Kidneys: function, location, anatomy and functioning of the kidneys

Where are your kidneys? Your kidneys are located at the back of your body, above the waistline. The kidneys are the body’s main excretory organs and are part of the urinary tract. The function of the two bean-shaped kidneys (see image) is vital. The kidneys filter the blood day and night, remove waste products from the blood and also maintain the water balance in the body. All waste and excess water is converted into a product called ‘urine’. Urine is in fact a mixture of waste products, foreign substances, excess water and excess salts, which are excreted by the kidneys. This excretory fluid from the kidneys is collected in the bladder and ultimately discharged through the urethra or urethra, in an action called ‘urination’.

  • Kidneys and urinary tract
  • Urinary system
  • Two kidneys
  • Location of the kidneys: where are your kidneys located?
  • Reddish-brown, bean-shaped organs
  • Vulnerable
  • Anatomy and function of the kidneys
  • Renal capsule, renal cortex and renal medulla
  • Nephrons
  • Diabetes
  • Hormones
  • Function of the kidneys
  • Removing waste products from the blood
  • Regulating the body’s fluid and salt balance
  • Production of hormones
  • Kidneys and blood pressure
  • Damage to the kidneys – kidney complaints
  • Loss of kidney function
  • Chronic damage to the kidneys
  • Hidden kidney damage


Urinary tract / Source: La Gorda/

Kidneys and urinary tract

Urinary system

The kidneys are part of the urinary tract. The urinary system consists of the kidneys, renal pelvis, ureters, bladder and urethra. The urinary system not only excretes excess water, but also carefully controls the composition of all fluids in the body to maintain balance. At the same time, harmful waste products are removed. The kidneys are the main excretory organs. The kidneys can be regarded as filters that work 24/7 to purify the blood of substances that do not belong in it, or that are present in too large quantities. The kidneys turn this into ‘urine’. Urine is a waste product. It consists of water and dissolved waste products.

Two kidneys

The design of the human body is based on two kidneys. The kidneys are located on the left and right, high in the side, just behind the lower ribs on the back. Each kidney has a renal pelvis: a kind of funnel in which urine collects. From the renal pelvis, urine flows via the ureter to the bladder, which is located in the lower abdomen. The urine is temporarily stored in the bladder; the bladder is essentially a temporary store of urine. When your urinary bladder is full, you feel the urge to urinate and urine ultimately leaves your body through the urethra.

Where are the kidneys located? / Source: Lennert B, Wikimedia Commons (CC BY-2.5)

Location of the kidneys: where are your kidneys located?

Reddish-brown, bean-shaped organs

The kidneys are two reddish-brown, bean-shaped organs about 10-12 cm long, 6-7 cm wide and 3 cm thick (about the size of a fist), and they are located on the back above the waistline. An average kidney weighs 150 grams. The kidneys owe their reddish-brown color to the many blood vessels that run through them. The inner edge is hollow and contains a notch in the middle, the kidney port. The kidneys receive their arterial blood from the aorta via the renal artery (via the renal portal). The renal vein drains the purified blood to the inferior vena cava. The beginning of the ureter is also located on the hollow side of the kidney. Approximately 1,500 to 2,000 liters of blood flows through the kidneys every 24 hours. The kidneys produce about 1.5 liters of urine every day. The kidneys can process up to 600 times their own weight in fluid every day.


The kidneys are less well protected against injury than organs such as the heart and liver. This seems to have been taken into account in the design of the human body, because we have two kidneys. This protects us against possible loss of a kidney due to an accident or intentional injury.

Construction and anatomy of the kidneys / Source: Public domain, Wikimedia Commons (PD)

Anatomy and function of the kidneys

The kidneys have a complex anatomy.

Renal capsule, renal cortex and renal medulla

The kidneys are carefully packaged. On the outside, the kidneys have a smooth brown capsule: the renal capsule. Within this is the second, fairly narrow layer, the renal cortex. This is followed by a layer of perirenal fat that protects the kidneys like a cushion against shock and impact. Beneath this layer is the renal medulla, which is striated and consists of cone-shaped structures that project into the renal pelvis, which is the cavity into which urine passes. The renal pelvis is connected to the urinary bladder via the ureter.
Striking in the renal medulla are the pyramid-shaped medulla pyramids. Each human kidney contains 10-18 medullary pyramids. The primary urine or foreurine produced by the renal corpuscles is processed in the medullary pyramids; for example, water and salts are extracted and added back to the body: this process is known as reabsorption. This means that only a small part of the primary urine actually enters the urinary bladder, most of it is recycled or reused. The medullary pyramids are tapered. Urine ‘drips’ from these papillae into the calyx and from there into the renal pelvis.


Each kidney is made up of a million microscopic filters called ‘nephrons’ (or ‘sieve bodies’). Each nephron consists of five parts. The nephrons filter the blood to produce ‘pre-urine’ and then substances are reabsorbed from the pre-urine to the blood, so that only urine remains. Each day, the two kidneys together produce approximately 180 liters of preliminary urine, of which 178.5 liters are reabsorbed. The reabsorption takes place by parts of the nephron other than the filter. The reabsorption of substances requires great precision, which requires a lot of energy and therefore oxygen.


The kidneys are adaptive. They can adapt well to changing circumstances. If you consume a lot of salts, your kidneys will also remove more salts from the blood. Kidneys will also always ensure that all glucose from the fore urine is reabsorbed. However, there are situations in which the supply of glucose in the blood is so high that the kidneys can no longer cope with this abundance, so that urine is still produced in which glucose is present (= glycosuria). This is, for example, the case with uncontrolled diabetes. When there is more sugar in the urine, extra water is drawn in. This is also the reason that a diabetic has to urinate a lot (= polyuria), causing him to feel excessively thirsty.

Pituitary gland / Source: Tefi/


The kidneys are assisted in their functioning by hormones in order to ensure adequate reabsorption and excretory processes. For example, the hormone ‘aldosterone’, which comes from the adrenal cortex, helps with the reabsorption of sodium. The ‘antidiuretic hormone’ (ADH), which is released into the bloodstream by the posterior pituitary gland, plays a role in the regulation of water and fluid in the body. ADH prevents the formation of urine (diuresis) by always retrieving the right amount of water in the nephron. A deficiency of this hormone causes excessive urine production (polyuria). A large amount of unconcentrated urine is produced, which can cause dehydration. A deficiency of ADH is called ‘diabetes insipidus’ (which has nothing to do with diabetes mellitus). People with diabetes insipidus urinate a lot and are constantly thirsty. The condition can be treated well with medication.

Function of the kidneys

The three main functions of the kidneys are:

  • Removing waste products from the blood;
  • Regulating the fluid and salt balance; and
  • The production of hormones.


Removing waste products from the blood

Your kidneys are the most important filter in your body. They filter waste products from the blood that end up there through the various metabolic processes in the body, and these waste products, together with water, form your urine.

Regulating the body’s fluid and salt balance

Above we have already described the process by which the kidneys remove waste products from the blood. Another important function of the kidneys is to regulate the body’s fluid and salt balance. The body (which consists largely of water) can only function properly if the amount of fluid remains as constant as possible. This is a job of the kidneys. For example, if you have drunk a lot, your kidneys produce extra urine and if you have to sweat a lot, less. The kidneys also ensure that the amount of salts in the blood remains within certain limits.

Production of hormones

Your kidneys also produce hormones. Hormones are (signal) substances that flow through the blood to other parts of the body and tell other organs what to do. Your kidneys produce a hormone (erythropoietin), which stimulates the bone marrow to make red blood cells. The kidneys also produce a hormone called ‘renin’, which plays a role in regulating blood pressure. The function of renin is to prevent low blood pressure. With impaired kidney function, too much renin is often produced, which results in high blood pressure (hypertension) and damage to the blood vessel wall (atherosclerosis). Your kidneys also ensure that the prehormone vitamin D3 is converted into active vitamin D. This vitamin has an important task; it ensures that the body can absorb calcium into the bones.

Kidneys and blood pressure

It is not only kidney disorders that can increase blood pressure. The reverse is also possible: high blood pressure can cause damage to the kidneys (high blood pressure damages the kidney filters, part of a nephron), resulting in a downward spiral. If your blood pressure is too high for a long time, the blood vessels can gradually damage and narrow. This causes reduced blood supply, also to the kidneys. This causes kidney damage, causing the kidneys to work less well and have increasing difficulty filtering waste products from the blood. This results in your kidney function deteriorating. Your kidneys then produce substances to maintain blood flow, but these substances increase blood pressure even more and then the circle is complete: due to the higher blood pressure, the blood vessels (including those in the kidneys) have to endure even more strain, etc. As a result of this process, the kidneys function increasingly poorly.

Blood collection / Source:

It turns out that about 1 in 5 people with high blood pressure suffer from chronic (that is, permanent and irreversible) kidney damage. If you suffer from high blood pressure, it is advisable to have your kidneys checked annually. If you catch it in time, kidney damage can often be limited. The doctor can perform this check by looking at the amount of protein in your urine. He may also have blood tests performed to determine kidney function. If chronic kidney damage is diagnosed, your doctor may decide to prescribe (other) medications.

Damage to the kidneys – kidney complaints

Loss of kidney function

Kidney damage can lead to loss of kidney function. A distinction can be made between acute and chronic kidney damage. In acute kidney damage, the kidneys suddenly (acutely) fail to function sufficiently, resulting in too little waste products being filtered from the body and therefore too much waste products remaining in the body. Acute kidney damage can be caused, for example, by:

  • Too little blood supply. Too little blood supply occurs due to low blood pressure. This can be caused, for example, by major bleeding elsewhere in the body. Another common cause is dehydration due to diarrhea or vomiting. This causes the pressure in the kidneys to drop and as a result the kidney filters can no longer perform their work (temporarily).
  • An infection. Inflammation can cause the kidney filters to become damaged.
  • A blockage. A blockage means that the kidneys cannot drain urine properly, causing congestion. An overly large prostate, stones or a tumor are possible causes.


Chronic damage to the kidneys

Chronic kidney damage is the most common. The progression is often insidious. Chronic kidney damage occurs gradually and can have various causes. The most common causes are:

  • inflammation of the kidneys (renal inflammation);
  • diabetes mellitus; or
  • high blood pressure.

Furthermore, the literature shows that smoking and obesity have an unfavorable influence on the disease process. It is known that smoking damages the blood vessels to and in the kidneys, which reduces the kidneys’ ability to function properly.

Moving is healthy! / Source:

Hidden kidney damage

It appears that in the Netherlands approximately 1 in 10 people suffer from ‘hidden kidney damage’. Kidney damage is evident from tiny traces of protein in the urine. Furthermore, it appears that approximately 1 in 20 Dutch people have kidney damage with loss of kidney function. Although chronic kidney damage cannot be cured, adequate treatment can slow the decline in kidney function. You can reduce the risk of kidney damage by:

  • eat healthy and maintain a healthy weight;
  • to use as little salt as possible;
  • not to smoke;
  • enough exercise; and
  • drink alcohol in moderation.

These lifestyle recommendations also help prevent high blood pressure and type II diabetes, two diseases that in themselves can also lead to kidney damage.

read more

  • Kidney Disease: Symptoms, Causes and Treatment
  • Kidney Stones: Symptoms, Causes, Treatment and Removal
  • Kidney function test: research into the functioning of the kidneys
  • Kidney pain: causes pain in the kidneys, kidney area and back pain
  • Kidney cysts (cysts in the kidneys): symptoms and treatment
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