ZLY 106DIGESTIVE SYSTEM

TOPIC TO COVER IN THIS LECTURE

• DIGESTION AND ALIMENTARY TRACT

• MODIFICATION OF THE ALIMENTARY TRACTS IN HERBIVORES

• FUNCTIONS OF THE LIVER

• ROLES OF ENZYMES IN DIGESTION

• At the end of the period, students should be able to:

• define alimentary canal & name its 5 main parts

• define ingestion, absorption and egestion.

• name the part of the alimentary canal where these occurs

• identify all the main parts of the digestive system

• list all the digestive enzymes in each part of the alimentary canal

•Digestion is the breakdown of complex food substance into simpler food substance which can be absorbed into the blood stream for body use.

• These are:

• Ingestion: placing food into the mouth (entry of food in the digestive system),

• Digestion: This can be defined as the mechanical, physical and chemical breakdown, mastication and the mixing of food into raw materials that is easily available for absorption by the body.

• Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport & diffusion.

• Egestion: Removal of undigested materials from the digestive tract through defecation.

• The liver is the heaviest organ in the body and is one of the largest.

• Lies in the upper right part of your belly under the ribs and is responsible for functions vital to life.

• The main function is to process nutrients from food, make bile, remove toxins from the body and build proteins.

• It's easy to see how inflammation of the liver, or hepatitis, interferes with these important functions and can lead to poor health.

• Fortunately, the liver is extremely resilient and most cases of liver inflammation don't even come to medical attention, but in cases of severe liver disease, there can be serious interruption of these essential liver functions.

FUNCTIONS OF THE LIVER

• Deamination is the removal of an amine group from

a molecule. Enzymes that catalyse this reaction are called deaminases.

• In the human body, deamination takes place primarily in the liver, however glutamate is

also deaminated in the kidneys.

• Deamination is the process by which amino acids are broken down if there is an excess

of protein intake. The amino group is removed from the amino acid and converted

to ammonia.

• The rest of the amino acid is made up of mostly carbon and hydrogen, and is recycled or

oxidized for energy. Ammonia is toxic to the human system, and enzymes convert it

to urea or uric acid by addition of carbon dioxide molecules (which is not considered a

deamination process) in the urea cycle, which also takes place in the liver. Urea and uric

acid can safely diffuse into the blood and then be excreted in urine.

Processing Nutrients from Food• The digestive system immediately begins to break down the food

that we eat into smaller and smaller pieces. Eventually these nutrients will enter the blood and travel to the liver through the hepatic portal system, the major pathway that blood takes from the digestive system to the liver.

• The liver will then process these nutrients in different ways, depending on the body's needs. It usually stores some of the nutrients in a form that the body can use for quick energy.

• The rest will be used to make other important chemicals the body needs.

• When the liver is severely damaged, such as in liver failure , it can't continue to process nutrients from the blood that the body must have. Without aggressive medical care, the absence of these essential liver functions can result in signs of serious illness like brain damage and coma.

• The liver is considered a gland—an organ that secretes chemicals—because it produces bile, a substance needed to digest fats.

• Bile’s salts break up fat into smaller pieces so it can be absorbed more easily in the small intestine.

• In addition to producing bile, the liver:

• Detoxifies the blood to rid it of harmful substances such as alcohol and drugs

• Stores some vitamins and iron

• Stores the simple sugar glucose

• Converts stored sugar to usable sugar when the body’s sugar (glucose) levels fall below normal.

• Breaks down hemoglobin as well as insulin and other hormones

• Converts ammonia to urea, which is vital in metabolism

• Destroys old red blood cells

MAKING OF THE BILE

• Bile is a thick, green-yellow fluid that the liver produces to help digest food, especially fat, as it passes from the stomach to the intestines.

• Bile is stored in a nearby sac called the gall bladder after its production.

• When a person eats a meal heavy in fat, the body will use its store of bile to help break down the fats for digestion. 

5 MAIN PARTS OF DIGESTIVE SYSTEM

•MOUTH

• EOSOPHAGUS

• STOMACH

• SMALL INTESTINE

• LARGE INTESTINE

THE MOUTH• Here lies the tongue between the dental arches

and the cheek.

• Produces alkaline saliva from salivary glands

• Secretes Ptyalin which convert starch to maltose (chemical digestion)

• Teeth cut masticate the food into smaller pieces

• Mouth opens into muscular pharynx

• During swallowing, the epiglottis flaps down to cover the trachea to prevent choking.

OESOPHAGUS

• It is a muscular band of muscles capable of contracting and relaxing• Food moves by peristalsis• Peristalsis occurs throughout the

walls of the alimentary gut which are muscular• No enzyme is secreted here

OESOPHAGUS• The oesophagus is a muscular tube

through which food is carried from the pharynx to the stomach.

• The oesophagus also has to accommodate a wide variety of food and drink (hot, cold, spicy etc).

• The oesophagus has a stratified squamous epithelial lining (SE) which protects the oesophagus from trauma.

STOMACH• This is a muscular organ located in the abdominal

cavity under the diaphragm.

• The diaphragm separates the abdominal cavity from the thoracic cavity.

• Stomach is a temporary storage organ capable of holding about 1.5L of food and water for approximately 3- 4hrs depending on the type of food.

• Holding capacity of food in the stomach relatively depends on the type of food eaten and also a function how active the animal is.

• The shape of the stomach changes as digestion continues to facilitates the pushing of food towards the small intestine.

• Food enters the stomach from the esophagus when the muscles of the cardiac sphincter on the upper region of the stomach are relaxed.

• Contraction of the muscular walls cause churning of food

• Secrete gastric juice (work best in acidic medium) which contain 2 enzymes. About two litres of gastric juices is secreted daily by the gastric glands

• Food enters the stomach from the esophagus when the muscles of the cardiac sphincter on the upper region of the stomach are relaxed.

• The chief cells of gastric glands produces two types of enzymes namely: pepsin and rennin.

• Pepsinogen is initially secreted but later activated to form pepsin by the action of dilute hydrochloric acid.

• Pepsin converts protein to peptones

• Rennin converts soluble milk, caseinogen to casein especially in infants

• Apart from activating pepsinogen to form pepsin, dilute HCl in the stomach helps maintain the pH of the gastric juice, also has antiseptic property capable of killing pathogens that may be present in the food.

• Food exits the stomach when the muscles of the pyloric sphincter are relaxed

• The stomach is a 'J'-shaped organ, with two openings- the oesophageal and the duodenal.

• Has four regions- the cardia, fundus, body and pylorus.

• Each region performs different functions; the fundus collects digestive gases, the body secretes pepsinogen and hydrochloric acid, & the pylorus is responsible for mucous, gastrin and pepsinogen secretion.

•Stomach has 5 major functions;

•Temporary food storage of food

•Control the rate at which food enters the duodenum

•Acid secretion and antibacterial action

•Fluidisation of stomach contents

•Preliminary digestion with pepsin, lipases etc.

• Divided into 3 parts viz Duodenum, Jejunum & Ileum

• Food enters duodenum(the first part)

• Pancreas secretes alkaline pancreatic juice which contains 3 enzymes viz

• Amylopsin (starch to maltose)

• Trypsin (protein to polypeptides)

• Lipase (fats to fatty acids & glycerol)

• Presence of numerous typically finger-like processes called Villi that project from the surface of the mucosa into the lumen.

• The villi are greatest and most numerous for a given area of the duodenum.

• Each villus consist of a central lymphatic vessel called Lacteal which may occasionally be doubled.

• The villi may sometimes be leaf-like or flattened.

• Presence of numerous depressions called Crypts that project from the surface of the mucosa into the lumen.

REGULATION OF DIGESTIVE SECRETIONS BY HORMONES

• It is important to note that hormones such as gastrin, secretin, cholecystokinin & enterogastrone a.k.a gastric inhibitory peptide (GIP) play vital roles in digestion.

• Gastrin is secreted in the mucosa wall of the stomach

• Secretin, cholecystokinin and GIP are all secreted in the mucosa walls of the duodenum.

REGULATION OF DIGESTIVE SECRETIONS BY HORMONES

• Gastrin signals gastric glands to release pepsinogen

• Secretin stimulates the release of sodium bicarbonate by the pancreas & bile to neutralize the acidic medium of chyme in the stomach

• Cholecystokinin stimulates the contraction of the gall bladder to release bile into the small intestine & the release of pancreatic enzymes.

• Enterogastrone inhibits peristalsis in the stomach in order to slow down the entry of chyme into the duodenum.

• Liver produces watery bile which emulsify fats into tiny droplet

• Bile contains no enzymes

• Help add water to chyme

• Chyme turns to chyle as it passes

• There are also intestinal juice which contains

• Maltase (Maltose to glucose + glucose)

• Lactase (Lactose to glucose + galactose)

• Sucrase (Sucrose to glucose+ fructose)

• Erepsin : Polypeptides to amino acids

• Lipase : Fats to carboxylic acids + glycerol

ABSOPTION IN ILEUM• Facilitated by villi, a finger-like projection& blind

lymphatic tube called Lacteal

• Villi is surrounded by network of blood capillaries (vascularised)

• Lymphatic vessels eventually empty their content into the blood vessels

• Protein is digested to amino acids, CHO to glucose and lipids to fatty acids and glycerol.

• Amino acids & glucose are transported to the liver by the hepatic portal vein

Absorption of food facilitated in Villi

Absorption of food facilitated in Villi

Villi increase the surface area of small intestine: absorption of food is facilitated

LARGE INTESTINE

• Undigested food passes into colon

• More water absorbed

• This makes it more concentrated

• Faeces are stored in rectum

• Leaves the body through the anus

The Large Intestine• This is divided into six regions namely caecum, colon,

rectum and anus.

• Colon is further subdivided into ascending, transverse & descending colons.

• Rectum is the last portion of the large intestine& terminates in the anal sphincter.

• A structure called the vermiform appendix extends from blind end of rectum & its inflammation causes appendicitis.

• Colon plays fewer roles in digestion but it is important for reabsorption of fluids which benefit the body.

• The large intestine consist mainly of columnar cells specialized for water & sodium reabsorption and mucus-secreting goblet cells to aid the passage of faeces.

• Lower organisms such as Sponges, Amoeba, Paramecium have no special digestive tracts at all.

• They feed on microscopic organisms by phagocytosis, then digestive the food particles intracellularly within the food vacuoles.

• Excess water and waste products are then liberated into the surrounding water.

• Cnidarians such as hydra, jellyfish feed on cylops, annelids, crustaceans, insect larvae. Their mouth serves for both ingestion and egestion as they lack anus.

• Cnidarians use tentacles to capture and direct the prey. The prey is killed extracellularly by the action of digestive enzymes from gland cells of gastrodermis.

• Intracellular digestion occurs in the gastrovascular cavity and excess materials are egested via the anus.

• Ruminants have the most elaborate adaptations for herbivorous diets & have a 4-chambered stomach: Rumen, Reticulum, Omasum & Abomasum.

• 3 of the 4 ruminant stomach compartments (rumen, reticulum & omasum) make up the forestomach & are extension of the lower oesophagus.

• The rumen, the first of the forestomach chambers, stores and processes plant material.

• Huge numbers of these bacteria are present in the rumen and the reticulum: with each gram of rumen fluid contains 10 - 50 billion bacteria

RUMINANTS DIGESTIVE TRACTS• Ruminant first chew & swallows its food in the

mouth, it enters the rumen & reticulum where symbiotic bacteria act on the cellulose-rich meal.

• Ruminants tend to swallow their food quickly, with only minimal mastication.

• When ruminants is resting after grazing, it regurgitates this partially chewed food, rechews it, then swallows it again making it more accessible to further bacteria actions.

• This process is called “chewing the cud” or rumination.

• Depending on the amount of  fibre in their food, cow may spend 3-6 hours per day chewing their cud.

• As by-products of their metabolism, the bacteria secrete fatty acids.

• The cow periodically regurgitates & rechews the cud(4) which further breaks down the fibres

• The cow then reswallows the cud which moves to the omasum, where the water is removed.

• The cud finally passes to the abomasum for digestion,

• The diet from which a ruminant actually absorb its nutrients is more richer than the hay or grass the animal originally eats.

• Infact, ruminants obtain many of its nutrients by digesting the symbolic bacteria which reproduce rapidly enough in the rumen..

COMPARTMENTALIZATION OF THE DIGESTIVE SYSTEM OF A RUMINANT eg Cow, Sheep, Goat, Deer

Thank you for listening

Dr. O. A. Iyiola