Comprehensive Overview of Human Digestive System Physiology and Function

DIGESTIVE SYSTEM (PHYSIOLOGY)
SAIRA NAWAZ
NURSING INSTRUCTOR

CONTENTS
 Define the term digestion
 Discuss the function of digestive system and role of accessory organs in digestion
 Describe function of salivary gland
 Explain digestion of food in mouth, stomach, small intestines & large intestines with the chemical processes of
enzymes
 Explain the absorption of various nutrients in the digestive system
 Discuss the process of defecation.

DIGESTION
 Digestion is the breakdown of large, complex organic molecules into smaller components that the body absorbs
for energy, growth, and cell repair. Molecules need to be small enough to diffuse across plasma membranes.
 Digestion is of two types:
 Mechanical/physical digestion physically breaks down the food in the mouth (chewing). Smaller pieces of food
increase surface area for digestion. It also takes place in the stomach (churning of food by the muscular stomach
walls)
 Chemical digestion uses enzymes to chemically break down complex food substances into their simplest form.
E.g. Starch is converted to maltose with the help of amylase enzyme
 Starch Amylase maltose

CONT……
 Chemical digestion:
 Starch (carbohydrate) digestion: in mouth and small intestine.
 Protein digestion: in stomach and small intestine
 Fat digestion: only in small intestine
 Why must food be digested???
 Large molecules of food are unable to pass through cell membranes, thus must be broken down into small
molecules so that they can diffuse through cell membranes into the blood stream

DIGESTIVE SYSTEM
 WHAT ISTHE DIGESTIVE SYSTEM ?
 The gastrointestinal tract (digestive tract, digestional tract,Gl tract,
GIT or alimentary canal) is an organ system within humans and
other animals which takes in food,digests it and absorb energy and
nutrients,and expels the remaining waste as feces.
 Major digestive organs
 ➤ Mouth.
 ➤ Pharynx.
 ➤ Esophagus.
 ➤ Stomach.
 ➤ Small Intestine.
 ➤ Large Intestine.
 ➤ Rectum
 Accessory digestive organs:
 ➤ Liver
 ➤ Gallbladder
 pancreas
 ➤ Salivary gland

FUNCTIONS OF DIGESTIVE SYSTEM/ PROCESS OF DIGESTION
 Ingestion: Food must be placed into the mouth before it can be acted on; this is an active, voluntary process
called ingestion.
 Propulsion: If foods are to be processed by more than one digestive organ, they must be propelled from one
organ to the next; swallowing is one example of food movement that depends largely on the propulsive process
called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
 Food breakdown/Digestion (Mechanical digestion)
 prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and
examples of mechanical digestion are: mixing of food in the mouth by the tongue, churning of food in the stomach,
and segmentation in the small intestine.

CONT ……
 Food breakdown/ Digestion (chemical digestion).The sequence of steps in which the large food molecules are
broken down into their building blocks by enzymes is called chemical digestion.
 Absorption:Transport of digested end products from the lumen of the GI tract to the blood or lymph is
absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or
passive transport processes.
 Defecation: Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of
feces.

DIGESTIVE ORGANS
 THE MOUTH
 Mouth ingests food
 Teeth masticates food into small pieces to increase surface area for digestion
 Saliva (pH 7) moisten and soften food.
 Tongue plays role in Speech and taste.
 Tongue mixes food with saliva and rolls food into a bolus before swallowing
 Saliva water, mucus, salivary amylase.

DIGESTIVE ORGANS
 THE ESOPHAGUS
 The esophagus or oesophagus, commonly known as the food pipe or gullet,The esophagus is a muscular tube
connecting the throat (pharynx) with the stomach.
 Functions include:
 Secrete mucus
 Moves food from the throat to the stomach using muscle movement called peristalsis
 What is Peristalsis?
 is a series of wave-like contractions of the muscles that are involved in the food movement and in the movement of
other liquid particles in the digestive tract to various processing organs that are located in the digestive system.”
 Reducing gastric reflux :Constriction of the upper and lower esophageal sphincters help to prevent reflux (backflow) of
gastric contents and acid into the esophagus, protecting the esophageal mucosa
 If acid from the stomach gets in here that’s heartburn.

DIGESTIVE ORGANS
 THE STOMACH
 J-shaped muscular bag that stores the food you eat, breaks it down into tiny pieces.
 Mixes food with digestive juices that contain enzymes to break down proteins and lipids.
 GASTRIC JUICE
 Gastric acid, gastric juice or stomach acid, is a digestive fluid formed in the stomach and is composed of hydrochloric
acid (HCl), potassium chloride (KCI) and sodium chloride (NaCl).
 The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel
so that digestive enzymes break down the long chains of amino acids.
 Acid in the stomach kills bacteria.
 Food found in the stomach is called chyme.
 Chyme is a thick, acidic, semifluid mixture of partially digested food, water, and stomach secretions (gastric juices)
formed in the stomach during digestion.

DIGESTIVE ORGANS
 THE SMALL INTESTINE
 The small intestine or small bowel is the part the
gastrointestinal tract between the stomach and the
large intestine, and is where most of the end
absorption of food takes place.
 PARTS
 The small intestine is divided into three structural
parts:
 (1)The duodenum
 (II)The jejunum
 (III)The ileum

CONT…..
 The majority of chemical digestion occurs in the first
of three sections of the small intestine known as the
duodenum.
 This section also contains an opening from the bile
duct and pancreatic duct through which bile and
pancreatic enzymes enter the small intestine

FUNCTIONS OF SMALL INTESTINE
 The small intestine is the primary site for nutrient absorption and final digestion. It absorbs over 90% of
nutrients (vitamins, minerals, carbohydrates, fats, proteins) and water into the bloodstream
 Key Functions of the Small Intestine:
 Final Digestion:The duodenum receives chyme from the stomach and mixes it with digestive juices from the
pancreas and bile from the liver/gallbladder.
 Nutrient and Water Absorption:The jejunum and ileum absorb vitamins, minerals, fats, proteins, and
carbohydrates into the bloodstream. Roughly 1- 3 gallons (6- 12 liters) of water and food pass through it daily.

CONT…..
 Increased Surface Area: The intestinal wall has folds, villi, and microvilli, creating a massive surface area (like a
tennis court) to maximize absorption efficiency.
 Motility:Through segmentation and peristalsis, the muscular walls churn food and move it forward to the large
intestine.
 Immune Function: The intestinal mucosa acts as a protective barrier against ingested pathogens.
 Endocrine Role: It releases hormones like cholecystokinin (CCK) and secretin, which regulate digestion and
enzyme release.

DIGESTIVE ORGANS
 THE LARGE INTESTINE
 The large intestine, also known as the large bowel
and colon, is the last part of the gastrointestinal tract
and end of the digestive system.
 Water is absorbed here and the remaining waste
material is stored as feces before being removed by
defecation.

PROCESS OF DEFECATION
 Defecation is the final act of digestion, by which
organisms eliminate solid, semisolid, or liquid waste
material (feces) from the digestive tract through the
anus.
 It is a combined involuntary and voluntary
process involving peristaltic waves in the colon,
contraction of rectal muscles, and relaxation of
anal sphincters, typically triggered by the
defecation reflex.

CONT…..
 Key Aspects of the Process:
 Physiology: Feces move into the rectum, causing it to stretch and triggering the defecation reflex.This reflex
causes the internal anal sphincter to relax involuntarily.
 Voluntary Control:The external anal sphincter is under voluntary control, allowing an individual to delay or
facilitate the process.
 Mechanism:The process involves the relaxation of pelvic floor muscles and contraction of abdominal muscles to
increase pressure and expel stool.
 Components: Stool consists of undigested food, bacteria, mucus, and dead cells from the
 intestinal lining.

ACCESSORY ORGANS
 Accessory organs are those that participate in the
digestion of food but do not come into direct
contact with it like the mouth, stomach or intestine.
 Accessory organs secrete enzymes into the digestive
tract to facilitate the breakdown of food.
 Accessory organs include the Salivary glands,
liver, the pancreas, and the gall bladder.

ACCESSORY ORGANS
 THE SALIVARY GLANDS
 The salivary glands are exocrine glands that produce
saliva through a system of ducts.
 Humans have 3 paired major salivary glands:
 Parotid
 Submandibular
 Sublingual

CONT……
 PAROTID GLANDS
 The two parotid glands are major salivary glands
wrapped around the mandibular ramus in humans.
 The largest of the salivary glands.
 They secrete saliva to facilitate mastication and
swallowing, and amylase to begin the digestion of
starches.
 It enters the oral cavity via the parotid duct
 SUBMANDIBULAR GLANDS
 The submandibular glands are a pair of major salivary
glands located beneath the lower jaws, superior to the
digastric muscles.
 The secretion produced is a mixture of both serous
fluid & mucus, and enters the oral cavity via the
submandibular ducts .
 SUBLINGUAL GLANDS
 The sublingual glands are a pair of major salivary glands
located inferior to the tongue, anterior to the
submandibular glands.
 Approximately 5% of saliva entering the oral cavity
comes from these glands.
 The secretion produced is mainly mucous in nature.

CONT……
 COMPOSITION OF SALIVA
 About 1.5 liters of saliva is produced daily & it consists of:
 Water
 Mineral salts
 An enzyme(Salivary Amylase)
 Mucus
 Lysozyme
 Immunoglobulins

ACCESSORY ORGANS
 THE LIVER
 Liver is the largest gland and the large accessory
organ. It is essential to the digestive system,
producing bile to emulsify fats in the small intestine
and processing nutrient-rich blood from the
gastrointestinal tract. It acts as the body’s metabolic
hub, regulating blood sugar, storing vitamins,
detoxifying harmful substances, and breaking down
nutrients into usable forms.

ACCESSORY ORGANS
 THE GALLBLADDER
 The gallbladder is a small, pear-shaped organ in the
upper right abdomen, under the liver, that stores
and concentrates bile produced by the liver to aid
in fat digestion. It releases bile into the small
intestine via the cystic duct when eating,
particularly triggered by fats and proteins.

ACCESSORY ORGANS
 THE PANCREAS
 The pancreas is a 6-inch, tadpole-shaped gland
located behind the stomach that acts as a vital part
of the digestive and endocrine systems. It
produces essential digestive enzymes (lipase,
protease) to break down food and releases
hormones (insulin, glucagon) to regulate blood
sugar levels. It is often referred to simply as an
accessory digestive gland or a part of the endocrine
system.

PHYSIOLOGY OF DIGESTION
 The mouth is the beginning of the digestive tract.

 Chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the
process of breaking it down into a form your body can absorb and use.

 From pharynx food travels to the esophagus or swallowing tube.

 By means of a series of contractions, called peristalsis, the esophagus delivers food to the stomach.

 The lower esophageal sphincter keep food from passing backwards into the esophagus.

CONT……
 The stomach secretes acid and powerful enzymes that continue the process of breaking down the food.

 When it leaves the stomach, food is the consistency of a liquid or paste.
 From there the food moves to the small intestine.

 The small intestine continues the process of breaking down food by using enzymes released by the pancreas and bile
from the liver.

 Bile is a compound that aids in the digestion of fat & eliminates waste products from the blood.

 Peristalsis is also at work in this organ, moving food through & mixing it up with digestive secretions.

CONT…..
 The duodenum is largely responsible for continuing the process of breaking down food, with the jejunum & ileum
being mainly responsible for the absorption of nutrients into the bloodstream.

 Pancreas secretes enzymes into the small intestine.

 These enzymes break down protein, fat & carbohydrates from the food we eat.
 Stool, or waste left over from the digestive process, is passed through the colon by means of peristalsis, first in a
liquid state & ultimately in solid form as the water is removed from the stool.

 A stool is stored in the sigmoid colon until a “mass movement” empties it into the rectum once or twice a day.

Comprehensive Overview of Human Digestive System Physiology and Function

Comprehensive Overview of Human Digestive System Physiology and Function

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