Biology 12 – Unit 3A

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The Digestive System

Anatomical Overview of the Digestive System

The organs of the digestive system fall into 2 main groups:

The digestive system consists of a long passageway that starts at the mouth and continues through more than 20 feet (7m) of muscular tubes and chambers where it ends at the anus. It

takes approximately 24 hours for the food we eat to travel along this passageway as it is mechanically and chemically broken down into the nutrients that build and fuel our cells.

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1) Gastrointestinal Tract (GI Tract)

The GI tract is a continuous muscular digestive tube that winds through the body

FUNCTION: food is digested – broken down into small nutrient molecules, nutrients are absorbed into the bloodstream, and indigestible wastes are eliminated from the body.

The organs of the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus

Food in the GI tract is technically OUTSIDE the body because the canal is open to the external environment at both ends.

2) The Accessory Digestive Organs

The accessory organs of the digestive tract do not have food pass through them,

but they are involved in the mechanical or chemical digestion of food e.g.

Digestive Processes

The processing of food by the digestive system involves six essential activities:

1) Ingestion – the intake of food and fluids (usually through the mouth)

2) Propulsion – the movement and mixing of food along the digestive tract

3) Mechanical Digestion – Physical preparation of food e.g. chewing, churning

4) Chemical Digestion – In a variety of catabolic steps macromolecules such as proteins, starches, and fats are broken down into their chemical building blocks by enzymes that are

secreted into the digestive tract

5) Absorption – The passage of digested end products, vitamins, minerals and water from the

GI tract through the walls of the small and large intestines and into the blood stream so they can be transported throughout the body

6) Elimination – Indigestible food material such as fiber and other wastes are eliminated from

the body

The Mouth (Oral Cavity)

The mouth (oral cavity) is the site of ingestion

mechanical and chemical breakdown of food begins when it is chewed and mixed with saliva

Teeth & Tongue

teeth tear and grind food and break it down into

smaller fragments (mechanical digestion) providing more surface area for digestive enzymes (chemical

digestion) to work

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We masticate (chew) by opening and closing our jaws and moving them side to side

During chewing, the tongue mixes food with saliva and constantly repositions it

between the teeth

food is formed into a compact mass called a bolus that can be easily swallowed

Salivary Glands

Three pairs of salivary glands secrete up to 1.5 L of saliva into the oral cavity every day!

Functions of Saliva:

1) Cleanses the mouth – Saliva contains antimicrobial peptides called defensins that kill bacteria and other microorganisms that are ingested

2) Dissolves food chemicals so they can be tasted

3) Moistens food and lubricates the mouth to make chewing and swallowing easier.

4) Produces Salivary Amylase -

a hydrolytic enzyme that breaks down starch molecules

into the disaccharide maltose

Swallowing

The process of swallowing begins as a voluntary action when a bolus of food is pushed by the tongue to the back of the mouth towards the pharynx

Deep at the back of the throat the pharynx branches off into 2 separate tubes:

1) Trachea – The passage way that carries respiratory gases (air) to and from the lungs

2) Esophagus – A muscular passage way approximately 25 cm long (10 inches) that carries food to the stomach

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The Steps of Swallowing

The swallowing process begins when a bolus is pushed towards the pharynx by the tongue.

As the bolus arrives at the pharynx the soft palate blocks the upper pharynx to prevent food from entering the nasal cavity.

Voluntary muscle contractions in the neck and throat continue to push the bolus safely down

the pharynx towards the epiglottis

At this time the epiglottis, a flap of cartilage, flips downward to cover the entrance to the trachea

The epiglottis functions to direct food into the esophagus and prevented from “going down the wrong tube” and into the lungs

Once inside the esophagus food is moved along its 10 inch length using a series of

involuntary wave-like muscle contractions in a process called peristalsis

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The Stomach

At the end of the esophagus there is a ring of circular muscle called the cardiac sphincter

which relaxes to allow the bolus to enter the stomach.

the cardiac sphincter remains closed to prevent the stomach’s acidic contents from

backing up the esophagus (acid reflux or heart burn)

the esophagus expands to form the J-shaped stomach

the stomach functions as a temporary “storage tank” where the chemical and mechanical digestion of food continues

when the stomach is empty, it collapses inward on itself creating specialized folds called rugae - the rugae give the stomach incredible expansive properties

Empty stomach = 50 mL

Full stomach 4 L and may extend nearly all the way to the pelvis!

The wall of the stomach is very muscular and functions to churn, mix and

mechanically break down the food into even smaller fragments in a process

called churning

The stomach wall is lined with mucus to protect the stomach from the HCl

The bolus entering the stomach causes cells in the stomach lining to release gastric

juice from gastric glands found deep within gastric pits.

Gastric Pit

Rugae

Goblet Cells

Muscular Wall of the Stomach

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Gastric juice contains:

1) Hydrochloric acid (HCl) pH 2 (strong acid)

HCl works to kill any bacteria present in food

HCl denatures the proteins found in the food we have eaten so they can be

chemically digested by hydrolytic enzymes.

HCl does NOT DIGEST FOOD, it breaks down the connective tissues of meat

2) Pepsinogen

HCl in gastric juice converts Pepsinogen into the active enzyme Pepsin

Pepsin, breaks down large proteins into smaller polypeptide chains. Pepsin is

secreted in an inactive form to protect the cells that secrete it from auto-digesting

3) Mucus

Goblet cells secrete alkaline mucus onto the stomach lining to protect it

from HCl burns

Digestive Processes that Occur in the Stomach

1) Protein Digestion

Protein to small peptide chains with the aid of pepsin

2) Absorption of lipid soluble substances such as alcohol and some drugs

NO FOOD is absorbed by the stomach however lipid soluble substances such as alcohol

and Aspirin can easily pass though the stomach wall and into the bloodstream

Homeostatic Imbalance

Anything that breaks through the stomach’s protective mucus lining causes inflammation of the

stomach wall, a condition called gastritis. Persistent damage to the underlying tissues can promote gastric ulcers, erosions of the stomach wall. The danger posed by ulcers is that they can potentially lead to massive hemorrhages that can be fatal.

For years the blame for stomach ulcers was put on things that caused high HCl production or low mucus production such as high doses of Aspirin, non-steroidal anti-inflammatory drugs (Ibuprofen), smoking, alcohol, caffeine, and stress.

Recently, the Nobel Prize winning research of one doctor has changed the way the doctors treat ulcers. It is now known that up to 90% of all gastric ulcers are caused by a corkscrew-shaped

bacterium called Helicobacter pylori that burrows beneath the mucus lining of the stomach and destroys its protective mucosal layer. This leaves these areas vulnerable to acid burns. Recent studies also show

that this bacteria may also be responsible for causing some types of stomach cancer.

A 1-2 week course of complementary antibiotics is usually all it takes to cure a patient of this troublesome microbe.

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3) Churning (Mechanical Digestion)

Contractions of the smooth muscle layer in the stomach wall cause a churning action

that mixes all of the stomach’s contents with gastric juices.

Thanks to churning, the bolus eventually becomes mixed with HCl, Pepsin and

mucus to form a semi-liquid mass called acid chyme

At the bottom end of the stomach the pyloric sphincter acts as a pump that

delivers small amounts of the acid chyme into the duodenum of the small intestine

The stomach usually empties completely within 4 hours after a meal, however

if you eat a meal high in fats, food may remain in the stomach 6 hours or more

The Small Intestine

The small intestine is named for its small diameter (2.5 - 4 cm) when compared to the large

intestine (5 – 8 cm); but is 5 - 6 m long

Function of the small intestine: chemical digestion, and virtually all nutrients are

absorbed into the bloodstream

The small intestine is divided into 3 sections:

1) Duodenum – is the first 25 cm of the small intestine.

Produces and secretes digestive enzymes:

Maltase breaks maltose

to glucose

Peptidase breaks peptides amino acids

Nucleosidase breaks nucleotides N-base,

phosphate group and pentose sugar

Receives digestive juices from the pancreas, liver and gallbladder

Completes chemical digestion

2) Jejunum – is approximately 2.5 m long

Absorption of nutrients

3) Ileum – approximately 2-4 m long, connects to the large intestine

Absorption of nutrients

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Absorption of Nutrients in the Small Intestine

The small intestine has an incredible amount of surface area and this makes it a

specialized organ for absorption

The majority of nutrient absorption occurs in the Ileum and the Jejunum

The following features of the small intestine contribute to its large surface area:

1) Small finger-like projections called villi cover the inner surface of the intestine

2) Each villi also contains microvilli – thousands of microscopic projections that further increase the overall surface area of the small intestine.

Each villus is richly supplied with capillaries, small blood vessels that absorb glucose,

amino acids, and dipeptides into the blood stream

At the center of each villus there is a lacteal – a small lymphatic vessel capable of

absorbing fats into the lymphatic system

Microvilli contain additional intestinal digestive enzymes that aid in the digestive

process and increase the efficiency of absorption

Much of the absorption of nutrients is via active transport (against the concentration

gradient) however, passive transport also occurs.

As a result, microvilli have many mitochondria and use a lot of ATP http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter26/animation__organs_of_digestion.html

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The Accessory Organs:

The Pancreas

As an accessory digestive organ, the pancreas produces enzymes that break down all major

macromolecules found in the food we eat i.e. carbohydrates, proteins, and fats.

The pancreas behaves like an exocrine gland when it releases pancreatic juice into the

small intestine.

The arrival of acid chyme (pH 2) from the stomach into the duodenum stimulates the

release pancreatic juice

Pancreatic juice is a basic fluid with pH 8 that is composed of:

1) Sodium bicarbonate (NaHCO3) - why do you think this is important??

2) water

3) pancreatic enzymes (proteases)

Pancreatic Digestive Enzymes (Proteases)

1. Trypsin

polypeptides to smaller peptide chains

2. Pancreatic amylase

starch to maltose

3. Lipase

fats (triglycerides) to fatty acids and glycerol

4. Nuclease

nucleic acids (DNA & RNA) to nucleotides

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The Pancreas & Maintenance of Blood Sugar Levels

In addition to being an important exocrine gland the pancreas is also considered to be an

endocrine gland because it produces hormones that travel throughout the body and affect particular “target organs” such as the liver.

Specialized cells in the pancreas called Islets of Langerhans secrete 2 important

hormones that help regulate blood glucose levels

1. Insulin

Lowers blood sugar levels by increasing a cell’s ability to uptake glucose

When blood sugar levels are HIGH, insulin promotes the storage of glucose in the form of

glycogen in the liver and muscle cells

2. Glucagon

Raises blood sugar levels by activating enzymes that hydrolyze glycogen stored in the liver and muscle cells to release glucose monomers

The glucose molecules are then released into the blood to raise blood sugar levels

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The Liver & Gallbladder

The liver is one of the body’s most important organs as it has many regulatory and

metabolic roles.

The liver sits in the upper right quadrant of the abdominal cavity underneath the ribcage

where it is protected from trauma

Important Functions of the Liver

1) Regulation of Blood Glucose

As we have already learned, the pancreas plays a vital role in the maintenance of blood glucose levels by secreting insulin and glucagon, 2 endocrine hormones that affect the liver

There are 3 main processes the liver uses to regulate blood sugar levels:

i) Glycogenesis

When there is excess glucose in the blood the pancreas releases insulin.

Insulin stimulates liver cells (hepatocytes) to combine glucose monomers into glycogen

molecules

The glycogen molecules are then stored in the liver until glucose is needed to raise blood

sugar levels again.

Gall Bladder

Large Lobe of the Liver

Esophagus

Small Lobe of the Liver

Duodenum

Stomach

Common Bile Duct

Ileum

Pancreas

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ii) Glycogenolysis

When blood glucose levels drop glucagon is secreted by the pancreas.

Glucagon stimulates liver enzymes to split molecules of glycogen into free glucose molecules.

Because glucose is a small molecule, it can readily diffuse from liver cells into the blood and raise blood sugar levels.

iii) Gluconeogenesis

When dietary sources and glucose reserves in the body are depleted and blood sugar levels

drop, the liver forms new glucose molecules from amino acids and glycerol in response to glucagon release from the pancreas.

This process occurs during periods of fasting, starvation, or intense exercise.

Gluconeogenesis protects the body, the nervous system in particular, from the damaging

effects of low blood sugar by ensuring that ATP synthesis can continue.

2) The liver destroys old blood cells

Red blood cells have an average life span of 4 months – millions die every day in our bodies.

The liver disassembles the RBC and recycles useful components, such as iron from

hemoglobin.

3) The liver produces bile salts that emulsify particles of dietary fat

The digestive function of the liver is to produce 500 – 1000 mL of bile per day for export to the duodenum of the small intestine

When protein-rich and/or fat-rich chyme arrives in the duodenum, the gall bladder secretes bile into the duodenum via the bile duct

Bile mechanically digests fat molecules by emulsifying them into microscopic

particles that have a greater surface area

Excess bile produced by the liver is sent to the

gallbladder where it is concentrated and stored

Homeostatic Imbalance

When red blood cells are broken down or recycled hemoglobin, the oxygen carrying protein found in red blood cells, is broken down and bilirubin is produced. In a healthy individual the liver removes bilirubin from the blood and excretes it as a component of bile. If the liver is not

functioning properly (there are many possible reasons) bilirubin will build up in the tissues. The build-up of bilirubin results in a condition called jaundice. Because bilirubin is a bright yellow-

green color, the skin and whites of the eyes appear yellow.

Did You Know?

Bile contains a pigment called bilirubin that gives it a yellow-green color. When bilirubin is

broken down it turns brown. This is what gives our feces a brown color.

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4) The liver makes non-essential amino acids and blood proteins

The liver uses amino acid monomers to build important blood proteins such as albumin and

fibrinogen

5) The liver helps regulate the level of cholesterol in the blood

When cholesterol levels are high the liver will convert cholesterol into bile salts

6) The liver detoxifies the blood

Unlike foods, alcohol needs no digestion and is quickly absorbed.

About 20% is absorbed directly across the walls of an empty stomach and the rest is rapidly

absorbed in the duodenum

Alcohol-laden blood travels to the liver via the veins and capillaries of the digestive tract,

which affects nearly every liver cell.

Liver cells produce alcohol dehydrogenase, an enzyme that oxidizes alcohol and

breaks it down into fatty acids

The Large Intestine (Colon)

The large intestine is shorter than small

intestine, but is almost 2.5 times larger in diameter.

By the time chyme has reached the end of the small intestine, most nutrients

have been absorbed into the bloodstream.

Indigestible materials such as water and cellulose, are moved into the

large intestine through the ileo-cecal valve and into the cecum

Homeostatic Imbalance

As alcohol is oxidized by the liver, fatty acids are released and deposited in liver tissue. Over

time these fatty deposits cause the liver to enlarge and scar tissue forms. Eventually, tough connective tissue will replace healthy liver cells where scar tissue was formed and liver function decreases. Many alcoholics have cirrhosis, and it can eventually lead to liver failure

and death.

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The colon is divided into 4 sections:

1) Ascending colon

2) Transverse colon

3) Descending colon

4) Sigmoid colon

The colon has 2 main roles in digestion:

1) Water is re-absorbed from food material

2) It is home to symbiotic bacteria such as E.coli that provide us with some

minerals, vitamins and essential amino acids

In the large intestine there is a tremendous number of goblet cells – cells that produce

mucus to protect the intestinal wall from irritating acids and gases released by the bacteria that live in the colon

The large intestine has a thriving flora of intestinal bacteria that help us with the

final stages of digestion

These bacteria ferment some of the indigestible carbohydrates such as cellulose and

produce about 500 mL of gas as a byproduct each day

One example of healthy bacteria found in the colon is E.coli. that metabolize some

macromolecules our body cannot and as a result they release minerals, vitamins (Vit K), and amino acids into the gut

Thanks to E.coli, these nutrients can be absorbed along with water into our circulatory

system

The Appendix

The appendix is a tiny worm-like projection that extends down from the cecum(large intestine) and is thought to be a vestigial organ – it has no known

function

Recent medical studies have shown there is some evidence that the appendix may play a

role in fighting infection.

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The Rectum

The sigmoid colon empties into the

rectum

The semi-solid product (¾ water and ¼

solid) delivered to the rectum via peristalsis as feces contains undigested food

residues, mucus, millions of bacteria and just enough water to allow its smooth passage out of the anus.

The rectum is usually empty, but when feces are forced into it, stretching of the

rectal wall initiates the defecation

reflex

As feces are forced into the anal

canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter

should remain open or be constricted to stop the passage of feces through the anus.

If defecation is delayed, the reflex

contractions end within a few seconds, and the rectal walls relax.

During defecation the muscles of the

rectum contract and the anal sphincter relaxes

Homeostatic Imbalance

Acute inflammation of the appendix, or appendicitis, results from a blockage (often by feces)

that traps infectious bacteria in its lumen. Because it is unable to empty its contents due to the blockage, the appendix swells, and squeezes off venous drainage. The lack of circulation that

results may lead to death and decay of the appendix. If the appendix ruptures, feces containing bacteria spray over the abdominal contents, causing inflammation of other organs. Loss of appetite, nausea, vomiting, and pain in the lower-right abdomen are common symptoms of

appendicitis.

Did you Know?

Some laxatives act as oily lubricants which literally grease up the lumen of the large intestine to allow feces to slide past.

Some laxatives such as Milk of Magnesia, act osmotically and prevent water from being absorbed by the large intestine. If people aren’t careful with the dosage, they can actually cause water to be deposited into the intestine from intestinal cells, and promote dehydration.

Some laxatives are irritants which cause inflammation in the intestinal wall. The irritation triggers peristalsis and feces are then eliminated from the body.

The safest and most effective laxative are the ones which contain natural sources of fiber such as

cellulose that simply increases the bulk of the feces and makes it easier to eliminate wastes.