Functions of the Urinary System

Protein catabolism produces ammonia, which reacts with CO₂ to produce [1], the most abundant of the [2] wastes. The accumulation of such wastes in the blood, called [3], results from kidney failure and may progress to a toxic and potentially fatal syndrome called [4]. The kidneys serve not only to prevent this, but also to maintain the [5] of the blood–its concentration of dissolved particles.

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Anatomy of the Kidney

The kidney parenchyma is divided into an outer layer called the renal [6] and about 6 to 10 conical [7], which together compose the renal [8]. The apex of each 7 empties into a cuplike urine receptacle called a [9]. These converge on 2 or 3 major calices, which unite at the renal [10], a single, funnellike collection point before the urine leaves the kidney. The [11] is a muscular tube that arises from the 10, exits the kidney through a slit on its concave surface called the [12], and leads to the urinary bladder below.

Urine is produced by about 1.2 million units called [13] in each kidney. To get to one of these, blood enters the kidney by way of the renal artery, flows through interlobar arteries between the pyramids, and then into a/an [14] artery, which turns 90° and travels along the corticomedullary junction. This artery gives off several interlobular arteries, which pass up into the cortex. Arising from these at intervals are the [15], one for each nephron. Each 15 supplies blood to a spheroid capillary bed called the [16], and a/an [17] carries blood away from it. This capillary bed is where urine production begins. Vessel 17 gives rise to another capillary bed, the [18], before blood enters the veins and begins its journey out of the kidney.

The 16 is enclosed in a double-walled [19] capsule. At its urinary pole, this capsule gives rise to a long coiled duct called the [20]. This continues as a long, narrow, U-shaped duct, the [21], which descends into the renal medulla, turns 180° and returns to the cortex, then gives rise to a short coiled duct called the [22]. This is the end of the nephron. The 22s of several nephrons empty into a [23], which passes down a renal pyramid and empties into a minor calyx. At the junction of the 15, 17, and 22, there is a [24] apparatus that monitors the salinity of the tubular fluid and adjusts nephron performance.

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Urine Formation I: Glomerular Filtration

Urine production begins with glomerular filtration. To get from the blood to the capsular space, substances must pass through three barriers: the pores (fenestrations) of the [25] cells, the basement membrane, and finally the filtration slits between the pedicels of the [26]. If these barriers break down, we may see blood in the urine, called [27], or protein in the urine, called [28]. Glomerular filtration is driven by blood pressure in the glomerular capillaries, which is unusually high because the [29] is smaller than the [30]. Filtration is opposed by the hydrostatic pressure in the capsular space and by the [31] of the blood. Nevertheless, there is a net filtration pressure of about 10 mmHg driving fluid out of the glomerular capillaries, and this is enough to produce a [32] of about 125 mL/min.

This value is controlled partly by the sympathetic nervous system and hormones, but also by renal [33]–self-control by means of the juxtaglomerular apparatus. This structure involves a patch of cells in the distal convoluted tubule (DCT) called the [34], which monitors the salinity of the tubular fluid, and cells of the afferent arteriole called the [35], which regulate the blood pressure of the glomerulus. The sympathetic nervous system innervates the afferent arteriole. Increased nerve signals have the effect of [36] the glomerular filtration rate (GFR) during exercise and certain other states. Low blood pressure stimulates the 35 to secrete the enzyme [37], which converts a protein in the blood plasma to [38]. ACE, an enzyme in the lungs, further converts this to [39], a strong vasoconstrictor that raises blood pressure.

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Urine Formation II: Tubular Reabsorption and Secretion

Since glomerular filtration produces about 180 L of tubular fluid per day and we cannot possibly lose that much water from the body, nearly all of this must be returned to the bloodstream. The process of doing so is called [40]. Most of this occurs in the proximal convoluted tubule (PCT), which is the reason for its great length and the abundance of [41] on its apical cell surfaces. Tubular reabsorption is favored by the low blood pressure and high colloid osmotic pressure in the [42] capillaries. To get from the tubular lumen to these capillaries, substances can follow a transcellular route through the tubule cells or a [43] route between them.

Epithelial cells of the PCT maintain a low internal Na⁺ concentration by means of [44] in the basal plasma membrane. Na⁺ then diffuses from the tubule lumen, where its concentration is high, into the epithelial cells, where it is low. Some of the Na⁺ carriers in the plasma membrane, called SGLTs, also bind [45] and carry it into the cell with the Na⁺. [46] follows Na⁺ by osmosis; the PCT reabsorbs it at a constant rate called [47]. The PCT absorbs protein from the tubular fluid by means of [48]. There is a limit, called the [49], to how fast the tubule can reabsorb any given solute. If the blood concentration of a substance is high and that substance enters the PCT faster than this maximum rate of reabsorption, the excess appears in the urine. In diabetes mellitus, this occurs with glucose and results in [50], glucose in the urine. Other parts of the nephron also reabsorb water and solutes. The nephron loop, for example, has transporters that simultaneously bind 1 Na⁺, 1 K⁺, and [51] and return them to the ECF. The distal convoluted tubule reabsorbs sodium and water, but differs from the PCT in that its rate of reabsorption is influenced by the hormone [52]. Another hormone, [53], has the opposite effect–it promotes sodium and water excretion. The renal tubules can also remove substances from the blood and add them to the tubular fluid, such as H⁺, uric acid, and penicillin and other drugs. This process is called [54].

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Urine Formation III: Concentrating the Urine

Urine concentration is controlled by the [55], which reabsorbs varying amounts of water depending on the body's state of hydration. Urine is hypotonic or at most [56] to the blood plasma when it enters this duct and can be as much as [57] times as concentrated when it leaves. Exactly how concentrated it becomes depends on a hormone called [58], which regulates the water permeability of this duct. The ability to concentrate urine at all, however, results from a gradient of salinity in the ECF of the renal medulla. This gradient is produced by a part of the nephron called the [59]. Because this structure multiplies the salinity of the ECF and because it works on the basis of fluid flowing in opposite directions through adjacent limbs of the duct, it is called a/an [60] mechanism. As tubular fluid flows down the proximal part of this tubule, it loses [61] to the ECF but retains [62]. Therefore, its osmolarity is relatively high by the time it reaches the bottom of the tubule, goes around the bend, and begins to ascend the other limb. The ascending limb is impermeable to water, but cells of its [63] segment pump Na⁺, K⁺, and Cl^- out of the tubular fluid into the ECF. Therefore the tubular fluid is very dilute when it reaches the upper end of this limb, while salt remains behind in the renal medulla and creates the salinity gradient needed for the collecting duct to function. The collecting duct adds to the osmolarity of the renal medulla by contributing [64] to the ECF. The renal medulla needs a blood supply, and yet there must be a way to ensure that blood capillaries do not carry away the salt of the ECF and destroy its salinity gradient. The blood capillaries of the medulla, called the [65], are arranged side by side to form a/an [66] system, which ensures that the osmolarity of the blood leaving the medulla is no greater than the blood entering it, and the salt in the medulla remains there.

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Urine and Renal Function Tests

Measurement of the physical and chemical properties of the urine, called [67], is one of the most routine tests performed on patients. This includes examination of the color and odor of the urine; measurement of its pH, osmolarity, and [68] (its density relative to distilled water); and tests of its chemical composition. The most abundant solute in urine is [69]. Most adults produce about [70] liters of urine per day. Scanty output (under 500 mL/day), called [71], can result from such conditions as dehydration or kidney disease. Excessive output, called [72], can result from certain drugs and diseases. Any chronic 72 with a metabolic cause is called [73]. Although it often involves glucose in the urine, this is not the case in [74], which results from hyposecretion of ADH and inability of the kidney to conserve water. Chemicals that increase urine output are called [75].

One of the important clinical measures of renal function is the volume of blood plasma from which a metabolic waste is completely removed in a given time, called [76]. If we can determine the concentration of the waste in the urine, U, its concentration in the blood plasma, P, and the rate of urine output, V, we can calculate renal clearance (C) from the formula [77]. The same formula can be used to calculate GFR if we use a solute that is neither [78] nor [79] by the renal tubules. A fructose polymer from garlic and artichokes, called [80], can be used for this measurement. In 80 were injected into a patient, and subsequently we found that it had a concentration of 0.45 mg/mL in the blood plasma and 20 mg/mL in the urine, and the patient produced 3 mL of urine per minute, that person's GFR would be [81] (include units of measurement).

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Urine Storage and Elimination

The urinary bladder has a muscular wall called the [82] muscle. Its mucosa has a/an [83] type of epithelium and exhibits folds called [84] when the bladder is empty. These folds are absent from the [85], a smooth triangular patch between the openings of the ureters and [86]. The urethra is only 3 to 4 cm long in the female, but in the male it is much longer and is divided into three regions: the [87] urethra immediately inferior to the bladder, the [88] urethra where it passes through the muscles of the pelvic floor, and the [89] urethra in the penis. In both sexes, there are two muscles that regulate the emptying of the bladder: a/an [90] of smooth muscle and, inferior to this, a/an [91] of skeletal muscle. When the bladder fills to about 200 mL, stretch receptors in the wall send signals to the spinal cord and activate the [92] reflex. This causes the 90 to relax and the 82 to contract. In infants, this is enough to produce urination, but in adults, a neuronal pool in the pons called the [93] controls the 91 and prevents the bladder from emptying except when one wills it. When the bladder is not full enough to activate the 92 reflex but you desire to urinate anyway, the [94] maneuver can be used to increase bladder pressure and trigger the reflex early.

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