Blood Flow and Filtered Fluid Flow PDF

24.4 Blood Flow and Filtered Fluid Flow At least 20% to 25% of the resting cardiac output (or about 1 liter per minute [L/min]) normally flows through the kidneys for the purpose of removing unwanted substances. A filtrate is formed when blood flows through the glomerulus, and some components of the...

24.4 Blood Flow and Filtered Fluid Flow At least 20% to 25% of the resting cardiac output (or about 1 liter per minute [L/min]) normally flows through the kidneys for the purpose of removing unwanted substances. A filtrate is formed when blood flows through the glomerulus, and some components of the plasma enter the capsular space. As a result, two fluid flow patterns must be noted: The flow of blood into and out of the kidney The flow of filtrate, tubular fluid, and urine through the nephron and other urinary system structures 24.4a Blood Flow Through the Kidney LEARNING OBJECTIVES 15. List the series of blood vessels for the path of blood flow into and out of the kidney. 16. Identify the three different capillaries within the kidney and describe the function of each as the site of either the filtration of blood or the exchange of gases and nutrients. The blood flow pathway into, through, and out of the kidney is shown in figure 24.8. Be sure to refer back to this figure often as you read this section on kidney blood flow. Figure 24.8 Blood Supply to the Kidneys. A coronal view of the kidney depicts kidney circulation. An expanded view shows circulation to a nephron. Pink boxes indicate vessels with arterial blood; lavender boxes indicate vessels where reabsorbed materials reenter the blood; blue boxes indicate venous blood returning to the general circulation (PCT = proximal convoluted tubule; DCT = distal convoluted tubule). APR Module 13: Urinary: Dissection: Lower Urinary- male: Sagittal view: Spongy Urethra Page 957 Arteries Blood is delivered to each kidney by way of a renal artery that arises from the abdominal aorta. The renal artery branches as it enters the renal sinus into segmental arteries. Within the renal sinus, the segmental arteries branch to form the interlobar (in-ter-lō′bar) arteries, which are located between the renal lobes (within the renal columns). The interlobar arteries extend to the corticomedullary junction, where they branch to form arcuate (ar′kū-āt; arcuatus = bowed) arteries. Arcuate arteries are arch-shaped and project parallel to the base of the medullary pyramid at the corticomedullary junction. The arcuate arteries extend branches called interlobular (inter-lob′ū-lăr) arteries (or cortical radiate arteries) that project peripherally into the cortex (where renal lobules are located). As the interlobular arteries extend into the cortex, numerous small afferent arterioles branch from them. Arterioles and Capillaries Each afferent arteriole supplies blood to a glomerulus. Some blood plasma is filtered at the glomerulus (this fluid enters the glomerular capsule of the renal corpuscle). After this filtration event, most blood (which was unfiltered and remains within the glomerulus) exits the glomerulus through an efferent arteriole. Each efferent arteriole now branches into a second capillary network, either the peritubular (per′ē-tū′byū-lăr; peri = around, tubule = small tube) capillaries or vasa recta (vˉa′să rek′tă; straight vessel). The peritubular capillaries are associated with, and intertwined around, both the proximal and distal convoluted tubules; thus, they primarily reside in the cortex of the kidney. In comparison, vasa recta capillaries are “straight vessels” associated with the nephron loop; thus, they primarily reside in the medulla of the kidney. Notice that all blood moves through two capillary beds as it flows through the kidney. Blood first enters from the afferent arteriole into the glomerulus, where it is filtered. When the blood reaches the second capillary bed of either the peritubular capillaries or the vasa recta, the exchange of gases, nutrients, and wastes occurs between the tissues of the kidney and the blood. The peritubular capillaries and vasa recta then drain into the network of veins. Veins Blood drains from both peritubular and vasa recta capillary beds into small veins. The smallest of these veins are the interlobular veins (or cortical radiate veins). Interlobular veins merge to form arcuate veins at the base of the medullary pyramids, and these merge to form interlobar veins, which extend through the renal columns. Interlobar veins merge in the renal sinus to form the renal vein. Note that there are no segmental veins; rather, the interlobar veins directly form the renal vein. The renal vein, which extends from the kidney at its hilum, drains blood into the inferior vena cava. WHAT DID YOU LEARN? 11 List the pathway that blood follows as it enters via the renal artery and later leaves via the renal vein. 12 What are the three major types of capillaries associated with the nephron? Describe the location and general function of each. Page 958 INTEGRATE LEARNING STRATEGY 24.2 The names of the blood vessels in the kidney give you clues as to their location or appearance: Interlobar vessels are positioned “inter” = between the lobes of the kidney. Arcuate vessels form “arcs” as these vessels run parallel to the corticomedullary junction. Interlobular vessels are located between the lobules of the kidney cortex. Afferent arterioles carry blood to the glomerulus (“afferent” means carrying toward). Efferent arterioles take blood away from the glomerulus (“efferent” means to carry away, or exit). Peritubular capillaries are “peri” = around the convoluted tubules in the cortex. Vasa recta means “straight vessels,” and these vessels run parallel to the long, straight components of the nephron loop in the medulla. 24.4b Filtrate, Tubular Fluid, and Urine Flow LEARNING OBJECTIVES 17. Differentiate among filtrate, tubular fluid, and urine, and list the urinary system structures that transport each of these fluids. 18. Trace the fluid from its formation at the renal corpuscle until it exits the body through the urethra. The flow of fluid through a nephron, other renal tubules, and then the rest of the urinary system is integrated in section. figure 24.9. Be sure to refer to this figure as you read through this Figure 24.9 Structures That Transport Fluids Through the Urinary System. (a) Microscopic view depicts flow of fluid through the nephron and the collecting tubules and ducts. (b) Macroscopic view depicts urine flow from the papillary ducts through the renal calyces and renal pelvis of the kidney, through the ureters, into the urinary bladder, and through the urethra. APR Module 13: Urinary: Animation: Micturition Reflex When blood flows through the glomerulus and is filtered, both water and solutes move from the blood plasma across the filtration membrane (described in section 24.5b) and into the capsular space to form filtrate. Filtrate, which is essentially plasma minus large solutes (e.g., most proteins), then enters the proximal convoluted tubule, where it is now called tubular fluid. It flows through the proximal convoluted tubule, nephron loop, and distal convoluted tubule. Tubular fluid from several distal convoluted tubules enters into small collecting tubules that empty into larger collecting ducts. Tubular fluid is not changed further after leaving the collecting ducts, and it is now called urine. It enters a papillary duct located within a renal papilla and then flows progressively through spaces within the renal sinus of the kidney. These spaces occur in the following order: minor calyx, major calyx, and renal pelvis. The renal pelvis transports urine from the kidney into a ureter, and a ureter from each kidney transports urine into the urinary bladder, where it is stored until it is excreted from the body through the urethra. Page 959 WHAT DID YOU LEARN? 13 List the pathway of fluid filtered by the kidney from the glomerulus to its eventual excretion.