It consist approximately 7 percent of body weight.

By definition organ is composed of multiple tissues. Blood is a fluid, a circulating tissue.

Therefore can we call this fluid system a liquid organ?

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    $\begingroup$ I think , blood is an organ due to the composed of different tissues $\endgroup$ – Ibrahim Khan Sep 24 '17 at 2:07
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    $\begingroup$ Hmm... Blood(a connective tissue) in my own opinion qualifies to be called an organ, if we are going by the definition which states that " Organ is a group of different tissues coming together to perform a specific function". And we know tissue is basically a group of similar cells performing one function. So group of red blood cells in the blood is one tissue, group of white blood cells in there is another tissue,etc. So I think with these two different tissues coming together (with others) to make up the blood, automatically makes the blood an Organ. $\endgroup$ – M. Awwal Nov 3 '17 at 21:36

Blood is considered a type of connective tissue (sometimes). However, an organ is formed of multiple different tissues. Thus, blood is a tissue, not an organ.

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  • $\begingroup$ Would it be fair to say that a counter to blood not being an organ is that blood has multiple cell types? But what is key to remember is that tissues are organised structures of different tissues. Blood has very little organisation due to its fluidity. $\endgroup$ – James Apr 23 '15 at 17:39
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    $\begingroup$ @Good Gravy I suspect that the fact that blood lacks structure is a reason why not everyone considers it to be a tissue. As far as I know, the multiple cell types themselves are not considered to be separate tissues by anyone (for instance, we don't consider blood to be composed of red blood cell tissue and white blood cell tissue), but then again this is not my area of expertise so I wouldn't be able to say for sure $\endgroup$ – C_Z_ Apr 23 '15 at 17:42
  • $\begingroup$ organs have to be solid or gelatinous to be organs OR tissues in my opinion. otherwise, they are just juices. blood is not a tissue, it is a juice! tissues are called tissues because they are meaty. your brain is tissue. your blood is merely liquid. $\endgroup$ – 4D Neuron Oct 8 '17 at 1:57

While organs are generally considered to have a single, specified function (or perhaps a group of closely-related functions), blood (the fluid inside the vessels, not the vessels themselves) has many different functions:

  • deliver $O_2$ from lungs to cells
  • remove waste $CO_2$ from cells to lungs
  • respond to injury by clotting
  • carry multiple different types of nutrients by multiple different means (free-floating, carrier protein-associated, lipid particles, etc.)
  • provide transport for the immune system, which in itself has diverse functions
  • transport waste products to liver and kidneys
  • move regulatory molecules around the body (hormones, chemokines, cytokines, antibodies, etc.)
  • regulate heat in the body
  • maintain $pH$ and $H_2O$ balance
  • and probably more I haven't thought of

So, I don't think blood should be classified as an organ, just because of its multitude of uses. Instead, it is vital for the proper functioning of pretty much all the organs in the body, and allows for connections among them.

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    $\begingroup$ Any reason for the downvote? $\endgroup$ – MattDMo Apr 24 '15 at 14:21
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    $\begingroup$ Sorry for the late update. I don't think multitude of functionality is an criterion of being an organ or not. Organs like the liver also have a multitude of uses which are not necessarily related. One could also argue that the functions you listed are also a group of closely related functions (transport and dissipation). $\endgroup$ – Sleepses May 6 '15 at 15:15

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