is responsible for segmental contractions and peristaltic movement in the GIT
• It contains an inner circular and outer longitudinal layer of smooth muscle
• The muscle layers cause food movement by churning with digestive enzymes in the GIT
• Serosa is the outermost, protective layer with vascular connective tissue and simple squamous epithelium

Organization of Digestive System

• There are two primary subdivisions: the alimentary canal and accessory organs
• The alimentary canal constitutes the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and anus
• Accessory organs include salivary glands, tongue, teeth, liver, gallbladder, pancreas, and vermiform appendix

Oral Cavity (Mouth)

• In humans, digestion initiates in the oral cavity, with food being chewed and mixed with saliva
• The oral cavity consists of the vestibule and the proper oral cavity
• The vestibule is surrounded by the lips and cheeks externally, and the teeth and gums internally
• The proper oral cavity is bounded superiorly by the palate(hard and soft), inferiorly by the oral diaphragm and tongue, and anteriorly/laterally by the teeth and gums; also connects with the pharynx
• The hard palate separates the oral cavity from the nasal cavity
• The uvula rises upwards as a conical appendage backwards from the soft palate
• The uvula separates the oropharynx

Tongue

• The tongue is a muscular structure located on the floor of the oral cavity
• It is made up of skeletal muscles
• The tongue manipulates food during chewing and swallowing
• It is also the main taste organ, and is covered in taste buds
• Papillae protrude from the mucous membrane on the dorsal surface containing capillaries and nerves
• There are four types of papillae: filiform, fungiform, circumvallate and foliate
• Filiform papillae provide the sense of touch, but lack taste buds
• Fungiform papillae are scattered among filiform papillae and contains taste buds
• Circumvallate papillae are dome-shaped and vary in number from 8 to 12 and contain taste buds
• Foliate papillae are present on each side of the tongue, and contain taste buds
• The tongue functions to provide food and drink taste
• Sensation of heat, pain and touch is also a function
• It helps to mix food during chewing and swallowing
• Pronunciation/speech is another function

Teeth

• Teeth are present and attached to both jaws in the oral cavity
• Teeth consist of a crown, neck, and root
• There are four types: canines, incisors, premolars, and molars
• Anatomically, teeth consist of a crown (covered in enamel), dentin (enclosing pulp cavity), and cement (covers root)
• Functionally, teeth aid in chewing and mastication of food

Salivary Glands

• Salivary glands are associated with the mouth and oral cavity
• Salivary glands are exocrine glands that produce saliva, which keeps the oral cavity moist
• 1 to 1.5 liters of saliva is secreted daily by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual)
• Saliva then mixes with the chewed food
• There are two types of saliva - thin watery, and thick mucous
• The thin watery saliva wets food. The thick mucous saliva acts as a lubricant causing food particles to stick together and form a bolus
• Saliva is composed of 97-99.5% water, salivary amylase (digests carbohydrates), mucus (softens food), and lysosomes/antibodies (destroy bacteria), and cleans the oral cavity

Pharynx

• The pharynx connects the oral cavity to the esophagus and the nasal cavity to the larynx
• It can be divided into three regions: nasopharynx, oropharynx, and laryngopharynx
• It is a common path because both air and food travel throught it
• Air passes through the pharynx to the larynx, while food passes through the pharynx to the oesophagus
• The lower pharynx, the Laryngopharynx, is closed by the epiglottis during swallowing
• The nasopharynx contains the Eustachian tube connecting it to the middle ear.
• The epiglottis is a tissue flap guarding the trachea entrance and closes when anything is swallowed

Oesophagus

• The oesophagus is a narrow muscular tube, about 25 cm long, connecting the pharynx to the stomach's cardiac portion
• Its wall consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer
• Chewed food moves down the oesophagus to the stomach via peristaltic contraction of these muscles in about seven seconds
• Digestion does not take place in the oesophagus

Stomach

• The stomach is a 'J'-shaped pouch with thick, el astic muscle walls, which stores and helps breakdown food
• Food enters the stomach through the cardiac portion
• It is then broken apart and thoroughly mixed with gastric juice containing hydrochloric acid (HCl), mucus, and digestive enzymes
• The stomach consists of three portions: cardiac, stomach body, and pyloric
• The cardiac portion has mucous cells
• The stomach body has mucous, parietal and chief cells
• The pyloric cells also has mucous cells
• Mucous cells secrete mucus to lubricate food and protect the stomach walls from HCl
• Parietal cells secrete HCl, making the medium acidic, aiding digestive enzyme reactions, and killing harmful microorganisms
• Chief cells secrete digestive enzymes (pepsinogen and renin) that break down the proteins
• Pepsin digests protein and is in the proenzyme form pepsinogen, activated by HCI
• Gastrin activates HCl secretion from parietal cells and is a hormone secreted by gastric glands
• Renin is an enzyme that curdles milk (for easier protein digestion) in new baby stomachs
• Intrinsic factor is secreted by parietal cells which enables vitamin B-12 absorption
• Food in the stomach is semi-liquid and known as chyme

Small Intestine

• After stomach processing, food passes to the small intestine via the pyloric sphincter
• Digestion and absorption mostly occurs here after chyme enters the duodenum
• The small intestine has three regions: duodenum, jejunum, and ileum
• The remaining protein, carbohydrate, and fat digestion occurs in the duodenum via pancreatic and hepatic juices
• Absorption occurs in the ileum via villi
• Pancreatic juice produced by the pancreas includes amylase (digests carbs), trypsin/chymotrypsin (act on proteins), and lipase (digests fats)
• Hepatic juice (bile) emulsifies fats, neutralizes chyme, is produced by the liver, and is stored in the gallbladder
• Bile in the gallbladder is more concentrated
• Brunner's gland in the duodenum secretes mucus rich in bicarbonates, which helps neutralize stomach acidity for digestive enzymes
• The ileum has finger-like villi and smaller microvilli that help absorption by increasing surface area and absorption speed
• Blood carries absorbed nutrients from the small intestine via the hepatic portal vein to the liver for filtering, toxin removal, and nutrient processing.

Large Intestine

• After the small intestine, food enters the large intestine; digestion is retained for fermentation by gut bacteria, breaking down substances
• In humans, the large intestine is ~1.5 meters long with three parts: cecum (junction with small intestine), colon, and rectum
• The colon has four parts: ascending, transverse, descending, and sigmoid
• The large intestine is longer in herbivores because more plant material is consumed
• The large intestine absorbs water from the bolus and stores feces in the rectum until egestion via the anal sphincter
• The large intestine has no villi, and no digestion, though it reabsorbs water and some ions like sodium and chloride
• The large intestine's role through microbial fermentation produces vitamins K/B-12 and forms feces

Vermiform Appendix

• The vermiform appendix is a vestigial organ
• It connects to the caecum providing immunity
• The vermiform appendix helps maintain gut flora as well as cellulose digesting bacteria

Liver

• The liver is the largest gland in the body with two lobes (right and left) and the right lobe is bigger
• Kupffer cells in hepatic sinusoids engulf harmful organisms and remove old red blood cells
• The livers functions include detoxification by removing/converting toxins into less harmful substances
• The liver:
  • Produces and excretes bile required to emulsify fats; some bile directly drains into the duodenum, and some is stored in the gall bladder
  • Produces certain proteins for blood plasma (albumin, globulin, and fibrinogen)
  • Produces coagulation factors
  • Converts excess glucose into glycogen for storage and later usage
  • Forms urea from amino acids and ammonia
  • Stores vitamins A, D, and K
  • Breaks down insulin and other hormones
  • Processes hemoglobin for its iron content

Gallbladder

• Function: Bile Storage
• Function: Emulsification of fats and absorption of Vitamin K, and other fat soluble vitamins

Pancreas

• It is the second largest gland.
• It is a mixed gland (both exocrine and endocrine).
• Secretes pancreatic juice (amylase, trypsin, chymotrypsin, and lipase) with pH 7.5-8.1.
• Exocrine functions: Digestive enzyme secretion: Trypsin and chymotrypsin digest protein, lipase digests fats and amylase digests carbohydrates
• Trypsin and chymotrypsin are secreted as proenzyme forms trypsinogen/chymotrypsinogen, respectively
• Enterokinase helps convert trypsinogen into trypsin, while trypsin helps chymotrypsinogen into chymotrypsin
• Endocrine functions(islets of Langerhans): Hormone secretion (Glucagon and Insulin)
• Glucagon: secretedfrom alpha cells of the islets of Langerhans converts the stored glycogen in the liver and muscles into glucose
• Insulin: made in the beta cells of the islets of Langerhans regulates blood glucose by converting extra glucose into glycogen

Digestion Process

• Digestion has three phases: cephalic, gastric, and intestinal

Cephalic Phase

• Cephalic phase occurs in the head region
• Thought, sight and smell of food stimulate the cerebral cortex, which is linked to the hypothalamus and medulla oblongata
• The vagus nerve is routed and releases acetylcholine
• Gastric secretion rises to 40% of maximum

Gastric Phase

• Gastric Phase last normally for 3 to 4 hours
• It is stimulated expansion of the stomach
• The presence of food/decrease in pH stimulates gastric juice secretion

Intestinal Phase

• The intestinal stage two parts, the excitatory and the inhibitory
• Partially digested food fills the duodenum.
• This triggers intestinal gastrin release.
• Enterogastric reflex and inhibits acetylcholine of vagus.
• The pyloric sphincter tightens to prevent more food and inhibits local reflexes.
• Digestion begins in the mouth with saliva and enzymes; food becomes a bolus via mastication; swallowed into the oesophagus
• Food enters the stomach via peristalsis and gastric juice contains hydrochloric acid and pepsin
• It can damage the walls of the stomach that mucus protects the walls
• Further enzymes break down food combined with churning
• Partially digested food (chyme) enters the duodenum
• Most digestion takes place in the small intestine aided by bile, pancreatic juice, and intestinal juice
• Intestinal walls are lined with villi/microvilli to improve nutrient absorption.
• Peristalsis is slower in the large intestine enabling fermentation by gut flora.
• Water is reabsorbed and waste material becomes faces for removal via the anus

Respiration

• Respiration is essential for any animal's survival to recieve oxygen and release carbon dioxide
• Living organisms require constant oxygen for cellular activity and release carbon dioxide as waste
• Respiration includes the physiological processes for oxygen uptake and carbon dioxide elimination
• Inspiration is inhaling, while expiration is exhaling

The Respiration Process

• Respiration is O2 intake accompanied with removing CO2 divided into: external and internal
• External includes gas exchange between the surrounding medium and respiratory organs
• Ventilation is the movement of gases in an out of the respiratory organs
• Inspiration is breathing in, while expiration is breathing out
• Breathing process brings oxygen into your lungs
• Pulmonary respiration: gas exchange between the alveoli and blood via diffusion.
• Internal is gas exchange between the blood and body cells, also oxidation of food via O2 to liberate CO2 and energy

Respiratory System

• It is composed of the nose, pharynx, larynx, trachea, bronchi, and lungs (with alveoli).
• All parts excluding alveoli are air passages, alveoli are the active respiratory components
• Air passages need to be open due to being characterized by a cartilaginous structure
• Lined with ciliated epithelium containing mucous cells
• The mucus works to trap dust and warm inhaled air and cilia removed/cleaned air passages
• The respiratory system includes: filter inhaled air, supply oxygen, remove carbon dioxide, and create the sounds of speech

Respiratory Organs

• The nose and mouth openings pull outside air into the system, with the nose starting with paired nostrils.
• The nasal cavity is a continuation of the two nostrils that are separated by the nasal septum to two chamber
• Air is warmed and humidified as inhaled air goes to the nasal cavity
• Wall hairs filter for dust/foreign particles; respiratory functions and chemoreceptors for smell
• Nasal Sinuses:Air-filled cavities in the skull that help act as shock absorbers, and humidify inhales air
• They include Frontal, Ethmoidal, Sphenoidal, and Maxillary.
• The pharynx is a tube connecting the nasal cavity and back of the mouth with the larynx
• Dual functions: moving passageway for both part of the respiratory and digestive systems (with air from the nasal cavity and food form through the mouth)
• The bottom part, the Laryngo-pharynx, closed by the epiglottis food swallowing
• Nasopharynx has Eustachian tube (connects the middle ear).
• Epiglottis: a tissue flap guarding the trachea entrance, it closes when anything swallowed
• In turn prevents the lungs from debris

Larynx

• The larynx connects air through and out of respiratory to the pharynx and trachea
• It is considered the voice box, and vibrates when the air flows
• The trachea carries passageway to the lungs it known as the wind pipe
• It is about 2.5cm wide/10-15cm supported in dorsally incomplete by "C" shaped through cartilage
• Lined with pseudo-stratified and ciliated epithelium
• Trachea branches via tubes

Bronchi and Bronchioles

• Two main bronchial in the right or left side tubes supported by cartilage
• Bronchi transport air between the trachea and lungs
• Each bronchus and lung breaks into tubules
• Tiny bronchioles in alveolar ducts terminate in air sac clusters referred as alveoli, which performs the in lungs respiration

Lungs

• These are the largest respiratory tract organs and they serve pairs, soft, elastic, and spongy
• Present in thoracic cavity from each side from the surface heart
• Conical shaped divided to the lobes, while the lung has split two, through split via threes
• Right lung smaller than large to allow heart (left in the center of the chest) that two layers (pleural membrane), is presented pleura has, helps, protecting shocks.
• Lung cells from alveoli (units/exchange) gas

Alveoli

• Air sacs in lungs where oxygen and carbon dioxide are exchanged and covered via blood
• Each time you breathe alveoli fill with the air increasing lung (make expand), oxygen
• The air blood absorbs inside capillaries covering around each alveolus and releases the air
• Each time you breathe the outside air is released, outside and carries the waste in the air.
• Two Mechanism of Breathing processes include inspiration 1st (fresh lungs air), and expiration(with muscle)
• This starts by center is the medulla oblongata, and contract diaphragm/inter-ribs, elevating for vertical diameter chest and sternum elevate leading increasing the lateral/anterior-posterior the wall
• In turn it increases the vertical and will lower pressure the air, which is the leads equal to the thoracic pressure
• Diaphragm: The abdominal separates both thoracic,abdominal cavities
• It's made of Mammalian voluntary.
• It contracts when, flattens (inspiration), and relaxes (expiration).

Mechanism of Expiration

• Expiration entails expelling carbon dioxide/air lungs
• It then passive (relax diaphragms) which turns positive and decrease volume causing push is in air/lung lungs
• Diaphragm become shaped

Summary of Respiration Steps

• Receieve oxygen and releasing carbon.
• Can be classified two categories, Internal/External
• (Ventilation and pulmonary):Carried Lungs /Structures
• Internal via respiratory/Cells
• Normal Breathing: Exhalation/Inhalation
• Rib Muscles: Inhale/Exhalation
• Gas helps the surfaces
• Pigments in carry blood the to controlled in the medulla oblongata autonomic