NEPHRON: CHARACTERISTICS, PARTS, FUNCTIONS, TYPES AND HISTOLOGY - BIOLOGY - 2025

The nephrons are structures found as part of the cortex and medulla of the kidney. They are considered the functional units of this filtering organ. Human kidneys possess, on average, 1 to 1.5 million nephrons.

Structurally, nephrons are made up of two main regions: the glomerular portion, known as Bowman's capsule, and the tubular portion. In the latter region, three subregions are distinguished: the proximal tubule, the loop of Henle, and the distal nephron.

Structure of the nephron.

Source: StormBringer, from Wikimedia Commons

In the kidney, not all the nephrons that form it are the same. They are classified as cortical, medium cortical, and juxtamedullary. The glomeruli of the nephrons are located in the cortex. In cortical nephrons, they are located in the outer region of the cortex and in juxtamedullary nephrons they are found in the corticomedullary zone.

Characteristics of nephrons

Nephrons are the functional unit of the kidneys. A nephron consists of an intricate epithelial tube that is closed at one end and open at the distal portion.

A kidney is made up of numerous nephrons that converge in the collecting ducts, which in turn form the papillary ducts and ultimately empty into the renal pelvis.

The number of nephrons that make up a kidney varies widely. In the simplest vertebrates we find hundreds of nephrons, while in small mammals the number of nephrons can increase by up to an order of magnitude.

In humans and other large mammals, the number of nephrons reaches more than a million.

Parts and histology

Madhero88, via Wikimedia Commons

The kidney of mammals is typical of vertebrates. They are paired organs, whose morphology resembles a bean. If we see them in a sagittal section we will see that it has two marked regions: the outer one called the cortex, and the inner one known as the medulla. The cortex is rich in Malpighi bodies and tubules.

Structurally, a nephron can be divided into three main zones or regions: the proximal nephron, the loop of Henle, and the distal nephron.

Proximal nephron

The proximal nephron consists of a tube with a closed initial end and the proximal tube.

The end of the tube is particularly wide and resembles a ball with one of its ends pressed inwards. The spherical structure is known as Malpighi bodies. The latter have a capsule with a double wall that encapsulates a series of capillaries.

This cup-shaped structure is called the Bowman capsule. The interior of the capsule forms a continuum due to the narrow light that is understood as the renal tubule.

In addition, in the internal portion of the capsule we find a kind of entanglement of capillaries called the renal glomerulus. This structure is responsible for the early stages of urine production.

Tubules of the nephrons

Starting from Bowman's capsule, we find the following tubules in the structure of nephrons:

The first is the proximal convoluted tubule, which arises from the urinary pole of Bowman's capsule. Its trajectory is particularly intricate and enters the medullary ray.

Next we find the proximal rectus tubule, which is also called the thick descending branch of the loop of Henle, which descends towards the medulla.

Then we find the thin descending limb of the loop of Henle, which has continuity with the proximal rectus tubule inside the medial. The continuation of the descending limb is the thin ascending limb of the loop of Henle.

The distal rectus tubule (also called the thick ascending limb of the loop of Henle) is the structure that continues the thin ascending limb. This tubule rises through the medulla and enters the cortex of the medullary ray, where it meets the renal corpuscle that gave rise to the aforementioned structures.

Renal corpuscle. Tieum from French Wikipedia / CC BY-SA (https://creativecommons.org/licenses/by-sa/1.0)

Subsequently, the distal rectus tubule leaves the medullary ray and meets the vascular pole of the renal corpuscle. In this region, epithelial cells form the macula densa. Finally, we have the distal convoluted tubule that leads to a collecting conductor.

Henle handle

In the previous section, an intricate and tortuous U-shaped structure was described. The proximal tubule, thin descending limb, ascending limb, and distal tubule are components of the loop of Henle.

As we will see in the types of nephrons, the length of the loop of Henle is variable within the components of the kidney.

Henle handle..

Source: StormBringer, from Wikimedia Commons

The fork of the loop of Henle is made up of two branches: one ascending and the other descending. The ascendant ends in the distal tubule that forms a collecting duct that serves multiple nephrons.

In mammals, the nephron is spatially located so that the loop of Henle and the collecting duct run parallel to each other. In this way, the glomeruli are located in the renal cortex and the loops of Henle do so deepening to the papilla of the medulla.

Features

The kidneys are the main organs responsible for the excretion of waste in vertebrates and participate in the maintenance of an optimal internal environment in the body.

As a functional structure of the kidney, the nephron is an indispensable element of the homeostatic mechanism, by regulating the filtration, absorption and excretion of water and the different molecules dissolved in it, from salts and glucose to larger elements such as lipids and proteins.

Functions of the glomerular and tubular zone

In general, the function of the zona glomerularis consists in the filtration of fluids and their components. The tubule, for its part, is related to functions of modifying the volume and composition of the filtrate.

This is achieved by reabsorption of substances into the plasma and secretion of substances from the plasma into the tubular fluid. Thus, the urine manages to have the elements that must be excreted in order to maintain the volume and stable composition of the liquids inside the organisms.

Functions of the loop of Henle

The loop of Henle is typical of the lineages of birds and mammals, and plays a crucial role in the concentration of urine. In vertebrates lacking the loop of Henle, the ability to produce hyperosmotic urine relative to blood is highly diminished.

Filtering capacity

The ability of the kidneys to filter is exceptionally high. Daily, about 180 liters are filtered and the tubular portions manage to reabsorb 99% of the filtered water and essential solutes.

Functioning

The kidneys have a very particular function in organisms: selectively eliminate waste substances that come from the blood. However, you must maintain body water and electrolyte balances.

To achieve this purpose, the kidney must perform four functions: renal blood flow, glomerular filtration, tubular reabsorption, and tubular secretion.

Modified from Kidney Nephron.png at Wikimedia Commons by Holly Fischer, via Wikimedia Commons

The artery in charge of supplying blood to the kidney is the renal artery. These organs receive about 25% of the blood that is pumped from the heart. The blood manages to penetrate the capillaries through the afferent arteriole, flows through the glomerulus and leads to the efferent arteriole.

The different diameters of the arteries are essential, since they help to create a hydrostatic pressure that allows glomerular filtration.

Blood travels through the peritubular capillaries and the rectus vessels, flowing slowly through the kidney. The peritubular capillaries surround the proximal and distal convoluted tubules, which achieve the reabsorption of essential substances and the last stage of adjustments in the composition of the urine occurs.

Types of nephrons

Nephrons are classified into three groups: juxtaglomerular, cortical, and mediocortical. This classification is established according to the position of your renal corpuscles.

Cortical nephrons

Cortical nephrons are also known as subcapsular. These have their renal corpuscles located in the outer portion of the cortex.

The loops of Henle are characterized by being short and extend specifically to the medullary region. They are considered the average type of nephron, where the loop appears close to the distal rectus tubule.

Corticals are the most abundant. On average, they constitute 85% - relative to the rest of the nephron classes. They are responsible for the elimination of waste substances and the reabsorption of nutrients.

Juxtamedullary nephrons

The second group is made up of the juxtamedullary nephrons, where the renal corpuscles are located at the base of a medullary pyramid. The handles of Henle are long elements, as are the thin segments that extend from the inner region of the pyramid.

The proportion of this type of nephron is considered to be close to one eighth. The mechanism by which they work is essential for the concentration of urine in animals. In fact, juxtamedullary nephrons are known for their ability to concentrate.

Midcortical nephrons

The mediocortical or intermediate nephrons have - as their name implies - their renal corpuscles in the middle region of the cortex. Compared with the two previous groups, the mediocortical nephrons present loops of Henle of intermediate length.

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