Download The Excretory System

RAT DISSECTION GUIDE
INTRODUCTION
Rats are often used for dissection because they are
readily available and they possess the typical
mammalian body plan. Most of what you learn on the
rat is applicable to the anatomy of other mammals, such
as humans.
External Features
Muscular System
The Mouth - The mouth is the most anterior part of the
digestive system. Within the mouth, the food is ground up by
chewing and mixed with saliva, which contains carbohydratesplitting enzymes and lubricating mucus within the Salivary
glands - There are three pairs of salivary glands. The other
glands are more ventral and extend anteriorly under the
lower jaw. The saliva, as previously mentioned, contains
enzymes, which begin the digestion of carbohydrates, and
mucus, which moistens food and sticks it together to
facilitate swallowing. The floor of the mouth and pharynx Locate the '''tongue'''. It plays a role in the swallowing
response. The food moves from the mouth into a chamber
shared by the respiratory system called the '''pharynx''' and
on into the ''esophagus'''. The esophagus can be seen under
the trachea (a tube recognized by its cartilage rings) in the neck region.
Circulatory System
The circulatory and respiratory systems are housed in the thoracic
cavity. The thoracic cavity are covered by various thin membranes
called pericardium. These lubricated surfaces allow the two sets of
organs to move vigorously without wearing each other away.
The Heart
The heart is basically two pumps: one circulates blood to the lungs for
oxygenation and the other circulates blood to the body cells. Each side of
the heart has an atrium (the anterior chamber that receives blood from a
vein) and a ventricle (a thicker walled chamber whose contractions drive
the blood into an artery).
Right side - The right side of the heart circulates blood to the lungs. The
''right atrium'' receives blood returning from the body via two large veins
called the ''superior and inferior vena cava''. When the right atrium
contracts, blood is forced into the ''right ventricle''. The contraction of this
chamber pushes the blood into the ''pulmonary artery'' and on to the lungs,
where it picks up oxygen.
Left side - The oxygen-rich blood is returned from the lung to the ''left
atrium'' of the heart through the ''pulmonary vein''. Contraction of the left
atrium moves the blood into the ''left ventricle''. The powerful contraction of
the muscles of the left ventricle drives the blood out of the heart through
the ''aorta'' to the rest of the body.
Valves - The effectiveness of these contractions is increased by the presence of valves that prevent the backflow of blood. Locate the ''semilunar valves''
between the aorta and the left ventricle. A similar set of valves is found at the junction of the pulmonary artery and the right ventricle. These valves prevent
blood from flowing back into the ventricles from the arteries. Between each atrium and its corresponding ventricle is a set of ''atrioventricular valves''. When
the ventricles contract, blood is forced against these valves, forcing them shut and preventing the flow of blood back into the atria.
Blood Vessels The circulatory system performs the essential duties of transporting oxygen and nutrients to metabolizing body tissue and carries off carbon
dioxide and other metabolic waste for eventual removal from the body. The rat has a closed circulatory system, which means that the blood remains within a
system of vessels through which it is pumped by the heart. A vessel that carries blood away from the heart to a capillary bed is an ''artery''. A ''vein'' carries
blood in the reverse direction, from the capillaries back to the heart. A ''portal vein'' carries blood from one capillary bed to another. Note that normally an
artery carries oxygenated blood, and a vein carried deoxygenated blood. However, the pulmonary artery and vein are backward in this regard because they
carry the blood between the heart and lungs.
Veins Vena cava - The vena cava returns blood from the body to the right atrium.
Pulmonary vein- This vein enters the left atrium with blood from the lungs.
Internal and external jugular veins - locate these veins in the neck region. These carry blood from the head region back toward the heart.
Arteries Aorta - Emerging from the left ventricle, near the midline of the heart, is the '''aortic arch'''. It bends around dorsally, giving rise to three branches
very near its origin.
Respiratory System
Food materials are broken down into simpler subunits by the digestive system, and these simpler substances
are delivered to the cells by the circulatory system. Within body cells, simple molecules may be broken down to
release energy. Some of this energy is used to form ATP, which can be used to supply energy for cellular
activities. The process of breaking down food molecules to form ATP is called cellular respiration. Part of this
energy transaction is to pass hydrogen ions along the electron transport chain. At the end of the chain, the
hydrogen is joined to its final acceptor, oxygen, forming water (H2O). In the absence of oxygen, the hydrogen
transfers are prevented and no ATP can be formed. This is the reason that oxygen must be delivered to the
cells. In many invertebrates, the size of the body is small enough for oxygen to reach the cells by diffusion. In
larger animals such as rats and humans, however, a respiratory system has evolved, creating an internal
surface area large enough to allow oxygen to diffuse into and be carried to the cells by the circulatory system.
The respiratory and circulatory systems must function together.
Trachea - To reach the lungs, air
travels through the nose or mouth to
the pharynx and then to the trachea.
Find the trachea in the neck region.
You will notice that it has C-shaped
rings of cartilage to prevent it from
collapsing as air rushes through it.
This initial portion of the trachea is the
larynx or voice box, within which lie
the vocal cords. The vocal cords are
folds of epithelium that vibrate,
producing sounds as air passes over
them.
Bronchi are the two tubes that branch off the
trachea.
Thyroid gland The small brownish
glandular mass found on either side of
the anterior end of the trachea.
Thymus gland. This is located directly above
the heart. The thymus gland is involved in the development of T cells in the immune system.
Lungs - Identify the lungs and note that there are four lobes on the right lung and only one on the left lung.
Within the lungs, the bronchioles carry the air to their endings, tiny air sacs. Inside, these air sacs are further
partitioned into chambers called alveoli. This greatly increases the surface area available for gas exchange. In
fact, our lungs have a greater surface area than our skin. The alveoli are only one cell layer thick and have
capillaries immediately outside of them. The gas exchange occurs across these moist surfaces by simple
diffusion.
Diaphragm - a muscular wall separating the thoracic and abdominal cavities.
The Abdominal Organs: Digestive System
1. The coelom is the body cavity within which the viscera (internal organs) are located. The cavity is
covered by a membrane called the peritoneum. Parietal Peritoneum - a very thin and shiny
membrane that lines the inside of the abdominal wall. Visceral Peritoneum - a very thin and shiny
membrane that covers the internal organs of the abdominal cavity.
2. The liver, which is a large, dark colored organ suspended just under the diaphragm. The liver has
many functions, one of which is to produce bile which aids in digesting fat. The liver also stores
glycogen and transforms wastes into less harmful substances. Rats do not have a gall bladder which
is used for storing bile in other animals.
3. The esophagus runs through the diaphragm and moves food from the mouth to the stomach. It is
distinguished from the trachea by its lack of cartilage rings.
4. The stomach on the right side (usually) just under the liver. The functions of the stomach include
food storage, physical breakdown of food, and the digestion of protein. The opening between the
esophagus and the stomach is called the cardiac sphincter.
5. The spleen is about the same color as the liver and is attached to the greater curvature of the
stomach. It is shaped like a banana and is associated with the circulatory system and functions in the
destruction and recycling of red blood cells. It stores white blood cells and platelets. A person can
live without a spleen, but they're more likely to get sick as it helps the immune system function and
keeps body fluids balanced.
6. The pancreas is not a clearly identifiable organ but a thin membrane that overlays the stomach
and spleen. The pancreas produces digestive enzymes that are sent to the intestine via small ducts
(the pancreatic duct). The pancreas also secretes insulin which is important in the regulation of
glucose metabolism. Find the pancreas by looking for a thin, membrane looking structure that has the
consistency of cottage cheese.
7. The small intestine is a slender coiled tube that receives partially digested food from the stomach
(via the pyloric sphincter). The term “small” refers to its diameter, not its length. It consists of three
sections: duodenum, ileum, and jejunum. The small intestine leads to the cecum.
8. The cecum is a pouch that connects the large and small intestines. Food is temporarily stored in
the cecum while helpful bacteria digest the cellulose found in plant cells. Most herbivores such as the
rat have a large cecum. Humans and other omnivores and carnivores have
a much smaller cecum which is referred to as the appendix.
10. The mesentery (connective tissue and network of blood vessels that
connects the small intestine) of the small intestine, but do not remove the
small intestine from its attachment to the stomach and rectum. If you are
careful you will be able to stretch it out and untangle it so that you can see
the relative lengths of the large and the small intestine.
11. The large intestine, which is the large greenish tube that extends
from the small intestine and leads to the anus. The large intestine is also known as the colon. This is
where the finals stages of digestion and water absorption occurs and it contains a variety of bacteria
to aid in digestion.
12. The rectum - the short, terminal section of the colon between the descending colon and the
anus. The rectum temporarily stores feces before they are expelled from the body.
The Excretory System
The excretory system functions in removing the nitrogenous waste products of cellular metabolism, as
well as in removing a number of other materials that may be present in the blood in excess of the
body’s needs. However, it conserves materials that are not in excess. In this way it plays a vital role
both in excretion and in maintenance of a fairly constant internal environment.
Kidneys - Find the pair of bean-shaped kidneys lying against the back muscles on the dorsal side of
the animal. These are often embedded in fat. The blood is brought into the kidney for processing and
taken out of the kidney via the vessels that can be seen near the medial indentation. Remove some of
the fat from around the kidneys and locate the adrenal glands. These are endocrine glands, not
involved in excretion and found just anterior to the kidney. The hormone they are most famous for is
adrenaline (epinephrine). This substance has a variety of effects on a body, preparing it for
emergency situations.
Ureter - The material removed from the blood by the kidney is gathered internally in collecting ducts
that empty into the larger ureter through which the urine leaves the kidney. Locate this duct as it
passes from the indentation on the medial side of each kidney close to the blood vessels. The path of
the ureter is toward the posterior end of the animal.
Urinary bladder- This sac stores the urine prior to its passage outside the body. The organ is usually
contracted in a preserved specimen and appears as a small pear-shaped muscular sac.
Urethra - This is the tube the urine flows through as it exits the body. It can be seen if the
mesenteries holding the bladder are dissected away and the bladder is displaced dorsally. In the
male, the urethra extends through the penis. It will carry sperm from the testes and secretions of
reproductive accessory glands as well as the urine. In the female, the urethra opens to the exterior
separately from the reproductive system.
The Reproductive System
The Male
Scrotum - You have already identified the scrotum, the sac housing the testes. In the nonreproductive season,
the testes may be withdrawn into the abdominal cavity.
Testes - Cut open the scrotum to reveal the testes. Slit one testis open and notice that it contains a mass of
tubules. These are the ''seminiferous tubules'', where the sperm are produced. In addition to sperm, the testis
produces male sex hormones.
Epididymis - The sperm pass from the seminiferous tubules into this highly coiled tubule. The epididymis covers
both ends and the lateral surface of the testis. It collects and stores sperm.
Vas deferens - This tubule carries the sperm from the epididymis to the urethra. It passes anteriorly and joins
the urethra very close to the spot where the latter leaves the urinary bladder.
Urethra - You have previously identified this tubule.
Accessory glands - The secretions of all the accessory glands form the seminal fluid that carries the sperm
during ejaculation, activates the sperm, provides some nutrients for them, and contains substances that help to
neutralize the acidity of the vagina. The products of the coagulating gland probably contribute to the vaginal
plug, which is formed in the female rat after copulation. There are several accessory glands, we will only
concern ourselves with "seminal vesicles" and the "prostate gland".
a. The vesicular glands and the coagulating glands comprise the "seminal vesicles". They are lumpy brown glands
located to the left and right of the urinary bladder shaped like wings.
b. The ''prostate gland'' is large and has lobes. Look for it where the urethra and vas deferens join. It usually
surrounds the bladder and it is partially wrapped around the penis. The seminal vesicles and the prostate gland secrete
materials that form the seminal fluid (semen).
Penis - You have already identified this structure. Its erectile tissue is composed of three cylindrical masses of
sponge-like vascular tissue.
The Female
Vagina- The short gray tube lying dorsal to the urinary bladder.
Uterus - Locate this structure in the region posterior to the kidneys. In the rat, the uterus is actually divided
into two complete uteri, which open separately into the vagina. The two uterus system is know as the unterine
horn. Open the uteri and examine any embryos you find.
Ovaries - These are located near the anterior end of the uterus. They are often embedded in fat, which must be
carefully dissected away. The ovaries produce the ova, which, if fertilized, will develop into embryos.
Oviducts - Each oviduct is a highly coiled tubule found on the surface of an ovary. The ova pass from the ovary
into the oviduct, which carries them to the uterus. The ovary is actually surrounded by the funnel-shaped
opening of the oviduct, so that the eggs are released directly into this tubule.
Rat Dissection Identification Sheet
You should be able to identify all of the following organs of the rat. In addition, you should be able to name the
organ system that each organ belongs to, as well as name at least one function for each.
Non-reproductive structures:
1. Salivary gland
2. Trachea
3. Esophagus
4. Thymus
5. Aorta
6. Heart
7. Lungs
8. Liver
9. Diaphragm
10. Mesentery
11. Stomach
12. Caecum
13. Small intestine
14. Kidney
15. Large intestine (colon)
16. Ureter
17. Pancreas
18. Urinary bladder
19. Spleen
Reproductive structures
Female
1. Ovary
2. Uterus
Male
1. Seminal vesicles
2. Vas deferens
3. Epididymis
4. Testes
5. Prostate