Digestive System

You are what you eat

The Old-School Food Pyramid

Digestive System

• Digestion is the mechanical and chemical breakdown of foods and the absorption of the resulting nutrients.

• Mechanical digestion breaks large pieces into smaller ones without altering their chemical composition.

• Chemical digestion breaks food into simpler chemicals.

Digestive System

• The digestive system consists of the alimentary canal, which extends about 8 m from the mouth to the anus, and the accessory organs which secrete substances used in the process of digestion.

Alimentary Canal

• The alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.

• The accessory organs of the digestive system include the salivary glands, the liver, gallbladder, and pancreas.

• Overall, the digestive system has a surface area of over 186 square meters.

Alimentary Canal

• Muscular tube that passes through body’s ventral cavity.

• Wall has four distinct layers:– Mucosa: carries out secretion and absorption.– Submucosa: carries away absorbed materials.– Muscular layer: produces movements of the tube– Serosa: outer covering.

Alimentary Canal

• Two movements:• Mixing – rhythmic

contractions. Mix food with digestive juices that mucosa secretes.

• Propelling – peristalsis wave pushes tubular contents ahead.

Mouth

• Mouth – receives food and begins digestion by mechanically reducing size of food.

• Oral cavity – chamber between palate and tongue

• Vestibule – space between teeth, cheeks, and lips. C. megalodon, extinct shark

Mouth

• Cheeks –pads of subcutaneous fat, muscles associated with expression and chewing

• Lips –surround the mouth opening. • Tongue – Nearly fills oral cavity. Frenulum

connects tongue to floor of mouth. Mixes food particles with saliva during chewing and moves food to pharynx during swallowing.

Mouth

• Papillae on tongue provide friction and bare tastes buds.

• Palate forms the roof of the mouth. Consists of hard and soft palate.

• Soft palate to back of mouth and forms uvula.• During swallowing, muscles draw soft palate

and uvula upward, allowing closure of nasal cavity and pharynx.– Why?

Mouth

• Palatine tonsils are masses of lymphatic tissue located at the back of the mouth.

• Pharyngeal tonsils, or adenoids, are also in the back of the mouth.– Why are all of these

lymphatic tissues in the mouth?– Have any of you had a tonsillectomy? What could

possible consequences be later in life?

Mouth

• Two different sets of teeth form during development: primary (deciduous) and secondary (permanent) teeth.

• Teeth begin mechanical digestion by breaking pieces of food into smaller pieces. – Why?– Increases surface area of food particles, allowing

digestive enzymes to react more effectively.

Salivary Glands

• The salivary glands secrete saliva. • Saliva moistens food particles, binds them,

and begins chemical digestion.• Amylase, a digestive enzyme splits starch and

glycogen down into disaccharides.• Mucus is secreted by mucosa cells that bind

food and lubricates during swallowing.

Salivary Glands

• What happens if your body quits making saliva (xerostomia)?

• “Meth Mouth” is the result of no spit, poor dental hygiene, tooth grinding, and clenching.

• Want to kiss this?• Didn’t think so

Pharynx (throat) [Look familiar?]

• Made of muscle and mucous lining into 3 divisions

• Nasopharynx – posterior nares to soft palate

• Oropharynx – soft palate to hyoid bone; adenoid tonsils

• Laryngopharynx – hyoid bone to esophagus; palatine and lingual tonsils

Esophagus

• A straight, collapsible tube about 25 cm long• Food passageway from pharynx to stomach• Secretions moisten and lubricate tube• The cardiac sphincter (lower esophageal

sphincter) remains contracting, closing the entrance to the stomach.– Why is this important?– To keep the stomach contents from regurgitating into

esophagus.– When peristaltic waves hit stomach, these fibers relax

and allow food to enter.

Stomach

• The stomach is an organ that:• receives food from the esophagus • mixes the food with gastric juices• begins chemical digestion • moves food on to the small

intestine.• The pH of the stomach, due to

gastric acids, can go as low as 1, and ranges up to 3-4 when food enters. – Why do you think it’s important to

have such a low pH in the stomach?

Stomach

• Gastric glands in the stomach excrete digestive enzymes such as:

• Pepsin, formed from pepsinogen upon contact with HCl from parietal cells. Pepsin begins digestion of proteins.

• Intrinsic factor which helps the small intestine absorb vitamin B12.

• When a person tastes, smells or sees appetizing food, the brain releases Ach, which stimulates gastric glands to secrete gastric juices.

Stomach

• After a meal, the mixing movements of the stomach wall and a mix of gastric juice and food particles form chyme.

• Chyme is what leaves the stomach and moves into the duodenum (the first part of the small intestine), where secretions from other organs are added.

Pancreas

• The pancreas has both endocrine and exocrine functions. It secretes a digestive juice – pancreatic juice.– What are the differences

between endocrine and exocrine glands?

– Endocrine glands secrete hormones or other chemicals into the local environment, while exocrine glands secrete their products into ducts which lead to other parts of the body.

Pancreas

• Pancreatic juice has several enzymes including:• Pancreatic amylase – splits molecules of srach

or glycogen into disaccharides.• Pancreatic lipase – breaks down triglycerides

into fatty acids.• Nucleases – break down nucleic acids into

nucleotides.• Proteolytic enzymes – break down proteins.

The Liver

• The liver is divided into lobes – left and right.

• The liver functions in many ways including with carbohydrate, lipid, and protein metabolism, storage, blood filtering, detoxification, and the secretion of bile.

The Liver

• The most vital function of the liver is its work in protein metabolism.

• This protein metabolism involves deaminating amino acids, forming urea, and synthesizing plasma proteins (such as clotting factors).

• The liver also stores iron, glycogen, vitamins, and breaks down fats that can be stored in adipose tissue throughout the body.

The Liver

• Bile – yellowish-green liquid secreted from hepatic cells in the liver. Bile contains water, salts, bile pigments (bilirubin and biliveridin), cholesterol, and electrolytes.

Gallbladder

• The gallbladder is a sac located on the liver’s surface.

• It stores bile between meals, reabsorbs water to concentrate the bile, and contracts to release bile.

Small Intestine

• The small intestine receives secretions from the pancreas and liver and completes the digestion of the nutrients in chyme, absorbs products of digestion and transports remains to the large intestine.

Small Intestine• Intestinal villi are tiny projections that greatly

increase the surface area of the intestine.– Why would this be important in this organ?– Aids the absorption of digestive products.

• Digestive enzymes are released from these villi:– Peptidase – breaks down peptides into amino acids– Sucrase – breaks down sucrose– Maltase – breaks down maltose– Lactase – breaks down lactose• People who are lactose intolerant lack lactase in intestine

Small Intestine

• Small intestine is the most important absorbing organ of the alimentary canal.

• Nutrients are absorbed into the villi and transported through the blood to various body parts.– Example: cholesterol

transported to liver.

Main Squeeze

• In snakes that feed as constrictors, the small intestine stretches after meals to increase its mass by over 40%, therefore increasing surface area to help with the absorption of nutrients.

Green Anaconda (Eunectus murinus) constricting a Caiman

Large Intestine

• The large intestine is so named because it has a larger diameter than the small intestine.

• The cecum is the beginning of the large intestine.

• From the cecum hangs the veriform appendix.

• The appendix is a vestigial organ.– What do you think the

appendix might have been used for in our ancestors?

Large Intestine• The large intestine absorbs

water and electrolytes from chyme remaining the alimentary canal.

• It also forms and stores feces.• 1 - cecum• 2 - ascending colon• 3 - transverse colon• 4 - descending colon• 5 - Sigmoid colon• 6 - rectum

Large Intestine

• The large intestine wall lacks villi. • The large intestine does not secrete enzymes.• Only secretion is mucus.• Instead, large intestine absorbs water and

electrolytes and remainder becomes feces.• Intestinal flora, the bacteria that inhabit the

large intestine, break down molecules in food, such as cellulose.

Large Intestine

Escherichia coli in human large intestine

• The human colon is home to over 100 trillion bacteria.• Flatus, intestinal gas, is the result of bacterial actions in the large intestine.

Large Intestine

• Peristaltic waves and mixing motions produce mass movements in the large intestine forcing the contents towards the rectum.

• Feces include material that were not digested or absorbed, plus water, electrolytes, mucus, and bacteria. Feces are usually about 75% water.

• …Or if you ask the girl in Donnie Darko, “What are feces?”– “Baby mice.”

Nutrition

• Four Basic Food Groups • Nutrition is the study of nutrients and how the

body uses them.• Nutrients include carbohydrates, lipids,

proteins, vitamins, minerals, and water.• Macronutrients – proteins, lipids,

carbohydrates. Required in large amounts.• Micronutrients – vitamins and minerals.

Nutrition

• Macronutrients provide potential energy that can be expressed in calories (unit of heat).

• Calorie – the amount of heat required to raise the temperature of a gram of water by 1° C.

• The “calorie” used to measure food energy is actually the kilocalorie, but is often written Calorie.

• 1 gram of carbohydrate or 1 gram of protein yields about 4 calories.

• 1 gram of fat yields about 9 calories.• Nutrients that human cells cannot synthesize, such as

some amino acids, are called essential nutrients.

Nutrition

• Carbohydrates are organic compounds used to supply energy for cellular processes.

• Starch, mono- and disaccharides, glycogen, and cellulose are all carbohydrates.• What is an example of a

cellular process in which a carbohydrate is being used?

Carbohydrates

• Monosaccharides absorbed from the digestive tract such as fructose, galactose and glucose are broken down and used in cellular respiration.

• Excess glucose is stored as glycogen in liver. Glycogen can be converted to glucose when needed. Some excess glucose is stored as fat.

• Ribose and deoxyribose are required for the production of these…– Nucleic acids RNA and DNA

Lipids

• Lipids are organic compounds that include fats, oil, waxes, and cholesterol.

• Main function is to supply energy.

• Saturated fats are found mainly in meat, eggs, milk but also in coconut and palm oil.

• Unsaturated fats are in seeds, nuts, and plant oils.

Lipids

• Saturated fats are “saturated” with hydrogen atoms because they lack double bonds.

• Unsaturated fats have a cis double bond.• Trans fats are unsaturated but have a trans double

bond. This adds a kink to the chain.• Often this occurs due to hydrogenation, a process

that adds hydrogen atoms and results in the trans bond. This allows the unsaturated fat to remain solid at room temperature, thus acting as a saturated fat and being able to be preserved longer.

Lipids

• So what?• Trans fats are not

essential to the diet.

• Also, they can raise LDL cholesterol levels and lower HDL cholesterol, causing problems.

Protein

• Proteins are polymers of amino acids.• Proteins have a variety of functions. Many are

enzymes. These and other proteins function in:– Metabolic rate– Clotting– Kertain in skin and hair– Elastin and collagen of connective tissue– Water balance– Muscle components (actin, myosin)– Hormones– Antibodies

Protein

• Food sources of protein:– Meat– Fish– Cheese– Milk– Nuts– Eggs– Cereal– Legumes

Protein

• Amino acids that the body can synthesize are called nonessential.

• Those that it cannot synthesize are called essential amino acids.

• Complete proteins supply all of the essential amino acids. These include: milk, eggs, meat. Eggs are the only 100% complete protein.

• Incomplete proteins must be combined.– Example: rice and beans.

Vitamins and Minerals

• Vitamins are organic compounds that are required in small amounts and which the cells cannot synthesize in adequate amounts.

• Fat-soluble vitamins: A, D, E, K• Water-soluble vitamins: B, C• Minerals are elements other

than carbon that are essential in human metabolism.

Stomach

• Stomach is divided into four parts: – Cardiac – near heart/esophagus– Fundic – balloons up– Body – main part– Pyloric – near small intestine

• Taste, smell, or see food, Ach stimulates gastric glands to secrete gastric juice (HCl, pepsin, etc).

• Gastrin also released which aids gastric juice• Churning of food produces chyme.

Pancreas

• As chyme enters duodenum, hormone secretin released, pancreatic juice excreted.

• Pancreatic juice includes:– Pancreatic amylase– Pancreatic lipase– Nucleases– Proteolytic enzymes

Small Intestine

• Three parts: duodenum, jejunum,Ileum• Villi – project from mucous membrane.– Increase surface area– Lacteal – lymphatic capillary carries away

absorbed nutrients