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Digestive System
When we eat such things as bread, meat, and vegetables, they are not in a form that the
body can use as nourishment. Our food and drink must be changed into smaller molecules
of nutrients before they can be absorbed into the blood and carried to cells throughout the
body. Digestion is the process by which food and drink are broken down into their smallest
parts so that the body can use them to build and nourish cells and to provide energy.
The main purpose of the Digestive system is to break down food and absorb nutrients.
There are two basic divisions to the digestive system; these are the gastrointestinal (GI)
tract, also known as the alimentary canal, and the accessory digestive organs.
Digestion is the process by which food substances are changed into forms that can be
absorbed through cell membranes. It allows the nutrients and energy of the plant and
animal cells that we eat to be released and absorbed.
Structure of the Digestive system
Foods must be broken down to their
building blocks to be absorbed. Building
blocks of carbohydrates are simple
sugars, or monosaccharides; Building
blocks of proteins are amino acids.
Building blocks of fats, or lipids, are
fatty acids and glycerol.
Gastrointestinal Tract
Mouth: Food is ingested into the
gastrointestinal tract through the
mouth, or oral cavity. Both mechanical
(chewing) and chemical food
breakdown begin in the mouth. Saliva
contains mucus, which helps bind food
together into a bolus, and salivary
amylase, which begins the chemical
breakdown of starch. Saliva is secreted
in response to food in the mouth,
mechanical pressure, and psychic
stimuli. Essentially no food absorption
occurs in the mouth.
Pharynx: From the mouth, food passes
posteriorly into the pharynx, which is
the common path for food, fluids, and
air. The pharynx is divided into three
sections: the nasopharynx (air from the
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nose passes here), the oropharynx (food and air from the mouth passes here), and the
laryngopharynx (air going to the lungs passes here). The walls of the pharynx contain two
skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer
(the constrictor muscles) run around the wall in a circular fashion. Alternating contractions
of these muscle layers propels food through the pharynx into the oesophagus below. This
propelling mechanism is called peristalsis.
Oesophagus: The Oesophagus is a straight, soft tube that connects the mouth to the
stomach. It has a layer of muscle running up and down its length. This allows the
oesophagus to ‘push’ mouthfuls of food down into the stomach immediately after
swallowing. This action is called peristalsis.
Stomach: When food enters the stomach, gastric secretion is stimulated by vagus nerves
and by gastrin (a local hormone). Hydrochloric acid activates the protein-digesting enzyme
pepsin, and chemical digestion of proteins begins. Food is also mechanically broken down by
the churning activity of stomach muscles. The enterogastric reflex controls movement of
chyme into the small intestine.
Small Intestine: Chemical digestion of fats, proteins, and carbohydrates is completed in the
small intestine by intestinal enzymes and, more importantly, pancreatic enzymes. Alkaline
pancreatic juice neutralizes acid chyme and provides the proper environment for the
operation of enzymes. Both pancreatic Juice (the only source of lipases) and bile (formed by
the liver) are necessary for normal fat breakdown and absorption. Bile acts as a fat
emulsifier.
Secretin and cholecystokinin, hormones produced by the small intestine, stimulate release
of bile and pancreatic juice. Segmental movements mix foods; peristaltic movements move
foodstuffs along the small intestine. Most nutrient absorption occurs by active transport
into the capillary blood of the villi. Fats are absorbed by diffusion into both capillary blood
and lacteals in the villi.
Large Intestine: The large intestine receives bacteria-laden indigestible food residue.
Activities of the large intestine are absorption of water and salts and of vitamins made by
resident bacteria.
Rectum / anus: The rectum and anus are the final place for storage and elimination of
faeces from the body during defecation. When faeces are delivered to the rectum by
peristalsis and the defecation reflex is initiated.
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Accessory Organs
Salivary Glands: Three pairs of major salivary glands (parotid, submandibular, and
sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is
mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase.
Functions of saliva include the following:
 It has a cleansing action on the teeth.
 It moistens and lubricates food during mastication and swallowing.
 It dissolves certain molecules so that food can be tasted.
 It begins the chemical digestion of starches through the action of amylase, which
breaks down starch and polysaccharides into disaccharides.
Liver: The liver is located primarily in the right hypochondriac and epigastric regions of the
abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface,
the liver is divided into two major lobes and two smaller lobes. The functional units of the
liver are lobules with sinusoids that carry blood from the periphery to the central vein of the
lobule. The liver receives blood from two sources.
Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of
the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive
tract is carried to the liver by the hepatic portal vein.
The liver has a wide variety of functions and many of these are vital to life. Hepatocytes
perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that
line the sinusoids are responsible for cleansing the blood.
Liver functions include the following:
 Secretion
 Synthesis of bile salts
 Synthesis of plasma protein
 Storage
 Detoxification
 Excretion
 Carbohyrate metabolism
 Lipid metabolism
 Protein metabolism
 Filtering
Gallbladder: The gallbladder is a pear-shaped sac that is attached to the visceral surface of
the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage
reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main
components of bile are water, bile salts, bile pigments, and cholesterol.
Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and
bile pigments from the breakdown of haemoglobin are excreted from the body in the bile.
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Pancreas: The pancreas has both endocrine and exocrine functions. The endocrine portion
consists of the scattered islets of Langerhans, which secrete the hormones insulin and
glucagon into the blood. The exocrine portion is the major part of the gland. It consists of
pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between
the cells. Pancreatic enzymes include amylase, trypsin, peptidase, and lipase. Pancreatic
secretions are controlled by the hormones secretin and cholecystokinin.
Food undergoes three types of processes in the body:
 Digestion
 Absorption
 Metabolism
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they
are available to all cells in the body and are utilized by these cells in metabolism.
Metabolism is the sum of all the chemical reactions that take place in the body including the
utilization of nutrients.
The digestive system prepares nutrients for utilization by body cells through:
Ingestion: The first activity of the digestive system is to take in food. This needs to take
place before anything else can happen.
Mechanical digestion: Large pieces of food need to be broken down into smaller ones so
that various enzymes can begin another activity in the chain of events. Mechanical
digestion begins in the mouth with chewing or mastication of the food and continues with
the churning and mixing actions within the stomach.
Chemical digestion: Complex molecules are transformed by chemical digestion into smaller
molecules that can be easily absorbed and utilized by the cells.
Movements: After ingestion and mastication, the food particles move from the mouth into
the pharynx, then into the oesophagus. This movement is deglutition or swallowing. The
movements that propel the food particles through the digestive tract are called peristalsis.
These are rhythmic waves of contractions that move the food particles through the various
regions in which the mechanical and chemical digestion takes place.
Absorption: The simple molecules that result from chemical digestion pass through cell
membrane of the lining in the small intestine into the blood or lymph capillaries. This
process is called absorption.
Elimination: The food molecules that cannot be digested need to be eliminated from the
body. The removal of indigestible wastes through the anus, in the form of faeces, is called
defecation.
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The functions of digestive system organs are summarised below:
Organ
Mouth
Function
Chewing, Salivating
Oesophagus
Stomach
Swallowing
Breakdown proteins
Some absorption
Small Intestine Breakdown all food
Some absorption
Large
Intestine
Rectum/Anus
Absorption of water and
electrolytes
Formation of faeces
Storage of faeces and defecation
Forms of digestion
Mechanical (chewing)
Chemical (Saliva)
Movements
Chemical
Movements
Chemical: enzymes, gastric
secretions
Absorption
Mechanical
Energy Expenditure
The amount of energy, measured in calories that a person uses. Calories are used by people
to breath, circulate blood, digest food, and be physically active.Daily energy expenditure
consists of three components:
Resting Metabolic Rate (RMR): Resting metabolic rate (RMR) is the minimum number of
calories your body needs to support its basic physiological functions, including breathing,
circulating blood and all of the numerous biochemical reactions required to keep you alive.
Your RMR is generally 60-75% of your total daily energy expenditure.
Thermic effect of food: Eating requires energy for the ingestion and digestion of food, and
for the absorption, transport, interconversion, oxidation and deposition of nutrients. These
metabolic processes increase heat production and oxygen consumption, and are known by
terms such as dietary-induced thermogenesis, specific dynamic action of food and thermic
effect of feeding. The metabolic response to food increases total energy expenditure by
about 10 percent of the BMR over a 24-hour period in individuals eating a mixed diet.
Thermic effect of exercise: The amount of energy the body burns during daily activities such
as exercise, recreation, work, and housework influences daily energy expenditure. Daily
physical activities account for 20 - 40% of calories burned each day. This part will vary
depending on the individual and how active they are each day. A sedentary person will
require fewer calories to maintain weight than a busy worker in a construction site. The
intensity, frequency and duration of any activity all have an effect on metabolism.
Adaptive thermogenesis: Adaptive thermogenesis (AT) represents the way the body
responds to stress whether it is emotional or environmental (for example, changes in
temperature). Typically, stressful situations result in an increase in calories burned.
Scientists are still not sure about how much it contributes to total daily caloric expenditure.
Many of them think it may be an important factor in explaining why two people can be
leading the same type of lifestyle in terms of exercise and diet and yet, have two completely
different body types.
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Medical Conditions
Diabetes
Diabetes is a condition in which there is an inability of insulin, a hormone
produced by the pancreas, to regulate the amount of glucose (sugars) in the
blood. Type 1 diabetes typically occurs in childhood and is characterized by
an inability of the pancreas to produce insulin; this type of diabetes often
requires injections of insulin. Type 2 diabetes, or adult onset diabetes, occurs
when the body "resists" insulin and glucose levels remain increased.
Gallstones
Gallstones form in the gallbladder when cholesterol or pigments crystallize out of bile. They
can vary in size from as small as grains of sand to as large as golf balls. The tendency to form
gallstones is partially inherited and is more common in people who are obese, overeat or
lose weight rapidly through extreme diets.
Cancer
Cancer is a general term for about 100 diseases characterized by abnormal and uncontrolled
growth of cells. Diet is thought to explain much of the variation in incidence of some
cancers, around the world and over time. This is a complex area of research because of the
long delays in cancer development after exposure to dietary causes. Major difficulties arise
when researchers try to estimate accurately what individuals ate many years ago.
Nonetheless progress in understanding the role of diet is being made.
It is now estimated that about 35% of deaths from cancer in economically developed
countries may be prevented if everyone had a healthy diet. The following are nutritional
issues associated with cancer.
Obesity increases the risk of colon, breast, gallbladder, ovary, prostate and uterine cancer.
High fat diets may contribute to the development of certain cancers, particularly those of
the breast, colon and prostate.
High fibre foods may help reduce the risk of colon cancer.
A varied diet containing plenty of vegetables and fruit may reduce the risk for many cancers.
Salt-cured, smoked and nitrate-cured foods have been linked to oesophageal and stomach
cancers.
Gout
Gout (old name: podagra) is a form of arthritis caused by the accumulation of uric acid
crystals (due to hyperuricemia) in joints. It is an immensely painful disease, which in most
cases affects only one joint, most commonly the big toe. The patient usually suffers from
two sources of pain. The crystals inside the joint can cause immense pain whenever the
affected area is moved.
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Generally thought to be a hereditary disease, there are many causes: increased purine
intake, sudden weight loss due to dieting, and/or diuretics can cause elevated uric acid and
mimic the symptoms of gout.
Nutritional deficiencies
Research has shown that optimum nutrient levels are required to maintain good health.
Nutrient deficiencies are associated with poor health such as iron deficiency in anaemia,
iodine imbalance with thyroid function, Vitamin D & Calcium with osteoporosis, Oxygen
deficiency associated with fatigue. Essential nutrients are involved in the formation of:
Antioxidants, blood supply, immune and nervous systems, brain function, digestion, skin
growth, bone building and energy levels.
Whilst nutrients are supplied by food, air & water modern farming & processing techniques
are depleting the essential nutrients found in these foods. Stress, poor diets and allergies
also have a contribution to nutritional deficiencies.
Iron deficiency: Iron deficiency is the most common known form of nutritional deficiency. In
the human body, iron is present in all cells and has several vital functions. Iron acts as a
carrier of oxygen to the tissues from the lungs in the form of haemoglobin, as a facilitator of
oxygen use and storage in the muscles as myoglobin, as a transport medium for electrons
within the cells in the form of cytochromes, and as an integral part of enzyme reactions in
various tissues. Too little iron can interfere with these vital functions and lead to morbidity
and mortality.
Calcium deficiency: One of the major symptoms of a deficiency of calcium is skeletal
abnormalities. Osteopenia, osteomalacia, osteoporosis and rickets may all be caused by
calcium deficiency. A constant supply of calcium is necessary throughout life, but is
especially important during phases of growth, pregnancy, and lactation (breast feeding).
Protein Energy Malnutrition (PEM): The lack of sufficient energy or protein to meet the
body's metabolic demands, as a result of either an inadequate dietary intake of protein,
intake of poor quality dietary protein, increased demands due to disease, or increased
nutrient losses.
Vitamin A deficiency: Vitamin A deficiency (VAD) is the leading cause of preventable
blindness in children of developing countries, and raises the risk of disease and death from
severe infections. In pregnant women VAD causes night blindness and may increase the risk
of maternal mortality. It exists when the chronic failure to eat sufficient amounts of vitamin
A or beta-carotene results in levels of bloodstream Vitamin A that is below a defined range.
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