Six Processes Of Digestion: Understanding The Digestive System And Its Organs

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 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).

Structure and Functions of Digestive System Organs

The digestive system

Structure and Functions of Digestive System Organs

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.

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.

Oral Cavity

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).

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 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)

Accessory Organs

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).   

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 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).

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

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 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).

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.