Simone Lawson
The kidneys are two bean-shaped organs that sit just below the rib cage, one on each side of the spine. Their primary function is to filter the blood, removing waste and balancing the body's fluids. Each kidney contains over a million filtering units called nephrons. Each nephron consists of a glomerulus, a cluster of tiny blood vessels that perform the first stage of filtering the blood, and a renal tubule, which returns needed substances to the blood and removes waste. The renal tubules carry urine to the renal pelvis, transferring it to the ureters, which carry urine to the bladder.
| Characteristics | Values |
|---|---|
| Number of filtering units in each kidney | 1 million to 2.5 million |
| Name of filtering units | Nephron |
| Composition of nephron | Glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct |
| Composition of renal corpuscle | Bowman's capsule and glomerulus |
| Glomerulus composition | Intricately tortuous inner capillary tuft formed by GECs |
| Layers of glomerular filtration barrier | Endothelial cells, podocytes, and extracellular matrix |
| Glomerular basement membrane thickness in children | 110 nm |
| Glomerular basement membrane thickness in adults | 310-350 nm |
| Blood filtered by kidneys per minute | Half a cup or 150 quarts per day |
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What You'll Learn
Filtering units: nephrons
The kidneys are two bean-shaped organs that sit in the back of the abdomen, just below the rib cage, one on each side of the spine. They are part of the urinary system and their primary function is to filter blood and create urine as a waste byproduct. Each kidney contains over a million filtering units called nephrons.
Nephron is the functional unit of the kidney, and each nephron consists of a renal corpuscle and an associated renal tubule. The renal corpuscle is composed of two distinct structures: the Bowman's capsule and the glomerulus. The glomerulus is a compact tuft of glomerular capillaries that invaginates Bowman's capsule. The visceral layer of Bowman's capsule surrounds the glomerular network and reflects to become the parietal layer, forming the encasement of Bowman's capsule. The glomerular capillaries are composed of endothelial cells and are surrounded by podocytes. The glomerular basement membrane (GBM) is a trilaminar structure that forms the only continuous, homogeneous layer of the filter barrier.
The nephron can be functionally subdivided into a filtration unit (renal corpuscle or glomerulus) and a segmented tubular resorption compartment. The renal tubule carries urine to the renal pelvis. The tubule comprises the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct system, which drains urine into the renal pelvis.
The nephrons work through a two-step process: the glomerulus filters the blood, and the tubule returns needed substances to the blood and removes waste. The glomerulus filters the blood through its intricate capillary network, allowing smaller molecules, wastes, and fluids to pass into the tubule. The tubule then reabsorbs water, nutrients, and minerals, while removing waste through a process called diffusion. The remaining waste is sent through the kidney's collecting chambers and eventually leaves the body as urine.
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Nephrons: structure and function
The kidneys are two bean-shaped organs that sit just below the rib cage, one on each side of the spine. Each kidney contains more than a million filtering units called nephrons. Nephrons are the functional units of the kidney, and they work tirelessly to ensure that blood is free from waste and excess fluids.
Each nephron consists of a glomerulus and a renal tubule. The glomerulus is a cluster of tiny blood vessels that perform the first stage of filtering the blood. The glomerulus filters the blood, allowing smaller molecules, wastes, and fluids to pass into the tubule. The renal tubule then reabsorbs and returns water, nutrients, and minerals that the body needs, such as sodium and potassium. This process is called glomerular filtration. The tubule also removes waste, including excess acid and fluids, through a process called diffusion. The remaining waste is sent through the kidney's collecting chambers and eventually leaves the body as urine.
The nephron is composed of a single layer of epithelial cells. It includes the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. The proximal convoluted tubule is part of the renal tubule, while the distal convoluted tubule is the segment that drains urine into the renal pelvis. The loop of Henle is a segment of the nephron that differs in vasculature between the juxtamedullary nephron and the cortical nephron.
The renal corpuscle is composed of two distinct structures: the Bowman's capsule and the glomerulus. The Bowman's capsule is a blind-ended tubule consisting of a single layer of epithelial cells, while the glomerulus is a compact tuft of glomerular capillaries that invaginates the Bowman's capsule. The visceral layer of Bowman's capsule surrounds the glomerular network and reflects to become the parietal layer, forming the encasement of the Bowman's capsule. The glomerular filtration barrier, composed of endothelial cells, podocytes, and the glomerular basement membrane, acts as a dynamic and highly selective filter that sieves molecules based on molecular size and electrical charge.
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Glomerulus: the nephron's filter
The kidneys are the body's primary filtration system, removing waste and balancing fluids. Each kidney contains between 1 and 2.5 million nephrons, which are the kidney's filtering units.
Each nephron consists of a glomerulus and a renal tubule. The glomerulus is a ball-shaped cluster of tiny blood vessels, or capillaries, that perform the first stage of filtering the blood. The word glomerulus refers to its intricate inner capillary tuft, from the Latin word 'glomus' for a ball of yarn.
The glomerulus filters the blood, allowing smaller molecules, waste and fluid to pass into the renal tubule. The renal tubule then reabsorbs water, nutrients and minerals back into the body, while removing waste, including excess acid and fluids, through a process called diffusion.
The glomerular filtration barrier is composed of three layers: endothelial cells, the glomerular basement membrane (GBM), and podocytes. The GBM is a stabilising element of the glomerulus, and its thickness changes with age and gender.
The nephrons work through a two-step process: the glomerulus filters the blood, and the tubule returns needed substances to the blood and removes waste. This waste and excess fluid become urine.
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Renal tubules: reabsorption and secretion
Each kidney contains more than a million filtering units called nephrons. Each nephron consists of a glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The nephron is the functional unit of the kidney involved in the critical interplay of fluid and electrolyte homeostasis by glomerular filtration, selective tubular reabsorption, and secretion.
The renal corpuscle filters the blood to create a filtrate that differs from blood mainly in the absence of cells and large proteins. From this point to the ends of the collecting ducts, the filtrate or forming urine is undergoing modification through secretion and reabsorption before true urine is produced. The first point at which the forming urine is modified is in the proximal convoluted tubule (PCT). Here, some substances are reabsorbed, while others are secreted.
Tubular reabsorption is a finely tuned process that maintains homeostasis of blood volume, blood pressure, plasma osmolarity, and blood pH. The mechanisms of reabsorption into the peritubular capillaries include passive diffusion (passing of solutes through the kidney epithelial cells by concentration gradients) and active transport (where membrane-bound ATPase pumps with carrier proteins transport solutes across kidney epithelial cells by consuming ATP).
Tubular secretion is responsible for the secretion of additional solutes and metabolic wastes, including unbound fractions of PBUTs and pharmaceutical drugs (antibiotics, antivirals, diuretics, and nonsteroidal anti-inflammatory drugs), from the peritubular capillaries into the lumen of the proximal tubule.
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Glomerular filtration barrier: a dynamic sieve
The kidneys are two bean-shaped organs that sit in the back of the abdomen and are part of the urinary system. Their primary function is to filter the blood, removing waste and balancing the body's fluids. Each kidney contains more than a million filtering units called nephrons. Each nephron consists of a glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
The glomerulus is a network of capillaries that resides in the Bowman's capsule and functions as a filtration unit of the kidney. The glomerular filtration assembly is composed of three main cellular barriers that are critical for the ultrafiltration process: the fenestrated endothelium, glomerular basement membrane, and highly specialized podocytes.
The fenestrated endothelium is a layer of endothelial cells that line the inner surface of the entire vascular tree and form an anticoagulant barrier between the blood and tissues. These endothelial cells contain numerous pores, or fenestrae, that allow for the filtration of fluid, blood plasma solutes, and protein while preventing the filtration of red blood cells, white blood cells, and platelets.
The glomerular basement membrane (GBM) is the only continuous, homogeneous layer of the filter barrier. It provides the primary structural support for the glomerular tuft and consists mainly of laminins, type IV collagen, agrin, and nidogen, which are synthesized and secreted by both endothelial cells and podocytes. The thickness of the GBM changes with age and gender, with adult males having a thicker GBM than females.
The podocytes are specialized cells that attach to the outer surface of the GBM. They have foot processes that form slit diaphragms, which are gaps spanned by a mat of proteins. The podocytes and their slit diaphragms form the basic backbone of the filtration assembly, ensuring that essential plasma proteins are retained in the blood while allowing the filtrate to pass on as urine.
Together, the fenestrated endothelium, glomerular basement membrane, and podocytes constitute the glomerular filtration barrier, a dynamic sieve that facilitates the flow of plasma water and small solutes while restricting the passage of larger molecules such as blood cells, proteins, and waste products. Damage to this filtration barrier can lead to glomerular diseases such as diabetic kidney disease, glomerulonephritis, and glomerulosclerosis.
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Frequently asked questions
The filtering units of the kidney are called nephrons.
Each nephron consists of a glomerulus and a renal tubule.
The glomerulus is a ball formed of small blood capillaries that filters the blood.
The renal tubule returns needed substances to the blood and removes waste.
