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    nephrons located close to the medulla with long nephron loops are called ________ nephrons.

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    Pre-Lab Reading

    Introduction

    The urinary system is comprised of the kidney, ureter, urinary bladder, and urethra. The kidney produces urine, which contains excess water, electrolytes and waste products of the body. It then flows down the ureter into the bladder where it is temporarily stored. The bladder is then emptied via the urethra.

    Kidney

    The kidney has several important homeostatic, hormonal, and metabolic functions that include:

    The maintenance of water and electrolyte homeostasis

    Regulation of acid-base balance in conjunction with the respiratory system

    Excretion of metabolic waste products, especially the toxic nitrogenous compounds

    Production of renin for blood pressure control and erythropoietin, which stimulates red blood cell production in the bone marrow

    Conversion of vitamin D into active form for the regulation of calcium balance

    The kidney is composed of an outer cortex and inner medulla. Portions of the medulla extend into the cortex as the medullary rays, collections of straight renal tubules. The medulla contains multiple cone-shaped lobes, known as medullary pyramids. These urinary lobes are fused in the cortex. The urine drains into the renal pelvis, which is the initial part of the ureter. The hilum of the kidney is the site of entry and exit for renal artery, renal vein, and ureter.

    Nephron

    The nephron is the structural and functional unit of the kidney. There are about two million nephrons in each kidney. Nephrons begin in the cortex; the tubules dip down to the medulla, then return to the cortex before draining into the collecting duct. The collecting ducts then descend towards the renal pelvis and empty urine into the ureter.

    The components of a single nephron include:

    renal corpuscle

    proximal convoluted tubule

    loop of Henle

    distal convoluted tubule

    Different sections of nephrons are located in different parts of the kidney:

    The cortex contains the renal corpuscle, proximal, and distal convoluted tubules.

    The medulla and medullary rays contain the loops of Henle and collecting ducts.

    Throughout the length of the nephron, capillaries called peritubular capillaries lie adjacent to all segments of the tubule. They originate from the efferent arteriole and are important for solute transport throughout the tubule.

    Renal Corpuscle

    The renal corpuscle is responsible for the filtration of the plasma. It contains two structures: the glormerulus and Bowman's capsule. The glomerulus is a cluster of capillary loops enclosed by Bowman's capsule, which is part of the renal tubule.

    Bowman's capsule has two layers:

    The visceral layer is in contact with the glormerulus, and is composed of specialized epithelial cells known as podocytes.

    The parietal layer is the outer layer, and is composed of simple squamous epithelial cells. This layer is continuous with the epithelium of the proximal convoluted tubule.

    The space between the two layers is named Bowman's space, and this space contains the ultrafiltrate of plasma. The plasma has to pass through a filtration barrier of three layers to enter Bowman's space: the capillary endothelium, the podocyte layer, and their fused basement membrane. Bowman's space is continuous with the proximal convoluted tubule.

    Blood enters the renal corpuscle via afferent arterioles and then leaves via efferent arterioles. The part of renal corpuscle where afferent and efferent arterioles are located is known as the vascular pole. On the opposite end of the vascular pole is where the renal tubule begins and is known as the urinary pole.

    Mesangial cells can also be found within the glomerulus. These cells secrete a matrix of basement membrane-like material to support the structure of the glomerulus.

    Promixal Convoluted Tubule

    The proximal convoluted tubule is the first segment of renal tubule. It begins at the urinary pole of the glomerulus. This is where the majority (65%) of the glomerular filtrate is reabsorbed. The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle.

    Loop of Henle

    The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the pars recta (the straight descending limb of proximal tubule), the thin descending limb, the thin ascending limb, and the thick ascending limb. The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.

    Distal Convoluted Tubule

    The distal convoluted tubule is shorter and less convoluted than the proximal convoluted tubule. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus.

    Juxtaglomerular Apparatus

    Source : medcell.med.yale.edu

    25.2 Microscopic Anatomy of the Kidney: Anatomy of the Nephron – Anatomy & Physiology

    25.2 MICROSCOPIC ANATOMY OF THE KIDNEY: ANATOMY OF THE NEPHRON

    Learning Objectives

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

    Distinguish the histological differences between the renal cortex and medulla

    Describe the structure 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

    Describe the structure and function of the juxtaglomerular apparatus

    Describe the histology and functional significance of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts

    From old 25.1:

    Nephrons are the “functional units” of the kidney; they cleanse the blood of toxins and balance the constituents of the circulation to homeostatic set points through the processes of filtration, reabsorption, and secretion. The nephrons also function to control blood pressure (via production of renin), red blood cell production (via the hormone erythropoetin), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).

    Each nephron consists of a blood supply and a specialized network of ducts called a tubule. For each nephron, an afferent arteriole feeds a high-pressure capillary bed called the glomerulus. Blood is filtered by the glomerulus to produce a fluid which is caught by the nephron tubule, called filtrate. The proximal end of the tubule that surrounds the glomerulus and catches the filtered fluid is the glomerular (Bowman’s) capsule. The glomerulus and glomerular capsule together form the renal corpuscle. Filtered fluid caught by the glomerular capsule (filtrate) travels through the rest of the tubule to the proximal convoluted tubule (PCT), loop of Henle and distal convoluted tubule (DCT), in this order, before exiting the nephron into common collecting ducts shared by many nephrons. Though all nephron glomeruli are in the cortex, some nephrons have short loops of Henle that do not dip far beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have very long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons.

    Blood exits the glomerulus into the efferent arteriole (Figure 25.2.1). The efferent arteriole then forms a second capillary network around the tubule, called the peritubular capillaries. For juxtamedullary nephrons, the portion of the capillary that follows the loop of Henle deep into the medulla is called the vasa recta. As the glomerular filtrate progresses through the tubule, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, this is another example of a portal system (also seen in hypothalamus-pituitary axis and hepatic portion of the digestive system).

    Figure 25.2.1 – Blood Flow in the Nephron: The glomerulus filters blood into the glomerular capsule; the peritubular capillary reclaims substances from the tubule. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta. EDITOR’S NOTE: ADD cortical & justamedullary nephrons to this image like the model in our lab; combine this figure with the next.

    External Website

    Visit this link to view an interactive tutorial of the flow of blood through the kidney.

    MICROANATOMY OF THE NEPHRON

    MICROANATOMY OF THE NEPHRON RENAL CORPUSCLE

    As discussed earlier, the renal corpuscle consists the glomerulus and the glomerular capsule. The glomerulus is a high pressured, fenestrated capillary with large holes (fenestrations) between the endothelial cells. The glomerular capsule captures the filtrate created by the glomerulus and directs this filtrate to the PCT. The outermost part of glomerular capsule is a simple squamous epithelium. It transitions over the glomerulus as uniquely shaped cells (podocytes) with finger-like arms (pedicels) that cover the glomerular capillaries (Figure 25.2.2). A thin basement membrane lies between the glomerular endothelium and the podocytes. The pedicels interdigitate to form filtration slits, leaving small gaps that form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters out of the fenestrations, through the basement membrane and between these sieve-like fingers to be captured by the glomerular capsule and funneled to the PCT. These features comprise the filtration membrane.

    Figure 25.2.2 – Podocytes: Podocytes interdigitate with structures called pedicels and filter substances into the glomerular capsule. In (a), the large cell body can be seen at the top right corner, with branches extending from the cell body. The smallest finger-like extensions are the pedicels.

    Source : open.oregonstate.education

    22.2. The Kidneys and Osmoregulatory Organs – Concepts of Biology – 1st Canadian Edition

    22.2. THE KIDNEYS AND OSMOREGULATORY ORGANS

    Learning Objectives

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

    Explain how the kidneys serve as the main osmoregulatory organs in mammalian systems

    Describe the structure of the kidneys and the functions of the parts of the kidney

    Describe how the nephron is the functional unit of the kidney and explain how it actively filters blood and generates urine

    Detail the three steps in the formation of urine: glomerular filtration, tubular reabsorption, and tubular secretion

    Although the kidneys are the major osmoregulatory organ, the skin and lungs also play a role in the process. Water and electrolytes are lost through sweat glands in the skin, which helps moisturize and cool the skin surface, while the lungs expel a small amount of water in the form of mucous secretions and via evaporation of water vapor.

    KIDNEYS: THE MAIN OSMOREGULATORY ORGAN

    The kidneys, illustrated in Figure 22.4, are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity. The adrenal glands sit on top of each kidney and are also called the suprarenal glands. Kidneys filter blood and purify it. All the blood in the human body is filtered many times a day by the kidneys; these organs use up almost 25 percent of the oxygen absorbed through the lungs to perform this function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine.

    Figure 22.4.  Kidneys filter the blood, producing urine that is stored in the bladder prior to elimination through the urethra. (credit: modification of work by NCI)

    KIDNEY STRUCTURE

    Externally, the kidneys are surrounded by three layers, illustrated in Figure 22.5. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters. The renal cortex is granular due to the presence of nephrons—the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight renal pyramids in each kidney. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.

    Figure 22.5.  The internal structure of the kidney is shown. (credit: modification of work by NCI)

    Which of the following statements about the kidney is false?

    The renal pelvis drains into the ureter.

    The renal pyramids are in the medulla.

    The cortex covers the capsule.

    Nephrons are in the renal cortex.

    Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries (which are each named based on the region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate “bow shaped” arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries, as the name suggests, radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent arterioles, and then enter the capillaries supplying the nephrons. Veins trace the path of the arteries and have similar names, except there are no segmental veins.

    As mentioned previously, the functional unit of the kidney is the nephron, illustrated in Figure 22.6. Each kidney is made up of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally. There are two types of nephrons— cortical nephrons (85 percent), which are deep in the renal cortex, and juxtamedullary nephrons (15 percent), which lie in the renal cortex close to the renal medulla. A nephron consists of three parts—a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries.

    Source : opentextbc.ca

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