The kidneys are essential homeostatic organs that filter approximately 150 litres of blood every day, eliminating around 2 litres of water and waste materials in urine. These waste and toxic materials would accumulate in the blood and cause damage to the body if the kidneys did not remove them. You can think of kidneys as our body's sewage treatment plants! As well as filtering our blood, kidneys also perform other functions, such as regulating the blood's water content and synthesising essential hormones.
Urine describes the waste excreted from the urethra. Urine contains materials such as water, ions and urea.
Kidney Location in the Human Body
The kidneys are two bean-shaped organs that are approximately the size of a clenched fist. In humans, they are located in the back of your body, directly below your ribcage, one on each side of your spine. You will also find the adrenal glands sitting on top of each kidney.
Fig. 1 - The location of the kidneys in the human body
The kidneys are paired retroperitoneal organs that are typically positioned between the transverse processes of the T12 - L3 vertebrae, with the left kidney being slightly superior to the right. This asymmetry is due to the presence of the liver above the right kidney.
Kidney Anatomy
The kidneys have three main structural regions: the outer cortex, inner medulla and renal pelvis. The outer cortex projects into the medulla, creating triangular segments called renal pyramids, while the renal pelvis serves as the region where blood vessels enter and exit the kidney.
Fig. 2 - This diagram shows the internal renal structures
Each kidney comprises around a million functional filtering units known as nephrons. Each nephron extends from the cortex to the medulla and is made of various components, each with its own set of functions.
The nephron is the functional unit of the kidney that is responsible for filtering the blood. Adults have approximately 1.5 million nephrons in each kidney.
Fig. 3 - A diagram depicting the structures and sections within a nephron
Nephrons are composed of the following main elements: the Bowman's capsule, glomerulus, proximal convoluted tubule, the loop of Henle, distal convoluted tubule and the collecting duct. You don't need to know the detailed structure of the nephron, but you should appreciate how it is responsible for filtration and selective reabsorption (which you will read in the following section)!
Kidney Functions
The kidneys' primary function is to maintain the water balance in the body, which is known as a homeostatic mechanism. The kidney can return the blood's water content to basal levels when it gets too high or too low, thus maintaining a constant internal environment. Additionally, kidneys are responsible for synthesising essential hormones necessary for red blood cell production, namely, erythropoietin and renin.
In embryos, erythropoietin is synthesised in the liver, but it is made in the kidneys in adults.
Kidney's Maintaining Water Balance
To maintain the blood's water balance, the kidneys produce urine which is excreted. This enables the removal of electrolytes, such as sodium and potassium, in excess in the body. Additionally, urine allows the excretion of metabolic waste products from the blood that would otherwise be toxic to the body.
The nephrons maintain water balance in two stages known as the glomerular stage and tubular stage. In the glomerular stage, ultrafiltration occurs whereby glucose, urea, salts and water are filtered at high pressure. Larger molecules, such as proteins and red blood cells, remain in the blood vessels supplying the kidneys and are filtered out.
Only useful substances are taken back into the blood in the tubular stage. This includes almost all of the glucose, some water and some salts. This 'purified' blood returns to circulation.
The substances that have not been reabsorbed travel through the nephron network, to the ureter and to the bladder where it is stored. The urine is then excreted through the urethra. Interestingly, the level of water reabsorption is influenced by the anti-diuretic hormone (ADH), which is released from the pituitary gland in the brain. When your body detects low water content in the blood, more ADH is released, which will promote water reabsorption to return your water levels to normal. Read more about this mechanism in our article ADH!
Ultrafiltration occurs within the Bowman's capsule. The glomerulus, an extensive network of capillaries, allows only small molecules, such as glucose and water to pass through into the Bowman's capsule. Meanwhile, selective reabsorption occurs within the tubules, including the proximal and distal convoluted tubules.
Producing Hormones in the Kidneys
The kidneys play an endocrine function by synthesising and producing several hormones, including renin and erythropoietin. Renin is an important hormone that is involved in the regulation of blood pressure. When blood pressure drops, the kidneys release renin, which activates a cascade of other effector molecules that constrict the capillaries to raise blood pressure; this is also known as vasoconstriction.
When the kidneys are not working correctly, they may secrete too much renin into the blood, raising blood pressure and occasionally leading to hypertension (high blood pressure). As a result, many individuals with kidney dysfunction suffer from hypertension.
Erythropoietin functions by acting on the bone marrow to stimulate the production of red blood cells. If kidney function deteriorates, an inadequate amount of erythropoietin is produced, significantly lowering the number of new red blood cells produced. Consequently, many individuals with poor kidney function also develop anaemia.
Anaemia is a condition in which an individual lacks sufficient numbers of red blood cells in their body, either in quantity or quality.
Another function of the kidneys is activating vitamin D into its active hormone form. This 'activated' form of vitamin D is required for calcium absorption in the gut, proper bone formation, and optimal muscular function. Low blood calcium and an insufficient quantity of vitamin D are shared in those whose renal function has become compromised, resulting in muscular weakness and diseases of the bone such as rickets.
Kidney Disease
When the kidneys fail, toxic wastes and excess fluid can accumulate in the body. This can result in ankle oedema (swelling caused by extra fluid accumulating in bodily tissues), weakness, poor sleep, and shortness of breath. Without treatment, the damage will deteriorate until it leads to complete kidney failure, which can be dangerously fatal. Kidney disease can broadly be classified into acute kidney injury (AKI) and chronic kidney disease (CDK).
AKI is a brief period of renal damage and is typically triggered by complications of another severe illness. This includes kidney stones or kidney inflammation. As a result, water products that would have otherwise been excreted accumulate in the blood. On the other hand, CKD is a long-term condition that describes the progressive loss of kidney function over several years. The most common causes of CKD include diabetes, obesity and high blood pressure.
CKD can only be identified after a blood or urine test. Patients usually show symptoms such as swollen ankles, shortness of breath and blood in the urine.
Kidney Disease Treatments
Individuals should be able to survive with just one healthy kidney, but if both fail, it can ultimately lead to death if left untreated. Those with very poor renal function need to undergo renal replacement therapy, which includes:
Although a kidney transplant is the best solution for complete renal failure, it requires the patient to satisfy all necessary criteria and be placed on a long waiting list. Meanwhile, kidney dialysis is a temporary solution for those awaiting a kidney transplant or ineligible for organ transplantation. There are three main types of dialysis: haemodialysis, peritoneal dialysis, and continuous renal replacement therapy (CRRT).
Have a read of our Dialysis article to learn about the advantages and disadvantages of each kidney dialysis treatment!
Kidney - Key takeaways
- The kidneys are two bean-shaped organs located in the back of your body, and they are essential for homeostasis.
- The nephron is the kidney's functional unit and extends from the outer cortex to the inner medulla.
- The kidneys' primary function is to maintain water balance and produce hormones, such as erythropoietin and renin.
- Kidney disease can broadly be classified into acute or chronic. Chronic kidney disease can be treated with dialysis or transplantation.
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