Chapter 23: The Urinary System
Functions:
(1) excretion , or the removal of organic waste products from body fluids,
(2) elimination , or the discharge of these waste products into the environment, and
(3) homeostatic regulation of the volume and solute concentration of blood plasma.
Urine leaving the kidneys travels along the urinary tract , which consists of paired tubes called ureters , to the urinary bladder , a muscular sac for temporary storage of urine. On leaving the urinary bladder, urine passes through the urethra , which conducts the urine to the exterior. The urinary bladder and the urethra are responsible for the elimination of urine, a process called urination , or micturition. In this process, contraction of the muscular urinary bladder forces urine through the urethra and out of the body.
Other functions:
Regulating blood volume and blood pressure , by adjusting the volume of water lost in urine, releasing erythropoietin, and releasing renin.
Regulating plasma concentrations of sodium, potassium, chloride, and other ions , by controlling the quantities lost in urine and controlling calcium ion levels through the synthesis of calcitriol.
Helping to stabilize blood pH , by controlling the loss of hydrogen ions and bicarbonate ions in urine.
Conserving valuable nutrients , by preventing their excretion in urine while excreting organic waste products–especially nitrogenous wastes, such as urea and uric acid .
Assisting the liver in detoxifying poisons and, during starvation, deaminating amino acids so that other tissues can break them down.
Each kidney is protected and stabilized by three concentric layers of connective tissue:
The renal capsule , a layer of collagen fibers that covers the outer surface of the entire organ.
The adipose capsule , a thick layer of adipose tissue that surrounds the renal capsule.
The renal fascia , a dense, fibrous outer layer. Collagen fibers extend outward from the renal capsule through the adipose capsule to this layer. The renal fascia anchors the kidney to surrounding structures.
Posteriorly, the renal fascia fuses with the deep fascia surrounding the muscles of the body wall. Anteriorly, the renal fascia forms a thick layer that fuses with the peritoneum. So, kidneys are held firmly in place.
This arrangement prevents the jolts and shocks of day–to–day living from disturbing normal kidney function. If the suspensory fibers break or become detached, a slight bump or jar can displace the kidney and stress the attached vessels and ureter. This condition, called a floating kidney , is especially dangerous because the ureters or renal blood vessels can become twisted or kinked during movement.
A typical adult kidney is about 10 cm (4 in.) in length, 5.5 cm (2.2 in.) in width, and 3 cm (1.2 in.) in thickness. Each kidney weighs about 150 g (5.25 oz). The hilus , a prominent medial indentation, is the point of entry for the renal artery and renal nerves and the point of exit for the renal vein and the ureter.
Sectional Anatomy of the Kidneys
The kidney itself has two layers: an outer cortex and an inner medulla. The renal cortex is the superficial portion of the kidney, in contact with the renal capsule. The cortex is reddish brown and granular. The renal medulla consists of 6 to 18 distinct conical or triangular structures called renal pyramids . The base of each pyramid faces the cortex, and the tip of each pyramid, a region known as the renal papilla , projects into the renal sinus. Each pyramid has a series of fine grooves that converge at the papilla. Adjacent renal pyramids are separated by bands of cortical tissue called renal columns , which extend into the medulla. The columns have a distinctly granular texture, similar to that of the cortex. A renal lobe consists of a renal pyramid, the overlying area of renal cortex, and adjacent tissues of the renal columns.
Urine production occurs in the renal lobes. Ducts within each renal papilla discharge urine into a cup–shaped drain called a minor calyx. Four or five minor calyces merge to form a major calyx , and two or three major calyces combine to form the renal pelvis , a large, funnel–shaped chamber. The renal pelvis, which fills most of the renal sinus, is connected to the ureter, which drains the kidney.
Urine production begins in microscopic, tubular structures called nephrons in the cortex of each renal lobe. Each kidney has roughly 1.25 million nephrons, with a combined length of about 145 km (85 miles).
Blood Supply and Innervation of the Kidneys
In normal, healthy individuals, about 1200 ml of blood flows through the kidneys each minute–a phenomenal amount of blood for organs with a combined weight of less than 300 g (10.5 oz)!
Each kidney receives blood from a renal artery , which originates along the lateral surface of the abdominal aorta near the level of the superior mesenteric artery.
The kidneys and ureters are innervated by renal nerves . Most of the nerve fibers involved are sympathetic postganglionic fibers from the celiac plexus and the inferior splanchnic nerves. A renal nerve enters each kidney at the hilus and follows the tributaries of the renal arteries to reach individual nephrons.
The Nephron consists of a renal tubule and a renal corpuscle. The renal tubule is a long tubular passageway which may be 50 mm in length. It begins at the renal corpuscle , a spherical structure consisting of Bowman's capsule , a cup–shaped chamber approximately in diameter, and a capillary network known as the glomerulus Blood arrives at the renal corpuscle by way of an afferent arteriole. This arteriole delivers blood to the glomerulus, which consists of about 50 intertwining capillaries. The glomerulus projects into Bowman's capsule much as the heart projects into the pericardial cavity.
The renal corpuscle is the site where the process of filtration occurs. In this process, blood pressure forces water and dissolved solutes out of the glomerular capillaries and into a chamber–the capsular space –that is continuous with the lumen of the renal tubule
Filtration produces an essentially protein–free solution, known as a filtrate , that is otherwise similar to blood plasma.
From the renal corpuscle, filtrate enters the renal tubule, which is responsible for three crucial functions:
A Representative Nephron.
The renal tubule has two convoluted (coiled or twisted) segments–the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT)–separated by a simple U–shaped tube, the loop of Henle. The convoluted segments are in the cortex, and the loop of Henle extends partially or completely into the medulla. For clarity, the nephron shown in Figure 26–6 has been shortened and straightened. The regions of the nephron vary by structure and function. As it travels along the tubule, the filtrate, now called tubular fluid , gradually changes in composition.
Each nephron empties into the collecting system , a series of tubes that carry tubular fluid away from the nephron.
Nephrons from different locations differ slightly in structure. Roughly 85 percent of all nephrons are cortical nephrons , located almost entirely within the superficial cortex of the kidney
1.Reabsorbing all the useful organic substrates that enter the filtrate.
2.Reabsorbing over 90 percent of the water present in the filtrate.
3. Secreting into the tubular fluid any waste products that failed to enter the renal corpuscle through filtration at the glomerulus
A Representative Nephron.
The renal tubule has two convoluted (coiled or twisted) segments–the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT)–separated by a simple U–shaped tube, the loop of Henle. The convoluted segments are in the cortex, and the loop of Henle extends partially or completely into the medulla. For clarity, the nephron shown in Figure 26–6 has been shortened and straightened. The regions of the nephron vary by structure and function. As it travels along the tubule, the filtrate, now called tubular fluid , gradually changes in composition.
Each nephron empties into the collecting system , a series of tubes that carry tubular fluid away from the nephron.
Nephrons from different locations differ slightly in structure. Roughly 85 percent of all nephrons are cortical nephrons , located almost entirely within the superficial cortex of the kidney
(a) The general appearance and location of nephrons in the kidneys. (b) The circulation to a cortical nephron. (c) The circulation to a juxtamedullary nephron. The length of the loop of Henle is not drawn to scale
The remaining 15 percent of nephrons, termed juxtamedullary nephrons, have long loops of Henle that extend deep into the medulla
The Renal Corpuscle includes (1) a region known as Bowman's capsule and (2) the capillary network of the glomerulus
The visceral epithelium consists of large cells with complex processes, or "feet," that wrap around the specialized lamina densa of the glomerular capillaries. These unusual cells are called podocytes. The podocyte feet are known as pedicels . Materials passing out of the blood at the glomerulus must be small enough to pass between the narrow gaps, or filtration slits , between adjacent pedicels.
The glomerular capillaries are fenestrated capillaries–that is, their endothelium contains large–diameter pores. Special supporting cells that lie between adjacent capillaries play a role in controlling their diameter and thus in the rate of capillary blood flow. Together, the fenestrated endothelium, the lamina densa, and the filtration slits form the filtration membrane . During filtration, blood pressure forces water and small solutes across this membrane and into the capsular space. The larger solutes, especially plasma proteins, are excluded.
Filtration at the renal corpuscle is both effective and passive, but it has one major limitation: In addition to metabolic wastes and excess ions, compounds such as glucose, free fatty acids, amino acids, vitamins, and other solutes enter the capsular space. These potentially useful materials are recaptured before filtrate leaves the kidneys, with much of the reabsorption occurring in the proximal convoluted tubule.
Principles of Renal Physiology
The goal of urine production is to maintain homeostasis by regulating the volume and composition of blood. This process involves the excretion of solutes–specifically, metabolic waste products. Three organic waste products are noteworthy:
Urea. Urea is the most abundant organic waste. You generate roughly 21 g of urea each day, most of it produced during the breakdown of amino acids.
Creatinine. Creatinine is generated in skeletal muscle tissue by the breakdown of creatine phosphate , a high–energy compound that plays an important role in muscle contraction. Your body generates roughly 1.8 g of creatinine each day, and virtually all of it is excreted in urine.
Uric Acid. Uric acid is formed by the recycling of nitrogenous bases from RNA molecules. You produce approximately 480 mg of uric acid each day.
These waste products are dissolved in the bloodstream and can be eliminated only while dissolved in urine. As a result, their removal is accompanied by an unavoidable water loss. The kidneys are usually capable of producing concentrated urine with an osmotic concentration of 1200–1400 more than four times that of plasma. If the kidneys were not able to concentrate the filtrate produced by glomerular filtration, losses of fluid would lead to fatal dehydration in a matter of hours.
Basic Processes of Urine Formation
Filtration. In filtration , blood pressure forces water and solutes across the wall of the glomerular capillaries and into the capsular space. Solute molecules small enough to pass through the filtration membrane are carried by the surrounding water molecules.
Reabsorption is the removal of water and solutes from the filtrate, across the tubular epithelium, and into the peritubular fluid. Reabsorption occurs after filtrate has left the renal corpuscle. Most of the reabsorbed materials are nutrients that your body can use. Whereas filtration occurs solely on the basis of size, reabsorption is a selective process. Solute reabsorption may involve simple diffusion or the activity of carrier proteins in the tubular epithelium. The reabsorbed substances pass into the peritubular fluid and eventually reenter the blood. Water reabsorption occurs passively, through osmosis.
Secretion is the transport of solutes from the peritubular fluid, across the tubular epithelium, and into the tubular fluid. Secretion is necessary because filtration does not force all the dissolved materials out of the plasma. Tubular secretion, which provides a backup for the filtration process, can further lower the plasma concentration of undesirable materials. Secretion is often the primary method of excretion for some compounds, including many drugs.
Together, these processes create a fluid that is very different from other body fluids.
Filtration
In the body, the heart pushes blood around the cardiovascular system and generates hydrostatic pressure. Filtration occurs across the walls of small blood vessels, pushing water and dissolved materials into the interstitial fluids of the body. In some cases, such as the liver, the pores are very large, and even small plasma proteins can be carried into the interstitial fluids. At the renal corpuscle, however, filtration occurs across a specialized filtration membrane that greatly restricts the passage of even the smallest circulating proteins.
Reabsorption and Secretion
The processes of reabsorption and secretion at the kidneys involve a combination of diffusion, osmosis, and carrier–mediated transport.
Normally, any plasma proteins and nutrients, such as amino acids and glucose, are removed from the tubular fluid by cotransport or facilitated diffusion.
The transport maximum thus determines the renal threshold –the plasma concentration at which a specific compound or ion will begin appearing in urine.
Nutrient reabsorption occurs primarily along the proximal convoluted tubules, but also elsewhere along the renal tubule and within the collecting system.
Active secretion occurs primarily at the proximal and distal convoluted tubules.
The loops of Henle–especially the long loops of the juxtamedullary nephrons–and the collecting system interact to regulate the final volume and solute concentration of the urine
Most of what follows applies equally to cortical and juxtamedullary nephrons. The major differences between the two types of nephron are that the loop of Henle of a cortical nephron is shorter and does not extend as far into the medulla as does the loop of Henle of a juxtamedullary nephron. The long loop of Henle in a juxtamedullary nephron extends deep into the renal pyramids, where it plays a vital role in water conservation and the formation of concentrated urine. Because this process is so important, affecting the tubular fluid produced by every nephron in the kidney, and because the functions of the renal corpuscle and of the proximal and distal convoluted tubules are the same in all nephrons, we shall use a juxtamedullary nephron as our example. Filtration occurs only in the glomerulus, with a combination of reabsorption and secretion occurring along the rest of the nephron and collecting system.
Renal Physiology: Filtration and the Glomerulus
Filtration occurs in the renal corpuscle as fluids move across the wall of the glomerulus and into the capsular space. The process of glomerular filtration involves passage across the filtration membrane. This membrane has three components: (1) the capillary endothelium, (2) the lamina densa, and (3) the filtration slits
(a) The filtration membrane. (b) Filtration pressure.
Glomerular capillaries are fenestrated capillaries with pores ranging from 60 to 100 nm (0.06 to ) in diameter. These openings are small enough to prevent the passage of blood cells, but they are too large to restrict the diffusion of solutes, even those the size of plasma proteins. The lamina densa is more selective: Only small plasma proteins, nutrients, and ions can cross it. The filtration slits are the finest filters of all. Their gaps are only 6–9 nm wide, which is small enough to block the passage of most small plasma proteins. As a result, under normal circumstances no plasma proteins other than a few albumin molecules (with an average diameter of 7 nm) can cross the filtration membrane and enter the capsular space.
Filtration Pressures
Hydrostatic Pressure
The glomerular hydrostatic pressure (GHP) is the blood pressure in the glomerular capillaries. This pressure tends to push water and solute molecules out of the plasma and into the filtrate. The GHP is significantly higher than capillary pressures elsewhere in the systemic circuit, due to the arrangement of vessels at the glomerulus.
Blood pressure is low in typical systemic capillaries, because capillary blood flows into the venous system, where resistance is relatively low. However, at the glomerulus, blood leaving the glomerular capillaries flows into an efferent arteriole, whose diameter is smaller than that of the afferent arteriole. The efferent arteriole offers considerable resistance, so relatively high pressures are needed to force blood into it. As a result, glomerular pressures are similar to those of small arteries, averaging 45–55 mm Hg instead of the 35 mm Hg typical of peripheral capillaries.
Glomerular hydrostatic pressure is opposed by the capsular hydrostatic pressure (CsHP) , which tends to push water and solutes out of the filtrate and into the plasma. This pressure results from the resistance to flow along the nephron and the conducting system. (Before additional filtrate can enter the capsule, some of the filtrate already present must be forced into the PCT.) The CsHP averages about 15 mm Hg.
The net hydrostatic pressure (NHP) is the difference between the glomerular hydrostatic pressure, which tries to push water and solutes out of the bloodstream, and the capsular hydrostatic pressure, which tries to push water and solutes into the bloodstream. Because glomerular hydrostatic pressure averages 50 mm Hg and capsular hydrostatic pressure averages 15 mm Hg, the net hydrostatic pressure is 35 mm Hg. This relationship can be written as an equation:
Colloid Osmotic Pressure
Recall that the colloid osmotic pressure of a solution is the osmotic pressure resulting from the presence of suspended proteins. The blood colloid osmotic pressure (BCOP) tends to draw water out of the filtrate and into the plasma. It thus opposes filtration. Over the entire length of the glomerular capillary bed, the BCOP averages about 25 mm Hg. Under normal conditions, very few plasma proteins enter the capsular space, so there is no opposing colloid osmotic pressure within the capsule. However, if the glomeruli are damaged by disease or injury and plasma proteins begin passing into the capsular space, a capsular colloid osmotic pressure is created that promotes filtration and increases fluid losses in urine.
Filtration Pressure (FP) at the glomerulus is the difference between the hydrostatic pressure and the colloid osmotic pressure acting across the glomerular capillaries.
The Glomerular Filtration Rate (GFR) is the amount of filtrate your kidneys produce each minute. Each kidney contains about 64 square feet of filtration surface, and the GFR averages an astounding 125 ml per minute . This means that roughly 10 percent of the fluid delivered to your kidneys by the renal arteries leaves the bloodstream and enters the capsular spaces.
In the course of a single day, your glomeruli generate about 180 liters (50 gal) of filtrate, roughly 70 times the total plasma volume. But as filtrate passes through the renal tubules, about 99 percent of it is reabsorbed. You should now appreciate the significance of tubular reabsorption!
Controlling the GFR
Glomerular filtration is the vital first step essential to all other kidney functions. If filtration does not occur, waste products are not excreted, pH control is jeopardized, and an important mechanism for regulating blood volume is lost.
Three interacting levels of control stabilize your GFR: (1) autoregulation , at the local level, (2) hormonal regulation , initiated by the kidneys, and (3) autonomic regulation , primarily by the sympathetic division of the autonomic nervous system.
The Response to a Reduction in the GFR.
(a) The general pattern of the renin–angiotensin system. (b) The specific mechanisms of action in the renin–angiotensin system.
Once released into the bloodstream by the juxtaglomerular apparatus, renin converts the inactive protein angiotensinogen to angiotensin I. Angiotensin I, which is also inactive, is then converted to angiotensin II by angiotensin–converting enzyme (ACE) . This conversion occurs primarily in the capillaries of the lungs.
Angiotensin II is an active hormone whose primary effects include the following:
In peripheral capillary beds , angiotensin II causes a brief but powerful vasoconstriction of arterioles and precapillary sphincters, elevating pressures in the renal arteries and their tributaries.
At the nephron , angiotensin II causes the constriction of the efferent arteriole, further elevating glomerular pressures and filtration rates. Angiotensin II also directly stimulates the reabsorption of sodium ions and water at the PCT.
At the adrenal glands , angiotensin II stimulates the secretion of aldosterone by the adrenal cortex. The aldosterone then accelerates sodium reabsorption in the DCT and cortical portion of the collecting system.
In the CNS , angiotensin II (1) causes the sensation of thirst, (2) triggers the release of antidiuretic hormone (ADH), stimulating the reabsorption of water in the distal portion of the DCT and the collecting system, and (3) increases sympathetic motor tone, further stimulating peripheral vasoconstriction.
Autonomic Regulation of the GFR
Most of the autonomic innervation of the kidneys consists of sympathetic postganglionic fibers. (The role of the few parasympathetic fibers in regulating kidney function is not known.) Sympathetic activation has one direct effect on the GFR: It produces a powerful vasoconstriction of the afferent arterioles, decreasing the GFR and slowing the production of filtrate.
For example, the dilation of superficial vessels in warm weather shunts blood away from your kidneys, so glomerular filtration declines temporarily. The effect becomes especially pronounced during periods of strenuous exercise. As the blood flow to your skin and skeletal muscles increases, kidney perfusion gradually decreases. These changes may be opposed, with variable success, by autoregulation at the local level.
At maximal levels of exertion, renal blood flow may be less than 25 percent of normal resting levels. This reduction can create problems for distance swimmers and marathon runners, because metabolic wastes build up over the course of a long competition. Proteinuria (protein loss in urine) commonly occurs after long–distance events because the glomerular cells have been injured by prolonged hypoxia (low oxygen levels). If the damage is substantial, hematuria (blood loss in urine) will occur. Hematuria develops in roughly 18 percent of marathon runners.
Walang komento:
Mag-post ng isang Komento