Chapter 8 - Digestive System

Nutrition and Digestive System

Rocky Python (Python sebae) eating an animal

 Heterotrophic nutrition  Adaptations of a herbivore, carnivore, liquid feeder, microphagous feeder to its diet  Structures of human alimentary/digestive tract

Learning Objectives Differentiate heterotrophic and autotrophic nutrition. •

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Define holozoic nutrition and their stages.

•

Explain 5 forms of heterotrophic nutrition.

• Plants and other autotrophs – Are the producers of the biosphere • Heterotrophs – Obtain their organic material from other organisms – Are the consumers of the biosphere – Cannot manufacture their own food

• The main stages of food processing are ingestion, digestion, absorption, and elimination • Ingestion, the act of eating – Is the first stage of food processing • Digestion, the second stage of food processing or food breaking – Is the process of breaking food down into molecules small enough to absorb – Involves enzymatic hydrolysis of polymers into their monomers – Mechanical and chemical digestion.

• Absorption, the third stage of food processing – Is the uptake of nutrients by body cells • Elimination, or egestion the fourth stage of food processing – Occurs as undigested or waste material that passes out of the digestive compartment

The four stages of food processing

• The four stages of Smallfood processing molecules Pieces of food

Mechanical digestion

Chemical digestion (enzymatic hydrolysis)

Nutrient molecules enter body cells

Undigested material

Food

1 INGESTION

2 DIGESTION

3 ABSORPTION

4 ELIMINATION

HETEROTROPHIC NUTRITION 

There are several form of heterotrophic nutrition:

• Holozoic nutrition – complex food is taken in and then broken down inside the organism into simple molecules and then reabsorbed by the organism. Most animals feed in this way and they have specialized digestive system.

• The stages of holozoic nutrition are: 1. Obtaining food 2. Ingestion 3. Physical or mechanical digestion – involves cutting or crushing action of the teeth and contraction of the gut muscles which pound the food. 4. Chemical digestion – involves enzymes and secretions (acid or alkali) 5. Absorption 6. Assimilation 7. Elimination/ egestion – is removal of ingested but unwanted material from the body.

Classification of holozoic organisms according to the type of food ingested. • Herbivores – eat plants, primary consumers. • Carnivores – eat animals • Omnivores – eat plants and animals • Liquid feeders – feed on plant or animal juices • Microphagous feeders – feed on small particles suspended in water, they are always aquatic. e.g. mussels

2. Saprobiontic/ saprophytic/ saprotrophic nutrition – complex organic food are obtained from dead or decaying organisms. The food may be digested externally outside the organism (saprobiont secretes various enzymes) into simpler molecules, which then diffuses into the saprobiont. The organisms have no digestive system. e.g fungi 3. Parasitism – complex organic food is obtained from other living organisms (hosts). The parasites get the benefit. The hosts get the harm. e.g. Plasmodium malariae

4. Mutualism – both species get the benefits from each other. e.g. root nodules of Papilionaceae (peas, bean) and Rhizobium (nitrogen-fixing bacteria). 5. Commensalism – one species benefit, the other neither benefits nor it is harmed. E.g. shark and remora fish. - Associations between organisms are never static. - E.g. Armillaria mellea can be a parasite of a tree which eventually kills the tree but after that it becomes a saprobiont, feeding on its remains.

A Spotted Hyena and Vultures fighting over the kill.

Learning Objectives Differentiate extracellular digestion and intracellular digestion and give examples. •

Extracellular and Intracellular digestion Extracellular digestion – occurs outside the cells, usually take place in the gut. Intracellular digestion – occurs inside the cells e.g. Amoeba. Amoeba has pseudopodia which invaginate to engulf food particles. These become enclosed in a phagocytic vesicle. Digestive enzymes are then secreted into the vesicle to digest the food particles. The soluble products of digestion are then absorbed into the cytoplasm.

Digestive cavity • Some invertebrates have a digestive cavity with a single

opening. • For example, sponge have no digestive system but digest

trapped food intracellularly. • Cnidarians and flatworms have gastrovascular cavities to

digest food extracellularly. Undigested materials are egested through the mouth. • Most animal digestive system have two opening. • Animal with a ‘tube-within-a-tube’ body plan have a

digestive tube with accessory glands. • The mouth leads to pharynx, the esophagus, the stomach,

the small and large intestine and anus.

• Animals with simple body plans – Have a gastrovascular cavity that functions in both digestion and distribution of nutrients Tentacles Mouth Food

Epidermis Mesenchyme Gastrodermis Nutritive muscular cells Flagella Gland cells Food vacuoles Mesenchyme

Gastrovasc ular cavity

Digestion in a hydra. The outer epidermis of the hydra has protective and sensory functions, whereas the inner gastrodermis is specialized for digestion. Digestion begins in the gastrovascular cavity and is completed intracellularly after small food particles are engulfed by the gastrodermal cells.



In some animals, both extracellular and intracellular digestion can occur.



They secrete extracellular enzymes into the gut to break the food down into smaller particles and these are then taken up by phagocytosis into the cells lining the gut.

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Digestion is completed by the intracellular enzymes.

Some Dental Adaptations • Dentition, an animal’s assortment of teeth – Is one example of structural variation reflecting diet • Mammals have specialized dentition – That best enables them to ingest their usual diet

Dentition Incisors

Canines (a) Carnivore

(b) Herbivore

(c) Omnivore

Molars

Pointed incisors and canines that can be used to kill prey and rip or cut away pieces of flesh. The jagged premolars and molars crush and shred food.

Premolars

Teeth with broad, ridged surfaces that grind tough plant material. Incisors and canines are generally modified for biting off pieces of vegetation. In some herbivorous mammals, canines are absent.

Relatively unspecialized dentition. Permanent (adult) - 32. Beginning at the midline of the upper and lower jaw are 2 bladelike incisors for biting, a pointed canine for tearing, 2 premolars for grinding and 3 molars for crushing.

1. Premolar and molar teeth – which have a large surface area for grinding. Ridges are formed on the surfaces of teeth due to uneven wear and tear of the hard enamel and softer dentine during grinding. 2. Long incisor teeth – for pulling plants. Between the incisors and premolars there is a pronounced gap called diastema, a space to keep newly nibbled food which is separated from that being ground at the back of the mouth.

3. Because of the wear and tear of teeth during grinding, the teeth grow continuously throughout the herbivore’s life. 4. Jaws easily move from side to side, allowing the food to be broken down between the ridged teeth. 5. Because the digestion of plant material is relatively difficult, herbivores have a long digestive tract. 6. Some herbivores have a multi-chambered stomach. Some chambers produce digestive enzymes to break down the food. Other chambers have bacteria which produce cellulase for digesting cellulose. 7. For herbivorous insects, they have a jagged-edge mandible cutting through the leaves of plants.

Learning Objectives Explain about the adaptation of herbivore, carnivore, liquid and macrophagous feeder. •

•

Deferentiate ruminant and non-ruminant.

Stomach and Intestinal Adaptations • Herbivores generally have longer alimentary canals than carnivores – Reflecting the longer time needed to digest vegetation

Stomach

Small intestine

Small intestine Cecum

Colon (large intestine) Carnivore

Herbivore

Symbiotic Adaptations • Many herbivorous animals have fermentation chambers – Where symbiotic microorganisms digest cellulose. • The most elaborate adaptations for an herbivorous diet – Have evolved in the animals called ruminants

Adaptations of a herbivore to its diet

• Plant material is relatively tough and indigestible without the aid of microorganisms. • Grinding the plant material is also important to increase the surface area, making chemical digestion easier. • Therefore, herbivores possess several adaptations for this kind of diet.

Ruminant herbivores – e.g. cow

• They have an enlarged stomach at the lower end of the esophagus which is divided into 4 chambers: 1. Reticulum 4. Abomasum

2. Rumen

3. Omasum

Rumen. When the cow first chews and swallows a mouthful of grass, boluses (green arrows) enter the rumen. 1

Reticulum. Some boluses also enter the reticulum. In both the rumen and the reticulum, symbiotic prokaryotes and protists go to work on the celluloserich meal. The microorganisms secrete fatty acids. The cow periodically regurgitates and rechews the cud (red arrows), which further breaks down the fibers, for further microbial action.

2

Intestine

Esophagus

Abomasum. The cud, containing great numbers of microorganisms, finally passes to the abomasum for digestion by the cow‘s own enzymes (black arrows). 4

Omasum. The cow then reswallows the cud (blue arrows), which moves to the omasum, where water is removed. 3

Non-ruminant herbivores – e.g. rabbit They also have fermentation chambers for digesting cellulose and these are the caecum and appendix. Since these structures are located at the end of the gut, the food cannot be regurgitated into the small intestine for absorption. So, to absorb the food, these animals have 2 types of faeces: 1. soft faeces – it contains digested food, which is eaten directly from the anus (called coprophagy) and absorbed when passes through the gut for the 2nd time. 2. hard faeces (pellets) – contain little food material.

Humans do not possess rumen but bacteria are present in the colon.

Adaptations of a carnivore to its diet Carnivores - catching and killing prey. These adaptations are: 1. High speed locomotion 2. Sharps claws 3. Long and pointed canine teeth for piercing and killing the prey and tearing the flesh from its body. 4. Sharp incisor teeth and carnassial teeth (last upper premolar and 1st lower molar) for biting flesh and pulling it off the bone. 5. Teeth of the upper jaw tend to overlap those of the lower jaw (like 2 blades of scissors), allowing the slicing of meat into pieces.

6. Vertical movement of the jaw is less restricted, allowing carnivores open a wide gap for capturing prey. 7. Well developed muscles of the jaw, allowing the carnivore to grip the prey firmly. Once captured, the prey may be swallowed as a whole without breaking it up into pieces. e.g. shark, pythons; or chewed up and then swallowed. e.g. majority of carnivores

Adaptations of liquid feeders to their diet There are 2 groups of liquid feeders: 1. Absorbers e.g. gut parasites such as tapeworms which absorbed digested food of the host straight through their integument. They do not have a gut or digestive enzymes. 2. Suckers e.g. insects such as mosquitoes, moths, butterflies). They possess proboscis for obtaining food.

Adaptations of microphagous feeders to their diet

• The food for microphagous feeders is already broken up, so digestion is not a problem. • Usually they have problems in collecting, sorting and concentrating the food particles and conveying them to the mouth. • Their adaptations for these problems are:  Have flapping appendages or cilia to draw water towards their body.  Have a sieve or sheet-like gills for filtering out the food particles.

Learning Objectives •

Describe the structure and process occurs in the human alimentary tracts.

DIGESTIVE SYSTEM Gastroenterology ? - The medical specialty deal with structure, function, diagnosis and treatment of diseases of the stomach and intestines.

Proctology ? - The medical specialty that deal with diagnosis and treatment of disorders of the rectum and anus.

• Animals with a more complex body plan – Have a digestive tube with two openings, a mouth and an anus • This digestive tube – Is called a complete digestive tract or an alimentary canal (gastrointestinal tract) • The digestive tube can be organized into specialized regions – That carry out digestion and nutrient absorption in a stepwise fashion

- Organ of the gastrointestinal tracts include the mouth, pharynx, esophagus, stomach, small intestine and large intestine. - The length of the alimentary tracts taken from the cadaver is about 9 m, longer compare to the living person. - Why? - It is because the alimentary tract organs are in the state of tonus or sustained contraction in living human.

Layer Of GI Tract - The wall of the GI tract, from the esophagus to the anal canal, has the same basic, four layered arrangement of tissues. - The four layers of the tract, from deep to superficial, are the mucosa, submucosa, muscularis and serosa.

MUCOSA - The lumen of the GI tract is lined by a mucous membrane, the mucosa. - There are three layers: -Epithelium -Lamina propia -Muscularis mucosae - Muscularis mucosae - throws the mucous membrane of the stomach and small intestine into many small folds that increases the surface area for digestion and absorption.

Submucosa - Consists of vascular and submucosal plexus or plexus of Meissner (the “brain of the gut”-neuron) - May contains glands and lymphatic tissue. Muscularis - Consists of smooth muscle –inner sheet-circular fibers and the outer sheets –longitudinal sheets - Consists of myenteric plexus or plexus of Auerbach Serosa - Consists of connective tissue and simple squamous epithelium. They also called as visceral peritoneum.

Oral cavity • In the oral cavity, food is lubricated and digestion begins – And teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers

Mouth - Refer to as oral or buccal cavity-form by the cheeks, hard and soft palates and tongue. Salivary glands - Release a secretion called saliva into the oral cavity. - Ordinarily, just enough saliva is secreted to keep the mucous membranes of the mouth and pharynx moist and to cleanse the mouth. - When food enters the mouth, secretion of saliva increase and begin the chemical breakdown of the food. - Three pairs of major saliva glands are parotid, submandibular and sublingual glands.

Salivary glands

Tongue - Is an accessory digestive organ composed of skeletal muscle covered with mucous membrane. -The lingual frenulum - a fold of mucous membrane in the midline of the undersurface of the tongue-limiting the movement. -The upper surface and lateral surface of the tongue are covered with papillae which contain taste buds.

Tongue

Teeth - Teeth or dentes are accessory digestive organs located in the sockets. - Human have two dentitions or set of teeth: deciduous and permanent. - The deciduous teeth also called as primary teeth, milk teeth or baby teeth. - All the deciduous teeth are lost between ages 6 and 12 years and are replaced by the permanent (secondary) teeth. - A typical tooth consists of three principal regions which is the crown, neck and roots.

- Crown is visible portion above the level of the gums. - Embedded in the socket are one to three roots. - The neck is the constricted junction of the crown and root near the gum line. - Teeth composed primarily of dentin, a calcified connective tissue that gives the tooth basic shape and rigidity. - The dentin of the crown is covered by enamel-hardest substance in the body and rich in calcium salts. - The dentin of the roots is covered by cementum, another bonelike substance, which attaches the root to the peridontal ligament.

Pharynx -The region we call our throat is the pharynx. - A junction that opens to both the esophagus and the windpipe (trachea) -Through chewing, the tongue manipulates food, the teeth grind it and the food is mixed with saliva. - As result, he food is reduced to a soft, flexible mass called bolus that is easily swallowed. - Swallowing (deglutition)is a mechanism that moves food from the mouth to the stomach.

Esophagus-eating gullet - Is the collapsible muscular tube that lies posterior to the trachea. - It begins at the end of the pharynx, pierces the diaphragm through an opening called the esophageal hiatus and end in the superior portion of the stomach. - Hiatal hernia is the condition where a portion of the stomach protrudes above the diaphragm through the esophageal hiatus. - The esophagus secretes mucus and transport food into the stomach. - It does not produces digestive enzymes and it does not carry on absorption.

From mouth to stomach The swallowing reflex and esophageal peristalsis Bolus of food Tongue Pharynx Glottis Larynx

Esophageal Epiglottis sphincter down contracted Esophagus To lungs

To stomach

Glottis up and closed

When a person is not swallowing, the esophageal sphincter muscle is contracted, 2 The swallowing the epiglottis is up, and the reflex is triggered glottis is open, allowing air when a bolus of food reaches the to flow through the trachea pharynx. to the lungs. 1

Epiglottis up Glottis down and open

Epiglottis up

Trachea

4 The esophageal sphincter relaxes, allowing the bolus to enter the esophagus.

Esophageal sphincter relaxed

Esophageal sphincter contracted

5 After the food has entered the esophagus, the larynx moves downward and opens the breathing passage.

Relaxed muscles

3 The larynx, the upper part of the 6 Waves of muscular respiratory tract, contraction moves upward and (peristalsis) tips the epiglottis move the bolus over the glottis, down the esophagus preventing food to the stomach. from entering the trachea.

Stomach

Contracted muscles Relaxed muscles

Peristalsis -Food is pushed through the esophagus by a progression of the involuntary coordinated contractions and relaxion called peristalsis. - Rhythmic waves of contraction of smooth muscles in the wall of the canal - The peristalsis occurs in others tubular structures as well. - Peristalsis is control by medulla oblongata. - In the sections of the esophagus lying just superior to the bolus, the circular muscle fibers contract, constricting the esophageal wall and squeezing the bolus towards the stomach.

-Meanwhile, longitudinal fibers in front the bolus also contract and pushes its walls outward so it can receive the bolus. -Mucus secretes by esophageal glands lubricates the bolus and reduce friction. -Before the bolus go in to the stomach, it has to pass through the lower esophageal sphincter

Stomach - Is typically J-shaped enlargement of the GI tract. - The stomach connects the esophagus to the duodenums, which is the first part of the small intestine. - The stomach serve as a mixing and holding reservoir. - It can accommodate a large quantity of food. - In the stomach digestion of starch continues, digestion of proteins and triglycerides begins. - Semisolid bolus is converted to a liquid and certain substances are absorbed.

-The stomach has 4 regions the cardia, fundus, body and pylorus. - When the stomach is empty, the mucosa lies in large folds called rugae. -The pylorus communicates with duodenum via pyloric sphincter. -Stomach has same 4 basic layers as like GI. -The mucosa layer form columns of secretory cell called gastric glands that line many narrow channels called gastric pits. -Secretions from several gastric glands flow into each gastric pit and then into the lumen

1. Body 2. Fundus 3. Greater curvature 4. Lesser curvature 5. Cardia 6. Pyloric sphincter 14. Rugae

Regions of stomach

-The gastric glands contain exocrine gland cells that secrete their product into the stomach lumen. -Parietal cell produce hydrochloric acid and intrinsic factor (needed for absorption of vitamin B12 -The chief cell secrete pepsinogen and gastric lipase. -The secretion of the mucous, parietal and chief cell form gastric juice. -G cell is a type of enteroendocrine cell that secrete the hormone gastrin into the bloodstream. -Gastrin stimulates growth of the gastric glands and secretion of large amounts of gastric juice.

Lower esophageal

• The lining of the stomach – Is coated with mucus, which prevents the gastric juice from destroying the cells Esophagus Cardiac orifice

Stomach

5 µm

Pyloric sphincter

Interior surface of stomach. The interior surface of the stomach wall is highly folded and dotted with pits leading into tubular gastric glands. Gastric gland. The gastric glands have three types of cells that secrete different components of the gastric juice: mucus cells, chief cells, and parietal cells.

Small intestine

Folds of epithelial tissue

Epithelium

3

Pepsinogen

2 HCl

Pepsin (active enzyme)

1 2 HCl converts pepsinogen to pepsin.

Mucus cells secrete mucus, which lubricates and protects the cells lining the stomach.

3 Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins.

Chief cells secrete pepsinogen, an inactive form of the digestive enzyme pepsin. Parietal cell

Parietal cells secrete hydrochloric acid (HCl).

1 Pepsinogen and HCI are secreted into the lumen of the stomach.

Chief cell

-Several minutes after food enter the stomach, mixing wave – gentle peristaltic movement pass over stomach every 15-25 sec. -This wave mix the food with the secretions of gastric glands and reduce it to a soupy liquid called chyme. -More vigorous wave begins at the body of the stomach and intensify as they reach pylorus. -The phyloric sphincter not completely close and several ml of chyme enter duodenum.

-Pepsinogen activate become pepsin, which broke peptide bonds in protein. -Lipase splits the short chain triglycerides (fats) in butter-fat molecules found in the milk. -Vomiting-involve squeezing the stomach between the diaphragm and abdominal muscles and expelling content through open esophageal sphincters.

• Gastric ulcers, lesions in the lining – Are caused mainly by the bacterium Helicobacter pylori – Initiates ulcers by destroying protective mucus and causing inflammation of the stomach lining. Then the acidic gastric juice can attack the stomach tissue. Bacteria

1 µm

Mucus layer of stomach

Liver and gallbladder -Liver is the heaviest gland of the body about 1.4 kg in adult. -Divided into two lobe-large right lobe and smaller left lobe by falciform ligament -The lobe of liver is made up of many functional units called lobules, which consists of specialized epithelial cellhepatocytes -Bile secreted by the hepatocytes enter bile canaliculi. -Bile is store temporarily in the gallbladder. -The gall bladder is a peer-shape sac that is located in a depression of the posterior surface of the liver.

Pancreas -Consist of a head, body and a tail and is connected to the duodenum by two ducts -The pancreas is made up of small clusters of glandular epithelial cells., they are 99 % arranged in clusters called acini. -The cell acini secrete a mixture of fluid and digestive enzyme called pancreatic juice. -The remaining 1 %are clustered as pancreatic islet-islet of Langerhans. -These cell secrete hormones glucagon, insulin and others.

-Pancreatic juice is clear, colourless consist of water, salts, sodium bicarbonate and several enzyme. -Sodium bicarbonate gives pancreatic juice slightly alkaline pH that buffers the acidic gastric juice in chyme. -It produce the proper pH for the actions of digestive enzyme in small intestine. -The pancreatic juice-pancreatic amylase, trypsin, pancreatic lipase, ribonuclease, deoxyribonuclease and others.

1 - head 4 - body 11 - tail

Pancreas region

- Pancreatitis is an inflammation of pancreas. - It may occur in association with alcohol abuse or chronic gallstone. - If the bile contain either insufficient bile salt or lecithin or excessive cholesterol. It may crystallize to form gallstones. - Jaundice is a yellow colouration of the sclerae (white eye), skin and mucous membrane-build up of yellow compound called bilirubin. - Bilirubin is formed from the breakdown of the heme pigment in aged red blood cell.

Functions of the liver: -Carbohydrate metabolism -Lipid metabolism -Protein metabolism -Processing of drugs and hormones -Excretion of bilirubin -Synthesis of bile salts -Storage -Phagocytosis -Activation of vitamin D

The Small Intestine • The small intestine-refers to its small diameter, compared to the LI – Is the longest section of the alimentary canal – The areas is further increase by circular folds, villi and microvilli. – It begins at the pyloric sphincter of the stomach and eventually open into the large intestine. – The SI is divided into 3 regions-duodenum, jejunum and ileum.

Enzymatic Action in the Small Intestine • The first portion of the small intestine is the duodenum – Where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself Liver

Bile

Gallbladder

Stomach

Acid chyme Intestinal juice

Pancreatic juice Pancreas Duodenum of small intestine

• The pancreas produces inactive proteases, proteindigesting enzymes into the duodenum. • Enteropeptidase, which is bound to the intestinal epithelium, converts trypsinogen to trypsin. • Trypsin then activates other proteases

Pancreas

Membrane-bound enteropeptidase Inactive trypsinogen Other inactive proteases Lumen of duodenum

Trypsin Active proteases

Endopeptidases and exopeptidases Endopeptidases - break peptide bonds in the interior part of a protein molecule producing polypeptide or small peptide chains. e.g. pepsin, trypsin Exopeptidases – attack polypeptide or small peptide chains to break the terminal part of amino acids therefore, releasing free amino acids. – e.g. aminopeptidases attack the end of polypeptide chain with free amino group (NH2); carboxypeptidases - attack the end of polypeptide chain with free carboxyl (COOH) group.

Digestion of cellulose by herbivores Cellulase • breaks beta-glycosidic links and split cellulose into monosaccharides. • it is secreted by bacteria, proctotist, fungi. • mammals cannot produce cellulose but it can be digested by mammalian herbivores.

-Enzyme from the liver, pancreas and sl combine to digest the nutrient -Carbohydrates digested into monosaccharrides Amylases hydrolyze glycosidic bond linking the glucose but cannot split the B-glycosidic bond in cellulose. -Protein digested to amino acids Chymotrypsin, trypsin and pepsin digest protein -Fat digested into fatty acids and monoacylglycerols Pancreatic lipase digest fat droplet Bile salt emulsify fat droplet Fat digestion is incomplete , undigested fat may be absorbed without complete digestion.

Protein digestion

Carbohydrate digestion

Nucleic acid digestion

Fat digestion

Oral cavity,Polysaccharides Disaccharides pharynx, (starch, glycogen) (sucrose, lactose) esophagus Salivary amylase Smaller polysaccharides, maltose

Stomach

Proteins Pepsin Small polypeptides

Lumen of Polysaccharides small intesPancreatic amylases tine Maltose and other disaccharides

Polypeptides Pancreatic trypsin and chymotrypsin (These proteases cleave bonds adjacent to certain amino acids.) Smaller polypeptides

DNA, RNA Pancreatic nucleases

Nucleotides

Pancreatic carboxypeptidase

Small peptides Disaccharidases

Monosaccharides

Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.) Amino acids

Bile salts Fat droplets (A coating of bile salts prevents small droplets from coalescing into larger globules, increasing exposure to lipase.) Pancreatic lipase

Amino acids

Epithelium of small intestine (brush border)

Fat globules (Insoluble in water, fats aggregate as globules.)

Glycerol, fatty acids, glycerides Nucleotidases Nucleosides Nucleosidases and phosphatases Nitrogenous bases, sugars, phosphates

Absorption of Nutrients • To enter the body, nutrients in the lumen must cross the lining of the digestive tract • A few nutrients are absorbed in the stomach and LI, but most absorption -SI • Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients into the blood occur in the SI. • Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine • The small intestine has a huge surface area – Due to the presence of villi and microvilli that are exposed to the intestinal lumen

• The enormous microvillar surface – Is an adaptation that greatly increases the rate of nutrient absorption Microvilli (brush border)

Vein carrying blood to hepatic portal vessel

Blood capillaries Epithelial cells Muscle layers

Villi

Epithelial cells

Large circular folds

Lacteal

Key Nutrient absorption

Intestinal wall

Villi

Lymph vessel

Layer of small intestine

SEM of small intestine

Enlarged view of the villi

SEM of the surface of epithelial cell showing microvilli

• The core of each villus – Contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal • Nutrients are absorbed across the intestinal epithelium & then across the unicellular epithelium of the capillaries or lacteals – Thus only 2 layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream – Intestinal juice is a clear yellow fluid secreted abt. 1-2 liter a day. – It has pH 7.6, slightly alkaline, which contain water and mucus.

• Amino acids and sugars – Pass through the epithelium of the small intestine and enter the bloodstream • After glycerol and fatty acids are absorbed by epithelial cells – They are recombined into fats within these cells – These fats are then mixed with cholesterol and coated with proteins – Forming small molecules called chylomicrons, which are transported into lacteals

Fat globule

1 Large fat globules are emulsified by bile salts in the duodenum.

Bile salts

Fat droplets coated with bile salts

Epithelial cells of small intestine

2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides, which Micelles made up of fatty acids, then form micelles. monoglycerides, 3 Fatty acids and monoand bile salts glycerides leave micelles and enter epithelial cells by diffusion.

Lacteal

4 Chylomicrons containing fatty substances are transported out of the epithelial cells and into lacteals, where they are carried away from the intestine by lymph.

The Large Intestine • The large intestine, or colon – Is connected to the small intestine

- The large intestine is the terminal portion of the GI tract. - Overall functions are the completion of absorption, the production of certain vitamins, formation of feces and expulsion of feces from the body. - The colon have cecum and vermiform appendix, colon, rectum and anus. - The colon subdivided into ascending colon, transverse colon, descending colon and sigmoid colon. - Normally the anus is closed except during the elimination of feces. - Appendicitis is the inflammation of the appendix.

• Peristalsis also occurs but in the slower rates. • The final stage of digestion occurs in the colon through the activity of bacteria that inhibit the lumen. • Mucus is secreted by the glands of the large intestine. • No enzymes are secreted. • Chyme –prepared for elimination by the actions of bacteria, ferment carbohydrates and release gases as flatus. • A major function of the colon – Is to recover water that has entered the alimentary canal • The wastes of the digestive tract, the feces – Become more solid as they move through the colon

• The terminal portion of the colon-rectum - where feces are stored until they can be eliminated • Between the rectum and the anus are 2 sphincters, one involuntary and the other voluntary • One or more times each day, strong contractions of the colon create an urge to defecate • Diarrhea-is and increase in the frequency, volume and fluid content of the feces caused by increase motility and decreased absorption by the intestine. • Constipation-refer to infrequent or difficult defecation caused by decreased motility of the intestine.