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Nephron The functional units of the kidneys are called nephrons. We will cover the anatomy and physiology of the nephrons in the next section. To understand how different regions of the nephron are able to have unique, spatial functions, we will first discuss how the multicellular epithelial structures found in these structures are organized for specific functions. Nephrons are made up of epithelial cells with an underlying non-cellular layer or basement membrane that separates the filter in the lumen fluid from the interstitial space. These epithelial cells differ in cellular anatomy along the length of the nephrons according to the filtering and processing functions of the epithelial cells. Functional Urinary Tissue – The Nephron As the chief functional organ in the urinary system, the kidneys excrete nitrogenous wastes and are involved in regulating the volume, composition, and pH of the blood. The kidneys receive one fourth of total cardiac output, a reflection of their function as blood processors. Each kidney contains about one million nephrons, the structural and functional units of the kidneys. Each nephron is made up of a highpressure capillary bed called a glomerulus and a renal tubule, segments of which included a proximal convoluted tubule, nephron loop (loop of Henle), and distal convoluted tubule. The distal convoluted tubules from multiple nephrons join a common collecting duct. The nephrons are involved in three functions: filtration, reabsorption, and secretion. Overview of the Nephron Structure The structure within each nephron that actually filters blood plasma is the renal corpuscle containing the glomerulus and glomerular capsule. Another nephron structure called the renal tubule receives the filtered fluid, called glomerular filtrate. Very thin and a little over an inch long, the renal tubule has three major consecutive segments that the filtrate flows through: a proximal convoluted tubule the nephron loop(loop of Henle), and a distal convoluted tubule. Nephron Functions Nephron Structure Function Description Glomerulus Filtration The glomerulus is a capillary network found in close proximity to the nephron that filters plasma into the nephron.Proteins and blood cells are retained in the glomerular capillary. Tubules and nephron loop (loop of Henle) Reabsorption Epithelial cells actively transport some substances from the tubular fluid back into blood. Other substances, such as water, are passively reabsorbed in some segments. Capillaries specifically Peritubular Secretion Epithelial cells actively secrete certain substances from the blood into the tubular lumen. Collecting duct Collection Accumulates any material that is not returned to blood in the preceding segments. Secretes or reabsorbs H+, HCO3+, and K+ ions. Reabsorbs water under the influence of anti-diuretic hormone. Anything left in the distal end of the collecting duct will l be excreted as urine The Glomerulus The renal corpuscle is made up of a tangled capillary network called a glomerulus and a cup-shaped structure called the glomerular capsule (Bowman's capsule) surrounding the glomerulus. The glomerular capsule has an external parietal layer made of simple squamous epithelium. Although this layer is not involved in the production of filtrate, it helps to maintain the shape of the capsule. An inner visceral layer adheres to the glomerular capillaries and is composed of a special type of simple squamous epithelial cells called podocytes. These podocytes have multiple projections called pedicels or foot processes. The pedicels of one podocyte interlock with the pedicels of adjacent podocytes. Filtrate from the glomerulus passes through filtration slits, the openings between the foot pedicels, to enter the capsular space(Bowman's space), the area between the visceral and parietal layers of the glomerular capsule. The Proximal Convoluted Tubule The proximal convoluted tubule (convoluted refers to the coiled shape) tubule connects the glomerular capsule to the nephron loop. The apical surface of the simple cuboidal epithelial cells making up the proximal convoluted tubule are covered in microvilli producing a brush border. The brush border and the length of the proximal convoluted tubule dramatically increase the luminal surface area available for reabsorbing water and solutes and for secreting substances into the filtrate. The Nephron Loop The renal corpuscle, the proximal convoluted tubule, and the juncture between the proximal convoluted tubule and the nephron loop are located in the renal cortex. The first part of the nephron loop, the descending limb of the nephron loop, drops into the renal medulla. In the renal medulla, the loop makes a sharp, almost 180-degree turn back toward the renal cortex as the ascending limb of the nephron loop. The ascending limb is continuous with the distal convoluted tubule. The ascending and descending limbs of the nephron loop have two distinct parts: a thin section of the limb and a thick section of the limb. In the thin section of the limb, the diameter of the tubule is distinctly smaller than the diameter of the rest of the nephron tubules. In the thin sections of the limbs, the epithelium is thinner simple squamous epithelium that is permeable to water. In the thick sections of the limbs, the epithelium is simple cuboidal epithelium that is highly impermeable to water. Regardless of being in the thin or the thick segments, the lumen is the same size as the lumen in the rest of the renal tubule. Distal Convoluted Tubule The final segment of the nephron is the distal convoluted tubule. As the ascending limb of the nephron loop reaches the renal cortex, it becomes the distal convoluted tubule. The distal convoluted tubule extends to the collecting tubule, the short connection with a collecting duct. The distal convoluted tubule is composed of simple cuboidal epithelium with very few microvilli and no brush border. Capillaries of the Nephron The glomerulus is not the only capillary bed associated with nephrons. Peritubular capillaries are branches of the efferent arterioles that drain the glomeruli and recover most of the filtrate produced in the renal corpuscle. Glomerular capillaries differ from other capillary beds in the body, because they are both supplied by and drained by arterioles. The feeder afferent arterioles are branches of the cortical radiate arteries. The draining efferent arterioles branch into the peritubular capillary network around the proximal and distal convoluted tubules or the vasa recta around the nephron loop. The diameter of the draining efferent arterioles is smaller than that of the afferent arterioles, giving the efferent arterioles higher resistance. Because of this, the glomerulus has a high blood pressure that allows it to filter high volumes of fluid and solutes out of the blood and into the glomerular capsule. The nephrons segments reabsorb approximately 99 percent of this filtrate. The peritubular capillaries adhere to neighboring convoluted tubules and drain into neighboring venules. These lowpressure and porous capillaries easily reabsorb the water and solutes that the tubule recovers from the filtrate. In some nephrons, rather than breaking up into peritubular capillaries, the efferent arterioles form clusters of thin-walled vasa recta. Important for the formation of concentrated urine, the vasa recta are long, straight capillaries that reach deep into the medulla alongside the longest nephron loops where they can collect reabsorbed substances from the loop segments. Because blood in the renal circulation flows through two arterioles (where a majority of the manipulation of vascular resistance is found), renal blood pressure drops from about 95 mm Hg in the renal arteries to less than 10 mm Hg in the renal veins. Resistance in the afferent arterioles fluctuates in order to help maintain a relatively constant glomerular capillary hydrostatic pressure even if there are substantial changes in systemic blood pressure. The resistance of the efferent arterioles also contributes to maintenance of glomerular capillary hydrostatic pressure and also contributes to a low hydrostatic pressure in the peritubular capillaries. Specialized Cells Associated With the Nephron In all nephrons, the last part of the ascending limb of the nephron loop transitions into the distal convoluted tubule and comes in contact with the afferent arteriole supplying the renal corpuscle. In this region the columnar epithelial cells at the beginning of the distal convoluted tubule are very crowded, leading to the name macula densa ("dense spot"). The macula densa is believed to monitor sodium chloride concentration of the filtrate entering the distal convoluted tubule. The wall of the afferent arteriole that is adjacent to the macula densa contains granular cells (also known as juxtaglomerular cells ). The granular cells produce and secrete the enzyme renin and are also capable of contracting when stimulated. These cells and the macular densa make up the juxtaglomerular apparatus. The action of the juxtaglomerular apparatus helps control glomerular hydrostatic pressure by sending signals to the afferent arteriole. There are also special smooth muscle cells called intraglomerular mesangial cells in in the spaces between the loops of the glomerulus. These cells help regulate blood flow through the glomerulus. Collecting Ducts As the functional units of the kidneys, the primary role of the nephrons is to filter plasma, reabsorb what the body would like to keep, and excrete the rest. Any substances not reabsorbed in the tubules of the nephrons flows into one of thousands of collecting ducts in the kidney. A short collecting tubule forms the juncture between a distal convoluted tubule and a collecting duct. Each collecting duct receives fluid from several nephrons and then transports it to the renal pelvis. The collecting tubules and ducts have two types of cells: intercalated cells, cuboidal cells with plentiful microvilli, and the more populous principal cells, cuboidal cells with limited, short microvilli. Principal cells help maintain water and sodium and potassium ion balance in the body. Intercalated cells help regulate the acid-base balance of the blood.