24.2 Gross Anatomy of the Kidney PDF

24.2 Gross Anatomy of the Kidney The kidneys are two symmetrical, bean-shaped, reddish-brown organs ( figure 24.1). Each kidney measures about 12 centimeters (4.7 inches) in length, 6.5 centimeters (2.5 inches) in width, and 2.5 centimeters (1 inch) in thickness and is about the same size as your ha...

24.2 Gross Anatomy of the Kidney The kidneys are two symmetrical, bean-shaped, reddish-brown organs ( figure 24.1). Each kidney measures about 12 centimeters (4.7 inches) in length, 6.5 centimeters (2.5 inches) in width, and 2.5 centimeters (1 inch) in thickness and is about the same size as your hand to the second knuckle (between the proximal and middle phalanges). A kidney weighs approximately 100 grams (g). Each has a concave medial border called the hilum (hī′lŭm; a small bit), where vessels, nerves, and the ureter connect to the kidney. The kidney’s lateral border is convex. An adrenal gland rests on the superior aspect of each kidney. 24.2a Location and Support LEARNING OBJECTIVES 3. Describe the location of the kidneys in the body. 4. List and describe the four tissue layers that surround and support the kidneys. INTEGRATE LEARNING STRATEGY 24.1 To understand the retroperitoneal position of the kidneys, imagine placing an eraser against a whiteboard, which represents the posterior abdominal wall. Then hang a sheet that represents the parietal peritoneum so that the eraser is between the whiteboard and the sheet. The eraser, which is located posterior to the sheet (the parietal peritoneum), is in a region called retroperitoneal. Structures that would be in front of (and enclosed by) the sheet are described as being intraperitoneal. Page 950 Kidneys are located along the posterior abdominal wall, lateral to the vertebral column. The left kidney is between the level of the T12 and L3 vertebrae, and the right kidney is about 2 centimeters inferior to the left kidney to accommodate the large size of the liver. Both kidneys are only partially protected by the rib cage, making them vulnerable to forceful blows to the inferior region of the back. The kidneys are positioned posterior to the parietal peritoneum, and so the kidneys are referred to as retroperitoneal (ret′rō-per′i-tō-nē′ăl; retro = back) ( figure 24.2). Thus, only the anterior surface of the kidneys is covered with parietal peritoneum. Figure 24.2 Position and Stabilization of the Kidneys. An inferior cross-sectional view (which is the view typically used in computed tomography [CT] scans) shows that the kidneys are located against the posterior abdominal wall and are covered on their anterior surface by the parietal peritoneum and surrounded by four concentric tissue layers (from innermost to outermost): fibrous capsule, perinephric fat, renal fascia, and paranephric fat. APR Module 13: Urinary: Imaging: Kidney: CT: Axial 1: Left kidney Each kidney is surrounded and supported by several tissue layers. From innermost (closest to the kidney) to outermost, these layers are the fibrous capsule, perinephric fat, renal fascia, and paranephric fat: The fibrous capsule (capsa = box) (or renal capsule) is directly adhered to the external surface of the kidney. It is composed of dense irregular connective tissue and maintains the kidney’s shape, protects it from trauma, and helps prevent infectious pathogens from penetrating the kidney. The perinephric (peri = around) fat, also called perirenal fat or adipose capsule, is external to the fibrous capsule and contains adipose connective tissue. It provides cushioning and stabilization for the kidney. The renal fascia (rē′năl fash′ē-ă; ren = kidney) is external to the perinephric fat and is composed of dense irregular connective tissue. It anchors the kidney to surrounding structures. Page 951 The paranephric (para = next to) fat, also called pararenal fat or paranephric body, is the outermost layer surrounding the kidney. It is composed of adipose connective tissue and provides cushioning and stabilization for the kidney. INTEGRATE CLINICAL VIEW 24.1 Renal Ptosis and Hydronephrosis The loss of adipose connective tissue in very thin elderly individuals or individuals with anorexia nervosa may result in renal ptosis (tō΄sēz), which is the “dropping,” or inferior movement, of the kidney within the abdominal cavity. Consequently, the ureter may kink, resulting in a decrease or blockage of urine flow from the kidney to the urinary bladder. Urine backs up into the proximal part of the ureter and kidney, which results in an enlargement of the renal pelvis and calyces, and swelling of the kidney. This buildup of urine in the kidney and resulting swelling is called hydronephrosis (hī΄drō-ne-frō΄sis). If the cause of hydronephrosis is not treated, renal failure may occur. WHAT DID YOU LEARN? 3 What tissue composes the fibrous capsule that directly adheres to the kidney, and what are its functions? 24.2b Sectional Anatomy of the Kidney LEARNING OBJECTIVES 5. Identify and describe the two distinct regions of the kidney and the components of each. 6. Explain the relationship among minor calyces, major calyces, and renal pelvis. When a kidney is sectioned along a coronal plane, the parenchyma, or “functioning tissue,” is visible. The two distinct regions of the parenchyma include an outer renal cortex and an inner renal medulla ( figure 24.3). Figure 24.3 Kidney. A coronal cut through the right kidney reveals the parenchyma and urine drainage areas of the kidney. Both a photo and illustration are shown. ©McGraw-Hill Education/Rebecca Gray APR Module 13: Urinary: Animations: Kidney Gross Anatomy Extensions of the cortex, called renal columns, project into the medulla and subdivide it into renal pyramids (also termed medullary pyramids) that appear striated, or striped. An adult kidney typically contains 8 to 15 renal pyramids. The wide base of a renal pyramid lies at the external edge of the medulla, where it meets the cortex: This is called the corticomedullary junction, or corticomedullary border. The medially directed apex (or tip) of the renal pyramid is called the renal papilla. The parenchyma of a human kidney can also be divided into 8 to 15 renal lobes. A renal lobe consists of a renal pyramid, portions of renal columns adjacent to either side of the renal pyramid, and the renal cortex external to the pyramid base. Each kidney contains a medially located space called the renal sinus, in addition to the parenchyma. This space serves as the urine drainage area. It is organized into minor calyces, major calyces, and a renal pelvis. Each of the 8 to 15 funnel-shaped minor calyces (kāl′i-sēz, sing. calyx = cup of a flower) is associated with a renal pyramid. Several minor calyces merge to form a larger major calyx. Each kidney typically contains two or three major calyces. The major calyces merge to form a large, funnel-shaped renal pelvis. The renal pelvis merges at the medial edge of the kidney with the ureter. Housed within the space around the renal pelvis are the renal artery, renal vein, lymph vessels, and nerves and a variable amount of fat. (Lymph vessels and nerves of the kidney are not shown in figure 24.3.) INTEGRATE CLINICAL VIEW 24.2 Kidney Variations and Anomalies Anatomic variations of the kidneys can occur during development. Renal agenesis (ā-jĕn΄e-sis) is the failure of a kidney to develop. If one kidney fails to develop, it is called unilateral renal agenesis, and it occurs in about 1 per 1000 births. Bilateral renal agenesis is the failure of both kidneys to develop, and it occurs in about 1 per 3000 births. Unilateral renal agenesis is often asymptomatic, whereas bilateral renal agenesis is invariably fatal. A pelvic kidney may occur if the developing kidney fails to migrate from the pelvic cavity to the abdominal cavity. A horseshoe kidney develops when the inferior parts of the left and right kidneys fuse as they ascend from the pelvic cavity into the abdominal cavity. Horseshoe kidneys are fairly common, occurring in about 1 per 600 births. Both pelvic kidneys and horseshoe kidneys typically are asymptomatic and function normally. Supernumerary (sū-per-nū΄mer-ār-ē) kidneys are extra kidneys that develop, but they are rare and usually have no clinical significance. In fact, some individuals find out that their kidneys have anatomic variations in structure or number only when seeing a physician regarding an unrelated health issue. A horseshoe kidney is a fairly common anatomic variation. Page 952 WHAT DID YOU LEARN? 4 What are the regions of the kidney that drain urine? 24.2c Innervation of the Kidney LEARNING OBJECTIVE 7. Explain the autonomic innervation of the kidney. Each kidney is innervated by both divisions of the autonomic nervous system. Sympathetic nerves extend from the T10–T12 segments of the spinal cord to the blood vessels of the kidney (see section 15.4a), including the afferent and efferent arterioles, as well as innervate the juxtaglomerular apparatus (these structures are described in section 24.3c). The general effect of sympathetic stimulation of the kidneys is to decrease urine production. Parasympathetic nerves to the kidney extend from the brain within the vagus nerve (CN X, see section 15.3a), but the specific effects of parasympathetic innervation to the kidney are not known. WHAT DID YOU LEARN? 5 What three anatomic structures of the kidney are innervated by the sympathetic division of the autonomic nervous system?