Download Human Digestive System

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 semiliquid 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 - digests proteins into peptides.
typsinogen)
(Secreted as
iii. lipase - digest fat into fatty acids and glycerol
iv. sodium bicarbonate - 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
environment.
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.