Fish Facts

Do Fish Have Kidneys?

Kidneys in fish are of different sizes in various species such as seawater, marine fish, saltwater, and cartilaginous.

Do Fish Have Kidneys? Kidneys in fish play a vital role in different excretory mechanisms like osmoregulation, excretion, homeostasis, and a few others. Fish kidneys regulate the water, salt, and nitrogenous waste like ammonia, urea, creatinine, and others, eliminating through urine.

Do Fish Have Kidneys?

The essential kidney function is to release out excess wastage substance and fluid from the body.

It does through the urine, which produces due to the excretion and reabsorption through a complex process.

This process helps maintain the essential function or parameters like hemostasis, osmoregulation, and many others.

The vital role includes removing nitrogenous waste potassium, acid content and developing or producing the beneficial hormone. 

Their leading role in your fish is to maintain the constant internal body environment balance the water, salt, and dissolved substance. 

These are different in structure to other mammals present in a paired form on both sides of the backbone.

It present in a retroperitoneal position dorsal to the abdominal cavity and ventral to the vertebral column.

It is surrounded by the connective tissue capsule and paired urinary duct, which joins the common urinary duct.

It opens into the urinary pore. The particular term pseudo-copulatory papilla uses both reproductive and excretory systems to leave through a fish’s shared vent.

This case happens mostly in males than in females. The posterior tubules region multiplication is known as Opisthonephros.

Types of fish kidneys

First, In some fish species like rainbow trout and salmon, two kidneys fuse altogether. There is no apparent distinction between the truck and the head of it. 

Second, in this case, the kidney middle and posterior arts fuse entirely. In the microscope, you can observe the clear head and trunk demarcation.

The fish types include carps, Ayo, and Cyprinidae, in which, from the middle fuse portion, the anterior tube-like structure appears.

These separate from each other, and the head kidney contains a sac-like shape located on the tube tip.

The significant feature of it is the tail portion, which fuses but the other parts separate. It is present at the anterior top most region, and the head kidney has a round shape.

Fourth, the morphological adaptation of fusion is less for the eel species, and no distinction is present. In this case, you observe that the two kidneys are separate, but the posterior part becomes not isolate and fused.

Another type, in which there is no clear distinction occur and shape also different. The front or anterior trunk kidneys are thick in appearance, while the tail-like have a thin tube-like structure.

Kidney mechanism

The blood supply from the kidney proceeds by the arterial and venous blood through the renal portal system. Glomerulus acts as an ultra-filtring device.

Water, salt, and excess waste product pass through the renal tubule by entering into the Bowman capsule. The hormonal action by the endocrine system controls the reabsorption and filtering process. 

A significant difference occurs in the marine and freshwater nephron part due to different salt and water absorption and intake. The size of this kidney organ different in both, like freshwater contain big as compared to the other. 

Why kidneys are essential for fish?

These have a vital role in the fish body system, removing the waste product from it. It also excretes out the excess fluid from the body and maintains the original level.

The functioning or its importance is different relates to the fish species like saltwater and marine fish. 

Homeostasis process in which internal fish environment of salt and water proper balance through this excretory organ.

Bothe type of fish surrounding water has different salt content, so their uptake and osmosis also change. 

Osmosis is the process in which exchanges of salt and water occur through a concentration gradient.

The mechanism is different for both species, and habitat like saltwater has high water loss while others absorb it. 

Another essential function is the diffusion process, in which various dissolved substance concentrations present on both sides of the membrane.

In this case, water will enter or diffuse into the more concentrated portion. However, the salts travel into the low concentration area.

If the kidney functions damage, resulting in hypertrophy, deformation, tubules compression and destruction, nephrocalcinosis, cyst, duct, and urinary bladder inflammation.

Granulomas also the leading factor of tubular color changes. 

Regeneration of kidneys

In this scenario, acute and chronic kidney structure damage like nephron. It can regrow this part though de novo in its entire life.

Fish nephron progenitor present in that excretory organ, which adds new nephron continuously during fish growth.

The kidney function is at the same rate as its body weight. The cascade of signals produces, which help the salt and fluid balance changes maintenance. 

Marine water and freshwater kidney difference

Freshwater fish

Freshwater fish contain in their blood more salt than the water around their bodies. 

Through the osmosis process, water will diffuse into the body due to the concentration gradient. The plenty of water will accumulate build up inside.

These fish also get some salt amount from the food, and they also absorb through the mouth when water passes out, and skin and gills absorb through it.

The process proceeds through a particular cell, but they drink less water and get through food.

This type of fish’s excretory organ increases the water amount in the urine and active salt reabsorption to maintain balance.

The freshwater type has diluted urine, which contains almost water amount. These have low blood urea concentration levels.

Some fish species like salmon move between freshwater and marine water and maintain the osmotic gradient reversal.

This mechanism is essential for the physiological processes through retaining in that intermediate brackish environment.

Marine water fish

The salt or marine water fish kidney function is the complete opposite of the other. In this case, water and liquid enrich with the high salt concentration and other solutes.

Through the osmosis process, fish water constantly stuck out of then and inside the water around than the fluid inside fewer concentrates than the saltwater surrounding the fish body.

To overcome water sticking, fish require water intake and use their excretory organ. In this way, water removes from their bodies but results in excess salt intake, which harms them.

The urine becomes more concentrated because water reabsorbs and active salt excretion is done through kidneys.

The nitrogenous waste releases out from the fish in ammonia by gills and salt from the special cell-like (chloride cells).

Shark, rays, and hagfish osmotic concentration equal to the seawater, and they do not require water intake for physiologic process maintenance. It becomes high because of blood urea retention. 

Morphological and functional characteristics of kidneys in fish

There are most of the fish species which have the various habitat, so their morphological and functional characteristics also different. 

Teleost fish

The teleost species control the sodium and chloride ion plasma concentration and osmolality of seawater one-third.

Seawater fish 

Marine fish like sea horses have a different feature than freshwater in which dehydration risk is high. In seawater(SW), the salt content is high, about 2%-3%, which highly hypertonic compared to the extracellular fluid.

Cartilaginous fish

The cartilaginous fish like shark, rays, stakes, and others contain a unique Osmo-regulatory strategy.

It helps to adopt the marine environment’s high salinity and controls the ion plasma concentration one-half of the surrounding seawater.

They also act as an osmolyte and store the urea as a nitrogenous component. This phenomenon considers as the Ureosmotic scheme.

This type of species’ fluid is more hyperosmotic than the seawater surroundings and high blood urea level, like a shark, around 2%-2.5%.

Another mechanism is urea retention, which maintains a high urea level. To overcome the urea loss, the fish adapt the specialized nephron cell through, which urea recover from the filtrate efficient way.

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