Chapter summary
Introduction
It is difficult to overestimate the significance of the urinary system. Certainly in human medicine, the numbers of patients on dialysis who are awaiting possible kidney transplants are stark reminders.
The kidney is a key to metabolic health, maintenance of electrolyte balance, control of blood and tissue fluid osmolarity, and elimination of multiple waste products. The capacity of the kidney to produce copious amounts of dilute urine or scant amounts of highly concentrated urine is a physiological marvel.Urinary system
The urinary system consists of the paired kidneys, two ureters, the urinary bladder (except in birds), and the urethra. When sufficiently distended, the urinary bladder discharges the stored urine to the outside of the body.
Anatomy and function
There is probably no better example of the concept of structure and function going hand in hand than the kidney. The primary functional component of the kidney is the nephron. This simple but elegantly organized tube and associated structures allow for (1) filtering and creation of fluid derived from the blood plasma, (2) addition (secretion) of waste products or recovery (absorption) of important nutrients, (3) adjustments in volume of urine produced, and (4) control of blood and tissue acid/base balance, pH, and osmolarity.
The nephron
The nephron begins in the area of Bowman's capsule as filtrate is formed. Sequential segments of the nephron include proximal convoluted tubule, descending loop of Henle, ascending loop of Henle, distal convoluted tubule, and collecting duct. The location of nephrons (cortical or medullary) varies as well as the relative length of the loop of Henle. These variations help determine the relative importance of individual nephrons in regulation of blood osmolarity.
Kidney blood flow
In usual circumstances, the kidneys receive about 25% of cardiac output.
The renal arteries enter the kidney in the hilus, then divide into interlobular arteries, which pass between the pyramids and close to the cortex where arched branches become the arcuate arteries. Each of these feeds interlobular arteries that supply blood to the afferent arterioles, which provide blood to the glomerulus. The glomerulus is housed within the renal corpuscle, the internal surface of which is Bowman's capsule and the entrance into the proximal convoluted tubule. Capillaries from the glomerulus combine to become the efferent arterioles that give rise to veins corresponding to the arteries described earlier.Blood pressure, hormones, and regulation
It is easy to recognize the significance of the heart and blood vessels in control of blood pressure, but the kidneys are especially important in long-term control of blood pressure. Fundamentally, the degree to which urinary filtrate volume is recovered is directly correlated with blood volume and therefore blood pressure. Determining how much (or how little) urine is produced depends on multiple structures and actions. For example, recovery of filtrate depends on the maintenance of an osmotic gradient in the tissue surrounding the loops of Henle and the collecting ducts of the nephrons that are located in the border region between the cortex and medulla. Near the beginning of the collecting ducts, interstitial fluid osmolarity is similar to blood plasma ~400mOsm∕L, but closer to the distal end of the collecting ducts (deeper in the medulla), and near the boundary between the descending and ascending loop of Henle, interstitial fluid osmolarity can reach 1200mOsm∕L or more (depending on the species). The creation and maintenance of this osmotic gradient depends on the unique structure of the loops of Henle and the capillaries that are a part of the descending and ascending vasa recta. The blood flow through the vasa recta is the opposite of the flow of fluid in the loops of Henle.
This is called a countercurrent flow. This arrangement, along with changes in the permeability of the loops of Henle (descending different from ascending), allows the osmotic gradient in the surrounding interstitial fluid to be maintained. The control of urine production depends on (1) the relative degree to which sodium and other ions and osmotically active agents are removed from the filtrate flowing in the collecting ducts and (2) the degree to which the collecting ducts are permeable to water.The secretion of antidiuretic hormone (ADH) has a major impact on the permeability of the cells of the collecting ducts to water. If ADH is not secreted or is low in concentration, the collecting duct permeability to water is poor, so there is minimal response to passage of filtrate through the region of high interstitial fluid osmolarity. In other words, the normal osmotic forces that you would expect would cause water to leave the collecting duct do not occur. This means a larger amount of dilute hypotonic urine is produced. On the other hand, when ADH is present, the cells of the collecting duct are highly permeable so the osmotic forces allow water to be recovered so that a smaller volume of concentrated hypertonic urine is produced. Secretion of ADH is regulated by osmoreceptors in the hypothalamus.
Another blood pressure control depends on the juxtaglomerular cells. Some nephrons have loops of Henle and corresponding distal convoluted tubules that pass in close apposition to the boundary between afferent and efferent arterioles near the renal capsule. In these regions, cells in the space between these structures are differentiated for special functions. Populations of cells in the wall of the afferent arteriole called juxtaglomerular or granular cells synthesize the enzyme renin. Modified cells of the adjacent nephron (the macula densa) along with the juxtaglomerular cells create a grouping called the juxtaglomerular apparatus. The release of renin is controlled by sympathetic nerve impulses and/or reductions in blood pressure at the level of the arterioles.
Renin acts on plasma precursors to produce angiotensin I, which in turn is cleaved to produce angiotensin II, which constricts other nonkidney arterioles and increases blood flow and pressure to the kidney. It also promotes the secretion of aldosterone.Produced by the adrenal gland, aldosterone acts on principal cells in the collecting ducts to promote sodium reabsorption from the filtrate. This promotes additional recovery of water and therefore enhances blood volume and pressure.
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References
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Haller, M., K. Rohner, W. Muller, F. Reutter, H. Binder, W. Estelberger, and P. Arnold. 2003. Single-injection inulin clearance for routine measurement of glomerular filtration rate in cats. J. Feline Med. Surg. 5: 175-181.
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