Download Human Digestive System

Human Digestive System
Human Digestive System
 Consists of:
Alimentary canal: Long tube from the mouthanus + 3 glands:
1. Salivary glands.
2. Pancreas
3. Liver
The human digestive system is adapted to deal successfully with a mixed diet of plant
and animal material.
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Human Digestive System
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Human Digestive System
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Human Digestive System
Diagram of the Human Digestive System: ( fig 29.1 p289)
Passage of ‘food’ from the Mouth to the Anus:
 Consists of:
Alimentary canal: Long tube from the mouthanus + 3 glands:
1. Salivary glands (3 pairs).
2
Pancreas
3
Liver
The human digestive system is adapted to deal successfully with a mixed diet of plant
and animal material.
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Human Digestive System
Mouth cavity
Physical Digestion:
Teeth
Chemical Digestion
Enzyme: Amylase (found in saliva)
The food is physically broken into smaller
pieces.

3 pairs of salivary glands secrete saliva
which contains the enzyme amylase.
(ptyalin). Amylase is produced in the
salivary glands and travels through
tubes into the mouth for enzyme
action.

Amylase acts on starch to break it
down to maltose (Polysaccharide to
disaccharides).

Optimum conditions for amylase: pH 7
& 37˚C.

Amylase acts for a short amount of
time as it is inhibited by stomach acid
This speeds up the chemical digestion by
increasing the foods surface area.
Four types of teeth:
1.
Incisors- Cutting and biting; chisel
shaped; sharp edges
2.
Canines- Gripping& Tearing; long
pointed
3.
Premolars- Crush and grind;
projections called cusps
4.
Molars- Crush and Grind.
Crown- Above the gum.
Neck- Surrounded by the gum.
Other info on Saliva:
Root- Embeded into the jaw bone.

Location of salivary glands: under
tongue; back of jaws; in cheeks.

Saliva consists of: Water+Salts+
Mucous+Amylase+Lysozyme. (See
page 291 for more info on saliva).

Saliva helps soften & dissolve the food
making it easier to taste, chew &
swallow.

Lysozyme: destroy microbes (osmosis)
also in sweat & tears
See Fig 29.3p 290
Human dentitionOmnivorous- Adapted for the physical digestion
of plant material and meat.
Dental Formula of an Adult Human: 2(I2/2;
C1/1; PM2/2;M3/3)
The Dental formula indicates the number and
relative positions of the different types of teeth
on one side of the upper and corresponding
lower jaw.
Adult Humans have 32 teeth- full set.
In mouth, food formed into bolus & with aid of tongue, pushed backwards into the pharynx.
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Human Digestive System
 Pharynx:
During swallowing, a flap called the epiglottis closes over the trachea. This closes the
opening to the trachea so that the food or drink only enters the oesophagus but does not enter
the trachea. From pharynx, food enters oesophagus
 Oesophagus:
The Oesophagus carries the bolus into the stomach by peristalsis.
Peristalsis:
 Is an involuntary wave of muscular contraction which moves food through the
alimentary canal See fig29.4
 Is assisted by mucous produced in the lining of the oesophagus and mucous from
saliva
 In the stomach, peristalsis helps mechanical digestion(& mixes food with stomach
secretions, & then forces food to small intestine)
 In the small intestine, peristalsis forces food backwards & forwards to aid absorption
of food
 Continues throughout the length of the alimentary canal & every 30 mins strong
waves of peristalsis force waste into rectum
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Human Digestive System
 Stomach

Muscular bag which temporarily stores & digests food. It can hold up to 1L of food for up to 4 hrs

Warms food to 37C

Food enters stomach from oesophagus by the action of the cardiac sphincter muscle at top of stomach
(Sphincter Muscle: A Circular Muscle which opens & closes).
Chemical Digestion:

Stomach Lining (Mucosa): heavily
folded forming millions of gastric
glands.

Gastric glands secrete gastric juice into
the lumen of the stomach- pH 1-2.

Gastric Juice consists of:
Physical Digestion:
 Contraction of stomach walls
physically churn and digest food,
mixing it with the gastric juice
turning it into a thick soupy mixture
called chyme;
1. Mucous (coats stomach & prevents
self-digestion)
2. Pepsinogen (inactive enzyme to
prevent digestion of the cells which
produce it in in the stomach lining.
Pepsinogen is converted to active pepsin
by HCl in the stomach. Pepsin (a
protease) acts on proteins converting
them to peptides
3. HCl (give stomach pH of 1-2; kills
bacteria; denatures salivary amylase; activates
pepsinogen; loosens fibrous & cellular foods)
 The Stomach is protected from self-digestion from gastric juice (especially pepsin & HCl) by:
o Producing alkaline mucous which lines the stomach; this reduced the acidity
near the stomach wall
o Inactive pepsinogen: pepsin is released in the inactive form therefore does not
come into contact with the cells of stomach wall as it enters stomach
cavity/lumen
o Stomach lining cells can be replaced v quickly & are tightly packed
 Chyme leaves the stomach in small amounts when the pyloric sphincter opens briefly and
enters the small intestine.
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Human Digestive System
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Human Digestive System
 Small intestine- duodenum, jejenum and ileum.

5-6 m in length; 3cm diameter

pH 7-9

food remains here for 1-6 hrs
1 Duodenum: See Fig 29.6 p 293

(25cm)

duodenum lining secretes many digestive enzymes, receives bile from the liver and pancreatic juice
from the pancreas. This is the area where most digestion takes place
2 Jejunum & Ileum:

The remaining 5.5m

Food entering is almost fully digested. Function: Absorption of digested foods into the body
 End Products of digestion: Carbohydrates →
Proteins→
Lipids→
3 Villi in the small intestine: Draw Fig 29.8 p 294

Inner lining of small intestine contains many infoldings called villi and each villus has 600
micro villi

Villi increase surface area for digestion (duodenum) and absorption (Jejunum & ileum)

Villi walls are only one cell thick & there is a rich blood supply inside each villus. The
capillaries in each villus absorb water and soluble nutrients eg glucose , amino acids, vitamins
and minerals and carry these nutrients to the hepatic portal vein which takes them to the liver.
The liver acts as a warehouse, storing some nutrients & releasing others. Amino acids can not
be stored in the body (toxic) and those that are not used by the body are broken down in the
liver-deamination; urea and other waste leaves the liver → kidney → forms part of urine

Intestinal glands located between the villi & produce a range of enzymes called intestinal
juice
4 Lacteal: Draw Fig 29.12 p 295

inside each villus is a lacteal which contains lymph

fatty acids & glycerol are absorbed into the cells of the villus lining where they re-form into fats.

These fats are coated with protein and pass into the lymph in the lacteals. The lymph transports these fats
and carries them to rejoin the bloodstream at the subclavian vein near base of neck (takes 18 hrs). In the
blood, the protein coat is dissolved & fats are absorbed into cells
5 Adaptations of small intestine:

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Very long; Many villi & microvilli; Villi walls are very thin; rich blood supply to transport water soluble products of
digestion; lymphy supply in each villus (lacteal) to transport fats
Human Digestive System
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Human Digestive System
Glands Associated with the small intestine:
1. The Pancreas:draw fig 29.6 p 293
 Exocrine gland-Produces enzymes pancreatic amylase & pancreatic lipase which act
on food in the duodenum.
Pancreatic Amylase acts on starch to convert it to maltose.
Pancreatic Lipase acts on lipids to convert it to fatty acids + glycerol.
Both enzymes are produced in the pancreas, enter duodenum through pancreatic duct
and act on their substrates in the duodenum. Optimum pH for both enzymes in 7-8this is the pH of duodenum; optimum temp is 37C.
Pancreatic Juice: digestive material containing sodium hydrogen carbonate (to
neutralise chyme) + enzymes
 Endocrine gland- contains cells (million) called Islets of Langerhans which produce
the hormone Insulin, which is carried away by the blood. Insulin reduces blood
glucose levels & causes cells (muscle & fat) to absorb glucose from the blood. This
absorbed glucose is either used in respiration or converted to glycogen.
Low insulin production/inability of cells to take up insulin=diabetes.
Symptoms of diabetes: high glucose conc in blood & urine, severe thirst & large
amounts of urine, weight loss, tiredness.
Control of diabetes: (severe) insulin injections* 1-4 times per day, control
carbohydrate intake, increase physical activity & obtain/maintain normal body
weight. *Injections rather than ingestion because insulin is a protein & would be
broken down by digestive enzymes.
If diabetes develops in young people, the usual cause is failure of islets of
langerhans.
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Human Digestive System
2. The Liver: draw fig 29.6 p 293

Detoxify body- eg breakdown of toxins such as alcohol

Breakdown of excess amino acids forming urea (deamination)

Converts glucose to glycogen-storage & converts excess carbohydrates to fat

Storage of vitamins eg vitamin D & storage of minerals eg Fe Cu & Zn

Making plasma proteins eg fibrinogen used in blood clotting

Makes cholesterol-needed to form hormones

Produces heat to warm the body

Makes Bile: yellow-green viscous liquid formed partly from remains of dead red blood cells;
stored in the gall bladder; Bile enters duodenum through bile duct, gallstones can form in bile duct
& prevent its release; consists of water, bile salts & bile pigments (no enzymes);
Functions of bile: emulsifies lipids (physical digestion)-increasing surface area for lipase; contains
NaHCO3 –alkaline salt to neutralise chyme; excretes biliverdin & bilirubin from dead blood cells
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Human Digestive System

6
 Large Intestine:

1.5 m in length

6cm diameter

food remains here between 10 hrs and a few days.
1. Caecum: Draw Fig 29.14 p 296
2.


Vestigial organs- lost their former use/function

Function in humans unknown

Herbivores eg rabbits-large appendix-contains bacteria to break down cellulose; evolution-our
ancestors may have needed them for same function but we now obtain carbohydrates supply
from more easily digestible carbohydrates like starch
Colon: Draw Fig 29.15

Function: reabsorb water

Liquid waste which enters large intestine is converted into semi-solid waste called faeces (colour
of faeces is due to bile)

Faeces stored in rectum & egested through anus

Diorrhea: unabsorbed material moves too quickly through colon; less water reabsorbed & faeces
contain more liquid

Constipation: unabsorbed material moves too slowly through colon; too much water reabsorbed.
Eat more fibre-stimulates peristalsis
Symbiotic Bacteria in the digestive system:

Bacteria in colon feed on waste & produce vitamin B & K which are absorbed from the colon
(symbiotic relationship)

Bacteria in digestive system break down food especially cellulose. Some of these digested
nutrients are absorbed into the body from the intestines (symbiotic relationship)

Beneficial bacteria prevent growth of pathogenic bacteria & fungi.
 Summary of the digestive process:
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Human Digestive System
See table p 294 (next page for overall summary)
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Human Digestive System
 Major functions of the digestive system:
1. Digestion: breaking down of complex food into their soluble absorbable subunits.
2. Absorption: the passage of the products of digestion into the blood or lymph
3. Movement of food: controlled by sphincter, longitudinal and circular muscles in
the walls of the alimentary canal.
 The four stages of human nutrition
1. Ingestion: placing food into the alimentary canal at the mouth.
2. Digestion: breakdown of complex food into their simple soluble
absorbable subunits, by physical/chemical means (see nxt page). Essential
for absorption.
3. Absorption: the passage of the final products of digestion across the
membranes of the alimentary canal into the blood or lymph.
4. Egestion: expulsion of the undigested and unabsorbed material from the
alimentary canal through the anus.
(Assimilation: conversion of the absorbed nutrients into complex molecules
for growth, repair and defence.)
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Human Digestive System
Physical (Mechanical) digestion:

The food is physically broken into smaller pieces.

This speeds up the chemical digestion by increasing the foods surface
area.

The teeth, muscles of the stomach wall churning and peristalsis are
involved in physical digestion.
Chemical digestion:

Specific enzymes act on the large complex food molecules into new
simpler molecules.

Note- for chemical digestion you must study 3 enzymes an amylase, a
protease and a lipase.

For the selected enzymes make sure you know the production site, the pH
at a named location of its action and the products.
Enzyme:
Substrate
Made
in
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Active in
Optimum
pH
Product(s)