Functional anatomy of kidneys

The kidneys lie outside the peritoneal cavity in the posterior abdominal wall, one on each side of the vertebral column, slightly above the waistline. In the adult human, each kidney is approximately 11 cm long, 6 cm wide, and 3 cm thick. These organs are divided into two regions: the inner renal medulla and the outer renal cortex. The functional unit of the kidney is the nephron (see Figure 19.1 and Figure 19.2). Approximately 1 million nephrons are in each kidney. The nephron has two components:

Vascular component. Filtration of the plasma takes place at the glomerulus (i.e., glomerular capillaries) located in the cortical region of the kidney. Water and solutes exit the vascular compartment through these capillaries to be processed by the tubular component of the nephron. Blood is delivered to the glomerulus by afferent arterioles. The glomerular capillaries then join together to form a second group of arterioles referred to as efferent arterioles. All cellular elements of the blood (red blood cells, white blood cells, and platelets) as well as the unfiltered plasma continue through these vessels. The efferent arterioles then lead to a second set of capillaries, the peritubular capillaries. These capillaries provide nourishment to the renal tissue and return the substances reabsorbed from the tubule to the vascular compartment. Peritubular capillaries are closely associated with all portions of the renal tubules and wrap around them. These capillaries then join together to form venules and progressively larger veins that remove blood from the kidneys.

Tubular component. Approximately 180 l of filtrate is processed by the kidneys each day. Depending upon the volume of fluid intake, about 99% of this filtrate must be reabsorbed from the renal tubule back into the vascular

Proximal tubule

Proximal tubule

Figure 19.1 The nephron. The functional unit of the kidney is the nephron, which has two components. The vascular component includes the afferent arteriole, which carries blood toward the glomerulus where filtration of the plasma takes place. The efferent arteriole carries the unfiltered blood away from the glomerulus. The tubular component of the nephron includes Bowman's capsule, which receives the filtrate; the proximal tubule; the Loop of Henle; and the distal tubule. The tubule processes the filtrate, excreting waste products and reabsorbing nutrient molecules, electrolytes, and water.

Figure 19.1 The nephron. The functional unit of the kidney is the nephron, which has two components. The vascular component includes the afferent arteriole, which carries blood toward the glomerulus where filtration of the plasma takes place. The efferent arteriole carries the unfiltered blood away from the glomerulus. The tubular component of the nephron includes Bowman's capsule, which receives the filtrate; the proximal tubule; the Loop of Henle; and the distal tubule. The tubule processes the filtrate, excreting waste products and reabsorbing nutrient molecules, electrolytes, and water.

compartment. The movement of substances out of the tubule is facilitated by its structure, which consists of a single layer of epithelial cells. As will be discussed, each region of the tubule plays a different role in the reabsorption process.

Upon leaving the glomerular capillaries, the filtrate enters the first portion of the tubule, Bowman's capsule. The glomerulus is pushed into Bowman's capsule, much like a fist pushed into a balloon or a catcher's mitt. From Bowman's capsule, the filtrate passes through the proximal tubule,

Afferent_

arteriole £

Efferent arteriole

Glomerula capillaries

Afferent_

arteriole £

Efferent arteriole

Glomerula capillaries

Peritubular capillary

Bowman's capsule

Peritubular capillary

Bowman's capsule

Excretion in urine

To venous system

Figure 19.2 Basic renal processes. These processes include filtration, reabsorption, and secretion. (1) Filtration is the movement of fluid and solutes from the glomerular capillaries into Bowman's capsule. (2) Reabsorption, which takes place throughout the nephron, is the movement of filtered substances out of the tubule and into the surrounding peritubular capillaries. (3) Secretion is the movement of selected unfiltered substances from the peritubular capillaries into the renal tubule for excretion. Any substance that is filtered or secreted, but not reabsorbed, is excreted in the urine.

which is also located in the cortex of the kidney. The next segment of the tubule is the Loop of Henle found in the medulla of the kidney. The descending limb penetrates into the medulla and the ascending limb returns toward the cortex. From the Loop of Henle, the filtrate passes through the distal tubule in the cortex of the kidney. Finally, up to eight distal tubules empty into a collecting duct that runs downward through the medulla. Any filtrate remaining within the tubule at the end of the collecting duct drains through the renal pelvis to the ureters and is excreted as urine.

Two types of nephrons are distinguished by their anatomical characteristics:

• Cortical nephron

• Juxtamedullary nephron

The glomerulus of each cortical nephron is located in the outer region of the cortex. Furthermore, the Loop of Henle in these nephrons is short and does not penetrate deeply into the medulla. In humans, 70 to 80% of the nephrons are cortical.

In contrast, the glomerulus of each juxtamedullary nephron is located in the inner region of the cortex, close to the medulla. The Loop of Henle in these nephrons is significantly longer, penetrating to the innermost region of the medulla. Within the medulla, the peritubular capillaries of the neph-rons are modified to form the vasa recta, or straight vessels. Similar to the Loop of Henle, the vasa recta descend deep into the medulla, form a hairpin loop, and then ascend back toward the cortex. In fact, these vessels run parallel and in close association with the Loop of Henle and the collecting ducts. The remaining 20 to 30% of nephrons in the human kidney are jux-tamedullary.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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