Medical brain notes 27

  The Urinary System T he first successful human organ transplant was a   kidney transplant performed in 1953. Because the donor and recipient were identical twins, rejection was not a problem. Thousands of kidney transplants have been performed since then, and the development of immunosuppressive medications has permitted many people to live normal lives with donated kidneys. Although a person usually has two kidneys, one kidney is sufficient to carry out the complex work required to maintain homeostasis of the body fluids.   The  urinary system  consists of two kidneys, two ureters, the urinary bladder, and the urethra (Fig. 18–1). The formation of urine is the function of the kidneys, and the rest of the system is responsible for eliminating the urine.   Body cells produce waste products such as urea, Figure 18–1.   The urinary system shown in anterior view.   QUESTION:  Why is blood pressure relatively high in the kidneys? What do you see...

  KIDNEYS   The two  kidneys  are located in the upper abdominal cavity on either side of the vertebral column, behind the peritoneum ( retroperitoneal ). The upper por-tions of the kidneys rest on the lower surface of the diaphragm and are enclosed and protected by the lower rib cage (see Fig. 18–1). The kidneys are embedded in adipose tissue that acts as a cushion and  is in turn covered by a fibrous connective tissue mem-brane called the  renal fascia , which helps hold the kidneys in place.   Each kidney has an indentation called the  hilus  on its medial side. At the hilus, the renal artery enters the kidney, and the renal vein and ureter emerge. The renal artery is a branch of the abdominal aorta, and the renal vein returns blood to the inferior vena cava (see Fig. 18–1). The ureter carries urine from the kidney to the urinary bladder.   INTERNAL STRUCTURE OF THE KIDNEY   In a coronal or frontal section of the kidney, three ar...

  INTERNAL STRUCTURE OF THE KIDNEY   In a coronal or frontal section of the kidney, three areas can be distinguished (Fig. 18–2). The lateral and middle areas are tissue layers, and the medial area at the hilus is a cavity. The outer tissue layer is called the  renal cortex ; it is made of renal corpuscles and convo-luted tubules. These are parts of the nephron and are described in the next section. The inner tissue layer is the  renal medulla , which is made of loops of Henle and collecting tubules (also parts of the nephron). The renal medulla consists of wedge-shaped pieces called  renal pyramids . The tip of each pyramid is its apex or   papilla.   Figure 18–2.  ( A ) Frontal section of the right kidney showing internal structure and   blood vessels. ( B ) The magnified section of the kidney shows several nephrons. ( C ) Vascular cast of a kidney in lateral view. Red plastic fills the blood vessels. ( D ) Vascular cast in medial view. Blo...

  THE NEPHRON   The  nephron  is the structural and functional unit of the kidney. Each kidney contains approximately 1 million nephrons. It is in the nephrons, with their associ-ated blood vessels, that urine is formed. Each nephron has two major portions: a renal corpuscle and a renal tubule. Each of these major parts has further subdivi-sions, which are shown with their blood vessels in Fig. 18–3.     Figure 18–3.  A nephron with its associated blood vessels. Portions of the nephron have   been magnified. The arrows indicate the direction of blood flow and flow of renal filtrate. See text for description. QUESTION:  How does the shape of a podocyte contribute to its function? How is the lin-ing of the proximal convoluted tubule specialized? Renal Corpuscle   A  renal corpuscle  consists of a glomerulus surroun-ded by a Bowman’s capsule. The  glomerulus  is a cap-illary network that arises from an  afferent art...

  BLOOD VESSELS OF THE KIDNEY   The pathway of blood flow through the kidney is an essential part of the process of urine formation. Blood from the abdominal aorta enters the  renal artery , which branches extensively within the kidney into smaller arteries (see Fig. 18–2). The smallest arteries give rise to afferent arterioles in the renal cortex (see Fig. 18–3). From the afferent arterioles, blood flows into the glomeruli (capillaries), to efferent arterioles, to peritubular capillaries, to veins within the kidney, to the  renal vein , and finally to the inferior vena cava Notice that in this pathway there are two sets of cap-illaries, and recall that it is in capillaries that exchanges take place between the blood and surrounding tissues. Therefore, in the kidneys there are two sites of ex-change. The exchanges that take place between the nephrons and the capillaries of the kidneys will form urine from blood plasma.   Figure 18–2 shows two views of a vascular...

  FORMATION OF URINE   The formation of urine involves three major pro-cesses. The first is glomerular filtration, which takes place in the renal corpuscles. The second and third are tubular reabsorption and tubular secretion, which take place in the renal tubules.   GLOMERULAR FILTRATION   You may recall that filtration is the process in which blood pressure forces plasma and dissolved material out of capillaries. In glomerular filtration , blood pressure forces plasma, dissolved substances, and small proteins out of the glomeruli and into Bowman’s cap-sules. This fluid is no longer plasma but is called  renal   filtrate .   The blood pressure in the glomeruli, compared with that in other capillaries, is relatively high, about 60 mmHg. The pressure in Bowman’s capsule is very low, and its inner, podocyte layer is very permeable, so that approximately 20% to 25% of the blood that enters glomeruli becomes renal filtrate in Bowman’s capsules. The blood c...