KIDNEY: PHYSIOLOGY, FUNCTIONS, HORMONES, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The kidneys are a pair of organs located in the retroperitoneal region, one on each side of the spinal column and the great vessels. It is a vital organ for life since it regulates the excretion of waste products, the hydro-electrolyte balance and even blood pressure.

The functional unit of the kidney is the nephron, a set of cellular elements made up of vascular cells and specialized cells responsible for fulfilling the main task of the kidney: to function as a filter that separates impurities from the blood allowing their expulsion through the urine.

In order to fully fulfill its function, the kidney is attached to different structures such as the ureter (pair, one on each side in relation to each kidney), the urinary bladder (odd organ that functions as a urine reservoir, located in the midline of the body at the level of the pelvis) and the urethra (excretory duct) also odd and located in the midline.

Together, all these structures form what is known as the urinary system, whose main function is the production and excretion of urine.

Although it is a vital organ, the kidney has a very important functional reserve, which allows a person to live with only one kidney. In these cases (single kidney) the organ hypertrophies (increases in size) in order to compensate for the function of the absent contralateral kidney.

Anatomy (parts)

1. Kidney pyramid
2. Efferent artery
3. Renal artery
4. Renal vein
5. Renal hilum
6. Renal pelvis
7. Ureter
8. Lesser chalice
9. Kidney capsule
10. Lower kidney capsule
11. Upper kidney capsule
12. Afferent vein
13. Nephron
14. Lesser chalice
15. Major chalice
16. Renal papilla
17. Renal spine

The structure of the kidney is very complex, since each of the anatomical elements that make it up is oriented to fulfill a specific function.

In this sense, we can divide the anatomy of the kidney into two large groups: macroscopic anatomy and microscopic anatomy or histology.

The normal development of structures at different levels (macroscopic and microscopic) is essential for the normal functioning of the organ.

Macroscopic anatomy

The kidneys are located in the retroperitoneal space, on each side of the spinal column and closely related above and ahead with the liver on the right side and the spleen on the left side.

Each kidney is shaped like a giant kidney bean that is about 10-12 cm long, 5-6 cm wide, and about 4 cm thick. The organ is surrounded by a thick layer of fat known as perirenal fat.

The outermost layer of the kidney, known as the capsule, is a fibrous structure made up mainly of collagen. This layer covers the organ around its perimeter.

Below the capsule are two well differentiated areas from the macroscopic point of view: the cortex and the renal medulla, which are located in the most external and lateral areas (looking outward) of the organ, literally enveloping the collecting system, that is closest to the spine.

Renal cortex

In the renal cortex are the nephrons (functional units of the kidney), as well as an extensive network of arterial capillaries that give it a characteristic red color.

The main physiological processes of the kidney take place in this area, since functional tissue from the point of view of filtration and metabolism is concentrated in this area.

Renal medulla

The medulla is the area where the straight tubules as well as the tubules and collecting ducts meet.

The medulla can be considered as the first part of the collecting system and functions as a transition zone between the functional area (renal cortex) and the collecting system itself (renal pelvis).

In the medulla, the tissue made up of the collecting tubules is organized into 8 to 18 renal pyramids. The collecting ducts converge towards the apex of each pyramid in an opening known as the renal papilla, through which urine flows from the medulla into the collecting system.

In the renal medulla, the space between the papillae is occupied by the cortex, so that it can be said that it covers the renal medulla.

Collecting system

It is the set of structures designed to collect urine and channel it to the outside. The first part is made up of the smaller calyces, which have their base oriented towards the medulla and the vertex towards the greater calyces.

The smaller calyces resemble funnels that collect the urine that flows from each of the renal papillae, channeling it towards the larger calyces that are larger in size. Each smaller calyx receives the flow from one to three renal pyramids, which is channeled into a larger calyx.

The larger calyxes resemble the smaller ones, but larger. Each one is connected at its base (wide part of the funnel) with between 3 and 4 minor calyces whose flow is directed through its apex towards the renal pelvis.

The renal pelvis is a large structure that occupies approximately 1/4 of the total volume of the kidney; The major calyces flow there, releasing the urine that will be pushed into the ureter to continue its way out.

The ureter leaves the kidney on its inner side (the one that faces the spine) through the area known as the renal hilum, through which the renal vein (which empties into the inferior vena cava) also emerges and the renal artery enters (direct branch of the abdominal aorta).

Microscopic anatomy (histology)

At the microscopic level, the kidneys are made up of different highly specialized structures, the most important of which is the nephron. The nephron is considered the functional unit of the kidney and in this several structures are identified:

Glomerulus

Integrated in turn by the afferent arteriole, the glomerular capillaries and the efferent arteriole; all this surrounded by the Bowman's capsule.

Adjacent to the glomerulus is the juxtaglomerular apparatus, responsible for much of the endocrine function of the kidney.

Renal tubules

They are formed as a continuation of Bowman's capsule and are divided into several sections, each with a specific function.

Depending on their shape and location, the tubules are called the proximal convoluted tubule and the distal convoluted tubule (located in the renal cortex), joined together by the straight tubules that form the loop of Henle.

The rectus tubules are found in the renal medulla as well as the collecting tubules, which form in the cortex where they connect with the distal convoluted tubules and then pass to the renal medulla where they form the renal pyramids.

Physiology

The physiology of the kidney is conceptually simple:

- Blood flows through the afferent arteriole to the glomerular capillaries.

- From the capillaries (of smaller caliber) the blood is forced by pressure towards the efferent arteriole.

- Because the efferent arteriole has a higher tone than the afferent, there is greater pressure which is transmitted to the glomerular capillaries.

- Due to the pressure, both the water and the solutes and waste are filtered through "pores" in the wall of the capillaries.

- This filtrate is collected inside Bowman's capsule, from where it flows into the proximal convoluted tubule.

- In the distal convoluted tubule, a good part of the solutes that should not be expelled are reabsorbed, as well as the water (the urine begins to concentrate).

- From there the urine passes to the loop of Henle, which is surrounded by several capillaries. Due to a complex counter-current exchange mechanism, some ions are secreted and others are absorbed, all with the aim of concentrating the urine even more.

- Finally the urine reaches the distal convoluted tubule, where some substances such as ammonia are secreted. Because it is excreted in the last portion of the tubular system, the chances of reabsorption are decreased.

- From the distal convoluted tubules, urine passes into the collecting tubules and from there to the outside of the body, passing through the different stages of the urine excretory system.

Features

The kidney is known mainly for its function as a filter (previously described), although its functions go much further; In fact, it is not a mere filter capable of separating solutes from the solvent, but a highly specialized one capable of discriminating between the solutes that must come out and those that must remain.

Due to this capacity, the kidney performs different functions in the body. The most prominent are the following:

- Helps control acid-base balance (in conjunction with respiratory mechanisms).

- Preserves plasma volume.

- Maintains the hydro-electrolyte balance.

- Allows control of plasma osmolarity.

- It is part of the blood pressure regulation mechanism.

- It is an integral part of the erythropoiesis system (blood production).

- Participates in the metabolism of vitamin D.

Hormones

The last three functions in the list above are endocrine (secretion of hormones into the bloodstream), so they are related to the secretion of hormones, namely:

Erythropoietin

It is a very important hormone since it stimulates the production of red blood cells by the bone marrow. Erythropoietin is produced in the kidney but has its effect on the hematopoietic cells of the bone marrow.

When the kidney does not function properly, erythropoietin levels decrease, which leads to the development of chronic anemia refractory to treatment.

Renin

Renin is one of the three hormonal components of the renin-angiotensin-aldosterone system. It is secreted by the juxtaglomerular apparatus in response to pressure changes in the afferent and efferent arterioles.

When the arterial pressure in the efferent arteriole falls below that of the afferent arteriole, renin secretion increases. On the contrary, if the pressure in the efferent arteriole is much higher than the afferent one, then the secretion of this hormone decreases.

The function of renin is the peripheral conversion of antiotensinogen (produced by the liver) to angiotensin I which in turn is converted to angiotensin II by the angiotensin converting enzyme.

Angiotensin II is responsible for peripheral vasoconstriction and, therefore, for blood pressure; likewise, it has an effect on the secretion of aldosterone by the adrenal gland.

The higher the peripheral vasoconstriction, the higher the blood pressure levels, while as the peripheral vasoconstriction decreases, the blood pressure levels fall.

As renin levels increase, so do aldosterone levels as a direct consequence of increased circulating levels of angiotensin II.

The objective of this increase is to increase the reabsorption of water and sodium in the renal tubules (secreting potassium and hydrogen) with the aim of increasing plasma volume and, therefore, raising blood pressure.

Calcitriol

Although it is not exactly a hormone, calcitriol or 1-alpha, 25-dihydroxycholecalciferol is the active form of vitamin D, which undergoes several hydroxylation processes: the first in the liver to produce 25-dihydroxycholecalciferol (calcifediol) and then in the kidney, where it is converted to calcitriol.

Once it reaches this form, vitamin D (now active) is capable of fulfilling its physiological functions in the field of bone metabolism and the processes of absorption and reabsorption of calcium.

Diseases

The kidneys are complex organs, susceptible to multiple diseases, from congenital to acquired.

In fact, it is such a complex organ that there are two medical specialties dedicated exclusively to the study and treatment of its diseases: nephrology and urology.

Listing all diseases that can affect the kidney is beyond the scope of this entry; however, the most frequent ones will be mentioned roughly, indicating the main characteristics and the type of disease.

Kidney infections

They are known as pyelonephritis. It is a very serious condition (as it can cause irreversible kidney damage and, therefore, kidney failure) and life-threatening (due to the risk of developing sepsis).

Kidney stones

Kidney stones, better known as kidney stones, are another of the common diseases of this organ. Stones are formed by the condensation of solutes and crystals that, when joined, form stones.

Stones are responsible for much of recurrent urinary tract infections. In addition, when they cross the urinary tract and get stuck at some point, they are responsible for nephritic or renal colic.

Congenital malformations

Congenital malformations of the kidney are quite common and vary in severity. Some are completely asymptomatic (such as the horseshoe kidney and even the single kidney), while others can lead to further problems (as in the case of the double renal collecting system).

Polycystic kidney disease (RPE)

It is a degenerative disease in which healthy kidney tissue is replaced by non-functional cysts. At first these are asymptomatic, but as the disease progresses and the nephron mass is lost, RPE progresses to renal failure.

Renal failure (IR)

It is divided into acute and chronic. The first is usually reversible while the second evolves towards end-stage renal failure; that is, the stage in which dialysis is essential to keep the patient alive.

IR can be caused by multiple factors: from recurrent high urinary tract infections to obstruction of the urinary tract by stones or tumors, through degenerative processes such as RPE and inflammatory diseases such as interstitial glomerulonephritis.

Kidney cancer

It is usually a very aggressive type of cancer where the best treatment is radical nephrectomy (removal of the kidney with all its related structures); however, the prognosis is poor, and most patients have a short survival after diagnosis.

Due to the sensitivity of kidney diseases, it is very important that any warning sign, such as bloody urine, pain when urinating, increased or decreased urinary frequency, burning when urinating or pain in the lumbar region (renal colic) consult with the specialist.

This early consultation is intended to detect any problems early, before irreversible kidney damage occurs or a life-threatening condition develops.

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