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Introduction to Kidney

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The kidney is an organ that is present in vertebrates and some invertebrates. The kidneys are the two reddish-brown bean-shaped organs that are found in all vertebrates. In adult human beings, the kidneys are twelve centimeters in length and they are located on the left and the right in the retroperitoneal space. The renal arteries are responsible for the transfer of blood in the kidneys and the blood exit into the paired renal veins. Both the kidneys are attached to a ureter which is a tube that carries excreted urine to the bladder. 

The functional and structural unit of the kidneys is called the nephron. There are one million nephrons in each adult human kidney. The number of nephrons in the kidney changes from one organism to another as in a mouse, there are about twelve thousand five hundred nephrons only. 

Kidney takes part or has various functions such as in the control of the volume of the various body fluids, acid-base balance, various electrolytes concentrations, fluid osmolality and also helps in the removal of the toxins from the body. 

The filtration of the blood takes place in the glomerulus where one-fifth of the blood volume which enters the kidneys gets filtered. Solute-free water, glucose, amino acids, sodium, and bicarbonates are examples of the substances that get reabsorbed during filtration. Substances such as uric acid, ammonium, potassium, and hydrogen get secreted during filtration. 

There are also few functions that kidneys carry, which are independent of the nephrons. Functions such as converting the precursor of the vitamin D to its active form which is calcitriol and also synthesizing the hormones erythropoietin and renin are a few of the functions which the kidneys carry which are independent of the nephrons. 

The study which deals with the study of kidney function is called Renal physiology. Nephrology is one of the medical specialties that deal with diseases that are related to kidney function such as nephrotic syndromes, acute kidney injury, chronic kidney disease nephritic, and pyelonephritis. 

In this article, we are going to discuss the definition of kidney, the kidney diagram or the kidney drawing, the Structure of the kidney, functions of the kidney, the diseases related to kidneys, and the various treatments which are present to treat the diseases. 


Kidney Definition 

Kidneys are the pair of reddish-brown, bean-shaped organs that are present on either side of a human spine, which is below the ribs and behind the belly region. 

Kidneys are present in all vertebrates and few invertebrates and the human kidney is about five inches long which is roughly equal to the size of a large fist. Filtering the blood is the main function of the kidneys. They also help in removing the waste, controlling the body’s fluid balance, and also helps in maintaining the right level of electrolytes in the body. All the blood which is present in our body passes through the kidneys several times a day just for filtration. 

When the unfiltered blood enters the kidney, the waste present gets removed and if required the salt, water, and minerals are adjusted. The filtered blood from the kidney then moves back into the body and the waste which was collected from the blood during the filtration process gets turned into the urine that is collected in the kidney’s pelvis. The kidney’s pelvis is a funnel-shaped structure that drains down a tube called the ureter to the bladder.

The ts of kidney of mammal or ts of mammalian kidney is given below:

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Structure of Kidney 

A Kidney Drawing is Given Below:

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  • In humans, the kidneys are located at the back of the abdominal cavity, one on each side of the spine and they lie in the retroperitoneal position at a slightly oblique angle. 

  • The asymmetry within the abdominal cavity is due to the position of the liver which results in the right kidney being slightly lower and also smaller than the left kidney. The left kidney is mostly at the vertebral column and the right kidney is placed slightly more to the middle than the left kidney. 

  • The right kidney is slightly smaller than the left kidney and also it sits just below the diaphragm and posterior to the liver. The right kidney is smaller as it allows the liver to be in that space. 

  • The left kidney sits posterior to the spleen and below the diaphragm. Each kidney weighs up to 125-175 grams in the males and 115 to 155 grams in the females. The kidney is surrounded by the tough and fibrous renal capsule. 

  • There is the presence of an adrenal gland on top of each kidney. The eleventh and the twelfth ribs help in protecting the upper part of the kidney.

  • The adrenal glands present on each kidney have two layers of fat on it which are the perirenal fat and the pararenal fat. The perirenal fat is present between the renal fascia and the renal capsule and the pararenal fat is present above the renal fascia. 

  • The kidney is a bean-shaped structure that has concave and convex borders. Renal hilum is the recessed area on the concave border. The renal hilum is the place where the renal arteries enter the kidney and the renal vein and the ureter leaves.

  • The kidney is surrounded by the tough and fibrous renal capsule which is in turn surrounded by layers of two layers of fat such as the perirenal fat and the pararenal fat and also the renal capsule is surrounded by the renal fascia. 


Anatomy of the Kidney 

  • The parenchyma is called the functional substance of the kidney. The parenchyma is divided into two major structures which are the outer renal cortex and the inner renal medulla. 

  • The renal cortex and the renal medulla take the shape of eight to eighteen cone-shaped renal lobes. Each of the renal lobes contains the renal cortex which surrounds a portion of the medulla called the renal pyramid. 

  • Between the renal pyramids, there are projections of the cortex called renal columns. 

  •  Nephrons are the urine-producing functional structures of the kidney that span the cortex and the medulla.

  • The renal corpuscle which is located in the cortex is the initial filtering portion of a nephron. The renal corpuscle is followed by the renal tubule that passes through the cortex deep into the medullary pyramids.

  • The medullary ray is a collection of renal tubules that drains into a single collecting duct. The medullary ray is a part of the renal cortex. 

  • The papilla or the tip of each pyramid empties urine into a minor calyx, the minor calyces empty into the major calyces and the major calyces empty into the renal pelvis, which becomes the ureter. At the hilum section, the ureter and the renal vein exit the kidney, and the renal artery enters the Hilar fat and lymphatic tissue with lymph nodes that surround these structures.

  • Basically, inside each kidney, there are numbers of pyramid-shaped lobes. These lobes consist of an outer renal cortex and the inner renal medulla. There is a flow of nephrons in these sections. These are considered to be the urine-producing structures of the kidneys. 

  • It is through the renal arteries that the blood enters the kidney and leaves through the renal veins. The kidneys are very small organs when compared to the heart but receive twenty-five per cent of the blood from the heart’s output. Each kidney excretes the urine through a tube called the ureter that leads to the bladder. Let us take a look at the kidney diagram.

  • A kidney figure is given below that explains the anatomy of a mammalian kidney

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Functions of the Kidney 

  • The main function of the kidney is it participates in whole-body homeostasis. The kidneys are responsible for regulating the acid-base balance, electrolyte concentrations, extracellular fluid volume, and blood pressure. These homeostatic functions are accomplished to keep the internal environment of the body consistent and comfortable. 

  • These homeostatic functions are accomplished by kidneys both independently and with the help of other organs, particularly those related to the endocrine system. Many endocrine hormones coordinate in these endocrine functions, these include renin, angiotensin II, aldosterone, and others. Kidneys serve a wide range of functions such as 

  • Waste Excretion: The kidneys help in the removal of a number of waste products and get rid of them in the urine. The two most important components that the kidney removes are urea and uric acid. Urea is the result of the breakdown of the protein and uric acid is the breakdown of the nucleic acid.

  • Reabsorption of the Nutrients: Kidneys are also responsible for the reabsorption of the nutrients from the blood and also they transport the reabsorbed nutrients to the place in the body which would support health. The kidneys also reabsorb other products that help in maintaining homeostasis. The reabsorbed products include bicarbonate, amino acids, glucose, sodium, water, phosphate, chloride, sodium, magnesium, and potassium ions. 

  • Maintaining the pH: The acceptable pH levels are between 7.38 and 7.42 for human beings. When the pH level of the human body falls below this boundary then the body enters a state of acidemia and above it would be alkalemia. Outside the optical pH level in humans, the proteins and the enzymes break down and can no longer function properly and in very extreme cases this condition could be fatal. Both the kidneys and the lungs help in keeping a stable pH level within the human body. The lungs are capable of maintaining the pH level by moderating the concentration of carbon dioxide whereas the kidneys manage the pH level by two processes which are Reabsorbing and regenerating the bicarbonates from the urine and excreting the hydrogen ions and fixed ions. In the process of reabsorbing and regenerating the bicarbonates from the urine, the bicarbonates help in neutralizing the acids. The kidneys have the option of either retaining their trusted source if the pH level is tolerable or releasing it if the acid level rises.

  • Osmolality Regulation: Osmolality is the measure of the body’s water-electrolyte balance or the ratio between the fluid and the minerals in the body. The primary cause of dehydration is considered to be electrolyte imbalance. The kidneys help in maintaining the water and salt level in the body. When the osmolality level rises in the blood plasma then the hypothalamus in the brain responds by passing a message to the pituitary glands. The pituitary glands release antidiuretic hormone(ADH). When ADH is released in the body the kidneys make a number of changes such as     

  1. An increase in urine concentration.

  2. Increase in the water reabsorption.

  3. There is a reopening of the portion of the collecting duct, which allows the water back into the body. 

  4. The urea in the medulla of the kidney is retained back rather than it getting excreted, as it draws in water. 

  • Blood Pressure Regulation: When it is required, the kidneys help in regulating the blood pressure of the body, they are also responsible for the slower adjustments. The kidneys help in adjusting the long-term pressure in the arteries by causing changes in the fluid outside of cells. The medical term of the fluid is extracellular fluid. The changes in the extracellular fluid occur after the release of a vasoconstrictor called angiotensin II. The blood vessels get narrowed only because of the hormone vasoconstrictors. These hormones also work with other functions to increase the kidney’s absorption of sodium chloride or salt. When there is an absorption of the salt, this effectively increases the size of the extracellular fluid compartment and raises blood pressure. Consumption of excessive alcohol, smoking, or obesity can alter the blood pressure which can lead to damage of the kidneys.

  • Secretion of Hormones and Active Compounds: The kidneys are responsible for the release of many active compounds such as Renin, erythropoietin, and calcitriol.

  1. The Erythropoietin: It controls erythropoiesis or the production of red blood cells. Even though the liver also produces erythropoietin, the kidneys are the main producers in an adult human being.

  2. Renin: Renin helps in managing the expansion of the arteries and the volume of the blood plasma, lymph, and interstitial fluid. The interstitial fluid is the main component of the extracellular fluid and the lymph is the fluid that contains the white blood cells and also it supports the immune activity.

  3. Calcitriol: The calcitriol is the hormonally active metabolite of vitamin D. Calcitriol helps in increasing the amount of calcium that the intestines can absorb and also helps in the reabsorption of phosphate in the kidney. 

  • Below a diagram that shows a mammal kidney slide is given 

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Diseases That Affect the Kidney Health 

There are a number of diseases that can actually affect the kidneys. Medical or environmental factors can also cause functional and structural problems from birth for some people. Here is the list of diseases with which the kidneys can be affected. 

  • Diabetic Nephropathy: The capillaries of the kidney get damaged in people who have diabetic nephropathy. The damage to the kidneys occurs due to long-term diabetes. The symptoms are not clearly seen until a year or so until the damage starts to develop. The symptoms include constant headache, tiredness in the whole body, nausea, itchy skin, and swollen legs.

  • Kidney Stones: When there is a build-up of the mineral in the kidney, the stones are formed. The kidney stones are capable of causing intense pain and if the stones block the ureter they can affect kidney function. 

  • Kidney Infections: Kidney infection is caused due to bacteria which is the result of bacteria present in the bladder which gets transferred to the kidneys. Painful urination, lower back pain, and fever are the symptoms of kidney infections. The kidney infection can also be detected by noticing a change in the urine such as the presence of blood, cloudiness, and the different odour. Kidney infections are seen to be most common in women than men and the kidney infection often responds to antibiotics. 

  • Renal Failure: In people who are suffering from renal failure, the kidney is unable to filter out the waste products from the blood effectively. Overusing a medicine can be considered a reason for kidney failure and the condition of kidney failure is often reversible with the treatment. If the kidney failure is caused by the disease then for the time being the condition has no cure. 

  • Kidney Hydronephrosis: Kidney hydronephrosis means ‘water on the kidney’.It occurs due to the presence of an obstruction that prevents the urine from leaving the kidney which causes very intense pain. At the time of this, the kidney might even shrink.

  • Duplicated Ureter: In this condition, there are two ureters formed between a kidney and the bladder. There are few complications due to this condition and it also increases the risk of urinary tract infections. The presence of the duplicated ureter affects around one per cent of people.

  • Kidney Tumour: This tumour could be benign or malignant. Malignant cancer spreads aggressively throughout the body whereas benign doesn’t spread. Renal cell carcinoma is the most common malignant kidney cancer. 

  • Nephrotic Syndrome: When there is any damage to the kidney function, it causes the protein levels in the urine to increase. Due to this, there is a protein shortage throughout the body, which in turn draws water throughout the tissues. The symptoms of nephrotic syndrome include puffy eyes, an increase in cholesterol levels, the presence of fluid in the lugs, and anaemia.


Kidney Treatments

  • Kidney Transplant: Transplanting the kidney into a person with end-stage renal disease(ESRD) can restore the function of the kidney. The transplantation of the kidney can be done from a living donor or from a deceased organ donor.

  • Antibiotics: Kidneys are mostly affected by the infection caused by the bacteria and this could be treated with antibiotics. By taking the blood or the urine sample can help to start the antibiotic treatment.

  • Nephrostomy: In this treatment method, a tube is placed through the skin into the kidney. Urine then gets drained from the kidney, bypassing any blockages in the urine flow. 

  • Lithotripsy: Kidney stones are a very common kidney disease that causes too much pain. Few of the kidney stones get shattered into small pieces that are able to pass through the urine. The stones which are not able to shatter are treated by lithotripsy. Lithotripsy is done with the help of a machine that projects ultrasound waves through the body. 

  • Dialysis: Kidney failure causes the blood to not filter the blood and as a result of this there is a requirement of the filtering of the blood and that is done by dialysis. Artificial filtration of the blood is done to replace the work that the damaged kidneys cannot do. Hemodialysis is the most common method of dialysis in the world. 

  • Hemodialysis: In hemodialysis, a person with complete kidney failure is connected to the dialysis machine which basically does what kidneys do, filter the blood. The blood was filtered artificially then returned back to the body. Hemodialysis is a very expensive treatment and it is done three days per week for patients with end-stage renal disease(ESRD).

  • Nephrectomy: It is the surgery that is performed to remove a kidney. A nephrectomy is performed for people who suffer from kidney cancer or severe kidney damage. 

  • Peritoneal Dialysis: In this treatment, large amounts of a special fluid are placed in the abdomen through the catheter which allows the body to filter the blood by using the natural membrane lining the abdomen. After the treatment is done the waste is drained and discarded. 

FAQ (Frequently Asked Questions)

Q1: Name a Few Kidney Tests.

Ans: There are few kidney tests that are done to determine the disease which is affecting the kidneys such as Urinalysis, Kidney ultrasound, Computed tomography (CT) scan, Magnetic resonance imaging (MRI) scan, Urine, and blood cultures, Ureteroscopy, and Kidney biopsy.

Q2: What Are the Important Functions of the Kidney? 

Ans: The kidney is mainly responsible for eliminating the toxic metabolites through urine that could be harmful to the body. It also helps in the regulation of blood homeostasis and blood pressure and produces some hormones that are required for the body.