Urinary System Basics
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Introduction The urinary system is responsible for removing toxins, metabolic wastes and excess ions from the blood stream by way of urine. The primary organs producing urine are the kidneys. About 200 liters of fluid is filtered by the kidneys on a daily basis. These two bean shaped organs are positioned posterior to the peritoneal cavity, anterior to the dorsal wall and at about the height of the 12th ribs and spans between the T12 and L3 vertebrae. In summary the kidneys regulate the total volume of water in the body and the total concentration of solutes in that water (osmolality). They regulate the various ion concentrations in the extracellular fluids. They help maintain long-term acid-base balance. They remove foreign substances such as drugs and toxins. They produce renin and erythropoietin, which regulate blood pressure and red blood production, respectively. They help produce sugars via gluconeogenesis and convert vitamin D into its active form. While the kidneys form the urine, they are not, however, responsible for removing the urine from the body. The urinary system has three other important components namely: ureters, which are responsible for draining the urine from the kidneys into the urinary bladder, which temporarily stores the urine produced in the kidneys. Lastly, the urine is drained from the urinary bladder and voided from the body by way of the urethra, the last major component of the urinary system. The remainder of this page will highlight the structural features of the components of the urinary system and some of their functional roles. -O. James The collage on the right contains a frontal section of a sheep's kidney (A) and a detailed frontal section of a right kidney (B). Click on image for larger representation. Photo by Orin James. As mentioned in the introduction, the kidneys are bean shaped organs used to filter the body's blood supply. It is positioned posterior to the peritoneal cavity, anterior to the dorsal wall and at about the height of the 12th ribs and spans between the T12 and L3 vertebrae. A frontal section of the kidneys will reveal the following important components:
-O. James © Orin James 2013The image on the right is that of a nephron. To view enlarged image in a new window, simply click on image. (image by Orin James). Millions of nephrons can be found in the kidney. These are the functional units of the kidneys that produce urine. Majority of the components that comprise the nephron can be found in the renal cortex. These include the renal corpuscle, proximal and distal convoluted tubules and parts of the nephron loop and collecting duct. The nephron loop and collecting duct continue into the renal medulla. It is important to note that although the major components of the nephron are found in the cortex, some nephrons may have these closer to the cortex-medulla junction than to the cortex. In the case where the major components are closer to the cortex-medulla junction, these will be called juxtamedullary nephrons. These tend to have a longer nephron loop/Loop of Henle and its efferent artery will feed into the vasa recta. In the case where the major components of the nephron may be found further from the cortex-medulla junction, these will be called cortical nephrons. They tend to have a shorter nephron loop/Loop of Henle than the juxtamedullary nephron. Their efferent arteries feed into peritubular capillaries rather than the vasa recta capillaries. See The Renal Corpuscle here.
-O. James © Orin James 2013
The image on the right is the juxtaglomerular complex and the renal corpuscle. To view enlarged image in a new window, simply click on image. (image by Orin James). As stated previously, the renal corpuscle is the site where filtration of blood will begin. Aiding this process is the juxtaglomerular complex. More on these structures below.
-O. James © Orin James 2013The image on the right is a photomicrograph of the tissue found in the renal cortex of the kidney (200x). It shows the renal corpuscle, proximal and distal convoluted tubles of the nephron. To view enlarged image in a new window, simply click on image. (image by Orin James).
-O. James © Orin James 2013The image on the right shows a photomicrograph of a cross section of the ureter. To view enlarged image in a new window, simply click on image. (image by Orin James). The ureters are responsible for conveying urine from the kidneys to the bladder. The bladder has three layers of tissue: Mucosa, which is composed of a transitional epithelium and lamina propia. The second layer is Muscularis, which is composed of internal longitudal and external circular smooth muscle layers. This layer allows urine to be actively transported to the bladder via muscular contractions. The third layer is Adventitia, which is the outermost covering of the ureter.
-O. James © Orin James 2013The image on the right shows various representations of the ureters, bladders and urethra in the male (A, B) and female (C). To view enlarged image in a new window, simply click on image. (image by Orin James). Urine produced in kidneys are then drained, of ureters (1), into the bladder, which is a smooth, collapsible, muscular sac that temporarily stores urine ( 2, 3, 4,5, & 6). The bladder can collect up to about 200ml, before stretch receptors are activated. At this point impulses caused by contractions are sent to the parasympathetic division of the nervous system. As contractions continue urine may be forced past the internal sphincter (7) into superior part of the urethra. At this point the individual may feel the need to void. The individual may, however, not find it convenient to void at that point and inhibit the opening of this sphincter. If voiding is inhibited, bladder contractions may cease temporarily and the bladder may store another 200 to 300 ml of urine. After this amount has been collected, the micturition reflex will be once again initiated. Once voiding has been initiated, urine will leave the body via the urethra. The length of the urethra differs between males and females. In males the urethra also runs the length of the penis and may reach a length of about 8 inches, while in females the urethra lies anterior to the vaginal opening and posterior to the clitoris, extending only about 1.5 inches long.
-O. James © Orin James 2013Urine Basics By performing a urinalysis, one can easily study the contents of their own urine, the voided product of kidney filtration. Some of the characteristics and substances tested for include pH (usually between 4.5 and 8, average about 6), color (usually clear to pale yellow to amber), specific gravity (usually between 1.001 and 1.030) inorganic substances such as sulfates, phosphates, chlorides, and nitrites. One can also test for organic components like urea, glucose, albumin, ketone bodies, bilirubin, leukocytes bilirubin and urobilinogen. If you are wondering what type information we can gain from such a test, here is a run down:
Physiological Mechanisms of Urinary SystemComing soon... -O. James © Orin James 2013What arteries branch off the arcuate arteries?The radial arteries come off the arcuate arteries at right angles and these supply blood to the cortex. The afferent arterioles that supply blood to the glomeruli are short lateral branches of the radial arteries.
Which arteries branch off the arcuate arteries and extend through the renal cortex?Interlobular arteries, branches of arcuate arteries, run perpendicular to the arcuate arteries and extend through the cortex toward the capsule (Fig. 1.1). Afferent arterioles branch from the interlobular arteries and give rise to glomerular capillaries (Fig.
Which artery lies on the boundary between the cortex and medulla?arcuate artery lies on the boundary between the cortex and medulla of the kidney. The cortex is the outer part of the kidney, while the medulla is the inner part of the kidney.
What are the function of arcuate arteries and veins?Arcuate veins deliver blood to the interlobar veins. Unlike their arterial counter-parts, there is a good deal of anastomosis between the branches of interlobar veins. The renal veins pass from the kidneys to the posterior vena cava: the left renal vein passes ventral to the aorta.
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