Next Lesson - The Glomerulus – Tubular Reabsorption and Secretion
Core
A nephron is the microscopic functional unit of a kidney - a single kidney contains an average of about 900,000 to 1 million nephrons, although the number varies widely between people (from about 200,000 to over 2.5 million in individual kidneys).
There are two types of nephrons: cortical and juxtamedullary, named based on where in the kidney they are found. The cortex is the outer part of the kidney and the medulla is the inner part (juxtamedullary means next to the medulla). Both types of nephrons function very similarly, with small differences which will be discussed later.
This article aims to give an overview of the structure of the nephron - for more information on relevant ion and water transporters please see the article Control of Plasma Volume.
The nephron can be split up based on different cell types and their functions.
This diagram shows the difference between juxtamedullary nephrons (on the left of the image) and cortical nephrons (on the right of the image). Cortical nephrons are situated more peripherally in the kidneys, with the tip of the nephron descending just into the medulla. In contrast, the juxtamedullary nephrons have a longer Loop of Henle that extends deep into the renal medulla.
For more information on the structure of these nephrons and their individual blood supplies, please see our article on the Anatomy of the Urinary System.
Diagram - An overview of a juxtamedullary nephron (left) and a cortical nephron (right)
Creative commons source by Holly Fischer [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The glomerulus is the ball of capillaries between the afferent (carrying blood in) and efferent (carrying blood away) arterioles. It is enclosed by the Bowman’s capsule.
As blood flows through the glomerulus, water and small dissolved substances (solutes) are filtered out of it. They move into the Bowman’s capsule and then flow into the proximal convoluted tubule (PCT, the coiled first section of the tubule). Blood cells and large proteins (such as albumin) are not normally filtered, remaining behind in the vessels. The fluid that is filtered into the Bowman’s capsule is called the ultrafiltrate. As it passes along the tubule, different components of the ultrafiltrate are reabsorbed (taken back into the blood) at different points of the nephron, so its composition changes.
More information on the structure and function of the glomerulus can be found in our article on the glomerulus.
The PCT is responsible for reabsorbing many solutes including sodium, chloride, potassium, bicarbonate, urea, amino acids, glucose, and water. Under normal physiological conditions, virtually all filtered amino acids and glucose are reabsorbed at the PCT. The PCT also reabsorbs about two-thirds of the filtered sodium, chloride, potassium and water, and most (about 85%) of the filtered bicarbonate.
The PCT is split into three segments, S1, S2, and S3 (in order along the tubule) - based on the transporters (proteins that carry substances across a cell membrane) which are expressed (present) on the cell membranes.
The LoH is the part of the nephron which is responsible for setting up and maintaining the vertical osmotic gradient (the concentration of dissolved particles rises steadily deeper into the medulla). This gradient is what allows water to be reabsorbed from the tubules, mainly in the descending limb and in the collecting duct, so that concentrated urine can be made. More information on the vertical osmotic gradient can be found in the article Control of Plasma Osmolarity.
The LoH can be split into the descending limb and ascending limb. The descending limb is permeable (lets things pass through) to water but largely impermeable (blocks them) to solutes like sodium, chloride, and potassium. The ascending limb is impermeable to water. Sodium and chloride leave its thin part passively (without using energy), and its thick part actively reabsorbs sodium, chloride, and potassium through the sodium-potassium-two-chloride transporter (NKCC2).
Distal Convoluted Tubule (DCT)
The DCT is responsible for reabsorbing sodium, chloride, calcium, and magnesium. It is split into the early DCT (DCT1) and the late DCT (DCT2) - again based on the types of transporters expressed on the cell membranes. The late DCT also secretes potassium into the tubule (secretion moves a substance from the blood into the tubule, the opposite of reabsorption).
Most filtered bicarbonate is reabsorbed earlier, in the PCT. The kidney fine-tunes serum pH (the acidity of the blood) later, mainly through the intercalated cells of the collecting duct (see below).
Thiazide diuretics block the sodium-chloride cotransporter (NCC, a protein that carries sodium and chloride into the cell together) in the early DCT. Amiloride, a potassium-sparing diuretic (it makes the body keep potassium), blocks the epithelial sodium channel (ENaC) in the late DCT and collecting duct. Both drugs reduce sodium reabsorption, which is why they increase urine output.
The CD is the last part of the nephron (some textbooks say it is continuous with the nephron rather than strictly part of it, because each duct collects fluid from several nephrons). It extends down into the renal medulla and eventually joins with other collecting ducts. These ducts merge and empty at a renal papilla (a bundle of collecting ducts in the medulla), and the urine then drains into the renal calyces (the start of the system that carries urine to the ureter). The collecting ducts are responsible for the reabsorption of sodium, chloride, and water from the ultrafiltrate, and of urea in their deepest (inner medullary) part. They also secrete potassium and hydrogen ions into the tubule.
The CD can be split into cortical and medullary segments based on their location. In the cortical CD there are two main cell types: principal cells which primarily reabsorb sodium (and secrete potassium), and intercalated cells which help to control acid-base balance by secreting hydrogen ions (acid) or bicarbonate into the tubule. This is how the kidney fine-tunes serum pH.
The reabsorption of water in the CDs can change in response to plasma osmolarity and is under the influence of antidiuretic hormone (ADH). The presence of ADH stimulates the insertion of aquaporin channels into the apical membrane (the side of the cell facing the inside of the tubule) of collecting duct cells - these channels allow water to pass through the membrane. This means that antidiuretic hormone (as indicated by the name) encourages water reabsorption and therefore reduces the amount of urine produced.
The ultrafiltrate left after passing through the CD becomes urine. By this stage, the vast majority of filtered water has been reabsorbed, so what remains is a relatively small volume containing water, ions, metabolites (waste products of the body’s chemical reactions), and nitrogenous waste (waste that contains nitrogen, such as urea).
All parts of the nephron have their own roles to play in the regulation of homeostasis. It is important to note that all the functions of all the parts of the nephron are happening simultaneously. The action of a drug or a hormone on one portion of the nephron can change how the other parts of the nephron work. For example, a loop diuretic blocks sodium reabsorption in the loop of Henle, so more sodium reaches the distal nephron and more potassium is lost in the urine.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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