Kidney - Function - Structure

Kidneys / urinary bladder: anatomy and function

In addition to cleaning and detoxifying the blood, the kidneys play an important role in the water and fluid balance, in regulating blood pressure and the acid-base balance. In addition, they are involved in the production of various hormones. They can only fulfill this wide range of tasks with the help of their special inner workings. It is not without reason that the kidneys with their special architecture are very important organs of the human body…

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• Kidneys - bean-shaped organs
• What is urine?
• Functions of the kidneys
• The inner workings of the kidneys
• The urinary bladder

Kidneys - bean-shaped organs

The kidneys are reddish-brown, paired organs with a bean-like shape. With a size of around eleven by seven by four centimeters, a kidney weighs between 120 and 200 grams. The kidneys are located on both sides of the spine. The right kidney is slightly lower because of its proximity to the liver. The kidneys are located approximately below the costal arch, depending on the breath, and are partially covered by ribs. On both kidneys there is an adrenal gland - like a cap.

The kidneys are very sensitive organs and are protected by a shell made of fiber and fat capsules as well as a connective tissue-like sac (fascia). These structures are made up of connective, muscle and fat tissue. The kidneys are not encapsulated on the inside towards the spine, the so-called hilus. As a rule, there is a supplying arterial blood vessel (renal artery) and a draining vein (renal vein). The ureter, a muscle tube up to 35 cm long, is also connected to the kidney and transports the urine produced by the kidneys into the urinary bladder. There it is collected and ultimately eliminated. The urinary bladder has both a storage and an emptying function.

What is urine?

Urine is actually a blood filtrate that contains water and electrolytes as well as substances that are subject to urination such as urea and uric acid, ammonia and externally supplied toxins (exogenous toxins). If, for example, proteins (amino acids, peptides), sugars (carbohydrates), fats (lipids) or blood cells are found in the urine, this can be an indication of a kidney malfunction. A normally functioning kidney partially filters these substances back because they are valuable for the body.

Functions of the kidneys

The tasks of the kidneys can essentially be assigned to the three functional areas of excretion, regulation and production:

• Blood purification: The kidneys purify the blood. Using different filter processes, they remove toxic and urinary substances that would otherwise poison the body. These substances occur in the metabolism, among other things. In particular in the metabolism of protein, substances that are subject to urine such as ammonia, creatinine, urea and urea are formed. The breakdown of drugs can also lead to toxic substances that need to be removed from the body. The end product of the complex process of blood purification is the urine. It transports urinary substances and toxins out of the body.
• Fluid and electrolyte balance: The kidneys regulate the excretion of water and salts (electrolytes) as required. In this way the volume and the concentration of the extracellular space are kept constant.
• Acid-base balance: The kidneys also have a buffer function in the body through compensatory processes such as the excretion or reabsorption of acidic and basic ions, including hydrogen (H) and bicarbonate (HCO ₃). Together with breathing, they maintain the acid-base balance. The lungs can react relatively quickly to changes in the pH value of the blood by exhaling carbon dioxide (CO ₂). The kidneys react more slowly, but can compensate for larger amounts.
• Blood pressure: The kidneys are involved in the regulation of blood pressure through the production of various hormones that affect the fluid and electrolyte balance (including the renin-angiotensin-aldosterone system, RAAS).
• Formation of hormones: In addition to the blood pressure-regulating hormones, other hormones are formed in the kidneys, such as erythropoietin (EPO), which is responsible for the formation of red blood cells (erythrocytes). The hormone-like vitamin D is also converted into its active form in the kidneys.
• Formation of vitamin D (calcitriol): The fat-soluble vitamin D can be produced by the body from cholesterol using UV light (self-synthesis). This process begins in the skin, continues in the liver, and finally ends in the kidneys. Here the inactive precursor of the liver is converted into active vitamin D (calcitriol) (hydroxylation to 1,25-dihydroxycholecalciferol).
• Recovery of essential salts, amino acids, sugars (gluconeogenesis).

For more information, see Vitamin D and Laboratory Medicine / Vitamin D.

The inner workings of the kidneys

The kidneys have an immense ability to act. About 1.2 liters of blood flow through both kidneys per minute, which corresponds to a total volume of 1,800 liters per day. The blood is filtered here. First of all, the so-called primary urine is produced (approx. 180 liters per day), many of which are valuable for the body, such as water, protein and sugar. Of the original 180 liters of primary urine, only around 1.8 liters of urine enter the urinary bladder and are excreted.

In order to be able to cope with these high volumes, a very complex construct is required inside the kidneys. Diverse fluid and mass transfer processes take place via different structures. The kidney is roughly divided into the renal cortex, medulla and pelvis.

In the renal cortex(Cortex renalis) are the actual filter stations of the kidneys, the nephrons (one to 1.5 million nephrons per kidney). These functional units are only 0.2 mm in diameter and each contain a glomerulum (spherical structure with many tiny blood vessels) that is surrounded by a capsule (Bowman's capsule). This is where the production of urine originates, which first occurs as primary urine and is concentrated through several exchange processes. A nephron also has a tube-like transport system made up of many urinary tubules, the tubules. When the first filtrate of the blood passes through these tubules, important substances and water are reabsorbed and fed back into the bloodstream. These transport systems are already located in the renal medulla (medulla renalis), which is composed of several conical pyramids. Between these pyramids there are always parts of the kidney cortex.

The tubes of the transport system open into the so-called collecting tubes, which in turn merge into kidney calyxes. The final filtrate reaches the renal pelvis (pelvis renalis) in concentrated form via these calyxes, where it enters the urinary bladder via the ureter and is excreted via the urethra.

The urethra is subject to anatomical differences in men and women:

In men, it is enclosed in the upper part of the prostate. The spermatic ducts converge in the prostate and open into the 17 to 20 centimeter long urethra. After passing through the pelvis, this is accompanied by the two erectile tissue of the penis (corpora cavernosa) and the so-called urethral erectile tissue (corpus spongiosum) and finally ends at the glans (glans penis). During ejaculation, the closure of the bladder neck enables the ejaculate to be ejected through the urethra and not to enter the bladder

Urinary system man © elvira fair

In women, the urethra opens into the so-called vaginal vestibule between the clitoris and vagina. The significantly shorter urethra in women (about four centimeters in length) sometimes causes more frequent urinary tract infections, as bacteria can more easily ascend into the urinary bladder

Urinary system woman © kocakayaali

For more information, see Female Anatomy and Male Anatomy.

Note In principle, humans can also live with only one kidney, provided that it is healthy and has enough functioning kidney corpuscles (20 to 30 percent active nephrons).

The urinary bladder

The urinary bladder (Vesica urinaria) collects the urine concentrated by the kidneys, which is transported to it via the two ureter tubes. It can be understood as a collecting organ that stores the urine to a certain extent. When the bladder is empty, it only contains a few milliliters of urine. As the degree of filling increases, it takes on a spherical shape. From a volume of approx. 130 to 150 ml, people feel the urge to urinate; from approx. 400 ml, this is normally perceived as intense and urination is initiated. The maximum bubble capacity is approx. 350 to 400 ml and varies from person to person. However, a larger bladder filling volume can also be trained, but this is usually associated with impairment of the urinary bladder.

The urinary bladder has five different wall layers, which essentially consist of mucous membrane, muscle and connective tissue. This tissue structure enables its elasticity on the one hand and the ability to contract to urinate on the other. The urethra and bladder are subject to a regulatory mechanism which, among other things, uses various muscle loops and elastic nets to prevent unintentional urine leakage. If this does not work, urination disorders such as incontinence can occur.

For more information on kidney and urine values, see Laboratory Medicine - Kidney / Urine.