Major Organs of Dogs and Their Functions

Modified: 9th Sep 2021
Wordcount: 5227 words

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There are a number of major organs in any animal’s body that will have a different structure and function. Each of the major organs belongs to a different system to allow the animal to live. For example the lungs are part of the respiratory system. My chosen animal is a dog.

Respiratory system

A dog’s respiratory system will need to have a trachea, bronchioles, bronchus, and the lungs. All the cells in the dog’s body will have an essential need for oxygen so they can react with the nutrients to produce energy. For the dog to be able to provide these cells with the oxygen the gases will need to be exchanged continuously between the ordog ganism and the environment that the cow is in. The oxygen will be taken from the environment and then transported to the cells whilst carbon dioxide which is the bi-product of respiration has to be excretes back in to the environment. The supply of the oxygen and elimination of the by-product carbon dioxide will be facilitated by the respiratory system and also the cardiovascular system working together. There are different functions of the respiratory system. These functions are:

Acid based balance. This means that the carbon dioxide will dissolve in the blood and will the form carbonic acid which will lower the pH. Respiration will help balance the body pH by adjusting the extent and the rate of the carbon dioxide removal from the animal’s body.

It will also help regulate the body temperature as it allows the animal to pant which will cool the body temperature. If the animal inhales warm air this will heat the body up.

It will also help olfaction (sense of smell) Olfaction receptors will be located on the upper respiratory tract. As air that containing chemical smells passes over the animal during the inspiration these receptors will sense the smell.

It will help voice vocalisation (phonation) immunity. It will have a role in the animals body defence by going through the alar fold and will turbinate where the special epithelium traps dust, dirt, and pathogens in the mucus and will then waft the out of the respiratory system.

Gaseous exchange facility respiration is the exchange of gases between an organism and its environment. External respiration will be the process of getting oxygen in to the body and to the cells and removal of carbon dioxide from the cells. Internal respiration is a complex series of chemical reactions by which the cells will break down fuel molecules, which will release carbon dioxide and energy which will require oxygen.

Respiratory pigments, this will combine reversible with oxygen and will help increase the capacity of blood to transport oxygen. Haemoglobin is the respiratory pigment that utilised in vertebrate species.

The respiratory system is divided into upper and lower portions. The upper tract will contain external nares, nasal cavity with paranasal sinuse, pharynx particularly nasopharnynx and larygopharynx,larynx and the trachea. The lower tract will contain lungs- every thing will be distal to the two main stem bronchi, bronchi, bronchiole, alveolar ducts, alveolar sacs and the aveoli.


The air will leave the larynx and will then enter the trachea. The trachea will be a tube that is permanently open. The tube will begin at the laryngeal cartilage and will run through the thoracic inlet where it will divide at the bifurcation in to the left and the right bronchi. Incomplete rings of hyaline cartilage will keep the tube from collapsing. They will be separated by fibrous connective tissue and smooth muscle fibres. The tube will be lined with ciliated epithelium mucous tissue. The trachea will need to be supported open as air is contained with in it and will be lower pressure than the atmospheric air during inspiration. The trachea is located in the ventral neck along the mid line with the oesophagus on the left of it. The trachea will divide in to two cartilaginous bronchi at the level of rib one

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The bronchi are a large air way that is formed by the bifurcation (split) of the trachea. One bronchus will go in to each lung and have a similar structure to the trachea and it also like the trachea will contain cartilage and smooth muscle. Bronchi are lined with ciliated mucous epithelium. When they enter the lobs of the lung it will divide in to smaller bronchi which will eventually give a rise to tiny bronchioles.


The bronchioles will be narrower than the bronchi and will only contain smooth muscle in the wall and no cartilage. They will branch repeatedly in to smaller passage ways through out the lung tissue. They will also eventually give a rise in clusters of alveoli. The cilia lining the respiratory tract move foreign bodies back up the tract to the pharynx to be swallowed. It is an intermittent smooth muscle wall that is broken up by communication with the alveolar ducts.


Lungs are large paired organ that occupy the thoracic cavity. They will lye on either side of the mediastunum. Each lung is divided in to several different lobes.

Right lung – cranial, caudal, middle and accessory.

Left lung – cranial and the caudal

Lungs are covered by a layer of smooth epithelium. The space between the pleura lining the thorax and also the pleura lining will be known as the pleural cavity. They are covered by a film of fluid which helps reduce the amount of friction during breathing. All of the structures will be supported by the connective tissue which is rich in elastic fibres. Lungs will have vast surface area which is used for gas exchange. Branches of the pulmonary artery and the veins will follow the bronchi in and out of the lobes of the lungs. This is to provide support for the gaseous exchange that will need to take place. The lungs supply of blood will be provides by the broncho-oesophageal artery.

Exchange of gases: the oxygen will diffuse into the capillaries and the carbon dioxide that will have been created will diffuse into the air that will be located in the alveoli. Oxygen binds that have haemoglobin molecules in the blood will be transported to cells. This will be used as oxyhaemoglobin.

Circulatory system

The circulatory system will transport the blood around the body. For the system to work it will need to have a heart, arteries, veins, capillaries, and blood constitutes.


The arteries will transport the oxygenated blood away from the heart. The only exception to this will be the pulmonary artery which is what carries the deoxygenated blood from the heart to the lungs. All of the arteries in the dog’s body will have a thick muscular wall which will contain elastic tissue. Arteries have a small lumen relative to the diameter. They are capable of construction but is not permeable i.e. substances will not be able to pass in and out of them. There are only two of the arteries that contain a valve. The blood that is in the arteries is under high pressure and will move in pulses and flow rapidly.


The blood will be delivered in to the microscopic vessels which are known as the capillaries by the arterioles. They will have to consist of single-celled epithelium which is the simple epithelium. This will permit the exchange of nutrients, gases and waste products between the blood and the tissues this will mean that they are permeable. They will not contain any muscles or elastic tissues and will have a large lumen which will have no valves. They are sites of exchange: blood will change from being oxygenated to deoxygenate across the capillary bed. The blood pressure will reduce due to the fact that there is a reduction in size of the vessel and the blood that is with in them will also be slow flowing. This is ideal conditions for diffusion to occur. Capillaries will not have any pulse as the blood will be under the low pressure and won’t be moving in waves.


These will be linked to the venules. Approximately 50% of the animal’s blood volume will be present in the veins. They will have a thinner and will have less elastic walls than the arteries. The blood will be under low pressure. Muscular activity that will have taken place will aid the venous blood flow by compressing veins although the veins cannot constrict for themselves. Veins will be equipped with a valve that will help to prevent back flow of the blood. The function of veins is to transport blood to the heart. This blood will contain deoxygenated blood. There will only be one exception which is the pulmonary vein. This is what will carry the oxygenated blood from the lungs to the heart. Veins will not have any pulse. This is due to the fact that the blood will be flowing through them slowly.

Blood constitutes

Blood is a red fluid that will flow around the dog’s body. The arterial blood will be a brighter red colour than the venous blood. This is due to the arterial blood will be carrying more oxygen than what the venous blood will be carrying. The blood that is circulating around the animal’s body will have a volume equivalent to 7% of the animal’s body weight. Blood will comprise of – blood cells and plasma.

Plasma is the liquid component of the blood it will normally be a straw coloured fluid. It will contain numerous chemical compound such as water (around 92%), plasma proteins, mineral salts, foodstuffs, gases, waste products, hormones and enzymes and antibodies. Plasma can be separated from the blood cells by a process that is known as centrifugation. In the body it will constantly be mixed with blood cells as it will circulate in the blood vessels.

Blood cells contain many different types of cells. These blood cells can be divided in to three different categories which are red blood cells or erythrocytes, white blood cells or leukocytes ( can also be spelled leucocytes) and thrombocytes or platelets.

Red blood cells or erythrocytes- these are a highly specialised cell which the function is to transport oxygen. In mammals they will not have a nucleus they are a biconcave discs and is small in size approximately 0.007cm. it is the haemoglobin which is an iron that contained protein that will give the cells the red colour. An average red blood cell will have a life span of around 21 days.

White blood cells are divided in to two groups. These groups are known as granulocytes and agranulocytes.

Granular leukocytes will comprise of around 70% of white blood cells and like the red blood cells they have a life span of around 21 days. They can also be known as poltmorphonuvlear leukocytes. They will possess in a large distinctive granule in the cytoplasm and is produced in red blood cells. There are three types which are neutrophils, eosinophils and basophils.

Agranular leukocytes have a clear cytoplasm and a rounded or kidney shape nucleus. There is two types of agranular leukpcytes which is known as monocytes and lymphocytes

Thrombocytes or platelets which are cell fragments that is essential for blood clotting. They will be produced in bone marrow from cells that is known as magakaryocytes. They will be small oval bits of cytoplasm that will lack nucleus and are enclosed by a membrane.

The function of the heart is to pump blood around the body under pressure. The dog’s heart is hollow and muscular and will have four chambers. It is a conical shape with the apex pointing downwards. It will lie at the left of the seventh rib. The heart will be enclosed by a connective tissue sac. The inner surface of the pericardium will be covered by a smooth layer of epithelial cells. Between the endocardium and the epicardium there is a pericardial cavity. This space will be filled with fluid that will help to reduce friction as the heart beats. The heart has a right and a left side. Both which is divided in to two chambers. the top chambers or the atria will have thinner walls than the thick walled bottom chambers which is known as the ventricles.

Lymphatic system

The fluid that has leaked from the capillary bed in to the surrounding tissue is known as tissue fluid. Some of this tissue fluid will be returned to the capillaries by osmosis but it will still leave a deficit that will need to be removed from the tissue and then returned to circulation. If this was not returned the volume of the blood would decrease. The lymphatic system is built up with vessels that will return the fluid to the circulation. Lymph is a fluid that will be with in the lymphatic vessels. Functions of the lymphatic system are to return excess tissue fluid back in to circulation, to filter any bacteria or foreign body out of the fluid in the lymph nodes, to produce lymphocytes and also to transport any digested food particularly fat. The system will consist of lymphatic capillaries, lymphatic vessels, lymphatic tissues (spleen and the lymph nodes) and last of all the lymphatic nodes.

Lymphatic capillaries

These are a series of fine channels that will collect tissue fluid. They will join together to form a lymph vessels. Lymphatic capillaries will also be found in the villi of the ileum where they are known as the lacteals. The lacteals will absorb the fatty acids and glycerol from digested fat in the intestine and transport it to the cistern chyli as minute fat droplets. It will then go to the thoracic duct and then back in to the blood stream.

Lymphatic vessels

These will have a similar structure to veins as they have thin walls which will have valves that will be distributed through out the length of them. The walls will not contain any smooth muscle and the passage of the lymphatic fluid will require milking or contraction of the surrounding tissues so they can push the fluid along the vessel past the non-return valves. The will be a number of lymphatic vessels in the body tissue.

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Lymphatic nodes

The vessels will return the lymph to the circulation via the lymph nodes. The nodes is a collection of lymphocytes that is joined together by connective tissue. The nodes will consist of cortical and medullary regions which all have different functions. The cortex will produce lymphocytes from lymph nodules whilst the medulla contains a network of sinuses packed full of phagocyte cells to filter the pathogens. The lymph nodes is located at various parts of the animals body mainly at the proximal end of the limbs and body portals, they will usually be where the inside meets the outside. For example the groin, some of the lymph nodes are palpable in normal animals but they will normally be easier to palpate when an infection or disease is present in the animals body. Lymph nodes that is in the animals body are: submandibular lymph nodes, prescapular lymph nodes, axillary lymph nodes, bronchial lymph node, mesenteric lymph nodes, popliteal lymph nodes, superficial lymph nodes, retropharyngeal lymph nodes and the parotid lymph nodes.

Lymphatic ducts

Once the lymphatic fluid has travelled through the lymph nodes it will travel to the lymphatic ducts. The smallest of the ducts which will drain right forelimb and the right hand side of the head and the neck. Whilst the larger of the duct will drain the rest of the body. Both right and left ducts will empty there contents in to either the right jugular vein or the right vena cava. There are also two smaller tracheal ducts that will aid in the drainage of the lymph from the head and neck.

Digestive system

The digestive system will comprise of a number of organs that is essential for digestion to take place. These organs are: oesophagus, stomach, small intestine- duodenum, jejunum, and ileum, and the large intestine- ceacum, colon, rectum and the anus. The function of the digestive system is ingestion which is to take food or liquid in to the body which is done by swallowing. Digestion which is the physiological act of breaking down food into a form that can be used or excreted. Absorption which is the physiological passage of materials through the lining of the intestines in to the blood which will allow the animal to get the nutrients that the animal needs. Metabolism which is a series of chemical reactions will take place to provide energy and nutrients that the dog will require to stay alive and the last main function which is excretion which is the act or the process of the dog discharging any waste matter from the tissues and organs.


This will be located caudal to the trachea and will follow approximately the same path. It will enter the stomach by going through the cardiac sphincter. It will comprise of smooth muscle tube which will have a capacity to expand when a food bolus is present. It will sit behind the trachea in the section where the hyaline rings which is incomplete. This will also follow it to expand without compromising respiratory function. The lining of the oesophagus is stratified squamous epithelium this is because it is subject to damage as the blouses pass along it. There will also be mucous glands present on the lining to help aid lubrication. The food bolus is moved along the length of the oesophagus through peristalsis it will also pass through the diaphragm via the oesophageal hiatus.


The stomach is an organ that is a bit like a sac. A dogs stomach is monogastric which means that it only has one compartment in the stomach. The stomachs function is to act as a food store, to break up the food that the animal has eaten, and to introduce digestive enzymes to begin the protein digestion. The stomach will be located on the left hand side of the cranial abdomen and consists of four parts. These four parts is cardia, which will control the entry of food, fundus which will secrete the gastric juices and mucus, corpus which is the body , continuation of the fundus and the last component the pylorus which is where the mucus and gastrin will be secreted. This will join the duodenum. The stomach will be coated in a visceral peritonem or mesentery. The layer that is in on the inner curve of the fundus is known as the lesser omentum and that on the outer curve is known as the greater omentum. With in the folds of the greater omentum the spleen can be found.

Small intestine

The small intestine consists of three different parts. These parts are known as the duodenum, jejunum, and the ileum. The function of the small intestines is digestion and absorption of nutrients. This is where the majority of enzymatic digestion will occur.

The duodenum is located on the right hand side of the abdomen it is a u shaped tube. It will incorporate the pancreas which will lie with in the “u”. Chime is present which will stimulate the production of pancreatic juices and bile which is added to the partially digested food through the bile duct and pancreatic duct.

It will be difficult to establish morphologically the two distinct last sections of the small intestine. These two parts is known as jejunum and the ileum. These two parts will make up a long mobile tube which will loop around the abdominal cavity filling the peritonem. The walls will contain crypts of lieberkuhn which will secrete the digestive enzymes. The ileum will end at the ileosacral junction where it will join the ceacum. This tube will be invaginated to increase the surface area and will be lined with villi which will incorporate lacteals. The products of fat, protein, carbohydrate digestion will be absorbed in to the blood stream by going through the lacteals. The chyme will continue to flow along via peristalsis and will head towards the colon.

Nutrients will break down in to there most basic components. So the fats will break down in to fatty acids and glycerol, carbohydrates/sugar will break down in to simple sugars and proteins will break down in to amino acids.

Large intestines

Any remaining contents will pass along the ceacum and in to the colon and then the rectum. When it gets to this stage only the indigestible material and fluid will remain. The water will be reabsorbed in to the body in the colon. The ceacum will have no role in a carnivore so therefore will not play apart in the dogs digestion. The colon will consist of three parts. These three parts is known as ascending colon, transverse colon, and the descending colon. The colons function is to absorb water and electrolytes and then to push any of the remaining waste in to the rectum. This will be done by peristalsis. The lining will be folded and will contain goblet cells so they can produce mucus for lubrication. There will be a commensally bacterial population which will help degrade the remaining undigested food. The rectum will be located in the pelvis. The function will be to store semisolid faeces before defecation. It will terminate in the anal canal.


  • Mixed with the saliva,
  • carbohydrate digestion will begin,
  • bolus will be formed


  • There will be hydrochloric acid in the stomach
  • The acidity will reduce any bacteria or pepsin

The protein break down will begin This diagram shows the process of digestion. When food enters the dogs mouth the saliva will mix with it and the carbohydrate digestion will begin and bolus will be formed. When the food reaches the stomach there will be hydrochloric acid in the stomach. The acidity of the stomach will reduce any bacteria or pepsins that is in the food therefore protecting the animal from becoming ill. The protein that has been eaten will begin to break down. The small intestines will then take over and pancreatic juices/ bile will be mixed with the chyme. The neutralises acid will continue the digestion and the nutrient components will be absorbed in to blood stream/ lymphatic system. Then the large intestines will allow any water to be absorbed in to the body and semi solid faeces will be formed.

Large intestines

  • Water will be absorbed in to body
  • Semi solid faeces will be absorbed

Small intestines

  • Pancreatic juices/ bile will be added to the chyme
  • Neutralisis acid will continue digestion
  • Nutrient components will be absorbed in to blood stream/ lymphatic system

Excretory system

There are two main organs in the excretory system. These two organs is known as the kidneys and the liver.


This organ is the largest organ in the animal’s body. It will be locate in the cranial abdomen and will have close proximity to the stomach, duodenum and the right kidney. It will be divided in to several lobes or folds surrounding the falciform ligament. The gallbladder will be situated in the lobs near the centre of the liver. Functions of the liver is

  • Carbohydrate metabolism which is conversion of the excess glucose to glycogen and will do this the opposite way around when it is needed.
  • Protein metabolism which is producing the plasma proteins which will combine amino acids to make the new proteins for growth and repair.
  • Deamination which is the removal of surplus amino acids by changing them to ammonia then to the urea which will be excreted in to the urine.
  • Fat metabolism which is the conversion of the fatty acids and glycerol in to the phospholipids which will be used for storage as excess fat.
  • The liver will create bile and will the store the bile in the dogs gall bladder.
  • It will also destroy all the old erythrocytes.
  • It will also create the new erythrocytes.
  • It will act like a vitamin store for the fat soluble vitamins A,D,E and K. It will also store iron
  • It will play a role in thermoregulation


The kidneys will filter the blood. They are located against the dorsal wall of the abdomen and will be attached by a fibrous covering which is known as the capsule. The right kidney will be situated about half its own length in front of the left kidney. Each kidney will have a pad of fat that will be located next to it for the protection and an energy store. The internal structure of the kidneys will comprise of three distinct areas. These areas are known as the cortex (outer), medulla (middle) and the renal pelvis (inner). Kidneys are surrounded by a dense fibrous tissue that will cover the capsule. The cortex will contain the renal corpuscle and convoluted tubules of the nephron. The medulla will contain the collecting ducts and the pyramids which is located between them and the loops of henle of the nethrons. The renal pelvis is a white colour and will be made if a dense connective tissue. It is effectively a funnel through which the urine will collect and drain from the kidneys in to the ureter. The functions of the kidneys are:

  • Formation of the urine (excretion)
  • Production of rennin
  • Conversion of vitamin D to its active form.
  • Production of the erythropoietin
  • It will play a role in homeostasis which is balancing the pH of the animals body.

The kidneys will produce urine from water, salts and wastes that they have filtered from the blood. By adjusting the amount if water and salts that the excrete, the kidneys will perform a vital homeostatic service in the maintenance of the internal chemical balance of the body. Urine will be excreted in a continuous trickle which will collect in the renal pelvis.

Sensory system

The sensory system is built up with a number of different sensory organs. A sense organ is a specialised structure that will consist of a number of receptor cells and often accessory cells. It will detect changes that are in the environment and will translate this information to the nervous system. For example rods and cones in the retina are receptor cells and the cornea and lens will act as an accessory structure which will enhance the versatility to the sense organ. Some of the sensory organs that is used by the dog is the skin, hair, claws and nails.


The skin will form the external layer over the body and merging with the mucous membranes at the body openings like the mouth. Skin will have several functions. These functions are:

  • Protect the surface of the body
  • It will protect against any invasion by microorganisms.
  • It also has a role in controlling the body temperature (homeostasis)
  • It will help prevent excessive loss of water.
  • It will produce vitamin D
  • The pigment in the skin and hair will protect the body against ultraviolet radiation.
  • The skin will contain a specialised receptor which is sensory cells that will detect the changes in the environment. For example pressure, temperature, and pain.

In dogs there will be hair/fur

Types of hair

  • Guard hairs

The long hairs of the top coat. There will be one from each follicle

Hackles raised

  • Wool hairs/undercoat

This is the insulating layer, it will be shorter and softer hair, and they will trap air between them. This will create warmth for the dog. There can be lots from one follicle

This may be on a puppy coat and in adult dogs there will be more of this coat in the winter rather than the summer so the dog does not over heat.

  • Vibrissae

Tactile hairs and is the thickest hairs. They will project beyond the rest of the coat. It will grow some specialised follicles there will be nerve endings at the base, sensory organs

This hair should never be cut. Will mostly be on the dogs head, upper lip and above the eyes

Sensory Organs

Other sense organs is organs like the ear which is what will allow the dog to hear any changes that has occurred in the environment. The eyes which will allow the dog to see any changes that has occurred in the environment that the animal is in.


The function of the ear is to allow hearing and the control the balance in the dog. Dogs have a pair of ears which will be located on the dorsal aspect of the head. The ear will be divided in to three chambers which is known as: the air filled outer ear, air filled middle ear, and the fluid filled inner ear. The outer ear will consist of the pinna which is for protection, the external auditory meatus at the base of the pinna there will be a ring of cartilage which is known as the annular cartilage this will lead to the external auditory meatus. The middle ear will be located with in the temporal bone of the skull. It will contain air, the auditory ossicles or bones in the tympanic cavity and the opening of the Eustachian tube. The ossicles is the three small bones that will have the synovial joints between them. The inner ear will be situated with in the temporal bone and this is what will contain the hearing apparatus. It will be a fluid filled chamber that will comprise of a membranous labyrinth which will be located in a bony labyrinth. This labyrinth will be made up of the vestibule. This will contain semicircular canals, the cochlea and a round window.


The eyeball will be located with in the orbit of the skull and will be embedded in a pad of fat which will mean that it is well protected with the out side of the eye being the only part that is exposed. The eye ball will be supplied wit a second cranial nerve. The eye ball will consist of three layers which are known as the outer fibrous coat- the sclera and also the cornea, the middle vascular layer which holds the choroids, tapetum lucidum, the ciliary body and finally the iris. The last layer is known as the inner nervous layer which will contain the retina.


Taste will be detected by specialised cells which are known as taste buds. The taste buds will normally be found in the mouth and on the tongue. They will be in tiny elevations or papillae. These will be replaced around every 10-30 hours. Taste buds should consist of an epithelial capsule that will have several taste receptors. Each taste receptors will be connected to a sensory neuron that will have complicated interlocking system. This interlocking system will allow the to be more than one neuron stimulated by a certain taste. Certain tastes will be detected in certain areas of the tongue.


Smell will occur in the olfactory epithelium which will be located on the upper surface of the nasal cavity. The nasal epithelium will contain around 20 million cells that is specialised olfactory cells which is connected to the axons of the olfactory nerve.


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