Internal organs of a dog

Below is a list of the main internal organs of a dog along with a description of each one and what function it has.

Tongue
The tongue is a muscular organ that is used for tasting food and grooming.

Stomach
Canine teeth are made for ripping, not chewing, so the stomach is the place where food starts to break down. The food enters the stomach by passing down the oesophagus where the salivary gland releases gastric juices and starts digestion. Then the food travels into the stomach through the cardiac sphincter. Once in the stomach the microscopic gastric glands which line the stomach release a mix of water and hydrochloric acid along with the enzyme pepsin. This starts the process of breaking down the protein molecules. The food then leaves the stomach via the duodenum and enters the small intestine in small quantities. When a dog eats a meal it passes relatively quickly into the stomach but leaves the stomach in smaller amounts over a long period of time, so the meal will sit in the stomach for hours before it eventually all makes its way out into the small intestine where the majority of digestion and absorption of nutrients takes place. The pyloric sphincter needs to relax so that food can pass from the stomach into the small intestine.

Pancreas
The pancreas has a dual function of being involved in the digestive process producing digestive enzymes; and as an endocrine gland releasing insulin. Pancreatic juices are added at the start of the duodenum via the pancreatic duct. They contain trypsin to digest protein, amylase to digest starch and lipase to digest fat. The pancreatic juices also contain bicarbonate, as do the bile and intestinal juice, to neutralise the stomach acid in order for the enzymes to work effectively.
Scattered throughout the pancreas are the islets of Langerhans, which secrete glucagon and insulin into the blood. Glucagon is released to increase the level of glucose in the blood and insulin is released to lower the level of glucose in the blood. When a dog eats carbohydrates this will be converted into glucose in the digestive tract and will trigger insulin to be released. The glucose can then be converted into glycogen and stored in the liver and muscles as an energy store, or used by the cells of the body as an energy source. Insulin also increases the synthesis of fats and protein.

Liver
The liver is the largest gland and organ in the dog’s body. It has a rich blood supply and is reddish brown in colour. The liver has many functions including the storage of glycogen, iron and vitamins A, B and D, the production of clotting factors needed for blood clotting, the production of plasma proteins, the conversion of fats, the breakdown of surplus amino acids and haemoglobin and the detoxification of harmful substances so that they can be excreted. The liver also produces bile pigments and bile salts which together with sodium bicarbonate and water enter the duodenum via the gall bladder, where it is stored, as bile to emulsify the fat in a meal so that lipase can work effectively to digest the fat.

Gall bladder
The gall bladder acts as a reservoir for bile, so that when food is present in the small intestine it releases it along the bile duct into the duodenum. It receives the bile from the liver and stores it until it is needed. 

Small intestine
This is where the food is digested and the nutrients are absorbed into the body. Then what ever is left makes its way into the colon.

Large intestine (Colon)
This is where the majority of the fluid is absorbed back into the body before the rest is excreted from the body via the anal sphincter.

Heart
The heart is essentially two pumps, responsible for pumping oxygenated blood around the body and deoxygenated blood to the lungs so that the waste gas carbon dioxide can be released and the blood can be oxygenated. There is one pump on the right to pump the blood to the lungs and one pump on the left to pump the blood to the rest of the body. Deoxygenated blood flows back to the heart via the veins and enters the right atrium. It is transferred then to the right ventricle where it is pumped around the lungs and oxygen is taken up by the blood vessels while carbon dioxide is given off. The oxygenated blood enters the left atrium of the heart and then moves to the left ventricle where it is pumped around the body.

Spleen
The spleen can be up to 25 cm long and weigh up to 150 gm and is reddish in colour. Red and white blood cells are produced in the spleen and red blood cells can be stored here. Aged red blood cells are broken down and their components recycled.

Thymus
The thymus is a solid organ that is responsible for producing most of the body’s lymphocytes. Lymphocytes are involved in antibody production.

Lungs
The right lung is larger than the left lung and they have a spongy texture. The lungs function is to bring oxygen into the body and excrete carbon dioxide out as the dog breathes.

Testicles
The dog’s testes are responsible for producing sperm and testosterone once the dog has reached puberty.

Ovaries
Each bitch has two ovaries and they contain eggs which will be released into the uterine tube where fertilisation takes place. The ovaries are also where oestrogen is produced once a bitch reaches puberty.

Womb (Uterus)
Four days after an egg is fertilised it leaves the uterine tube and enters the uterine horns then after 18 days after fertilisation attachment to the uterine wall starts.

Kidneys
The kidneys are responsible for regulating water in the body and play an important role in controlling blood pressure. Other functions include converting vitamin D3 into a useable form and production of erythropoietin releasing factor, which promotes red cell production in the bone marrow.

Bladder
The bladder stores urine before it is excreted from the body.

Brain
The brain is a complicated organ with 6 divisions. The cerebral cortex has 2 cerebral hemispheres and 4 lobes in each hemisphere. It receives sensations such as vision and touch and then allows them to reach a conscious level. There are large areas responsible for motor activity and 20 per cent of the cerebral cortex is concerned with association, making decisions based on previous outcomes of actions by the dog. The frontal lobe and temporal lobe are behaviour areas in relation to the alertness, intelligence and temperament of the dog.

The thalamus receives the sensory information such as vision and touch and relays it to the cerebral cortex. It is also responsible for regulating the motor activity which is initiated in the cerebral cortex.

The hypothalamus controls the release of the pituitary hormones and regulates things like thirst, hunger and body temperature. It makes up an important part of the autonomic nervous system and has general control over homeostasis. It is thought that the emotions of rage and aggression originate in the hypothalamus.

The pons includes a micturition centre responsible for the control of urine and it is a pathway for nerve fibres passing to the cerebellum. It sits underneath the brain.

The cerebellum is at the back of the brain and is responsible for precision of movement by checking information about planned motor activity in other areas of the brain, then comparing this with information about stretch receptors in muscles and tendons.

The medulla oblongata sits at the back of the brain and runs into the spinal column providing a pathway to and from the spinal column. The medulla contains many neurons such as the respiratory centre and cardiac centre, which are all responsible for specific body functions.

There is also the olfactory lobe at the front of the brain which connects to the olfactory system at the back of the nasal cavity, which is responsible for the dog’s incredible sense of smell.

Further reading - Turner, 2006, Veterinary Notes for Dog Owners