what structures in the glomerular capsule interdigitate to form filtration slits?

Learning Objectives

By the end of this section, you will be able to:

• Distinguish the histological differences betwixt the renal cortex and medulla
• Depict the construction of the filtration membrane
• Identify the major structures and subdivisions of the renal corpuscles, renal tubules, and renal capillaries
• Discuss the function of the peritubular capillaries and vasa recta
• Place the location of the juxtaglomerular apparatus and describe the cells that line information technology
• Describe the histology of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts

The renal structures that acquit the essential piece of work of the kidney cannot exist seen past the naked eye. Only a light or electron microscope can reveal these structures. Even and then, serial sections and reckoner reconstruction are necessary to give us a comprehensive view of the functional anatomy of the nephron and its associated blood vessels.

Nephrons: The Functional Unit

Nephrons take a simple filtrate of the blood and alter it into urine. Many changes accept place in the unlike parts of the nephron before urine is created for disposal. The term forming urine will be used futurity to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to rest the plasma to homeostatic set points and excrete potential toxins in the urine. They exercise this by accomplishing three principle functions—filtration, reabsorption, and secretion. They likewise accept additional secondary functions that exert control in three areas: claret force per unit area (via production of renin), ruddy blood cell product (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active class of vitamin D).

Renal Corpuscle

As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowman's (glomerular) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowman's capsule surrounds the glomerulus to grade a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowman's capsule, the parietal layer, is a elementary squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to course the visceral layer of the sheathing. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like artillery (pedicels) to embrace the glomerular capillaries. These projections interdigitate to form filtration slits, leaving small gaps between the digits to course a sieve. Every bit blood passes through the glomerulus, 10 to xx percent of the plasma filters between these sieve-similar fingers to be captured by Bowman's capsule and funneled to the Pct. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte "fingers," the but matter separating the capillary lumen and the lumen of Bowman's capsule is their shared basement membrane. These three features comprise what is known as the filtration membrane. This membrane permits very rapid move of filtrate from capillary to capsule though pores that are simply 70 nm in diameter.

Podocytes

Podocytes interdigitate with structures called pedicels and filter substances in a way similar to fenestrations. In (a), the large prison cell body can be seen at the top right corner, with branches extending from the jail cell torso. The smallest finger-like extensions are the pedicels. Pedicels on one podocyte ever interdigitate with the pedicels of another podocyte. (b) This capillary has three podocytes wrapped around it.

Fenestrated Capillary

Fenestrations permit many substances to diffuse from the blood based primarily on size.

The fenestrations prevent filtration of claret cells or big proteins, just permit near other constituents through. These substances cross readily if they are less than four nm in size and near pass freely upwardly to 8 nm in size. An additional factor affecting the ability of substances to cross this barrier is their electric charge. The proteins associated with these pores are negatively charged, so they tend to repel negatively charged substances and let positively charged substances to pass more readily. The basement membrane prevents filtration of medium-to-big proteins such as globulins. There are also mesangial cells in the filtration membrane that tin can contract to help regulate the rate of filtration of the glomerulus. Overall, filtration is regulated past fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The upshot is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.

Lying simply outside Bowman's sheathing and the glomerulus is the juxtaglomerular apparatus (JGA). At the juncture where the afferent and efferent arterioles enter and go out Bowman's capsule, the initial part of the distal convoluted tubule (DCT) comes into direct contact with the arterioles. The wall of the DCT at that point forms a role of the JGA known as the macula densa. This cluster of cuboidal epithelial cells monitors the fluid composition of fluid flowing through the DCT. In response to the concentration of Na⁺ in the fluid flowing past them, these cells release paracrine signals. They too take a single, nonmotile cilium that responds to the charge per unit of fluid movement in the tubule. The paracrine signals released in response to changes in flow rate and Na⁺ concentration are adenosine triphosphate (ATP) and adenosine.

Juxtaglomerular Apparatus and Glomerulus

(a) The JGA allows specialized cells to monitor the composition of the fluid in the DCT and accommodate the glomerular filtration rate. (b) This micrograph shows the glomerulus and surrounding structures. LM × 1540. (Micrograph provided past the Regents of University of Michigan Medical School © 2012)

A second cell blazon in this appliance is the juxtaglomerular jail cell. This is a modified, smooth muscle cell lining the afferent arteriole that can contract or relax in response to ATP or adenosine released past the macula densa. Such contraction and relaxation regulate blood menses to the glomerulus. If the osmolarity of the filtrate is too high (hyperosmotic), the juxtaglomerular cells will contract, decreasing the glomerular filtration charge per unit (GFR) then less plasma is filtered, leading to less urine germination and greater retentiveness of fluid. This volition ultimately decrease blood osmolarity toward the physiologic norm. If the osmolarity of the filtrate is too low, the juxtaglomerular cells will relax, increasing the GFR and enhancing the loss of water to the urine, causing blood osmolarity to rise. In other words, when osmolarity goes up, filtration and urine formation decrease and h2o is retained. When osmolarity goes downward, filtration and urine formation increase and h2o is lost by manner of the urine. The net event of these opposing actions is to keep the rate of filtration relatively constant. A second function of the macula densa cells is to regulate renin release from the juxtaglomerular cells of the afferent arteriole. Agile renin is a protein comprised of 304 amino acids that cleaves several amino acids from angiotensinogen to produce angiotensin I. Angiotensin I is not biologically active until converted to angiotensin II by angiotensin-converting enzyme (ACE) from the lungs. Angiotensin II is a systemic vasoconstrictor that helps to regulate blood pressure past increasing it. Angiotensin Two also stimulates the release of the steroid hormone aldosterone from the adrenal cortex. Aldosterone stimulates Na⁺ reabsorption by the kidney, which besides results in water retentivity and increased blood pressure.

Conversion of Angiotensin I to Angiotensin 2

The enzyme renin converts the pro-enzyme angiotensin I; the lung-derived enzyme ACE converts angiotensin I into agile angiotensin Ii.

Proximal Convoluted Tubule (Percent)

Filtered fluid collected past Bowman'southward capsule enters into the Percentage. It is called convoluted due to its tortuous path. Elementary cuboidal cells grade this tubule with prominent microvilli on the luminal surface, forming a brush border. These microvilli create a big surface area to maximize the absorption and secretion of solutes (Na⁺, Cl^–, glucose, etc.), the most essential function of this portion of the nephron. These cells actively transport ions across their membranes, so they possess a high concentration of mitochondria in guild to produce sufficient ATP.

Loop of Henle

The descending and ascending portions of the loop of Henle (sometimes referred to as the nephron loop) are, of form, just continuations of the same tubule. They run next and parallel to each other later on having made a hairpin turn at the deepest bespeak of their descent. The descending loop of Henle consists of an initial curt, thick portion and long, thin portion, whereas the ascending loop consists of an initial short, thin portion followed by a long, thick portion. The descending thick portion consists of simple cuboidal epithelium similar to that of the Percent. The descending and ascending thin portions consists of elementary squamous epithelium. As you lot will see later, these are important differences, since different portions of the loop have different permeabilities for solutes and water. The ascending thick portion consists of simple cuboidal epithelium similar to the DCT.

Distal Convoluted Tubule (DCT)

The DCT, like the PCT, is very tortuous and formed past elementary cuboidal epithelium, simply information technology is shorter than the PCT. These cells are not as agile as those in the PCT; thus, there are fewer microvilli on the apical surface. However, these cells must also pump ions against their concentration gradient, then yous will observe of large numbers of mitochondria, although fewer than in the Per centum.

Collecting Ducts

The collecting ducts are continuous with the nephron merely non technically role of information technology. In fact, each duct collects filtrate from several nephrons for terminal modification. Collecting ducts merge as they descend deeper in the medulla to grade about 30 last ducts, which empty at a papilla. They are lined with simple squamous epithelium with receptors for ADH. When stimulated by ADH, these cells will insert aquaporin channel proteins into their membranes, which every bit their name suggests, permit water to pass from the duct lumen through the cells and into the interstitial spaces to be recovered by the vasa recta. This procedure allows for the recovery of large amounts of h2o from the filtrate dorsum into the blood. In the absence of ADH, these channels are non inserted, resulting in the excretion of h2o in the class of dilute urine. Virtually, if not all, cells of the torso comprise aquaporin molecules, whose channels are so minor that only water can laissez passer. At least 10 types of aquaporins are known in humans, and half-dozen of those are found in the kidney. The function of all aquaporins is to allow the movement of water across the lipid-rich, hydrophobic jail cell membrane.

Aquaporin Water Channel

Positive charges within the channel foreclose the leakage of electrolytes beyond the jail cell membrane, while assuasive water to movement due to osmosis.

Chapter Review

The functional unit of the kidney, the nephron, consists of the renal corpuscle, Pct, loop of Henle, and DCT. Cortical nephrons have brusk loops of Henle, whereas juxtamedullary nephrons take long loops of Henle extending into the medulla. Nigh 15 percent of nephrons are juxtamedullary. The glomerulus is a capillary bed that filters blood principally based on particle size. The filtrate is captured by Bowman'south sheathing and directed to the PCT. A filtration membrane is formed by the fused basement membranes of the podocytes and the capillary endothelial cells that they embrace. Contractile mesangial cells farther perform a role in regulating the rate at which the blood is filtered. Specialized cells in the JGA produce paracrine signals to regulate blood menstruation and filtration rates of the glomerulus. Other JGA cells produce the enzyme renin, which plays a central role in claret pressure regulation. The filtrate enters the PCT where absorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and sparse segments. Absorption and secretion proceed in the DCT but to a lesser extent than in the PCT. Each collecting duct collects forming urine from several nephrons and responds to the posterior pituitary hormone ADH by inserting aquaporin water channels into the cell membrane to fine melody water recovery.

Disquisitional Thinking Questions

1. Which structures brand up the renal corpuscle?
2. What are the major structures comprising the filtration membrane?

Answers: Disquisitional Thinking

1. The structures that make up the renal corpuscle are the glomerulus, Bowman's capsule, and Percent.
2. The major structures comprising the filtration membrane are fenestrations and podocyte fenestra, fused basement membrane, and filtration slits.

Glossary

angiotensin-converting enzyme (ACE)enzyme produced past the lungs that catalyzes the reaction of inactive angiotensin I into active angiotensin II

angiotensin Iprotein produced by the enzymatic action of renin on angiotensinogen; inactive precursor of angiotensin Ii

angiotensin IIprotein produced by the enzymatic action of ACE on inactive angiotensin I; actively causes vasoconstriction and stimulates aldosterone release by the adrenal cortex

angiotensinogeninactive poly peptide in the apportionment produced by the liver; forerunner of angiotensin I; must be modified past the enzymes renin and ACE to be activated

aquaporinprotein-forming water channels through the lipid bilayer of the cell; allows h2o to cantankerous; activation in the collecting ducts is under the control of ADH

castor edgeformed by microvilli on the surface of sure cuboidal cells; in the kidney it is found in the Percent; increases surface area for absorption in the kidney

fenestrationssmall windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents

filtration slitsformed by pedicels of podocytes; substances filter betwixt the pedicels based on size

forming urinefiltrate undergoing modifications through secretion and reabsorption before true urine is produced

juxtaglomerular apparatus (JGA)located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood menses and GFR

juxtaglomerular jail cellmodified shine muscle cells of the afferent arteriole; secretes renin in response to a drop in claret pressure

macula densacells found in the part of the DCT forming the JGA; sense Na⁺ concentration in the forming urine

mesangialcontractile cells plant in the glomerulus; can contract or relax to regulate filtration charge per unit

pedicelsfinger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane

podocytescells forming finger-similar processes; form the visceral layer of Bowman's sheathing; pedicels of the podocytes interdigitate to form a filtration membrane

reninenzyme produced by juxtaglomerular cells in response to decreased claret pressure level or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I

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