Anatomy and Physiology of the Dog and Cat: Organ Systems
For Veterinary Assistants

Prerequisite Knowledge

 Software Used: This activity was written using SoftChalk 4 No other software is require to complete this activity..

Learning Objectives

 After successful completion of this activity, you should have:

  1. Recognized the anatomical location of the important structures in the circulatory system of the dog and cat.
  2. Located and properly identified the specific veins used for injection in the dog and cat.
  3. Identified the main muscle groups and the specific muscles used for injections in the dog and cat.
  4. Recognized the location and function of organs involved in respiration in the dog and cat.
  5. Identified the location and described the function of organs contained in the abdominal cavity of the dog and cat
  6. Recognized the location and basic physiology of the genital/urinary system components of the dog and cat.
  7. Identified the location and described the function of the thyroid, parathyroid and adrenal glands in the dog and cat.
  8. Compared the location and function of the various components of the male reproductive system of the male dog and male cat.
  9. Recognized the location and function of the various components of the female reproductive tract in the dog and cat.


Introduction to Activity

 As a veterinary assistant you will be intimately involved in the care of animals both sick and healthy. One of your main functions as a veterinary assistant is restraint of an animal for specific procedure to be performed. For example, a procedure may be venipuncture (drawing blood from a vein). You will most likely be involved in collecting urine and stool specimens as well. In order to be a part of the veterinary team and to better take care of the animal, you should be familiar with the location and function of all the major organ systems.

For this activity, we will focus on the following systems:

For the most part, all of these organ systems are the same in the dog and cat. So you may only see pictures of the dog but you should assume the cat is the same and vice versa. We will NOT be covering the special senses such as the ears or eyes. I have provided many pictures to help you. As you make your way through these lessons, make sure to scroll over the pictures. There may be additional information contained in pop-up textboxes.



Circulatory System

 We will now focus on the major organ systems of the cat and dog. The first system we will review is the circulatory system. This will be a brief overview as learning the entire circulatory system is beyond the scope of this class. As a veterinary assistant you will be restraining animals for blood draws. As such you should know the names and locations of the major vessels used for blood draws.

The heart in located in the thoracic cavity and is protected by the ribcage. The heart in the dog and cat (and human for that matter) consists of 4 chambers: these are the right and left ventricle and the right and left atrium. Blood is constantly being brought to and pumped from the heart. The general route of circulation in the body goes something like this......Heart>>>Aorta>>>Artery>>>.Arterioles>>>Capillary Beds>>>Venules>>>Veins>>>Vena Cava>>>Heart

For this activity, focus on learning the following:

Circulatory System: Aorta


This picture shows the anatomy of the heart.

Circulatory System: Artery

So the pulmonary artery carries blood to the lungs to pick up oxygen. Then the pulmonary vein returns oxygen rich blood to the heart where it is pumped out through the aorta. This is often a confusing point for students. Here is another good picture of the heart (cardiac) anatomy.



Circulatory System: Arteriole

Circulatory System: Jugular Vien

This is a drawing to give you an idea of the location for the jugular vein. There is a jugular vein on either side of the neck.The trachea is a very important structure and you do NOT want to injure it in any way. So the technician must be aware of the trachea so as not to stick it with the needle when obtaining blood from the jugular vein. You must be aware of where the jugular vein and trachea are located so as not to put too much pressure on the trachea when restraining the animal.


Circulatory System: Cephalic Vein

Here is a great picture showing you the location of the cephalic vein. Note how the person restraining is holding the vein down with her thumb. This is actually making the vein more visible.So this vein is located on the DORSAL (top) surface of the antebrachium. The vein moves across the leg to the medial (inside) aspect of the carpus (wrist). The restrainer has to almost reposition the vein for the person that is drawing the blood. When you are restraining for a blood draw from this vien, you will use your thumb to "roll the vien". This will bring the vein into view for the phlebotomist (person drawing the blood).



Circulatory System: Lateral Saphenous Vein

Here are some good pictures of the lateral saphenous vein in a dog. I realize you need to get your bearings....the paw is at the lower part of the picture. Notice the hair has been shaved so you can see the vein clearly. Note also how the restrainer is pushing down on the vein to make it more visible This is called "holding off the vein".

The picture to the left shows the phlebotomists thumb stabilizing the vein as she obtains a blood sample. The lateral saphenous vein is very mobile and often the thumb is used so the vein doesn't move as much. Remember, a phlebotomist is a person that is trained in drawing blood.




Circulatory System: Medial Saphenous Vein

To the right is a picture of a dissected cat cadaver showing the location of the medical saphenous vien. The head is to the left and the bottom part of the leg to the lower right.

cat_med_saph OK picture showing vein location.jpg


Circulatory System: Femoral Artery and Vein

For now, you need to familiarize youself with the femoral artery. Do you know where the femur is located? Right....this is the large thigh bone, so the femoral artery is located along the medial aspect of the thigh. Depending on the size of the animal, it can be quite large or very small. The femoral vein is beside the artery and it is the femoral vein that turns into the saphenous vein which we have already discussed.

When you are taking the heart rate of an animal, the femoral artery is the vessel you feel with your fingers. This is like to taking your own pulse by feeling your heart rate at your wrist. Don't worry, it's a lot easier than it sounds!

Look at the cat model below. The femoral arteries and veins are shown at the inner thigh. See how these vessels branch off from the aorta? The aorta is like the main trunk line serving the body and then there are branches serving each organ and different parts of the body.

Remember this picture from the previous page? Well the femoral artery and vein (arteries are in red and veins in blue) are located just cranial or above the instrument.



vesselslegcat cadavershowingmedialsaphenousvein.jpg


Musculature System

We are now moving into the musculature system of the dog and cat. Obviously we do not have enough time to review all the muscles, nor do you need to know every muscle. As a veterinary assistant you will be restraining for the veterinary technician or veterinarian to administer injections into the muscle. As such it is important that you know the main muscles used for injections and any important underlying anatomical features.

Muscles in general are named using a variety of techniques. Usually a muscle is named by it's location, for instance, if it is near a bone, the name of the bone might be included in the name of the muscle. Commonly you will see numerical prefixes such as "bi" or "tri" to indicate how many muscles are included in a group. One of the things that you might find difficult to understand is that muscles move in only one direction. That is muscles can only contract. So there are groups of muscles contracting all the time to move the skeleton. That is the basis of movement.

For this class you need to know the following muscles.

 For each of these muscles, focus on their location, some of these muscles will be very large (especially in large well muscled dogs such as the Boxer) and some of these muscles may seem very thin to  you.

Musculature System: Biceps Femoris

This is a picture of the major muscle groups in a dog. Notice the biceps femoris muscle runs along the lateral part of the thigh.The Biceps femoris is number 9 in this picture


Musculature System: Quadriceps Muscles

This is a picture of an Injection being given into the quad muscles of a dog

The quadriceps are not really clearly defined in this picture but they are number 8 

Imquad.jpg DogMusclesInRed.gif

 Musculature System: Epaxial Muscles

   This is a picture of an IM injection being given into the epaxial muscles of a dog

im_lumbar.jpg lumbago02.jpg


Musculature System: Triceps Muscles

This is a picture of an IM (Intramuscular) injection being given into the triceps muscle of a dog.

You should have noticed by now that in each picture the person giving the injection is holding the animal with one hand and giving the injection with the other hand.

 Referring back to our picture of the dog, the triceps mucle group is number 5

imbiceps.jpg DogMusclesInRed.gif


 Quiz on identifying muscles



Musculature System: Semimbranosous and Semitendinosus

This is a picture of an IM injection being given into the Semitendinosus muscle of a dog. Notice how this injection is being administered at an angle. This is to avoid inserting the needle too deeply into the muscle and injuring the sciatic nerve.



Respiratory System

 For this activity, we will only cover the very basics of the respiratory system. The physiology of respiration is beyond the scope of this class. The respiratory system functions to deliver oxygen to the body and rid the body of carbon dioxide. You need to understand the basics as you will frequently see animals in some sort of respiratory distress and it is important for you to be able to recognize normal from an emergency. For this class we will not discuss the physiology of the process of gas exchange but limit our review of the respiratory system to the following:

Respiratory System: Larynx

When looking at this picture, ignore all terms, just focus on the vocal folds. These are the doors of the Larynx.


Respiratory System: Trachea

Trachea: The trachea is a tubular structure that carries oxygen (air) to the lungs. The trachea has rings of cartilage which give it a firm shape. Otherwise the trachea would "collapse" and you or the animal wouldn't be able to get air to the lungs. Feel your own throat area. These are the rings of the trachea. Sometimes in dogs, these tracheal rings aren't strong enough to hold the trachea open and the trachea "collapses" during inspiration and then snaps open loudly. These dogs often have a loud "honking" cough, sounds almost like a goose.

Dogs with a collapsing trachea are usually small, like terriers. Often just the act of pulling on the collar can cause enough problems with the trachea and the dog will cough. The picture to the right show the trachea extending fom the throat area to the lungs.

 Reprinted with permission by the copyright owner, Hill's Pet Nutrition, Inc

lateral picture of a dog.jpg

To the right is a picture of a dogs trachea showing the difference in the tracheal rings. Number 1 shows a normal tracheal ring and number 2 shows an abnormal ring. Scroll over the picture for more information.




Respiratory System: Lungs

Lungs: As we have discussed, the trachea is the organ or structure that delivers air to the lungs. The lungs are the actual organs of gas exchange (where oxygen is taken in and carbon dioxide is expelled) and are located in the thoracic cavity. The lungs surround the heart. In a live animal, the lungs are a soft spongy pink tissue. On x-rays, the lungs appear black as air is black on x-ray film. Please refer to the picture to the right for the location of the lungs. Note how the lungs surround the heart. The diaphragm is not shown in this picture, but is located between the lungs and the liver.

Reprinted with permission by the copyright owner, Hill's Pet Nutrition, Inc.

lateral picture of a dog.jpg

 VD lung xray.jpg

This is an X-ray of a dog lying on its back. Note how the heart is white and the lungs are black as they are filled with air.

The bones appear white as well. You can see the ribs in this view.



Abdominal Organs: Body Cavities


We have now made it through the circulatory and musculature systems. Next we are going to be discussing the contents of the abdominal cavity. The dog's body is divided into different cavities. The picture to the left shows you the different body cavities.  

The thoracic cavity contains the organs of respiration and the abdominal cavity houses many important organs belonging to different organ systems.such as organs of digestion, reproduction, urine production and the spleen. The abdominal cavity is separated from the thoracic cavity by the diaphragm which is a large thin muscle. As a veterinary assistant,  you need to have a basic knowledge of the location of these organs. Additionally, you should know the basic function of each organ.

 You should know the basic location and function of the following:

Abdominal Organs

Chart of Organ Location and Function

Study this table and then review the pictures on the following slides. NOTE: Provided is a review of the medical term for disease of each particular organ.


Organ Location Function Disease
Esophagus Left side of the trachea Carries food to the stomach Esophagitis
Stomach Mostly left side of abdomen, very cranial in abdominal cavity Stores food/starts digestion Gastritis
Liver Most cranial, near diaphragm extends
on both right and left sides of abdomen on both right and left sides of abdomen
Detoxifies blood, makes proteinsStores glycogen, many functions
Stores glycogen, many functions
Gallbladder Tucked in between lobes of the liver Storage of bile/secretion of bile for digestion of fats Cholangitis
Small Intestines Middle of the abdomen  Absorbs digested nutrients Enteritis
Kidneys Right and left side of mid abdomen, near spine Filters blood/produces urine Nephritis/pyelonephritis
Bladder Caudal abdomen, between colon and uterus in females Stores Urine Cysitis
Ureter  Extends from each kidney to bladder Carries urine from the kidneys to the bladder  
Urethra Extends from the bladder to the the vagina or through the penis Carries urine from the bladder to outside the body Urethritis
Spleen On left side of abdomen Stores red blood cells  
Pancreas Tucked closely to stomach and small intestines Secretes digestive enzymes such as amylase and lipase
Secretes hormones such a insulin and glucagon
Ovaries Sit on the cranial pole (border) of each kidney Produces hormones/produces eggs (ova)  
Uterus Ventral to the bladder in the caudal abdomen, has 2 horns Carries feti Metritis
Adrenal glands Sit very close to each kidney Secretes/produces various hormones  
Large Intestines/Colon Run through the abdomen, divided into 3 sections Removal of water from ingesta and production of feces Colitis


 This is a quiz on the abdominal organs


Abdominal Organs


Ventral view of Digestive Organs in the Dog and Cat

This picture does not have the kidneys, bladder, ovaries or uterus.

Abdominal Organs


This drawing illustrates the male feline anatomy

Enodcrine System

 For this class we are going to present a very brief overview the endocrine organs. The endocrine system refers to an intricate interplay of hormonal regulation occurring in the body. Hormones are responsible for controlling metabolism, digestion, reproduction, as well as other functions. As a veterinary assistant you will frequently see both cats and dogs with endocrine diseases. We will limit our discussion to the two most commonly encountered endocrine organs in the cat and dog.

Endocrine System: Adrenal Glands

kidneyandadrenalgland.jpg adrenal_dog.gif

You may see Addisons disease, it is fairly rare, but when you see it, you'll never forget it. This is one of those diseases that can actually be fatal very quickly. As such, dogs that are very sick are said to be in an "Addisonian Crisis".


Endocrine System: Throid and Parathyroid Glands 

The thyroid gland secretes thyroid hormone which controls our metabolism. Again the terms hypo and hyper and used to distinguish over production (hyperthyroid) and underproduction (hypothyroid) of the thyroid hormone. Think of the thyroid hormone as "driving the bus". If there is something wrong with the thyroid gland, the entire body is affected. We commonly encounter two very classic diseases in cats and dogs.

Cats have hyperthyroidism. These cats have an enlarged thyroid gland at the neck region. This is called a thyroid goiter. These cats are ravenous and drink a lot of water. These cats are often vocal, howling at night. Even though they are eating a tremendous amount of food, they lose weight, often times vomit or have diarrhea. If left untreated, hyperthyroidism may be fatal to cats.

Dogs on the other hand have hypothyroidism. So these dogs eat very little food yet still gain weight. Their metabolism just isn't burning any calories. Hypothyroidism may also affect a dogs behavior. The thyroid gland is also responsible for proper hair growth, so dogs with hypothyroidism will have very strange looking hair coats!


This is a nice picture showing the location of the thyroid gland in the cat.

Note: number 4 is enlarged to show the difference between a normal thyroid (number 1) gland and thyroid goiter.

The numbers 2 and 3 refer to the parathyroid gland which sits on top of the thyroid gland.



Ventral view of the thyroid glands in a cat



Lateral view of the thyroid gland location in a cat.


Endocrine System

Photographs of dogs with hypothyroidism

Here are a couple of pictures of dogs with hypothyroidism. Aa a veterinary assistant, you will see this disease frequently.


Dog with hypothyroidism, note complete hair loss



Dog with hypothryodism, note unkempt uneven hair coat and the typical "rat tail"

Genital/Reproductive System

The reproductive organs of the female cat and dog are contained in the abdomen and are fairly similar. Male dogs and cats have vastly different reproductive organs. They both have a penis and testicles,but the location of the testicles and the shape of the penis is very different between the two species. You should note that the correct scientific term for a female dog is the "bitch" and the correct term for the female cat is the "queen". While the term bitch may sound offensive to you, I assure you, this is the correct term to use when discussing a breeding female dog. A male dog is called a "dog" and a male cat is called a "tom". In fact, true dog breeders will be offended if you do not use the correct terminology of bitch when discussing anything related to a breeding female dog.

For this class, we will review the following structures:

Genital/Reproductive System: Ovaries and Uterus

 Here is a good drawing of the female reproductive tract.

Now you have to use a bit of imagination here.

Remember the ovaries are located at the caudal end of the kidneys. Note all the blood vessels that are associated with the ovaries and uterine horn.

When a veterinarian performs a spay (medical term is ovariohysterectomy), he/she must be very careful when handling these arteries and veins.


Drawing of female dog and cat reproductive tract

Reprinted with permission by the copyright owner, Hill's Pet Nutrition, Inc.


Genital/Reproductive System: Vulva



Female kitten 8 weeks old view of anus and vulva.


Genital/Reproductive System: Mammary Glands

This is a picture of a dog with swollen mammary glands. This dog is most likely near whelping a litter of puppies. The best mammary glands are the most caudal. Therefore, the first puppies born have the best chance of nursing from the most caudal mammary glands. The last puppy born often gets pushed to the "back of the line" and has to nurse from a more cranial mammary gland. Hence those puppies might not grow as quickly and be the "runts" of the litter.


lab with swollen mammary glands.jpg caninemammaryglands.gif



Genital/Reproductive System: Male Genitalia

  • Penis: the penis is a male reproductive organ and is very different in the dog and cat. In the dog, the penis is located along the belly. The penis is housed in a tube of tissue called the prepuce. The penis of the dog has an actual bone in it called the "os penis". This bone provides structure and rigidity to the penis for intercourse.

The cat penis is housed internally below the anus. The cats penis does not have a bone, but has instead many many sharp little spines called "barbs". These barbs function to stimulate the female cat to ovulate (release an egg) during intercourse. Once a male cat has been neutered (the testicles removed), the barbs disappear. Sometimes you can not find the testicles in a male cat. If you find barbs on the penis, you know that the cat has testicles. The drawing to the right clearly illustrates the male cats' reproductive anatomy

Both male cats and dogs should have 2 testicles. If you only see 1 testicle, the animal is called "cryptorchid", The prefix "crypt" refers to something being hidden and "orchid" refers to testicle. Very rarely you will see a cat or dog with both testicles hidden. The penis also contains the urethra which functions to carry urine from the bladder to the outside of the body.  


To the right is a picture of a person retracting the skin around a male cat's penis. The penis is the pink structure that seems to be projecting out of the picture. Note how the penis has to pushed out of the skin. You normally can't see a male cats' penis. The barbs are the small little white "dots" on the penis. Remember, if you ever wonder if a cat has been neutered, look at the penis. The barbs disappear after the cat has been neutered.



A male dogs' penis points towards the head of the body (cranially) while the tom cat's penis points towards the rear of the body (caudally). Note in the picture above how the penis of the tomcat seems to point towards the back of the body. Note how the testicles are located close to the body directly under the anus.

Now compare the male cat with the picture to the right of the male canine anatomy. Note how the testicles are suspended between the legs and much lower than a male cats testicles Also note how the penis is pointed towards the head of the dog. The pouch of skin that contains the testicles is called the scrotum.


Reprinted with permission by copyright owner, Hill's Pet Nutrition, Inc.

Genital/Reproductive System: Prostrate Gland

 Here's the same picture. Note the location of prostate gland. If this gland becomes enlarged it can impact the urinary system.

The urethra which is the tube that carries urine from the bladder out of the penis is located inside the penis.

dogurinary2 HillsColorAtlas.jpg  

Picture of dogs urinary and genital organs

Reprinted with permission by the copyright owner, Hill's Pet Nutrition, Inc.

Genital/Reproductive System: Testicles



Male kitten 8 weeks old


Note the anus is at the top of the picture (most dorsal), then 2 testicles and then the opening (prepuce) for the penis.

Note how the penis seems to come out of the body towards the read of the cat whereas the penis in the dog points towards the head of the dog


Remember the picture of the female kitten at 8 weeks of age? Here it is again.

See how easy it is to get confused between a male and female kitten.

Often times people think they are getting a male kitten and it's a female or vice versa!!



Here's the picture of the male dog's genital anatomy one more time. Note the location of the testicles in the male dog compared to the male cat.



Picture of dog's urinary and genital organs

Reprinted with permission by the copyright owner, Hill's Pet Nutrition, Inc.

Genital/Reproductive System: Anal Glands

The anal glands contain a very foul smelling substance. We believe these glands are used as marking or "scent glands". When dogs sniff each other to say hello, they are, in fact, sniffing these glands. As a veterinary assistant, you will deal with anal gland issues on a daily basis.

Anal glands may become enlarged or impacted causing irritation to the animal. Animals with anal gland problems may "scoot" their bottom (perianal area) across the floor in an attempt to relieve the pressure caused by these enlarged glands. If anal glands become too enlarged, they may actually abscess or form an infection which drains to the outside of the body. Cats with painful or enlarged anal glands may not use the litter box.





There are about 36 flash cards for you to work through.

Use these cards as frequently as needed to help you learn this material.






This concludes our whirlwind tour of the organ systems of the dog and cat. Note we did not cover the skin or the special senses such as the eyes or ears.

You should now be familiar with the major organs and organ systems. For each organ or organ system, you should know the location of the organ as well as its' function.

Here are the organ systems we reviewed:

  1. Circulatory
  2. Musculature
  3. Respiratory
  4. Abdominal Organs
  5. Genital/Urinary
  6. Endocrine

I realize this is a lot of information. Try to review a little bit of material every day. Start using the correct medical terminology in your everyday conversations. I hope you have found this activity fun and informative.

Just a few more inches.....