Anatomy and Physiology of Domestic Animals
A. Introduction of Anatomy and Physiology
The mammalian body is a complex unit containing tissues, organs, and systems, each performing
specific functions; however, these entities must also work collectively in order to maintain life. The
body never really sleeps; it is busy day and night constructing new cells, warding off illness,
processing nutrients, and orchestrating a host of biochemical actions, all of which contribute to the
health and well-being of the animal.
Since any structure is relatively meaningless unless we know how it works, structure and
function are often considered together. Anatomy, the study of structure, is necessary to understand
physiology, the study of function; and physiology is necessary to understand anatomy. Medicine,
the practical application of anatomy and physiology, is constantly evolving from a “trial and error”
approach of yesteryear toward practice founded on a rational understanding of cell and tissue
function.
A working knowledge of anatomy and physiology is essential for veterinary technicians in order
to communicate intelligently with clients as well as to understand and apply the jargon and
instructions of the veterinarian. This will reduce miscommunications between you, the veterinarian,
and the client which will provide and maintain a professional atmosphere.
1. Levels of Organization
a. Cells
b. Tissues
c. Organ
d. Organ System
e. Organism
2. Cytology (the study of cells)
a. The cell is the smallest unit of life. The mammalian body is composed of between
75 to 100 trillion cells.
b. There are many shapes and sizes of body cells. For example, red blood cells are round
whereas skin cells are flat.
3. Histology (the study of tissues)
a. When cells come together for a common goal, they become a tissue.
b. There are 4 major tissues of the body:
1. Epithelial tissue covers body and organ surfaces. Examples include:
a. the skin covers the outermost parts of the body
b. epithelial tissue lines the inner lining of the intestines
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2. Connective tissue serves to connect and support structures. It is the most
diverse and abundant tissue in the body. Examples include:
a. Adipose tissue stores fats.
b. Blood is the transport medium of the circulatory system.
c. Bone is a hard tissue of the skeleton is give support and protection.
d. Cartilage protects the ends of the bones at joints.
3. Muscular tissue generates movement. There are 3 types of muscle tissue:
a. Skeleton muscle attaches to the bones to move the arms and legs.
b. Smooth muscle makes up some internal organs and serve to move
materials throughout the body.
c. Cardiac muscle makes up the heart to pump blood around the body.
4. Nervous tissue generates messages to control all body activities.
a. The central nervous system is made up of the brain and spinal cord.
b. The peripheral nervous system are the nerves of the arms and legs.
4. When tissues come together, they form organs.
5. When organs come to work together, they form an organ system. There are 10 organ systems
(or physiological systems) of the mammalian body:
1. Cardiovascular system
2. Digestive system
3. Endocrine system
4. Integumentary system
5. Muscular system
6. Nervous system
7. Renal (urinary) system
8. Reproductive system
9. Respiratory system
10. Skeletal system
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B. Skeletal System
The study of bones that collectively makes up the skeletal framework of the body is called
osteology. The skeleton is constructed of two of the most supportive tissues found in the body-bone and cartilage. Mammalian skeletons are classified as endoskeletons; that is, it is a living
structure capable of growth, adaptation, and repair, and lies within soft tissues. This is in
comparison to exoskeletons which are external and requires the animal, such as the beetle and
crayfish, to shed its nonliving outer skeleton and grow a new one in order to increase its size.
In mammals, cartilage is a specialized, flexible, fibrous connective tissue usually attached to,
or in association with, the joint surfaces of bones. However, there are other areas in which cartilage
provides a structural framework (such as the trachea and bronchi) as well as furnishes the skeletal
base for the external ear. Cartilage is the substance from which most bones develop.
Joints are created where bones meet, or articulate, to permit body movements. When two
bones meet, they are connected by fibrous, elastic, and/or cartilageous tissue.
1. Bones have many functions of the body which include:
a. They give support of soft tissue and gives rigidity and form to the body.
b. The work with the muscles to provide movement and leverage for locomotion,
defense, and grasping.
c. They protect vital organs such as the brain and heart.
d. They provide a storage reservoir for minerals such as calcium and phosphorus.
e. Many bones produce blood cells (a process called hemopoiesis). [hēme-ō-pō-ē-sis]
2. The Axial Skeleton comprises the main supportive framework of the body.
a. The bones of the skull protect the brain.
b. The bones of the vertebral column protect the spinal cord.
c. The bones of the thorax (chest) protect the heart and lungs. This includes the ribs
and sternum (breast bone).
3. The Appendicular Skeleton comprises the bones of the arms and legs.
a. The pectoral girdle helps to attach the front legs to the body. This includes the shoulder
blades and clavicles.
b. The pectoral appendage are the bones of the front legs of the animal. This includes the
humerus, radius, ulna, carpus (wrist), metacarpus (hand), and phalanges [fil-an-gees]
(fingers).
c. The pelvic girdle helps toe attach the hind legs to the body. This includes the pelvis.
d. The pelvic appendage are the bones of the hind legs of the animal. This includes the
femur, patella (kneecap), tibia, fibula, tarsus (ankle), metatarsus, and phalanges (toes).
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4. Cartilage is the connective tissue located where two bones come together to form a joint.
This serves to protect and cushion the joint.
5. Types of joints
a. A synarthrosis [sin-ar-thrō-sis] joint is an immovable joint comprised of fibrous
material. Examples include the skull sutures, teeth sockets, and pelvic symphysis.
b. An amphiarthrosis [am-fē-ar-thrō-sis] joint is a partially movable joint comprised of
cartilage. Examples include the intervertebral discs of the spine and costal cartilage
(attaches the ribs to the sternum).
c. A diarthrosis [dī-ar-thrō-sis] joint is a freely movable joint with a capsule containing
fluid that acts as a lubricant. There are 4 main types of disarthrosis joints:
1. ball and socket (such as the shoulder and hip)
2. hinge (such as the elbow, knee, and phalanges)
3. pivot (such as turning the head in the “no” movement)
4. gliding (such as the bones of the wrist and ankle)
6. Bones are made up of cylindrical units called osteons. Bone is an active tissue that is
constantly remodels itself. There are 3 types of bone cells:
a. osteoblasts build new bone tissue
b. osteoclasts break down old bone tissue
c. osteocytes are mature bone cells that function to store calcium and phosphorus
7. A fracture is when there is enough stress put on a bone that it breaks. When this happens,
a cast is place to immobilize the ends of the fracture to give the bone time to mend, which
usually takes about 6 weeks.
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C. Muscular System
The beat of the heart, blink of an eye, and breath of fresh air are all brought about by muscle
contraction. The muscular system, through contraction and relaxation, brings about changes in
shape or form and aids in locomotion, heat production, respiration, and excretion. The muscles are
intimately associated with practically every organ system in the body including the skeleton for
movement and protection, cardiovascular for circulation of blood and nutrients, gastro-intestinal for
digestion of foodstuffs, and renal for elimination of waste products.
Muscles are either voluntary (conscious control) or involuntary (autonomic) in action and there
are three types in the body: skeletal muscle comprises the major muscle mass of the body and
gives the body its form and shape; smooth muscle is found in many internal organs (such as walls
of the intestinal tract, urinary bladder, and blood vessels) and is innervated by the autonomic
nervous system; and cardiac muscle is found only in the heart and is under autonomic control.
Muscle is a type of tissue composed of contractile cells or fibers which affect movement of an
organ or part of the body. Muscle cells, like neurons, can be excited chemically, electrically, or
mechanically to produce the actions of elongation and contraction. Muscles of the body vary a great
deal in shape and size; however, the primary function of all muscles is to shorten in order to move a
body part, provide resistance to a movement, or to propel material through a structure. The result of
muscle contraction depends upon its arrangement, such as in sheets, tubes, rings (e.g. sphincters),
etc. Since muscle fibers are associated with many, if not all, structures in the body, knowledge of the
muscular system is vital in understanding not only muscular disorders and injuries but also the
normal and abnormal functions of essentially all other organ systems of the body.
1. All muscles have the ability to contract or shorten in length to generate movement. The
muscles make up about 30 to 40% of the animal’s body weight.
2. Skeletal muscle
a. these muscles move the appendages for movement in the environment
b. these muscles are under voluntary control (conscious) of the brain
c. examples include the biceps brachii (bends the elbow) and the quadriceps (moves the
hind leg)
3. Smooth muscle
a. these muscles are part of internal organs that move material throughout the body
b. these muscles are under involuntary control (unconscious) of the brain
c. examples include the stomach, small and large intestines, and uterus
4. Cardiac muscle
a. this type of muscle is only found in the heart and functions to pump blood around
the body
b. the heart is under involuntary control
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5. Muscle cells are called fibers.
6. The working unit of a fiber is the sarcomere [sar-kō-mere]. This is where the muscle
shortens to create movement.
7. Muscle tissue can be very active and requires lots of blood from which it gets oxygen.
a. Without oxygen, a muscle cannot work.
b. Physical activity during athletic training increases the number of blood vessels to
the muscle which gets more oxygen to the fibers so they do not tire out as quickly.
8. A muscle cramp is a spontaneous contraction without relaxation.
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D. Nervous System
A living organism’s ability to survive in a constantly changing environment is directly related
to its ability to perceive external signals and respond to them in an appropriate manner. For
humans, stimuli perceived as warnings are fire alarms, flashing red lights at railroad crossings, or
a POISON label on a bottle. Other stimuli merely appraise us of a noteworthy change in the
environment, such as a cool breeze on a hot day, a favorite song on the radio, or a glimpse of an
attractive member of the opposite sex. Of course, all of these stimuli require some degree of
cognitive evaluation prior to responding.
The body must have very extensive and refined mechanisms for control and coordination of
extracellular activities.
The structure of the multicellular mammal provides two basic
mechanisms whereby integrating control messages might be transmitted to and from various
organs and tissues. One mechanism involves transport of messages through the extracellular fluid
via hormones (endocrine system) and the other involves the use of specialized transport structures,
such as nerves and sensory receptors (nervous system). Reflex activity of the nervous system and
hormones of the endocrine system are the basis of all steady-state control. Every part of the
animal’s body is under their influence. They coordinate and integrate the functions of cells,
tissues, and organs so they act harmoniously as a unit (body).
In early development, lower life forms possessed a fine network of individual nerve cells
without a brain or special coordinating center. As development proceeded, an integrative
mechanism (CNS) emerged to coordinate activities according to stimuli received by sensory
neurons. To appreciate the importance of the nervous system, consider the problems faced by
those who have just become blind or deaf, or the difficulties encountered by victims of brain stroke
or spinal injury.
1. The overall function of the nervous system is to receive and send messages to control the
activities of the body. These messages are called impulses.
2. The structural division of the nervous system is defined by the location of the nerves:
a. the Central Nervous System (CNS) is located along the midline of the body which
includes brain and spinal cord
b. the Peripheral [per-if-er-al] Nervous System includes the nerves that leave the
brain and spinal cord as extensions of the CNS
3. The functional nervous system is defined by the function of the nerves:
a. somatic [sō-mat-ik] nerves are under voluntary or conscious control which sends
messages to the skeletal muscles for movement
b. autonomic [auto-nom-ik] nerves are under involuntary or unconscious control which
sends messages to internal organs such as the heart, smooth muscle, and glands
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4. A nerve cell is called a neuron. There are 3 main areas of a neuron:
a. the cell body which contains the nucleus and other organelles
b. dendrites [den-drītes] are short branches off the cell body that receive messages
c. the axon is a long branch that sends messages to other cells
5. The 3 main sections of the brain include:
a. the cerebrum is the largest structure and functions in the highest types of mental
activity such as voluntary movement, sight, hearing, memory, and emotions
b. the cerebellum is responsible for balance and equilibrium
c. the brainstem is responsible for vital life processes such as heart rate, blood pressure,
and respirations (breathing)
6. Neurons do not touch each other but are separated by a synapse [sin-apse] which is a small
gap. Because of this, messages can only go in one direction.
7. In order for the message to “jump” across the synapse, a chemical called a
neurotransmitter is used. The neurotransmitter is released from the axon of one
neuron which travels across the synapse to attach to receptors on the dendrites of
another neuron which allows the message to continue.
8. The special senses of the body are part of the nervous system since it takes messages to
and from the brain to react to the environment. There are 5 senses that send messages to
the nervous system:
a. visual (sight)
b. auditory (hearing)
c. tactile (touch)
d. gustatory [goo-sta-torē] (taste)
e. olfactory (smell)
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E. Cardiopulmonary System
Beyond a few cell diameters, diffusion is not effective to meet the metabolic requirements of cells.
If multicellular organisms were to evolve to a size larger than a small cluster of cells, some
mechanism of transportation would be needed to properly nourish all cells. The cardiovascular
system (circulatory system) evolved for this purpose. It utilizes a pump (heart) and a series of tubes
(blood vessels) to reach practically every cell in the body. The respiratory system works closely with
the circulatory system. Oxygen is inhaled through the respiratory passages and eventually diffuse
into the bloodstream. The combination of the respiratory system and the circulatory system is called
the cardiopulmonary system.
The principal function of the cardiopulmonary system is the distribution of materials within the
body. Nutrients (such as water, proteins, carbohydrates, and vitamins) are absorbed from the
intestine into the blood vessels to be distributed throughout the body. Oxygen and carbon dioxide
diffuse into the blood vessels to be transported to and from every body cell. The circulatory system
also contributes to the body’s immune response. Specific cells within the blood vessels protect the
body from foreign invaders (such as bacteria and viruses). Through immunization (vaccination), a
vaccine deliberately provokes these cells to respond and produce an immunity against particular
invaders.
The transport medium of the cardiovascular system is blood. Blood is a mixture of red blood
cells, white blood cells, and platelets. The red cells carry oxygen and carbon dioxide while the white
cells are part of the immune system (fight infection). The platelets initiate blood clotting to control
hemorrhage upon injury. This mixture of cells is suspended in the plasma, the liquid portion of
blood. The relative amount of plasma can dictate the hydration state of the animal. Each type of cell
has a specific function and allows the veterinarian to diagnose ailments, such as bacterial and viral
infections, anemia, allergic reactions, and clotting disorders. A knowledge of the cardiopulmonary
system is necessary to understand many concepts of veterinary medicine, including distribution of
drugs and anesthetics, fluid therapy, proper nutrition, disease and vaccination programs, treatment
of shock and hemorrhage, heart disorders, and lung diseases.
1. The Cardiovascular Portion consists of the heart, blood vessels, and blood.
a. the heart is a 4-chambered organ located in the thorax (chest) that contracts to pump
blood around the body
1. the right heart receives blood from the body after the oxygen was removed (called
deoxygenated) and sends it to the lungs to put more oxygen in it (called
oxygenated)
a. the right atrium [ā-trē-um] receives the deoxygenated blood
b. the right ventricle [ven-trik-al] pumps the blood to the lungs for
oxygenation
2. the left heart receives blood from the lungs and send it to the rest of the body
a. the left atrium receives blood from the lungs
b. the left ventricle pumps blood to the rest of the body
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3. there are valves between all of these chambers to keep the blood going in
one direction
b. the blood vessels act as a conduit to pipe the blood to all body cells
1. arteries [ar-ter-ēs] take blood away from the heart
2. capillaries [cap-ill-ar-ēs] are one-cell layer thick to allow for nutrient exchange
with cells
3. veins take blood back to the heart
c. blood transports nutrients and waste products around the body
1. plasma is the fluid of the blood
2. erythrocytes [ē-rith-rō-sites] carry oxygen to body cells and help take carbon
dioxide back to the lungs for exhalation
*red blood cells
3. leukocytes [luke-ō-sites] function to help rid the body of foreign invaders such as
bacteria, viruses, and parasites
*white blood cells
4. thrombocytes [throm-bō-sites] function to clot the blood after injury to a blood
vessel
2. The Respiratory Portion consists of the lungs and airways.
a. the upper airway begins with the nostrils or nares [nar-ēz] which are the external
openings into the respiratory system
b. the nasal cavity warms and moistens the air
c. the larynx [lar-nix] is the voice box which makes the various sounds of the animal
d. the trachea is a rigid tube that transports the air in the throat area
e. the trachea branches into 2 bronchi [bron-kī] (singular = bronchus) that delivers
air to each lung (right and left)
f. each bronchus continues to branch into bronchioles [bron-kē-ols] which progressively
get smaller the deeper they get into the lungs
g. the alveoli [al-vē-ō-ī] are microscopic sacs where the exchange of oxygen and carbon
dioxide occurs
h. each alveoli is associated with a capillary which helps with the exchange of gases
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3. The heart has a conduction system which sends out the message for cardiac muscles to
beat together.
a. the pacemaker is a group of cells that sets the pace for the heart beat
1. the pacemaker sends an electrical message over the atria (plural for atrium) so that
all the atrial cells contract at the same type
b. another group of cells between the right atrium and ventricle picks up the signal and
sends it to the ventricular cells so that all of these cells contract as a unit
c. this system is set up so that when the atria contract, the ventricles are relaxed; then
when the ventricles contract, the atria relax
d. this electrical activity is what is seen on the ECG (electrocardiogram) tracing
*the ECG does not show the actual beating of the heart
4. The physiology of the lungs is rather simple and works due to pressure differences and
concentration differences.
a. in order to inhale, the pressure inside the lungs is lower than the atmospheric
pressure therefore air goes from high pressure to low pressure
b. as the lungs fill with air, the pressure reverses (i.e. inside the lungs now is higher than
outside the lungs) so air is then exhaled
c. to exchange oxygen and carbon dioxide, a concentration difference occurs where there is
more oxygen in the alveoli compared to the surrounding capillary therefore oxygen goes
from high concentration (alveoli) to low concentration (capillary)
d. the opposite occurs with carbon dioxide since the capillary has more carbon dioxide than
the alveoli therefore carbon dioxide moves from high concentration (capillary) to low
concentration (alveoli) where it can be exhaled and removed from the body
5. Pneumonia occurs when the lung tissue gets inflammation and the exchange of respiratory
gases cannot occur as readily which creates the difficult breathing in animals afflicted with
pneumonia.
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F. Digestive System
Animals need to obtain nutrients from food for maintenance of its body tissues, growth of new
tissues, and a source of energy. Deprived of food, the animal will literally digest itself in an attempt
to make nutrients available. Prolonged starvation eventually results in death.
The gastrointestinal system (digestive system) consists of a continuous muscular tube that
extends from the mouth to the anus. The animal consumes food products which are passed along the
tract and broken down into smaller particles that can be used by body cells. Various organs are
involved in producing enzymes and other chemicals which help with this digestive process. There
are 2 main types of gastrointestinal tracts in domestic animals: monogastric (nonruminant) and
ruminant. The monogastric digestive system is seen in the pig, horse, cat, dog, and pig whereas the
ruminant digestive system is seen in cattle, sheep, and goats.
Animals are heterotrophs; that is, they cannot produce their own food like autotrophs (plants).
Herbivores, such as cattle, horses, rabbits, sheep, and goats, eat plant material so their diet is very
fibrous. Carnivores, such as cats, are meat-eaters; their diet is primarily protein-based.
Omnivores, such as dogs, pigs and humans, consume both plants and meat. It is important to know
the diet required by each species in order to formulate proper and balanced rations. This is
especially critical for animals in the veterinary clinic. For whatever reason the animal is in the
clinic, their body requires extra nutrition in order to recuperate and recover.
1. The components of the digestive system are essentially the same in all domestic animals
but the length or size may vary.
2. The Oral Cavity
a. the lips are important for some animals, such as horses, to help grasp food
b. the tongue is a muscular organ used to move the food around so it can be chewed
c. the teeth function to grind or reduce the food into smaller particles
1. incisors
2. canines
3. premolars
4. molars
3. Salivary glands
a. salivary glands produce a fluid that contains enzymes to start the digestion process
b. the 3 main salivary glands are the parotid, sublingual, and mandibular
4. the pharynx is a common passageway for food and air
5. the epiglottis [epa-glot-tis] is a flap that covers the trachea when food is swallowed
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6. the esophagus [ē-sof-a-gus] is a smooth muscular tube that transfers food from the mouth
to the stomach
7. the stomach is an enlarged smooth muscular organ that continues the digestive process
*a sphincter [s-fink-ter] is a muscle that, when relaxed, it is closed; since the stomach
is a harsh environment, 2 sphincters control what enters and leaves the stomach
a. the cardiac sphincter lies between the esophagus and stomach
b. the pyloric sphincter lies between the stomach and small intestine
c. a variety of stomach cells produce products that enhance the digestive process
1. parietal cells secrete HCl (hydrochloric acid) that breaks down food
2. chief cells secrete a protein enzyme called pepsinogen
3. goblet cells secrete mucus to protect the inner lining of the stomach
8. the small intestine is where the majority of digestion and absorption of food takes place
and consists of 3 sections:
a. duodenum [do-odd-num] is the first section which acts to neutralize the acidic
contents as it leaves the stomach as well as accepts products from the liver and
pancreas
b. jejunum [ja-jū-num] is the middle and longest section where food continues to be
broken down
c. ileum [ill-ē-um] is the last section and mostly acts to absorbs nutrients
9. the large intestine is a combination of several digestive organs
a. the cecum is important to digest fibrous material in herbivores and omnivores
b. the colon fine tunes the contents by adjusting the water content
1. absorption of too much water leads to constipation
2.. not enough water absorption leads to diarrhea
c. the rectum temporarily holds the unusable material which is called feces
d. the anus is the external opening to allow the feces to exit the body
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10. the accessory organs for the digestive system are those that secrete products for digestion
but the food does not pass through them
a. the liver produces bile to aid in fat digestion and, if present, the gallbladder stores it
b. the pancreas produces many products
1. bicarbonate is secreted into the duodenum to neutralize the acidic material
leaving the stomach
2. enzymes are also secreted into the duodenum to begin digestion various nutrients
3. insulin is released into the bloodstream to help the body cells absorb glucose
to be used for energy
4. glucagon is released into the bloodstream when glucose levels are low because this
hormone promotes the breakdown of glycogen (storage form of glucose)
11. Ruminant animals, such as cattle, sheep, and goats, have a compartmentalized stomach.
a. the rumen is the largest compartment and functions as a fermentation vat which
means it houses millions of microorganisms that break down grasses and hays
b. the reticulum participates in the rumination process by allowing small mouthfuls of
food to return to the mouth to be rechewed (i.e. “chewing the cud”)
c. the omasum filters large particles coming out of the rumen and returns them to the
rumen for further digestion
d. the abomasum functions exactly like the stomach of a monogastric animal by secreting
acids and enzymes to continue the digestion process
12. Nutrients are chemicals necessary for the body to meet its metabolic needs. There are 6
basic nutrients:
a. carbohydrates are used for quick energy
b. lipids (fats) are used for concentrated energy
*there is 2.5 times more energy in lipids compared to carbohydrates
c. proteins are used to repair body tissues
d. vitamins are necessary to help breakdown carbohydrates, lipids, and proteins
e. minerals participate in many metabolic and structural functions
f. water moistens tissues, acts as a transport media, helps regulate body temperature,
and is involved in almost every chemical reaction in the body
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G. Renal/Urinary System
The urinary (renal) system represents a pathway to eliminate metabolic end-products and
unessential chemicals. It also provides a way to precisely control the constituents of the body under
normal and adverse or changing conditions, such as water deprivation, dehydration,
hyper/hypoglycemia, and hypernatremia. The process by which the urinary system accomplishes
this extraordinary feat is dynamic, and what is excreted as excess or a waste product one moment
may be retained the next. In mammals, birds, and reptiles this process is called osmoregulation; the
kidney is responsible for osmoregulation which controls the water level and concentration of various
chemicals (especially salts) in the body water.
Of the structures that make up the urinary tract, the kidneys are by far the most interesting
because they process vast amounts of blood and produce very little waste fluid (urine). A series of
exchanges occurs between blood, parts of the kidney, and the interstitial fluid to achieve balance and
preserve body water volume. Kidney function is due to the collective action of numerous microscopic
units called nephrons. The nephrons control the composition of extracellular fluid by processes
known as filtration and absorption. By filtration, large amounts of substances in the blood plasma
are forced through a specialized capillary (glomerulus) into a cuplike capsule. After this nonselective
process, most substances are selectively reabsorbed back into the blood-stream. Substances in
excess, waste products, and toxins remain in the nephron which eventually become part of the fluid
to be excreted from the kidney. From the kidneys, the fluid, now known as urine, is collected in the
urinary bladder before it is voided from the body.
1. Mammals have 2 kidneys which are usually bean-shaped.
a. each kidney contains up to one million nephrons which are the functional unit of the
urinary system
b. the nephron is responsible for filtering the blood to precisely control water and
electrolyte levels in the body
2. Once the blood has been filtered, the waste material, now called urine, leaves each kidney
via a ureter (2 ureters are present) to transport the urine to the urinary bladder.
3. The urinary bladder temporarily stores the urine until it can be voided from the body via
the urethra.
*the voiding of urine from the body is called micturition [mick-tor-ish-on]
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H. Reproductive System
The reproductive system is essential for the perpetuation of the species. “Survival of the fittest”
is the norm in the wild where the strongest males and females pass along their blood-lines to
enhance the thrivability of the species. However, with domestic animals man has assumed the role
of determining the matings of outstanding males and females in the pursuit of producing champion
progeny. In theory, these techniques are very useful; however, many problems have occurred due to
irresponsibility and profit incentives. In the commercial cattle industry, the desire to produce larger
calves at birth to increase market weight at slaughter created serious dystocia problems. The
American Kennel Club has warned breeders against producing pups of very closely related animals
as this inbreeding has caused many congenital defects. Matings of distantly related individuals
decreases the incidence of these problems.
The male reproductive system is made up of essentially four structures. The testes produce the
male gametes (sperm) and the hormone testosterone. Accessory ducts store these cells while they
mature into viable gametes. Accessory glands add various fluids to the sperm cells to aid in
transport and nourishment. The penis deposits this fluid (semen) into the female reproductive
tract. Maturation and proper functioning of the male reproductive system relies upon the influence
of several hormones, including testosterone, FSH, LH, and GnRH.
The female reproductive system consists of the ovaries, Fallopian tubes, uterus, vagina, and
mammary glands. Collectively, these structures produce gametes (ova), provides an area for uniting
sperm and ova (fertilization), serves as a site for the development of young, and produces food (milk)
for the offspring. Like the male system, a variety of hormones aids in the proper functioning and
timing of these events and include estrogen, LH, FSH, GnRH, progesterone, oxytocin, and prolactin.
During pregnancy, the fetuses are housed, protected, and nourished within the maternal body
(uterus). In domestic animals and man, the placenta is the area of exchange between mother and
fetus. Not only is there an exchange of necessary nutrients for growth and development of the
fetuses, but a variety of potentially harmful substances may also cross the placental barrier.
Examples include many drugs (prescription, over-the-counter, and illegal), anesthetics, alcohol,
nicotine, caffeine, and many toxins that may be present in feedstuffs (grain, hay). The effects of
these substances vary with amount and exposure frequency but may cause birth defects, fetal and
neonatal addictions, and fetal death.
1. The male reproductive system is responsible for producing sperm (male gametes or sex
cells) delivering these cells into the female reproductive system for fertilization. The
difference between sex cells and other body cells is that they only contain half of the
genetic material so that when it unites with an egg cell (which also only contains half of
the genetic material), the new cell has a full set of genes.
a. the testes or testicles (2) are responsible for spermatogenesis (production of sperm cells)
as well as the production of the male hormone testosterone
1. immature sperm cells are produced in the seminiferous tubules
2. testosterone is produced by Leydig cells
3. the epididymis is a extension of the seminiferous tubules that allow the immature
sperm cells to mature before they are released from the male’s body
*the epididymis remains within the scrotum
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4. the ductus deferens (vas deferens) is an extension of the epididymis after the tube
leaves the scrotum and houses mature sperm cells
b. various accessory glands produce products that help with the survival of the sperm cells
1. seminal vesicles produce an energy fluid for the sperm cells to use
*these glands are present in all domestic male animals except the dog and cat
2. the prostate gland produces an alkaline solution to buffer the sperm cells against
the slightly acidic structures of the female reproductive tract
3. the bulbourethral gland also secretes an alkaline solution which will also act
as a lubricant inside the female tract
*these glands are present in all domestic male animals except the dog
4. when the sperm cells and these fluids are mixed together, the solution is now
called semen
c. the ductus deferens joins with the urethra which serves to ejaculate the semen
d. the scrotum is the outer sac that houses the testes
1. the cremaster muscle is responsible for raising and lowering the testes to control
the temperature within the testes
*sperm cells need a slightly lower than body temperature environment
to survive
2. the spermatic cord consists of the testicular blood vessels, ductus deferens, and
the cremaster muscle
*the spermatic cord is the structure that is tied off (sutured) when the
veterinarian castrates the male animal
e. the penis is the structure that transfers the semen into the female reproductive tract
1. in the bull, ram, and boar, the penis has a sigmoid flexure configuration which is
S-shaped when not erect and straightens during erection to insert into the female
2. the male dog has a bulbis glandis which enlarges during copulation and is
responsible for “the tie”
*this structure physically binds the male and female together for a few minutes
after mating
f. the prepuce is the skin that covers the penis
g. the surgical removal of the testes is called castration
h. the surgical transection of the ductus deferens is called a vasectomy
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2. The female reproductive system is responsible for producing the female gametes (eggs or
ova), house the developing offspring, bear the young, and nourish the young until weaning.
NOTE: an egg (singular) is called an ovum (singular)
eggs (plural) are called ova (plural)
a. the ovaries (2) produce the eggs as well as the female hormone estrogen
1. animals that give birth to one offspring at a time and they are referred to be a
monotocous [mo-not-ō-kus] animal; examples include the horse, cow, sheep,
and goat
*as with humans, twins or triplets may occur
2. animals that give birth to numerous offspring at once (called a litter), are
referred to be a polytocous [po-lit-ō-kus] animal; examples include dogs, cats,
and pigs
b. the oviducts “catches” the egg (or eggs) released by the ovary and directs it (or them)
to the uterus
*this is where the sperm cell and the egg cell unite in the process called
fertilization
c. the uterus is a smooth muscular structure that will sustain the offspring until birth
1. the fertilized egg cell will implant in the inner lining of the uterus
2. at the time of parturition (birth), the uterus contracts to expel the offspring out
d. the cervix acts like a sphincter to protect the developing offspring from disease, etc
e. the vagina is a muscular tube that serves to receive the sperm from the male
f. the vulva is the external opening of the female reproductive tract
g. after implantation of the fertilized egg, a placenta develops around each embryo in
order to nourish and sustain the embryo
h. the connection between the mother and the placenta is the umbilical cord
i. as mammals, domestic animals produce milk to feed the offspring after birth
j. during the first few days after giving birth, the mother’s milk contains nutrients as
well as antibodies that the offspring can use to help protect against disease
*this milk is called colostrum [kō-loss-trum]
k. the surgical removal of the ovaries, oviducts, and uterus called an
ovariohysterectomy (commonly called a spay)
l. the surgical removal of fetuses out of the uterus is called a caesarean section
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I. Endocrine System
The complexity of the animal body requires precise organization and control between internal
structures as well as with the external environment. This homeostatic condition is controlled by two
systems that serve as a communication network that reaches practically every cell in the body. The
nervous system functions very rapidly and specifically to adjust activities of organs or tissues within
seconds. Examples are changes in heart rate, blood pressure, and respirations; these changes occur
very quickly but are also very short in duration. On the other hand, the endocrine system acts more
slowly to have profound and longer-lasting effects on tissue activity. Chemical messengers, called
hormones, travel via the blood from their production site (endocrine gland) to a target tissue. The
effects of hormones are generally longer in duration, lasting from 30 seconds to several days. Target
tissues possess receptors for specific hormones; any tissue may have receptors for several different
hormones therefore the endocrine system has a more diffuse influence on the body (compared to the
nervous system).
To exert their effects appropriately, hormones must be precisely controlled. Basically, there are
two methods to regulate hormone secretion and inhibition. Many hormones are secreted due to
releasing factors. When a particular effect is desired, a releasing factor stimulates the secretion of
the specific hormone that will cause the effect. When the effect is registered, the releasing factor is
no longer secreted which stops the secretion of the hormone. The reason for this method is that an
endocrine gland may secrete a variety of hormones so if the gland is stimulated directly to secrete its
products, all of its hormones will be released (not just the one desired). A second method is called
negative feedback. The level of the hormone itself dictates its release; that is, if the concentration of
the hormone exceeds a particular level, if inhibits more of the hormone from being released. Many
disorders and diseases of the body are caused by abnormal hormonal function, production, and/or
secretion. Supplemental hormone therapy often relieves the symptoms of the disorder and returns
the body to normal.
1. Pituitary gland
a. The pituitary gland is located at the base of the brain behind the bridge of the nose.
b. It is called the “master gland” because it produced many hormones that control and
dictate many other organs in the body.
c. The anterior pituitary produces:
1. Growth hormone (GH) – stimulates the growth of body cells by enhancing
anabolism (building up of tissues by protein synthesis)
2. Adrenocorticotrophic hormone (ACTH) – stimulates the adrenal cortex
(outside) to release its hormones
3. Thyroid-stimulating hormone (TH) – stimulates the thyroid gland to
release its hormones
4. Follicle-stimulating hormone (FSH) – stimulates follicular development
in the ovaries and spermatogenesis in the testes
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5. Luteinizing hormone (LH) – stimulates the development of the corpus luteum in
the ovaries and the secretion of testosterone in the testes
6. Prolactin – stimulates the mammary glands to produce milk
b. The posterior pituitary produces:
1. Antiduiretic hormone (ADH) – stimulates the kidneys to absorb water
2. Oxytocin – stimulates the mammary gland to release milk and the uterus to
contract during birth
2. Thyroid gland
a. The thyroid gland is located in front of the trachea and below the larynx.
b. Thyroid hormones include T3 and T4 which control the metabolism of the body.
c. Calcitonin acts to decrease blood calcium levels by inhibiting osteoclast activity.
3. Parathyroid glands (4)
a. The parathyroid glands are located in the corners of the thyroid gland.
b. Parathyroid hormone acts to increase blood calcium levels by enhancing the
activity of osteoclasts and increasing absorption of calcium by the kidney
4. Adrenal glands (2)
a. The adrenal glands are located just above the kidneys.
b. The adrenal cortex produces:
1. Cortisol and cortisone - enhance the metabolism of glucose stores
(usually during stressful situations)
2. Aldosterone – stimulates the kidney to absorb sodium
c. the adrenal medulla produces:
1. Epinephrine – responsible for the “fight or flight” syndrome
*The “fight or flight” syndrome prepares the animal to either fight a
threat or run away from it. This includes increasing the heart rate,
dilating the pupils for better sight, and increased breathing to get
oxygen to muscle to fight or run.
2. Norepinephrine – works with epinephrine to enhance the “fight of flight”
syndrome
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5. Pancreas
a. The pancreas is located along the fold of the duodenum.
b. Insulin promotes the uptake of glucose into body cells.
c. Glucagon elevates blood glucose levels when needed by other tissues.
6. Gastrin is produced by stomach cells that cause the release of HCl.
7. Secretin acts on the pancreas to release pancreatic enzymes.
8. Cholecystokinin [coal-a-cist-ō-kī-nin] causes contraction of the gallbladder.
9. Testes
a. The testes produces testosterone which promotes anabolism and maintains libido.
10. Ovaries
a. the ovaries produce estrogen which stimulates the development of ova and thickens
the uterine wall in preparation for pregnancy,
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