Six Processes of Digestion and Structure of Digestive System Organs

This article discusses the six processes of digestion, the structure and functions of digestive system organs, and the stages involved in aerobic and anaerobic respiration. It also covers gastrectomy and the effects of zero gravity on digestion.

Biology
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1.1 Six Processes of Digestion
Large molecules like starch cannot pass through cell membrane. Hence, it needs to be broken
down into smaller particles. Breakdown of starch molecule in the smaller molecules occur in
the first step of digestion which is called as ingestion. Starch digestion starts in the mouth
which is called as ingestion. Teeth, saliva and tongue play important role in mastication of
the ingestion process. Saliva secreted by salivary glands in the mouth is useful in moistening
the consumed starch. After moistening starch is chewed. During this process, salivary glands
secret enzyme like salivary amylase. This enzyme is useful in breaking down starch in
simpler carbohydrates like maltose and glucose. Breaking down and moistening with saliva
results in soft mass of starch. It is easy to swallow and travel this soft mass through entire
length of oesophagus. This process is known as propulsion. Tongue and pharyngeal muscles
propel soft mass of starch into the oesophagus. This is last voluntary process in the digestive
process until defecation. In this process, there is movement of starch throughout entire
digestive tract. This movement include both voluntary swallowing and involuntary peristalsis.
In peristalsis, there is sequential and discontinuous waves of contraction and relaxation of
gastrointestinal tract wall smooth muscle. It leads to propel food along the gastrointestinal
tract. Peristalsis moves starch back and forth and segmentation is helpful in mixing starch
with digestive juices. Both together facilitates absorption of starch.
After ingestion, digestion of starch begins. During digestion process, soft mass of starch
enters stomach which mixed with chyme. There is no further digestion of starch because
bacteria in the stomach inactivate action of salivary amylase. Digestion include both
mechanical and chemical digestion. Mechanical digestion produces smaller starch particles.
Mechanical digestion also occurs after starch leaves mouth. Digestion starts in the mouth and
it continues in stomach and small intestine. Mechanical and chemical digestion occurs in co-
ordination with each other. Formation of soft mass through mechanical digestion is helpful in
providing more surface area for digestive juices which is chemical digestion process.
Segmentation mostly occurs in the small intestine. In segmentation there is local contractions
of circular muscle of the muscularis layer of the alimentary canal. These contractions produce
small segments in small intestine. Starch moves back and forth in these small intestine
segments. It results in subdividing, breaking up and mixing of starch. Movement of starch
back and forth in the intestinal lumen result in the mixing of digestive juices and facilitation
of absorption. Starch broken down in the small pieces need to enter in the bloodstream for its
utilization. Until food reaches bloodstream, its nutrients are not useful for the body. Entry of
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starch in the bloodstream occurs in the small intestine and this process is called as absorption.
Absorption of nutrients in the bloodstream occurs through epithelial cells of small intestine.
Epithelial cells of small intestine make up the mucosa. Defecation is the last step in the
digestion process and undigested starch is eliminated from the body during this process
(Brett, 10-14).
2.1a : The digestive system
2.1b Structure and Functions of Digestive System Organs
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Oral cavity: Oral cavity consists of two regions like vestibule and oral cavity proper.
Vestibule is zone among teeth, lips and cheeks. Front and sides of oral cavity are surrounded
by alveolar process and isthmus of the fauces. Roof of the oral cavity contains hard plate and
soft plate at front and back side respectively. Floor of oral cavity possess mylohyoid muscles
and tongue. Oral cavity is the beginning of the digestive tract and digestion process starts at
oral cavity. Teeth breaks down starch into pieces and broken-down pieces mix with saliva in
the oral cavity.
Accessory organs: Teeth, tongue, salivary glands, liver, and pancreas are the accessory
organs of the oral cavity. Teeth are made up of a material like bone called as dentin and it is
covered by enamel which is hardest tissue in the body. Teeth break down starch in small
pieces. Tongue is a fleshy and muscular sensory organ. Lingual septum is fibrous tissue
which separates tongue into right and left side. Taste buds in the papillae are present on the
surface of the tongue which receive sensory information which stimulates saliva secretion.
This saliva mixes with the broken pieces of starch to produce soft mass during ingestion
process of digestion. Tongue movement is also helpful in breaking down starch into pieces.
Three pairs of salivary glands like parotid, submandibular and sublingual are wrapped around
the mandibular ramus, underneath the lower jaws, superior to the digastric muscles and
inferior to the tongue, anterior to the submandibular glands respectively. Serous secretion
secreted by salivary gland contains alpha-amylase. It breaks down starch into maltose and
glucose.
Liver is the second largest organ of the body and it contains four lobs of unequal size and
shape. Each of the four lobules are made from millions of hepatic cells and these are the main
metabolic cells. Four lobules are composed by fine, dense, irregular, fibroelastic connective
tissue layer which is known as Glisson's capsule. Main function of liver is to process starch
absorbed from small intestine.
Anatomically pancreas is divided into head, neck, body and tail of pancreas. Pancreatic juice
can act as chyme for softening broken starch food.
Oesophagus: Oesophagus is a long and muscular tissue between oral cavity and stomach. It is
approximately 25 cm long in the adults. After swallowing of food, oesophagus walls contract
and relaxes rhythmically in peristalsis and food moves into the stomach.
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Stomach: Stomach is the J-shaped organ which connects oesophagus at its upper end and
duodenum at its lower end. Greater curvature of the stomach has greater omentum beneath it.
Lower oesophageal sphincter and pyloric sphincter both together keep contents of stomach
contained. It is mainly divided into four parts like cardia, fundus, body and pylorus. In
stomach, there is mixing of enzymes with starch which results in breaking down of starch in
the usable form. Stomach plays role in digestion step of digestion process. Bolus enters
stomach through oesophagus via lower oesophageal sphincter.
Small intestine: Small intestine is a 22-foot muscular tube and it is segmented into three parts
like duodenum, jejunum, and ileum. Small intestine plays role in both ingestion and
absorption step of the digestion process. In small intestine, there is breakdown of starch using
enzymes released from pancreas and bile from the liver. Peristalsis also occurs in small
intestine in which moving starch food get mixed with the secretions from the pancreas and
liver. Duodenum performs the function of breaking down starch and; jejunum and ileum
perform function of absorption of nutrients.
Large intestine, rectum and anus: Large intestine is 6-foot long muscular tube and it is a
connecting tube between small intestine and rectum. It is segmented into three parts like
ascending, transverse, descending and sigmoid colon. Large intestine plays important role in
processing waste and pass through colon through peristalsis process. Rectum is 8-inch
chamber which connects colon with anus. Rectum receives stool from the colon, evacuate it
and hold it before evacuation. Anus is 2-inch long canal which consists of pelvic floor
muscles and two anal sphincters. Sphincter muscle of anus is helpful in controlling stool
(Brett, 10-14).
2.1c Gastrectomy
There are two different methods to carry out gastrectomy like open surgery and laparoscopic
surgery. Both the procedures need to be performed under general anaesthesia.
Single and large incision usually between 15 to 20 cm long need to be given through
abdominal wall in open surgery. Surgeon pull back skin, muscle and tissue to get access to
abdominal organs. Incision need to be closed with staples or sutures.
Laparoscopic surgery is with minimal incision. In laparoscopic gastroscopy, multiple small
incisions need to be given. These incisions are 5 to 12 mm long. Through, one of the
incisions, insert small camera which is useful in viewing gastrointestinal contents. Remaining
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incisions are useful for laparoscopic surgical tools get inserted to access to the abdominal
cavity. After the completion of surgery, close all the incisions with dissolvable stitches and
steri-srips.
After removal of part or total stomach, remaining part of gastrointestinal system joined. This
procedure is called as resection. After removal of stomach, consumed food and fluid get
digested in the small intestine. Post-surgery, there is possibility of feeling full even after
consuming little quantity of food. Stomach removal leads to weight loss. There is also
possibility of dumping syndrome because consumed food gets dumped in the small intestine
post 15-20 minutes of its consumption. In the presence of stomach, food slowly moves from
stomach to the small intestine. Removal of stomach affects ingestion and digestion steps of
digestion process. There is possibility of injury to other gastrointestinal organs like spleen,
pancreas, liver and common bile duct during surgery. Without stomach, person is able to
digest foods and liquids. However, person should make alteration in the diet like small and
frequent meals instead of three large meals. Person need to consume, vitamin on regular basis
to get proper nutrition. There is possibility of loss of control in movement of food through
digestive system. It is mainly due to improper healing of vagus nerve. It leads to severe
diarrhoea in few patients after gastrectomy. Filling full after removal of stomach produces
vomiting sensation in few of the gastrectomy patients (Escott-Stump, 373)
3.1a Peristalsis and segmentation
Peristalsis is a wave-like rhythmic muscular contractions of gastrointestinal tract which
results in propelling food downward. Segmentation is local contractions of circular muscle of
the muscularis layer of the alimentary canal which results in small segments in small
intestine. Peristalsis occurs in oesophagus and segmentation occurs in large and small
intestine. Peristalsis occur in downward direction and segmentation occur is both the
directions. Peristalsis occurs in circular and longitudinal muscles and segmentation occurs in
circular muscles. Peristalsis occurs at high speed and segmentation occurs at low speed.
Breaking down and mixing of food are important steps in ingestion. In peristalsis, breaking
down and mixing occur to lesser extent. In segmentation, both breaking down and mixing are
more (Brett, 10-14).
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a -- Peristalsis
b -- Segmentation

3.1b Digestion in Zero Gravity
Oesophagus is lined by smooth muscle which produces continuous contraction and relaxation
which is called as peristalsis. Peristalsis is responsible for the movement of food from the
oesophagus to the stomach. Peristaltic movement is so strong that food reaches stomach even
if food is consumed upside down. It reflects, zero gravity doesn’t have effect on the
swallowing during food ingestion and digestion. Oesophagus is also not so broad that food
would move only with gravity (Brett, 10-14).
4.1a Stages Involved in Aerobic and anaerobic respiration
There are four steps in aerobic respiration like glycolysis, Acetyl-CoA, Krebs Cycle and
Electron Transport Chain. Glycolysis take place in cytoplasm. In glycolysis, there is
breakdown of glucose to yield four molecules of ATP, two three-carbon molecules called
pyruvate and two molecules of nicotinamide adenine dinucleotide, or NADH. Acetyl-
coenzyme A formation occurs in mitochondria. Pyruvate produced during glycolysis get
converted to two-carbon acetyl group. Acetyl-coA produced by combining acetyl group and
coenzyme A. Krebs cycle also occurs in mitochondria. In Krebs cycle, Acetyl-coA combined
to produce oxaloacetate. Subsequently it produces citrate, isocitrate, alpha ketoglutarate,
succinyl-CoA, succinate, fumarate and malate, one molecule of guanosine triphosphate
(GTP), three molecules of NADH and one molecule of flavin adenine dinucleotide (FADH2).
In Electron Transport Chain (ETC), NADH and FADH2 converted into ATP. ETC occurs in
the mitochondria. Energy generated through electron release is useful in pumping all the
released hydrogens in the Krebs cycle. Mixing of hydrogen and oxygen in the mitochondria
results in the formation of water.
Anaerobic respiration occurs in two steps like glycolysis and fermentation. In glycolysis, two
three carbon compound pyruvic acid and two ATP molecules produced from the breakdown
of glucose molecule. In the fermentation stage, incomplete oxidation of pyruvic acid takes
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place which result in the ethanol or lactic acid production. Pyruvic acid which contains one
carbon atom which is removed from each is useful in formation of two ethanol and two
carbon dioxide molecules. Lactic acid fermentation occurs when there is inadequate oxygen
necessary for anaerobic respiration. In this step, one three carbon containing molecule like
pyruvic acid gets converted into another three-carbon containing molecule lactic acid and
there is no formation of carbon dioxide (Coad and Dunstall, 15-16).
4.1b Difference between aerobic and anaerobic respiration
Aerobic respiration Anaerobic respiration
It occurs in presence of oxygen. It occurs in the absence of oxygen.
Complete oxidation of glucose occurs. Incomplete oxidation of glucose occurs.
Final products are CO2 and water. Final products are lactic acid or ethyl
alcohol and CO2.
More energy is liberated in the form of 38
ATP.
Less energy is liberated in the form of 2
ATP.
C6H12O6 + 6O2 6CO2 + 6H20 + 36
ATP
C6H12O6 2CH3CH2OH + 2CO2 +
2ATP
4.1c Significance of Aerobic and Anaerobic Respiration
Aerobic respiration comprises of continuous and maintainable oxygen intake; hence, this type
respiration is more useful in long distance running like in Mo Farah. Endurance running
which requires more than 2 minutes uses aerobic respiration. Long distance running is
considered as steady-state exercise and it requires sustainable oxygen intake. It is established
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that heart rate remains almost constant during long distance running which allows optimal
functioning of cardiovascular system without much exertion. Distance runners with heart rate
between 60 to 80 percent of maximum heart rate usually use aerobic respiration as the
primary source of energy. In long distance running, there is increase in the breathing rate. It is
helpful in bringing more amount of oxygen which is necessary for running for the longer
distance and duration. Hence, in long distance running, aerobic respiration is more useful.
In anaerobic respiration, body produces necessary energy in the absence of oxygen also.
Short distance sprinters need extensive physical work. Hence, they are considered as
explosive athletes which requires immediate energy for a short burst. Anaerobic respiration
provides this immediate energy for a short burst. This energy is useful in the first two minutes
of sprinter relying. In short distance sprinters, heart rate reaches above 80 percent of
maximum heart rate which can be stressful to the body. Anaerobic respiration is helpful in
reaching this heart rate zone to produce this final burst of energy. Sprint burns lot of calories
of energy. It is evident that, short anaerobic exercise also increases cardiovascular endurance
almost equivalent to the aerobic exercise. In sprinting, there is requirement of readily
available energy. Every individual doesn’t have readily available energy which can be used
whenever required. Anaerobic respiration is useful in fulfilling this type of energy demand.
Anaerobic exercise like sprinting need to put hard effort; hence, anaerobic respiration is
useful in providing energy to fulfil demand of hard effort (Coad and Dunstall, 15-16).
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References:
Brett, Flora. Your Digestive System Works. Capstone Young Readers: North Mankat, 2015,
pp. 10-14.
Escott-Stump, Sylvia. Nutrition and Diagnosis-related Care. Lippincott Williams & Wilkins:
Pennsylvania, United States, 2008, P. 373
Coad, Jane, and Dunstall, Melvyn. Anatomy and Physiology for Midwives. Elsevier Health
Sciences: Amsterdam, Netherlands, 2011, pp. 15-16.
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