Do fish have blood? What type of blood does a fish have?

Have you ever caught yourself at a dinner table eating a finely cooked fish wondering:

Where is the blood?

If so, you’re no different from many others that have asked the same question to themselves or others.

So, do fish even have blood? Well, fish do have blood. In fact, due to the presence of hemoglobin, their blood is also red. But the reason we don’t normally see blood in fish is that most of the fish we see are on the dinner table. Meaning, either their blood is coagulated because of being stored, or the store employees might have drained its blood before storing it.

However, if you happen to catch a living fish, you will see their gills are red because of the blood.

If you are in the mood to discover a lot of other interesting facts about the blood circulatory system of the fish, you should definitely read further.

Circulatory System in Fish (Video)

Comparison of blood circulation in fish and humans

We all learned the functioning of the human blood circulatory system in our high school biology classes. So, let us take a quick recap:

The human heart has four chambers; viz. the right and left atrium and the right and left ventricle. Here is a brief introduction to the functioning of these four chambers:

The right atrium receives deoxygenated blood from the body through the veins and pumps it to the right ventricle.

The right ventricle collects the blood from the right atrium and pumps it into the lungs.

The left atrium receives oxygenated blood from the lungs and transfers it to the left ventricle.

The left ventricle pumps this oxygenated blood to the rest of the body.

As we can imagine, this type of circulation is known as double circulation because the oxygenated blood and de-oxygenated blood circulated in different loops.

On the other hand:

The fish blood circulatory system is a single loop system in which the oxygenated and de-oxygenated blood travel in the same loop.


The fish heart contains a single atrium and a single ventricle.


The atrium has thin walls and consists of a few muscles. The deoxygenated blood from the fish’s body comes into the atrium, which pumps it to the ventricle.


The ventricle consists of a lot of cardiac muscles and has thicker walls. From here, the blood goes into the gills, gets oxygenated, and flows into the rest of the body.

Apart from the atrium and ventricle, following are two more parts that comprise a fish’s heart:

Sinus venus

Sinus venus is a part of the fish’s heart that has thin walls. The blood from the veins is collected in the sinus venus and then transferred to the atrium.

Bulbus arteriosus

Bulbus arteriosus is the fourth part of the fish’s heart that looks similar to a pear. The main purpose of bulbus arteriosus is to maintain blood flow into the gills.

It is important since the blood pressure needs to be enough to transfer it to the gills and from there to the rest of the body.

Fish blood

The fish blood mainly consists of the following:

  1. Plasma
  2. Blood cells

Plasma is the part of blood that makes it fluid. Plasma mainly consists of water, salts, and other substances such as glucose, waste (to throw out), etc.

The blood cells comprise 30% to 50% of the blood and are further divided into the following:

  1. Red blood cells
  2. White blood cells
  3. Thrombocytes

The red blood cells carry a protein called hemoglobin that carries the oxygen throughout the body.

The white blood cells attack harmful organisms such as viruses and bacteria. The white blood cells are part of the fish’s immune system.

Thrombocytes, similar to platelets in humans, keep the blood from clotting.

Blood Vessels

Blood vessels perform the task of carrying blood throughout the body. They can be classified as follows:

  1. Arteries
  2. Veins
  3. Capillaries

Arteries carry the oxygen-rich blood from the gills and take it throughout the fish’s body.

Then there are arterioles that are smaller and have thinner walls as compared to arteries. Arterioles link regular arteries with capillaries.

Did you know?

Arterioles can also dilate to ease circulation by minimizing resistance.

Veins take the deoxygenated blood from the body and carry it to the heart.

Capillaries are smaller than both arteries and veins and are situated in the body tissues. These small blood vessels connect the arteries and veins.

Capillaries are not only smaller but also thinner than other blood vessels. Due to the thinner capillary walls, diffusion is possible.

Diffusion is the process by which oxygen and other useful nutrients, present in the oxygenated blood from the arteries, enter into the cells. Simultaneously, carbon dioxide and other waste, present in the cell, is transferred to the capillaries.


Where humans have lungs, fish have gills.

This is where all the action happens, i.e. oxygen from the water is absorbed while the carbon dioxide is thrown out.

Blood circulation

Now that we have a basic understanding of all the main components of the blood circulatory system of a fish, it is time to discuss the whole process to get a comprehensive view.

The fish blood circulation system is a closed-loop system, i.e. all the blood flows between the blood vessels.

After the exchange of gases occurs in the gills, i.e. oxygen from water enters the blood, while the waste from the blood diffuses into the water, the blood enters the arteries.

The blood from these arteries goes into the arterioles. The arterioles transfer the blood into the capillaries where the oxygen is taken in by the cells, and the waste enters the capillaries.

Now that the blood is depleted of all the oxygen, it has turned into deoxygenated blood, which enters the venules. The venules take these blood to the veins that transfer it to the heart.

Function of blood

Like in all animals, the fish blood performs the following functions:

  1. Respiration – the transfer of oxygen to all the body cells
  2. Nutritional – the transfer of nutrients (vitamins, glucose, etc.) to all the body cells
  3. Excretion – the blood takes waste (urea, carbon dioxide, etc.) from the cells to the gills where it is thrown out
  4. Other – the blood contains hormones and exchanges electrolytes and other substances

Related Questions

Do fish have colorless blood?

Antarctic icefish is a type of fish that has colorless blood.

The reason is that its blood does not contain hemoglobin or red blood cells.

Experts believe that such odd blood may be because of genetic mutation.


Because there is no hemoglobin in its blood, the blood of Antarctic icefish carries only 10% oxygen as compared to fish with hemoglobin in their blood.

One might ask:

How can these fish survive with such little quantity of blood?

The thing is:

The Antarctic waters are known to carry a lot more oxygen as compared to the rest. Furthermore, these fish have bigger hearts which pump more blood throughout their bodies; thus, compensating for the lack of hemoglobin.

Why do fish have low blood pressure?

Fish need to maintain low blood pressure because the gills are thin-walled. Higher blood pressures can potentially damage the thin-walled gills.

The task of maintaining blood pressure is assigned to conus arteriosus that is consists of a lot of muscles and valves.

Leave a Comment