Hhs 1012 (7) - Digestive System (Part II)

7/30/2019 Hhs 1012 (7) - Digestive System (Part II)

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HHS 1012:

ANATOMY & PHYSIOLOGY 1(NURSING)

Lecture 7: The Digestive System


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THE SMALL INTESTINE

The small intestine plays theprimary role in the digestion and absorptionof nutrients.

The small intestine averages 6 7.5m in length and has a diameter ranging

from 4 cm (1.6 in.) at the stomach to about 2.5 cm (1 in.) at the junction

with the large intestine.

It occupies all abdominal regions except the right and left hypochondriac

and epigastric regions.

90% of nutrient absorption occurs in the small intestine, and most of the

rest occurs in the large intestine.

The small intestine has three subdivisions:

(1) the duodenum

(2) thejejunum

(3) the ileum.


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Duodenum

The duodenum, 25 cm (10 in.) in length, is the

section closest to the stomach.

This portion of the small intestine is a "mixing bowl"that receives chyme from the stomach and digestive

secretions from the pancreas and liver.

From its connection with the stomach, the

duodenum curves in a C that encloses the pancreas.


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Figure 12.27 The duodenum and its associated structures.


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Jejunum

A rather abrupt bend marks the boundary between

the duodenum and thejejunum.

At this junction, the small intestine reenters theperitoneal cavity, supported by a sheet of mesentery.

The jejunum is about 2.5 meters (8 ft) long.

The bulk of chemical digestion and nutrient

absorption occurs in the jejunum.


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Figure 12.28 The jejunum and ileum and their related structures.

Copyright Elsevier Ltd 2005. All rights reserved.


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Ileum

The ileum is the third and last segment of the small

intestine.

It is also the longest, averaging 3.5 meters (12 ft) inlength.

The ileum ends at a sphincter called the ileocecal

valve, which controls the flow of materials from the

ileum into the cecum (the 1st section of large

intestine).


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Figure 12.29 Section of a small piece of small intestine (opened out), showing the permanent

circular folds.


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Absorption of Nutrients

The surface are via which absorption takes

place in the intestinal is greatly increased by

the mucous membrane & by the very large

number of villi & microvilli.

Absorption of nutrients occurs by 2 possible

processes:

Diffusion

Active transport


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Diffusion

Monosaccharides, amino acids (AA), fatty acids

(FA) & glycerol diffuse slowly down their concgradients into the enterocytes from the intestinal

lumen.

Active transport Monosaccharides, AA, FA & Glycerol may be

actively transported into the villi (which is faster

than diffusion).

Disaccharides, dipeptides & tripeptides are also

actively transported into the enterocytes

transfer into the capillaries of the villi.


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Figure 12.30 A highly magnified view of one

complete villus in the small intestine.Figure 12.31 The absorption of nutrients.


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THE LARGE INTESTINE

The large intestine begins at the end of the ileum andends at the anus.

The large intestine lies inferior to the stomach andliverand almost completely frames the small

intestine.The major functions of the large intestine include

(1) the reabsorption of waterand compaction ofintestinal contents into feces,

(2) the absorption of important vitamins liberatedby bacterial action, and

(3) the storing of fecal material prior to defecation.


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The large intestine, or the large bowel , has anaverage length of about 1.5 meters (5 ft) and a

width of 7.5 cm (3 in.).

We can divide it into three parts:

(1) the cecum, the first portion of the largeintestine;

(2) the colon, the largest portion; and

(3) the rectum, the last 15 cm (6 in.) of thelarge intestine and the end of the digestivetract.


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16Figure 12.34 Interior of the caecum.


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Figure 12.35 Arrangement of muscle fibres in the colon, rectum and anus. Sections have been

removed to show the layers.


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We can subdivide the colon intofour regions:

i. ascending colon

ii. transverse coloniii. descending colon

iv. sigmoid colon


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Figure 12.33 The parts of the large intestine (colon) and their positions.


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Absorption in the Large Intestine

The reabsorption of water is an important function of thelarge intestine.

Although roughly 1500 ml of material enters the colon each

day, only about 200 ml of feces is ejected. The remarkable efficiency of digestion can best be

appreciated by considering the average composition of fecalwastes:

75 percent water 5 percent bacteria

a mixture of indigestible materials, small quantities ofinorganic matter, and the remains of epithelial cells.


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ACCESSORY DIGESTIVE

ORGANS


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HEPAR

Is the largest gland in the body.

Weighing between 1 2.3kg.

Situated in the upper part of the abdominal

cavity.

There are 4 lobes:

Right lobe (anterior ; the largest)

Left lobe (anterior) Caudate lobe (posterior)

Quadrate lobe (posterior)


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25Figure 12.37 The liver: anterior view.


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Figure 12.38 The liver, turned up to show the posterior surface.


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Blood supply of the hepar

The hepatic artery & the portal vein take

blood to the liver. Hepatic veins leave the posterior of the liver &

immediately enter the inferior vena cava.


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28Figure 12.40 Scheme of blood flow through the liver.


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Figure 12.39 A. A magnified transverse section

of a liver lobule. B. Direction of the flow of blood

and bile in a liver lobule.


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The main cells of the lobules are epithelial

cells known as the hepatocytes.

These are sometimes referred to as

parenchymal cells.

The hepatocytes in each lobule are arranged

in radial hepatic plates.

The blood flows between the hepatic plates in

large sinusoids and as a result all the

hepatocytes have a rich blood supply.


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The 4 main cell types of the sinusoids are :

Endothelial cells (Hepatocytes)

structural support

Kupffer cells

are fixed macrophages can detect and engulf bacteria in the

blood and lead to their breakdown.

Fat-storing cells (Ito cells)

are also known as lipocytes or Ito cells. have the ability to accumulate lipid droplets.

are the main source of vitamin A storage in the body and also play

a role in wound healing (hepatic fibrogenesis).

Pit cells (NK cells)

belong to the immune system.

are believed to be a form of Natural Killer cells (NK cells) detect

& destroy cancer cells!


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Hepar

The liver is responsible for metabolic regulation,

hematological regulation, and bile production.

Metabolic regulation

Carbohydrate metabolism Lipid metabolism

Amino acid metabolism

Removal of waste products

Vitamin & mineral storage

Drug inactivation (Phase I & II of Drug Metabolism)

Hematological regulation

Heat production


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Phagocytosis Plasma protein synthesis

Removal of circulating hormones

Removal of antibodies Removal or storage of toxins

Synthesis & secretion of bile salts


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36Figure 12.43 Summary of the source, distribution and use of glucose.

Glucose glycogen

Insulin

glucagon


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Bile salts production

About 500ml of bile salts are secreted by the

liver daily.

The bile acids are synthesized by hepatocytes

from cholesterol.

Bilirubin is one of the products of haemolysis

of RBC by Kupffer cells (& other macrophages

in the spleen & bone marrow).


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The original form of bilirubin is insoluble & iscarried in the blood by albumin.

In hepatocytes, the bilirubin is conjugatedwith glucoronic acid becomes water soluble excreted in bile.

Bacteria in the intestine change the form ofbilirubin & most is excreted as stercobilinogen(it gives the brownish color to the faeces) inthe faeces.

A small amount is reabsorbed & excreted inurine as urobilinogen.


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39Figure 12.41 Fate of bilirubin from breakdown of worn-out erythrocytes.


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Gallbladder

The Functions of BileMost dietary lipids are not water-soluble!!!

Mechanical processing in the stomach creates large drops containing avariety of lipids.

Pancreatic lipase is not lipid-soluble so the enzymes can interact withlipids only at the surface of a lipid drop!!!

The larger the droplet, the more lipids are inside, isolated andprotected from these enzymes.

Bile salts break the droplets apart, a process called emulsification.


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41Figure 12.42 Direction of the flow of bile from the liver to the duodenum.


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PANCREAS

The specific pancreatic enzymes involved include the following:

Carbohydrase , an enzyme that breaks down certain starches =is almost identical to salivary amylase.

Lipase breaks down certain complex lipids, releasing fattyacids and other products that can be easily absorbed.

Nucleases break down nucleic acids (RNA). Proteolytic enzymes break certain proteins apart.

Proteases (break apart large protein complexes)

Peptidases (break small peptide chains into individual aminoacids)


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Figure 12.36 The pancreas in relation to the duodenum and biliary tract; part of the anterior wall of

the duodenum has been removed.

Hhs 1012 (7) - Digestive System (Part II)

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