Internal Anatomy of Koi Fish

internal anatomy of koi fish

Here we will cover the parts of the internal anatomy of koi fish:


The swimbladder, a gas-filled sac located at the top of the koi’s body cavity below the vertebral column, is called a “swimbladder.” The swimbladder is a two-chambered structure. The posterior kidneys are located astride the constriction that separates the chambers. The swimbladder’s primary function is to regulate buoyancy and allow koi to stay at all depths in the water while using minimal energy. The swimbladder in koi retains a primitive connection to the stomach. Fish can gulp air from the surface to increase the gas content. Usually, several blood vessels control the gas content in the swimbladder.

As in many other fish, the swimbladder plays a secondary function in amplifying sound and transmitting that to the inner ear through a series of small bones.

Structure & Function of Gills

Gills are the central organ of respiration. They are delicate because they allow oxygen to flow from water through the tissue to the blood and carbon dioxide gas to be emitted. Koi fish are remarkable at extracting oxygen out of water. The primary lamellae of the gill filaments have a large area. This is extended by out folds and tissue called secondary lamellae. The blood and water are in close contact with the gills. They are separated by one layer of epithelial cells. To extract as much oxygen as possible from the water,

The blood flows through the gills in the opposite direction of the water flow. This is called a countercurrent system. The secondary lamellae allow blood cells to pass through, stretching to increase their surface area and transfer the most oxygen to red respiratory protein hemoglobin.

The gills are a crucial site for the elimination of nitrogenous waste. Eighty-two percent is eliminated as ammonia, and eight percent as urea. Dilute urine is the remaining 10%, which is expelled by the kidneys in the form of urea. Because blood is so close to the water at the kidneys, it must be either warmed or cooled depending on the temperature of the surrounding water. The oxygenated blood flow from the gills can be divided into two parts.


Koi fish have a three-chambered heart (sinus venosus, atrium, and ventricle). This pumps blood that is deoxygenated to the gills.  The ventricle, the muscular section of the heart, gets the oxygen-rich blood from the coronary artery. 


Koi have a huge liver.  Once the food is digested in the intestines, it is passed to the liver, distributed, and stored into the tissues. Breaking down the unwanted protein into ammonia, processing old red blood cells to help form bile, and breaking down any toxins or poisons. 


Reproductive organs, known as gonads, are located on either body side.  The ovaries can also produce huge eggs.  Sperm is produced by the testes (milt).


This is the area where the intestines open up at the anus.  Here urine from the kidneys is released, and eggs or sperm are released into the pond through the urogenital opening. 


The spleen will take immature red blood cells and store them.  And also produce cells of the immune system. 

Those are the significant parts of the internal anatomy.  View the external anatomy of koi fish here.