The systems of a horse

People who are directly involved with horses should know basic information about the species’ systems for at least two reasons: to call the systems and their parts by the most common names and to be able to identify possible health issues early.

Although all the systems that conform a horse work together and depend one from each other, to make it easier, here they are described individually. Each system is analyzed based on its parts, functions, and what can be perceived about it from the horse’s outside.

1. Except for the eyes, the INTEGUMENTARY system (or COVERING system) is what can be seen while looking at the horse. The Integumentary system is formed by:

• Hair (from the coat, mane, tail, and whiskers)
• Skin (epidermis, dermis, and hypodermis)
• Sweat glands
• Sebaceous glands
• Hooves (the end of the horse’s feet)

The functions of the Integumentary system are:

• Covering the body.
• Keeping the horse isolated and protected from the outside environment, for example, from pathogens (e.g., bacteria, virus).
• Helping with body temperature regulation.
• Synthetizing vitamin D when being exposed to the sunshine.

Most parts of the Integumentary system can be seen from the horse’s outside: The hair from the mane and tails, the whiskers around the muzzle, and the coat that covers most parts of the animal, except the muzzle, inner thigs, anus, vulva and udder (mare), and testicles and prepuce (stallion). Epidermis is the visible (external) layer of the skin. The hooves are the hardest structures of the Integumentary system that protect the horse’s feet, allowing the animal to run on a variety of ground surfaces.

2. The NERVOUS system is formed by:

• Brain
• Spinal cord
• Nerves (including those connected to the five specialized sensory organs: vision, hearing, smell, taste, and touch).

Although the parts of the Nervous system are not seen directly from the horse’s outside, most of their functions can be recognized while looking at the animal’s behavior, which may allow detecting some health problems. The functions are:

• Perceiving changes in the environment by means of the sensory organs (for example, dangerous conditions, cold, heat, finding water and food).
• Leading to movement.
• Perceiving changes in the horse’s metabolism and recovering balance.
• Controlling the other systems to have them working together.

3. The SKELETAL system is made up of:

• Bones (usually 205 in total)
• Cartilage
• Joints

Most parts of the Skeletal system can be recognized by either looking at the horse or sensing them thought the horse’s skin. Likewise, some injures to the skeleton can be identified when a bone is misplaced. However, radiographies are a better tool to identify bone injuries. The functions of the Skeletal system are diverse:

• Providing the ‘bony’ framework of the horse.
• Protecting the vital organs (e.g., the brain, heart, liver, lungs).
• Allowing locomotion (legs).
• Forming blood cells (in the bone marrow).
• Storing minerals (such as calcium).

4. The MUSCULAR system is made up of:

• Striated muscles (they are the major contributors to the horse’s movement).
• Tendons (attach the end of a muscle to a bone).
• Ligaments (surround the joints to hold them together; some also connect bones).
• Smooth muscles (withing the gastrointestinal tract, blood vessels, etc.) are involuntary.
• Cardiac muscle (heart) although ‘striated’ is involuntary.

The striated muscles, tendons, and ligaments together are known as ‘Skeletal muscles’. They are voluntary at different levels and allow movement to the animal. When the horse has difficulties to move normally or shows signs of pain to forced pressure, extension, or contraction on any skeletal muscle through the skin, it may indicate the existence of an injury.

The smooth muscles are not voluntary and have different functions in the inner systems to keep the horse alive and healthy.

5. The CIRCULATORY system is made up of:

• Blood Vessels (arteries, veins, and capillaries).
• Blood (plasma, red blood cells, and platelets; white blood cells are also transported in the blood).
• Heart (a muscular pump that ignites blood circulation with every beat).
• Spleen (removes damaged red blood cells from the blood; also stores red blood cells until they are needed).
  
  The functions of the Circulatory system are:

• Transporting oxygen and nutrients to the cells.
• Removing waste products from the cells, including carbon dioxide.
• Transporting white blood cells where they are needed to attack infections.
• Transporting hormones.
• Helping with body temperature regulation.

From the outside, some peripheric veins (close to the skin) can be recognized under the skin on certain areas of the horse, such as the neck, belly, thighs, and forearms during physical activity. The reason is that peripheric veins dilate to allow greater amounts of blood to flow and cool down while passing closer to the skin. No other parts of the Circulatory system can be perceived from the horse’s outside. However, the cardiac and pulse rates can be detected on various areas of the horse by using different methods.

6. RESPIRATORY system is made up of:

• Air passages (nostrils, nasal cavity, pharynx, larynx, epiglottis, and trachea or windpipe).
• Lungs (bronchial tubes or bronchus, bronchiole, and alveoli).
• Diaphragm (although not usually considered a part of the Respiratory system, it is vital to respiration. This muscle separates the thoracic cavity from the abdominal cavity from inside. It allows more space on the thoracic cavity during inspiration for the lungs to expand and be fulfilled by air; it does the opposite during expiration).

The functions of the Respiratory system are:

• Drawing air into the lungs.
• Capturing oxygen from the inhaled air and supplying it to the blood.
• Removing carbon dioxide and expelling it.

Of the Respiratory system, only the nostrils can be seen from outside the horse. When the horse is at rest in a place with a comfortable temperature and normal concentration of oxygen in the air, the respiratory rate is low and may require good attention from a person to notice the horse breathing unless a hand is placed in front of the animal’s nostrils to feel the air coming out of them while exhaling. The respiratory rate increases with exercise and, also in places or conditions where the concentration of oxygen in the air is reduced (due to elevated temperature and/or humidity, and/or high altitude). As the respiratory rate increases, the nostrils dilate to allow better air flow and there is a slight movement of opening and closing that allows a person to determine the rate easily. The expansion of the thoracic cavity can also be noticed easily on the horse’s thorax/belly area.

Note: The circulatory and respiratory systems have complementary functions which common denominators are the need of providing oxygen to the cells and eliminating carbon dioxide from the body.

7. DIGESTIVE system is formed by:

• Gastrointestinal tract (mouth, esophagus, stomach, and intestines that include the small, large, cecum, and rectum portions).
• Liver (among many other functions in the body, the liver releases bile into the small intestine to help digest fats; also regulates various chemicals in the blood by producing, transforming, or eliminating certain substances).

The functions of the digestive system are:

• Eating and digesting the food.
• Absorbing the nutrients and sending them to the bloodstream.
• Evacuating the non-digested material in the feces.

The only visible part of the Digestive system from the horse’s outside is the mouth.  Inspecting the mouth frequently to make sure there are no issues with the teeth (e.g., abnormal wearing, sharp points, and/or cavities), the tongue (e.g., lacerations and/or ulcers), and the gums (e.g., lacerations) is important because a great deal of the horse’s health depends on his ability to eat and chew his food properly. Also, because some of those mouth issues may lead to undesired behaviors of the horse during the training/riding sessions due to discomfort.

Intestine movements can be heard on different areas of the horse’s belly either with a bare ear placed directly over his skin or by using a stethoscope. This is especially helpful to diagnose colic and its progress.

After food has been digested and the nutrients absorbed, the non-usable materials are eliminated in the form of manure. Therefore, a simple way to deduce how the horse’s Digestive system is working consists of watching the frequency, amount, and appearance of his manure. It should be a pile of two-to-three-inch round pellets, neither too liquid (like cow’s manure) nor too hard and dry; its color, a green-to-brown shade that varies depending on the diet. It should not have pieces of food incompletely chewed, worms, or blood in it.

8. EXCRETORY system or URINARY system is formed by:

• Kidneys (filter the blood to remove waste and fluid excess, which make up urine).
• Ureters (conduct urine from both kidneys to the bladder).
• Bladder (collects urine until it fills up).
• Urethra (allows urine from the bladder to be expelled out of the body).

The functions of the Urinary system are:

• Controlling the water balance and the level of electrolytes in the horse’s body.
• Excreting waste substances out of the body.

A simple way to deduce how the horse’s Urinary system is working consists of watching the frequency, amount, and appearance of his urine, when possible. Frequent little amounts of urine (except in a mare in heat), no urine in a long period of time, or frequent attempts to urinate may be signs of dehydration, kidney problems, obstructions, or metabolic issues. Pink to red urine may be a sign of metabolic issues, kidney or urinary tract infection, bladder or kidney stone, or a secondary effect of medication.

9. ENDOCRINE system is formed by glands that produce hormones for various purposes:

• Pituitary (connected to the brain, produces various hormones, some of which influence the functions of other glands throughout the body; other of its hormones have immediate effects in specific tissues).
• Adrenal glands (release hormones that affect metabolism; also affect behavior during stressful conditions).
• Thyroid (regulates iodine in the body and produces hormones that help regulate metabolism and the growth of tissues and organs).
• Parathyroid glands (the hormone released by these two pairs of glands help maintain the required calcium levels in the bloodstream by resorbing bone when needed).
• Pancreas (produces insulin and glucagon, both of which help regulate the blood sugar levels; it also releases enzymes into the small intestine to break down sugars, fats, and starches).
• Gonads (also parts of the Reproductive systems, the two testicles of the stallion and the two ovaries of the mare produce sex hormones that affect reproduction and sexual behavior; also generate the gametes or sex cells).

10. The IMMUNE and LYMPHATIC systems are so interdependent that they are often described together. Here, the Immune system is explained based on all its functions and the Lymphatic system as its major contributor to fight infections within the body.

IMMUNE system: Its main function is to protect the body from external damaging agents including infectious diseases.

Although usually not considered parts of the Immune system by most authors, here we do include the mechanical barriers of the horse’s body against invaders as the first and most efficient parts of this system. They are the skin, the cornea (clear outer layer of the eye), and the mucous membranes inside the respiratory and digestive systems, and the reproductive tract of the mare. Any possible cut to either of these mechanical barriers may allow the entrance of pathogens.

Some substances with antimicrobial properties are released by certain external organs, which, depending on where they are produced, have other important functions. Examples are the sebum produced by the sebaceous glands (of the skin), tears (eyes), and saliva (mouth).

When invaders trespass the mechanical barriers, White blood cells (of various types) use different mechanisms to fight bacteria, virus, and fungi, and some potentially damaging proteins. White blood cells are produced in the Bone Marrow and the Thymus (only produces the Thymus-derived cells, commonly known as ‘T-cells’).

Fever is an additional mechanism used by the body to fight infections in the attempt to kill the pathogens by increasing the temperature.

The parts of the LYMPHATIC system are:

• Lymph (a clear-to-white fluid containing white blood cells, proteins, and fats that circulate through the Lymphatic system).
• Lymphatic vessels (capillaries, larger vessels, and ducts).
• Lymphoid nodules or lymph nodes (filter waste substances, pathogens, and cancer cells from the Lymph).
• Spleen (filters the blood to remove pathogens and damagedrRed blood cells; also stores white blood cells and releases them when needed)

Note: White blood cells are also transported by the blood.

The functions of the Lymphatic system are:

• Filtering waste substances and pathogens from the Lymph.
• Helps regulating body fluid levels.
• Absorbing and transporting fats from the intestine to the blood.

Except for the Skin and Cornea, no other parts of the Immune and Lymphatic systems are seen from outside the horse. However, a horse having fever is considered a clear sign of being fighting an infection. When lymphoid nodules are swollen, it is considered another sign of infection, although in rare cases it could be due to fighting cancer, instead.

A greatly swollen lymphoid nodule (also called an ‘abscess’) is usually full of pus (a mix of dead tissue, white blood cells, and the pathogen). The most common abscesses in horses relatively easy to identify occur around the throat, the mandible, or at the base of the ear. An abscess into the hoof is much harder to identify because it can neither be seen nor sensed with the fingers. Furthermore, because of the symptoms it causes (lameness and hoof fever), a hoof abscess is often confused with laminitis and mistakenly treated as such.

11. REPRODUCTIVE system: Its final purpose is perpetuating the species. Each gender has different parts and functions:

Stallion’s Reproductive system:

• Penis (its erection occurs due to arterial blood filling the sinuses, alike a man’s penis).
• Testicles or Testes (produce both testosterone hormone and spermatozoa).
• Ducts (epididymis, vas deferens, and urethra).
• Accessory Sex Glands (prostate gland, seminal vesicle glands, and bulbourethral glands. Each one produces a different type of seminal fluid).

Mare’s Reproductive system:

• Vulva (external part of the mare’s reproductive tract located below the anus).
• Vagina (immediately after the vulva toward the inside; ends at the cervix).
• Cervix (a thin duct of about 2 to 3 inches long that separates the vagina from the uterus).
• Uterus (the place where a fetus develops).
• Fallopian Tubes or Oviducts (catch the egg when it is released by the ovary).
• Ovaries (produce hormones that control the estrous cycle; also produce and release the egg).
• Udder (produces milk to feed the foal).

Only the testicles and penis of the stallion and the udder and vulva of the mare can be seen from the horse’s outside. When planning to include a stallion and/or a mare in a breeding program it is recommended to make an evaluation of their Reproductive systems. Besides an external observation, a Veterinarian with expertise in equine reproduction can use additional methods for a thorough evaluation.