The kidneys are a pair of bean-shaped organs that are found in all vertebrates. They are regulatory in nature and carry out a great number of tasks, including the removal of waste products from the body, maintaining the balance of electrolyte levels and regulating blood pressure.
The kidneys have been regarded as important organs since early history. In biblical times, the kidneys were considered to be the seat of conscience and reflective thought. God is described as inspecting the heart and kidneys of men.
Tellingly, ancient Egyptian embalmers left only the brain and kidneys in position before permanently fixing bodies, inferring some higher value.
In the Jewish text, the Talmud, it is claimed that one kidney prompts man to do what is good, and the other evil.
Modern nephrologists still have huge respect for the wonders of the kidney, although the exact reasons for this respect have changed since historical times. In the following article, we will look at the structure and function of the kidneys, diseases that affect them and how to keep the kidneys healthy.
Contents of this article:
Fast facts on the kidneys
Here are some key points about kidneys. More detail and supporting information is in the main article.
- The kidneys are a pair of organs within the abdomen
- Much of the kidneys' work revolves around maintaining the body's internal equilibrium
- Despite being relatively small organs, the kidneys use a high percentage of the body's cardiac output
- Kidneys help maintain the body's pH balance
- Dialysis is used if the kidneys have lost the majority of their function
- Blood pressure is partially maintained by the kidneys
- The kidneys secrete a number of hormones
- The adrenal glands sit on top of the kidneys
- Certain analgesics can damage the kidneys.
Structure and location of the kidneys
The right kidney is often smaller than the left.
The kidneys are located at the back of the abdominal cavity, one on each side of the spine. Due to the asymmetry caused by the liver, the right kidney is generally slightly smaller and lower than the left.
Each kidney weighs 125-170 g in males and 115-155 g in females.
Surrounding the kidneys is the tough, fibrous renal capsule and, beyond that, two layers of fat that serve as protection. On top of each kidney are the adrenal glands.
Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Flowing between these sections are nephrons, the urine-producing structures of the kidneys.
Blood enters the kidneys through the renal arteries and leaves through the renal veins. Despite being relatively small organs, together the kidneys receive up to 25% of the heart's entire output.1 Each kidney excretes urine through a tube called the ureter that leads to the bladder.
Function of the kidneys
The primary role of the kidneys is that of homeostasis. In other words, they manage a whole host of variables to ensure that the internal environment of the body is kept within constant parameters.
Here, we will discuss some of the kidneys' main functions in this regard:
A number of waste products are removed via the kidneys and expelled in the urine. Two of the major compounds to be removed are urea, produced from the breakdown of proteins, and uric acid from the breakdown of nucleic acids.
Reabsorption of nutrients
Nutrients from the blood are reabsorbed and transported to where they are needed. Other products are reabsorbed to help maintain equilibrium.
This process is referred to as reabsorption rather than absorption because the compounds have already been absorbed once, normally in the intestines.
Reabsorbed products include:
- Amino acids
- Chloride ions
- Sodium ions
- Potassium ions
- Magnesium ions.
Keeping pH at a tolerable level is essential. In humans, the acceptable level is between 7.38 and 7.42. Outside of the normal boundaries (states of acidemia or alkalemia), proteins and enzymes break down and can no longer function. In extreme cases, this can lead to death.
Both the kidneys and the lungs help keep a stable pH within the human body. The lungs carry out this role by moderating carbon dioxide concentrations; the kidneys manage it through two processes:
- Reabsorbing and regenerating bicarbonate from urine: bicarbonate is used to neutralize acids; the kidneys can either retain it if the pH is tolerable or release it when acid is on the rise2
- Excreting hydrogen ions and fixed acids: fixed acids are also known as nonvolatile acids and the name refers to any acids not produced as a result of carbon dioxide. They are a result of the incomplete metabolism of carbohydrates, fats and proteins. They include lactic acid, sulfuric acid and phosphoric acid.
The kidneys maintain the correct levels of electrolytes and water in the body.
Osmolality is a measure of the body's electrolyte-water balance. In other words, it is the ratio between fluid and minerals in the body.3 Dehydration is a key cause of electrolyte imbalance.
If a rise in plasma osmolality is detected, the hypothalamus in the brain responds by passing a message to the pituitary gland which, in turn, releases antidiuretic hormone (ADH).
In response to ADH, the kidney makes a number of changes, including:
- Increasing urine concentration
- Increasing water reabsorption
- Portions of the collecting duct that are normally not permeable to water are reopened, allowing water back into the body
- Retaining urea rather than excreting it; this compound is retained in the medulla of the kidney and attracts in water.
Regulating blood pressure
Rather than regulating blood pressure on an ad hoc basis, the kidneys are responsible for slower adjustments. The renin-angiotensin system adjusts arterial pressure over the long term by impacting the extracellular fluid compartment (fluid outside of cells).
The kidneys complete this task by releasing a vasoconstrictor called angiotensin II. This hormone, as part of an incredibly complex web, increases the kidney's absorption of sodium chloride, effectively increasing the size of the extracellular fluid compartment and raising blood pressure.
Anything that alters blood pressure can damage the kidneys over time, including excess alcohol, smoking and obesity.
Secretion of active compounds
The kidneys release a number of physiologically important products, including:
- Erythropoietin: controls erythropoiesis (production of red blood cells). The liver also produces erythropoietin, but during adulthood the kidneys are the primary producers. This hormone also plays an important part in wound healing4 and the response to neuronal injury5
- Renin: helps mediate arterial vasoconstriction and the volume of blood plasma, lymph and interstitial fluid
- Calcitriol: this is the hormonally active metabolite of vitamin D. It increases the level of calcium absorbed by the intestines and ups the reabsorption of phosphate in the kidney.
On the next page, we look at different types of kidney disease and their causes.