The importance of understanding equine physiology
Physiology is the scientific study of the mechanisms of living things. Physiology reveals how cells, tissues, organs, and systems help to maintain normal bodily functions in healthy animals, as well as examining how an animal responds to changes in its environment. A physiologist is an expert in physiology. Changes to normal physiology imply that disease is present in the animal. A good knowledge of physiology, therefore, can help an owner to maintain their animal’s health.
The horse is an athletic animal, which means that it is ever more crucial to understand equine physiology. A physiologist can help to increase your horse’s athletic performance, prevent injuries and identify problems in their early stages. Doing so can prevent injuries from becoming severe and can increase the strength of the animal. This can be of great economic value to the owner of a sporting horse! Physiological analysis can also be a helpful factor in assessing new horses before buying them. Following the advice of a physiologist, a horse owner can ensure that they choose and purchase an animal that is healthy and strong.
Basic equine anatomy
Anatomy is the scientific study of the body structure of a healthy animal. Anatomy can be subdivided into gross anatomy and microscopic anatomy. Gross anatomy is the study of healthy structures in the body which can be seen using the naked eye, whilst microscopic anatomy is the study of healthy body structures that cannot be seen with the naked eye and require the use of microscopes. In basic anatomical terms, the horse’s body is made up of skin, the musculoskeletal system, the central nervous system, the cardiovascular system, the gastrointestinal system, the lymphatic system, the endocrine system, and the urinary system.
Skin is the largest organ in the horse, made up of haired areas, non-haired areas, pigmented areas, and non pigmented areas. The skin is divided into three layers.
The epidermis is the outermost layer of the horse’s skin. It is a keratinised stratified squamous epithelium. ‘Stratified’ implies that there is more than one cell layer. The outer cell layers are keratinised. Keratin is a protein which is an essential part of the epithelial cells in the epidermis. Keratin helps the skin cells form a barrier and forms the outermost layer of the skin. The only living layer of the skin is the basal layer, which lies on the basal membrane. The basal layer continuously forms new cells, and these cells replace those cells which are sloughed off due to friction and physical damages in the outermost layers. The epidermis acts as a barrier and prevents pathogens entering into the body.
The dermis lies directly under the epidermis, and is divided into the papillary and the reticular layers. The papillary layer is located directly under the epidermis. The dermis is a connective tissue layer consisting of blood vessels, nerve endings, hair follicles, glands, collagen fibres, and elastic fibres. The blood vessels in the dermis have a thermoregulatory function. The thickness of the dermis differs by body region and horse breed.
The hypodermis is located at the very bottom of the skin, and is a loose connective tissue storing a large amount of adipose tissue. The hypodermis is absent in the lips, cheeks, and eyelids of the horse.
A Horse’s Skeleton
The horse’s skeleton consists of two parts.
- The Axial skeleton
- The Appendicular skeleton
The axial skeleton of the horse is made up of the skull, vertebral column, sternum, and ribs.
The skull is formed of connecting skull bones called the frontal bone, parietal bone, interparietal bone, temporal bone, ethmoid bone, occipital bone, sphenoid bone, incisive bone, palatine bone, pterygoid bone, mandible, and the maxilla.
The vertebral column of the horse consists of 7 cervical, 18 thoracic, 6 lumbar, 5 sacral, and about 20 caudal vertebrae.
The sternum is formed from the interconnecting of sternebrae. Ribs are connected to the sternum via cartilage.
Two forelimbs and two hindlimbs form the appendicular skeleton of the horse. A horse’s limbs are highly adapted for fast running, and the horse can make long strides via the straightening and lengthening of its limbs.
The skeleton of the forelimbs contain the scapula, humerus, radius, ulna, carpal bones, metacarpal bones, phalanges, and sesamoid bones. The scapula connects the forelimb to the axial skeleton. The humerus connects with the scapula, forming the shoulder joint. The radius and ulna form the antebrachial skeleton. The radius supports the humerus to form the elbow joint. The ulna is fused with the radius, but this fusion is interrupted at an interosseous space.
There are eight carpal bones arranged in two rows, 4 bones per row. The radial carpal bone, the intermediate carpal bone, the ulnar carpal bone, and the accessory carpal bone located in the proximal row from medial to lateral. There are four carpal bones in the distal row arranged from medial to lateral.
There are also 3 metacarpal bones, called metacarpal 2,3 and 4. Metacarpals 1 and 5 have disappeared over time whilst metacarpals 2 and 4 have significantly reduced in size. These are called splint bones. Metacarpal 3 is the prominent metacarpal bone in a horse, and is called the cannon bone. It is well adapted to carry weight.
The horse has three phalanges called the proximal, middle, and distal phalanges. There are two sesamoid bones called the proximal sesamoid bone and the distal sesamoid bone. The distal sesamoid bone is also called the navicular bone.
The skeleton of the pelvic limb/hind limb of a horse contains the pelvic girdle, femur, tibia, fibula, tarsal bones, metatarsal bones, phalanges, and sesamoid bones. The pelvic girdle connects the hind limb to the axial skeleton.
The pelvic girdle is formed by connecting three bones; the ilium, the ischium, and the pubis. The femur connects with the pelvic girdle, forming the hip joint. The hip joint of the horse is well adapted to weight-bearing. The tibia connects with the femur and forms the knee joint. The fibula articulates with the lateral condyle of the tibia.
There are six tarsal bones; the talus, the calcaneus, the central tarsal bone, and the three distal tarsal bones. Metatarsal bones, phalanges, and sesamoid bones are similar to their corresponding bones in the forelimb.
The nervous system
The nervous system of the horse is anatomically divided into two parts.
- The central nervous system
- The peripheral nervous system
The central nervous system (CNS) consists of the brain and the spinal cord. The CNS integrates information received and coordinates the activity of all parts of the body. The brain is protected by a bony structure called the skull, and the spinal cord is similarly protected by a bony structures called vertebrae.
The horse’s brain weighs between 400 and 700 grams; compared to its body weight, this constitutes a ratio of 1:800. (For reference, the brain to body weight ratio of a dog is about 1:100. It is fair to say, therefore, that the horse has a relatively small brain (Dyce, Sack, & Wensing, 1987).
There are no connective tissues in either the brain or the spinal cord. Both the spinal cord and the brain are anatomically divided into white matter and grey matter. The white matter contains myelinated axons and oligodendrocytes. The grey matter contains neuronal cell bodies, dendrites, glial cells, and initial unmyelinated axons’ portions.
In the brain, peripherally located grey matter is called the cerebral cortex, whilst the inner white matter area is called the medulla.
In the spinal cord, peripheral white matter and central grey matter form an “H” shaped area. The central canal is located in the middle of the “H” area, and is lined by ependymal cells. Sensory fibre from the spinal ganglia enters through the dorsal horns of the “H” shaped area, and motor neurons spread from the ventral horns of this area.
Meninges are connective tissues that cover the brain and spinal tissue to protect them. There are three types of meninges.
- Dura mater
- Pia mater
The dura mater is a dense connective tissue layer located externally. The arachnoid has two parts; the layer which contacts the dura mater and the trabeculae system that connects it to the pia mater. There are cavities between the trabeculae which form the subarachnoid space, and are filled by cerebrospinal fluid. Arachnoid villi penetrate the dura mater, where they absorb cerebrospinal fluid into venous sinuses. The pia mater is a loose connective tissue which travels between the brain and the spinal cord.
The peripheral nervous system (PNS) transmits signals between the central nervous system and all the other parts of the body. The main components of the peripheral nervous system are cranial nerves, spinal nerves, ganglia, and nerve endings. The PNS has two divisions.
- Sensory division/Afferent division
This division is composed of sensory neurons and conveys information from the sensory receptors to the central nervous system.
- Motor division/Efferent division
The motor division carries signals from the central nervous system to muscles, glands, and other tissues. The motor division can be divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system innervates skeletal muscle cells and leads to muscle cell excitation. The somatic nervous system consists of a single neuron between the central nervous system and skeletal muscle cells.
The autonomic nervous system can be either excitatory or inhibitory. Nerves in the autonomic nervous system innervate smooth and cardiac muscle, glands, gastrointestinal neurons but not skeletal muscle cells. The autonomic nervous system encompasses two neurons connected by a synapse between the central nervous system and the effector organ. The preganglionic neuron is the first of these neurons; passing between the central nervous system and the ganglia. The cell body of the preganglionic neuron is located in the central nervous system. The postganglionic neuron is the second of these neurons which passes between the ganglia and the effector cells.
There are two divisions in the autonomic nervous system.
- Sympathetic division/Thoracolumbar division
The neurons of the sympathetic division are attached to the thoracic and lumbar regions in the spinal cord. This division prepares the body for fight or flight responses.
- Parasympathetic division/Craniosacral division
Neurons of this parasympathetic division are attached to the brainstem and the sacral portion of the spinal cord. This division controls the body’s processes during normal situations such as digestive responses.
The autonomic nervous system controls the internal body processes such as blood pressure, heart and respiratory rates, body temperature, digestion, metabolism, urination, defecation, sexual responses, production of body fluids, water, and electrolyte balance.
The cardiovascular system (CVS)
The cardiovascular system consists of 3 components.
- The pump (Heart)
- The circulatory fluid (Blood and lymph)
- The circulatory system (arteries, arterioles, veins, venules, capillaries)
The CVS is the body’s transport system. It distributes oxygen from the lungs to cells and tissues, removes carbon dioxide from cells and tissues, distributes nutrients that are absorbed in the gastrointestinal tract, removes metabolic by-products in cells and tissues, and transports hormones from their origin sites to their target locations. The ultimate role of the cardiovascular system is to maintain homeostasis throughout the body.
The weight of the heart depends on the horse’s breed and the amount of training it has had.
The heart is located in the thoracic cavity within the mediastinum between the left and right pleural cavities and is protected by the ribs from about the third to the sixth intercostal spaces. The dorsal aspect is horizontally in line with the middle of the first rib, and the ventral aspect is located on the sternum. The long axis of the cardiac silhouette is oriented vertically in the horse (Reece, Erickson, Goff, & Uemura, 2015).
The base is the dorsal part of the heart, and the major blood vessels enter and leave from this section. These major blood vessels tend to hold the heart in a fixed position, though the ventral side of the heart is relatively free within the pericardial sac.
The cardiovascular system has two circulations.
- Pulmonary circulation
Pulmonary circulation carries deoxygenated blood to the lungs to remove carbon dioxide from the blood and absorb oxygen. Pulmonary circulation starts from the right ventricle, where deoxygenated blood is transported to the lungs via the pulmonary artery. Deoxygenated blood is converted into oxygenated blood in the lungs and flows back to the left atrium via the pulmonary veins.
- Systemic circulation
Systemic circulation distributes oxygenated blood in the left ventricle to the whole body through the aorta and carries deoxygenated blood to the right atrium via the cranial and caudal vena cava.
The Lymphatic System
The lymphatic system is a subsystem of both the circulatory system and the immune system. This provides a drainage function, removing excess fluid within tissues and returning it into the bloodstream. Nutrients leave the arterial end of the capillary bed, while tissue fluid containing metabolic waste reabsorbs back in at the venous end. Not all of the fluid is drawn back into the bloodstream, however. If this fluid is allowed to accumulate in the body, it will lead to the forming of an edema. This is where the lymphatic system comes in to play. It picks up this excess fluid and returns it to the circulatory system via lymphatic vessels. Whilst the circulatory system is a closed loop, the lymphatic system travels in one direction and is open-ended.
The lymphatic system also assists the absorption of dietary fat from the intestine. The lymphatic system has an immune function. The lymph vessels are connected to lymph nodes, and lymph is filtered within lymph nodes before reaching the bloodstream. Lymph nodes contain macrophages, dendritic cells, and lymphocytes. These cells destroy the pathogens in the circulatory lymph. Lymphocytes produce antibodies which enter into the bloodstream. These are immunoglobulins that can destroy specific antigens.
The lymphatic system is made up of lymphoid organs. There are two types of lymphoid organs.
- Primary lymphoid organs. EX Thymus and Bone marrow
(Lymphocyte production, maturation, and selection occur in the primary lymphoid organs.)
- Secondary lymphoid organs. EX Lymph nodes, spleen
(Matured lymphocytes enter into the secondary lymphoid organs, and these lymphocytes react with pathogens while the lymph is being filtered.)
The gastrointestinal system
The horse’s gastrointestinal system comprises the oral cavity, oesophagus, stomach, intestines, and other associated structures such as the liver, pancreas, and salivary glands.
The horse is quite distinct when it comes to feeding behaviour. Horses use both lips to collect feed and introduce it into their mouth. Therefore both of a horse’s lips are sensitive and mobile. A hairy integument extends across the upper lip of the horse.
The oesophagus is a narrow tube-like structure that runs between the pharynx and the stomach which carries food between these two organs. The stomach lies between the oesophagus and the small intestine, entirely within the rib cage, mostly to the left of the median plane. The stomach is the most dilated part of the digestive tract and has a capacity of 5-15 litres. When moderately filled, the stomach lies opposite the 9th to the 14th intercostal spaces. The stomach divides into cardia, fundus, body, and pylorus. The fundus region of the horse’s stomach is known as the blind sac (Saccus cecus). The horse has both glandular and non-glandular regions in its stomach. The margo plicatus is a boundary that lies between two regions.
The non-glandular part of the stomach is located towards the blind sac and the body lies near the lesser curvature. The glandular part of the stomach is located in the rest of the body and the pyloric part. Feed enters into the stomach via cardia, and once digested then enters into the duodenum through the pylorus opening.
The intestinal tract of the horse is divided into small and large intestines. The small intestine has three parts.
- Duodenum – 1m long
- Jejunum – 25m long
- Ileum – 50cm long
The large intestine also has three parts.
As hindgut fermenters, horses have an expanded cecum. This is about 1m long and has a capacity of 35 litres. Microbial fermentation occurs in the cecum. The cecum is made up of three parts: the base, the body, and the apex. The colon has ascending, transverse, and descending parts. The rectum is the terminal part of the large intestine. This is about 30cm long.
The pancreas opens with its pancreatic duct on the major duodenal papilla. The accessory pancreatic duct opens on the minor duodenal papilla. The pancreas has both exocrine and endocrine functions.
The exocrine function of the pancreas is the production of digestive juice, which is released into the proximal duodenum. The endocrine part of the pancreas comprises of the pancreatic islets, which produce insulin and glucagon.
The horse’s liver is asymmetrical, and the bulk of the organ is displaced to the right. The most distinctive aspect of a horse’s liver is the absence of a gallbladder. Liver cells produce bile, and this bile enters into the duodenum via the bile duct, through the major duodenal papilla.
The endocrine system
The endocrine system releases hormones directly into to the circulation of the body. These hormones are then transported to their target sites through the bloodstream. The pituitary gland, pineal gland, thyroid gland, parathyroid glands, and adrenal glands are the main endocrine glands in horses.
The pituitary gland has two parts.
- The Adenohypophysis
Adenohypophysis produces growth hormone, follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone, thyroid-stimulating hormone, and prolactin.
- The Neurohypophysis
The neurohypophysis does not produce hormones, rather, it stores and eventually releases the hormones oxytocin and vasopressin.
The pineal gland produces melatonin. It assists in the maintenance of an animal’s biological clock. The thyroid gland of the horse lies on the trachea, and paired lobes are widely dissociated but connected by an insubstantial isthmus.
The thyroid gland produces thyroxin. Thyroxin is mainly involved in the metabolism and growth of the animal. The parathyroid glands produce parathyroid hormone. This regulates calcium metabolism.
The entire body structure and its functions are absolutely amazing and can be very interesting to study. It can be fascinating to see how the body’s normal structure is formed to allow optimal functionality!
Dyce, K. M., Sack, W. O., & Wensing, C. J. (1987). TEXTBOOK OF VETERINARY ANATOMY (Fourth Edition ed.). 3251 Riverport Lane: SAUNDERS ELSEVIER.
Reece, W. O., Erickson, H. H., Goff, J. P., & Uemura, E. E. (2015). Dukes’s Physiology of Domestic Animals. Hoboken, New Jersey: John Wiley & Sons.