From an anatomical point of view, the domestic koi is a carp with a few modifications brought about by modern day breeders. Of course, with all the changes and modifications to the Koi of today, general anatomy is obviously the same but shape and proportion will depend on variety. Understanding the anatomy of a Koi may assist you in the general health and treatment of your fish. Below is a diagram of the Koi's anatomy with descriptions of the koi Anatomy and Physiology.

 

1	Eyes	The eyes are just forward of the gills. Koi can see in two directions at same time - to either side of the body as well as above or below on each side.
2	Nostrils	The nostrils are just forward and slightly above the eyes. The nostrils are used purely for scent.
3	Barbells	The barbells located are located on the upper lip of the koi and contain many sense receptors to help locate food.
4	Gills	Gills have a similar function to the lungs. The gills are served by a series of fine blood vessels. As the water passes over the gills, oxygen is absorbed through the blood vessels and then transported directly to the body. Carbon Dioxide is return to the water via the gills.
5	Fin (Pectoral)	The pectoral fins are paired and located on the lower sides of the koi. They can be used to rotate the koi almost on the same axis, this is done by one fin working in the other direction to the other. The also act as the main braking fins, they achieved this by placing the fins out to provide a large surface area to the water. They also use them when looking for food to stir up the bottom.
6	Liver	The liver helps remove waste from the blood and controls the use of digested food. It also produces bile which is used in the digestion / absorption of fat.
7	Gall Bladder	Located just below the liver, the Gall Bladder stores bile and releases it to help with digestion.
8	Fin (Pelvic)	The pelvic or ventral fins are paired and located on the lower sides of the koi approx mid body. The pelvic fins enable the koi to rise or descend as it swims, you could say they act as hydrofoils.
9	Spleen	The spleen produces lymph cells (a yellow fluid consisting mostly of blood plasma and white blood cells) and stores red blood cells.
10	Intestines	See Digestive System for further details.
11	Reproductive organs	The internal sex organs of the male are the testes and the ovaries of the female. In both the male and female they are located below the swim bladder. Eggs and sperm exit the body via the gonophores which is located just in front of the urinary opening. The gonophores are connect by the gonoduct.
12	Anal Pore	The anal pore is located just forward of the anal fin. The waste products of the koi's digestive system are expelled via the anal pore. Water in the form of urine is also expelled via the anal pore.
13	Urinary Bladder	The urinary bladder plays an important part for the koi. As the salt content of the koi is higher then that of the water in which it lives the koi's body is continually taking in water which tries to equalize the salt concentration, this is known as osmosis. As a result of this process the koi must release the excess water, otherwise it would blow up like a balloon.
14	Anal Fin	The anal fin located just forward of the tail and is primary for stabilization.
15	Caudal or Tail	The caudal (tail) acts as the koi's rudder and can be used to gain maximum speed / thrust.
16	Kidney
17	Swim Bladder	The swim bladder, located just below the backbone consists of 2 different size chambers. Fish adjust their position by inflating or deflating these chambers, this changes the density relative to the surrounding water. In conjunction with the auditory system it controls the fishes orientation, level at which they swim etc.
18	Dorsal Fin	The dorsal fin located on the top of the koi is the major stabilizing fin. It works in a similar fashion to a keel on a ship by keeping the koi upright. Koi can lower the dorsal fin to create a more streamlined effect when the koi needs to move at faster speeds.
19	Stomach	See Digestive System for further details.
20	Lateral Line	The lateral line runs roughly along the mid-body of the koi. The lateral line is a row of special pores that open into a channel which runs to the head and brain of the koi. The channel is filled with a viscous solution which is extremly sensitive to vibrations in the water.
21	Scales
22	Ears	Fish have internal ears that respond to vibrations within the water. The auditory canal is connected to swim bladder and is used for balance.
23	Mouth	The mouth of Koi are located in an inferior position - not quite at the tip of the head but slightly below. This indicates that it has bottom feeding habits.
24	Color	The color variations in koi is determined by the amount of  (reflective tissue) in the skin below the scales (dermis). The reflective tissue  contain waste by-products of the bodies metabolism. The out layer (epidermis) cells contain color pigments, these are Erythrophores (contain red or orange pigment granules), Melanophores (contain the black pigment melanin), Xyanthophores (contain yellow pigment granules). Their placement in the skin will determine the color of a koi. The more complete the  layer, the more metallic the appearance of the koi as and if this layer is partially or completely missing more color are visible deeper.
25	Mucus Layer	The mucus layer covers the entire external area of the koi. The mucus layer provides protection for bacteria and fungus and gives the koi there slippery feel. It is therefore important that you hands are wet before handing koi, to ensure that the mucus layer is not damaged.
26	Digestive System	The digestive system of koi is more or less like that of any higher animals but differs from many as the koi doesn't have a stomach as such. Food enters via the mouth and is crudely crushed by the pharyngeal teeth (bony projections from the gill supports). From there it passed into esophagus and then into the intestines. The anterior part of the in intestines are swollen and look a lot like a stomach. The intestines long and coiled, usually 4-5 times the length of the koi. The is due to the fact the the vegetable matter eaten by the koi require more time within the body to be broken down so that the goodness is released. The intestines exit the body at the anal pore.
27	Nervous System	The nervous system of a koi consists of optic and other sensory nerves that radiate from the head. Fine fibers at the nerve endings transmit and receive message to the koi's brain, which is relatively simple. The koi's spinal cord helps protect the central nervous system which extends to all parts of the body.