Human Digestive System

Digestion

• Defn: the mechanical and chemical breakdown of foods and the absorption of the resulting nutrients by cells • Two Types: – Mechanical: changes size not composition – Chemical: breaks food into simpler chemicals

Alimentary Canal ~8m tube from mouth to anus

• General Characteristics – Structure of wall 4 layers • Mucosa: mucous membrane on surface of epithelium (connective tissue) and smooth muscle —may be folded in lumen (increases surface area) • Submucosa: consists of loose connective tissue, glands, blood vessels, lymphatic vessels, and nerves organized into network called plexus • Muscular layer: produces movements of the tube, consists of two coats of smooth muscle tussue and some nerves organized into a plexus. Fibers of inner coat encircle the tube, circular fivers contract causing diameter to decrease —longitudinal fibers contract, causing the tube to shorten • Serosa: the visceral peritoneum comprises the serous layer, or outer covering of the tube. Cells protect underlying tissues and secrete serous fluid that lubricates tubes outer surface

General Characteristics continued

– Movement of the tube • Motor functions of the alimentary canal either mixing movement or propelling movement – Mixing: occurs when smooth muscles in small segmentsof the tube contract rhythmically (stomach churning) – Propelling: include wavelike motion called

Peristalsis:

a ring of contraction appears in the wall of tube. At same time, muscular wall just ahead of ring relaxes---pushes contents ahead

Mouth

•

Cheeks and lips (labia

): Cheeks outer layer of skin, pads of subcutaneus fat, muscles associated with expression and chewing, and inner linings of moist, stratified squamous epithelium. Lips mobile structures that surround mouth —skeletal muscles with useful receptors for judging temperature and texture of food.

•

Tongue

: nearly fills mouth cavity. Covered in mucous membrane, held to floor of mouth with membranous fold called frenulum. Mostly skeletal muscle, covered with papillae —provides friction and houses taste buds. Root of tongue anchored to hyoid bone and is covered by lingual tonsils.

Mouth continued

•

Palate

: forms the roof of oral cavity and consists of hard anterior part and soft posterior part. Soft palate forms muscular arch that extends posteriorly and downward as cone shaped uvula. Back of mouth on either side of tongue are masses of lymphatic tissue called palatine tonsils. Above border of soft palate are pharyngeal tonsils (adenoids) • Teeth

Mouth

•

Functions of Saliva

1. It contains an enzyme (salivary amylase) that breaks down starch to maltose.

2. It moistens the food and forms a food ball 3. It lubricates the esophagus so swallowing is made easier

Pharynx and Esophagus

Swallowing • The top of the esophagus is under voluntary control. eg. swallowing a pill is a conscious effort • rest of the esophagus is under involuntary control, - swallowing is a a reflex action -Peristalsis is the involuntary contraction of the muscles which cause the food to move along the entire digestive tract. Swallowing is at this point an involuntary muscle contraction.

• The stomach and esophagus are separated by a ring of muscle called the cardiac sphincter. This prevents the contents of the stomach from entering the esophagus (reflux)

Stomach

holding and churning • Gastric glands are found on the surface of the stomach, they produce gastric juice -Gastric juice contains two substances, HCl (hydrochloric acid) and Pepsinogen

i. Functions of HCl in the stomach

- kills bacteria and other living cells in food.

- causes pepsinogen to be converted its active form pepsin.

- denatures proteins which make it easier for enzymes to act on them - causes the stomach to have a low pH which is optimum pH for pepsin to function

ii. Functions of Pepsinogen

(inactive pepsin plus HCl yields Pepsin.) -Pepsin causes proteins to be broken down into peptides.

- After eating a meal high in protein, the hormone Gastrin is produced by the stomach - the secretion of Gastrin causes increased secretion of pepsinogen.

Stomach

• The churning action of the stomach, the action of HCl, mucus, and pepsin eventually converts the food into a semi liquid mass called acid chyme. The acid chyme gradually empties into the small Intestine through a valve called the pyloric sphincter.

Small Intestines

• Complete digestion and absorption of food.

-Upper part (Duodenum) is most active in digestion and the lower parts are concerned with absorption.

•

Intestinal juice

(produced by digestive glands in the small intestine) contains: 1

. peptidases

- which digest peptides into amino acids (finish of protein digestion) 2

. maltase

- which digest maltose into glucose (finish of carbohydrate digestion)

Absorption

• The features of the small intestine that increase the surface area are: a. it is a long, convoluted organ.

b. the lining of the intestine is thrown into numerous ridges and folds c. small fingerlike projections called villi cover the inner surface of the intestine.

• The features of the small intestine that facilitate absorption of nutrients are: – The villi are richly supplied with blood capillaries and lacteals. The capillaries absorb the monosaccharides, amino acids, and dipeptides into the blood system. The lacteals absorb fats d. To facilitate the absorption of of nutrients the folds and villi are covered with tiny projections called microvilli, which further increase the surface area.

Pancreas

• produces pancreatic juice which contains 2 things important to digestion: - enzymes which act on every group of food and sodium bicarbonate.

• These enzymes are:

i. pancreatic amylase

- digests starch into maltose

ii. trypsin

typsinogen) - digests proteins into peptides.

iii. lipase

- digest fat into fatty acids and glycerol (Secreted as

iv. sodium bicarbonate

environment.

- neutralizes stomach acids and changes the pH from acid to alkaline. This change in pH makes the enzyme from the stomach (pepsin) inactive and allows the pancreatic enzymes which work at a basic pH to function in their optimum

Large Intestine

• - absorption of water -water is reabsorbed from the fecal material as it passes through the large intestine. The waste is then eliminated through the anus (anal sphincter).

-E. coli bacteria living here produce vitamin K (which is involved in blood clotting), amino acids and other growth factors which is absorbed and used in the body.

6 functions of the liver:

• • • 1. destroys red blood cells and converts hemoglobin to the breakdown products in bile (bilirubin and biliverdin) 2. produces bile that is stored in the gallbladder. When bile enters the small intestine it emulsifies fats. (Breaks fat into drops increasing surface area so digestive enzymes can digest them more efficiently.) 3. stores glucose as glycogen after eating and breaks down glycogen to glucose between eating to maintain the glucose concentration of the blood constant 4. produces urea from the breakdown of amino acids 5. makes blood proteins (albumin, fibrinogen, immunoglobulins) 6. detoxifies the blood by removing poisonous substances and metabolizing them

gallbladder

• is a small organ that aids mainly in fat digestion and concentrates bile produced by the liver • the loss of the gallbladder is, in most cases, easily tolerated • The surgical removal of the gallbladder is called a cholecystectomy.