How the kidneys function

How the kidneys function

We all have listened about the importance of the kidneys for the proper functioning of our body and, consequently, for our health. But, do we really know how kidneys function in our bodies? We tell you everything in this article!

What are the kidneys and where are they located?

The kidneys represent the main functional organ of the urinary system, and they have a size of approximately 4 or 5 inches long (approximately like a fist). Although most people have two kidneys, in isolated cases, a person may be born with anatomical abnormalities, thus having one or three kidneys.

Kidneys are bean-shaped organs located in the abdomen, specifically on either side of the spine, approximately in the level from the penultimate dorsal vertebrae to the second lumbar vertebrae. As kidneys are located behind the peritoneum (the membrane lining of the abdominal cavity), they are considered retroperitoneal organs.

kidneys function

The right kidney is usually a little lower than the left kidney because it is located in the back of the liver, while the left kidney is located in the back of the spleen. At the top of each kidney, there is an adrenal gland which is a primarily endocrine organ which is involved in some renal functions.

The kidneys are protected by the lower part of the ribcage. Likewise, they are covered by a fibrous connective tissue (called the renal capsule or fascia) and two layers of fat (the perirenal and pararenal fat), which surround, cushion and protect the kidneys from trauma.

What are the functions of the kidneys?

The kidneys are considered the “natural filter of the body” because one of their main functions is to filter the blood to remove waste materials from food, medications, and toxic substances to later excrete them through the urine. In fact, the kidneys filter about 120 to 152 quarts of blood to create 1 to 2 quarts of urine every day.

They are also in charge of performing other functions of the renal system, such as controlling the body’s salt and water balance in order to maintain the overall fluid balance, regulating the blood pressure, and maintaining the homeostatic functions. For example, the kidneys are in charge of maintaining proper levels of the different electrolytes and minerals, including glucose, protons, bicarbonate, and amino acids, which are necessary to carry out the functions of our body and to maintain the acid-base balance.

Finally, certain hormones and enzymes which are involved in the production of red blood cells (the hormone erythropoietin), the bone health (the hormone calcitriol), and the blood pressure regulation (the enzyme renin) are produced in the kidneys.

What is a Nephron?

A nephron is defined as “the basic structural and functional unit of the kidneys”, so it is considered by many authors as the most important part of them. In fact, to perform its function, each kidney has more than a million of nephrons.

The nephrons are the structures in charge of filtering the water and soluble substances from the blood, selecting what is needed for the body to reabsorb it, and detecting what is toxic or found in excess to excrete it through the urine. Likewise, the nephrons have a very important role in metabolizing nutrients, regulating the pH of the blood, maintaining the blood volume balance, and controlling the blood pressure.

Each nephron has a complex internal set of structures, which are the renal corpuscle and the renal tubules. The renal corpuscle is formed by the Bowman’s capsule and its tubules, which include the proximal tubule (with a contoured part and other straight parts), the descending and ascending branches of the Henle’s loop, and the distal contoured tubule. Finally, the urine is drained from the contoured tubule into the collecting duct to continue to the rest of the urinary system and to be excreted by the bladder.

The function of the nephrons is regulated by the antidiuretic hormone, the aldosterone, and the parathyroid hormone, which are produced by the neuroendocrine system (including the adrenal glands).

How does every part of the nephron work?

The glomerulus is the part of the nephron which receives the blood from the systemic circulation. In this part of the nephron, the fluid and small molecules (such as glucose and ions like sodium) in the blood are filtered to the space called Bowman’s capsule. Likewise, a group of specialized cells in this area, called juxtaglomerular apparatus (JGA), secret an enzyme called renin under certain stimuli to maintain the blood volume and the body’s homeostasis.

In normal circumstances, large proteins and red blood cells can’t pass to the Bowman’s capsule. So, the presence of those molecules in the urine is a sign of problems in the kidney. On the other hand, the proximal tubule is the part of the nephron where most of the water, salt, glucose, and other solutes are reabsorbed.

In the loop of Henle, the urine osmolarity increases in the descendant part, while it decreases in the ascendant part. In other terms, in this part of the nephron the osmolarity of blood and urine is determined.

Finally, the nephron finishes with the distal convoluted tubule and the collecting duct. In these parts of the nephron, the permeability to the water can vary by the stimuli of the anti-diuretic hormone. For example, if a person is dehydrated, the antidiuretic hormone is released to avoid losing fluid unnecessarily.

What are the Ureters and what are their functions?

The ureters are narrow and hollow ducts that carry the urine from the kidneys to the bladder. Most people have one ureter for each kidney and each one of them measures between 9 and 12 inches long. However, in isolated cases, there are two ureters in the same kidney. The upper part of the ureters is in the abdomen, emerging from the kidneys, while their lower part is in the pelvic area, crossing the wall of the bladder to empty the urine from there.

To fulfill its function, the ureters have thick contractile and muscular walls, and their diameters vary at different points. The involuntary muscle fibers are stimulated by the nervous system to get contractions between 1 and 5 times per minute, which causes regular peristaltic waves that push the urine in small quantities to the bladder for storage and for later elimination. The muscle fibers in the ureter’s and bladder’s walls also get contractions when the bladder is full to prevent the urine from returning to the kidneys.

In conclusion, it is very important that kidneys fulfill their functions to maintain a healthy body, including a good balance of liquids, an adequate pH, and blood pressure numbers within normal. So, any abnormality in the structures of the urinary system can alter the kidneys’ function and cause severe damage to health.

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Author: Dr.Aurelysmar GrimandecollantesDr.Aurelysmar is 29 years old MD and medical writer. Medical degree from the Universidad Centroccidental "Lisandro Alvarado", Venezuela, 2014. Diploma in Occupational Health and Safety, Venezuela, 2015. Course: Specialized Health Training for Physicians, Spain, 2017.