Chapter 6

### The Renal System

#### Overview

The kidney is a homeostatic organ that varies the excretion of water, electrolytes, and other hydrophilic molecules to maintain constancy of the composition of the extracellular fluid (ECF). This organ plays the most important role in the long-term regulation of blood pressure through its control of ECF volume. The kidney is also the major effector that varies excretion of metabolic acids and many electrolytes, including K+, Ca2+, and Mg2+.

#### Anatomy of the Kidney

The paired kidneys are bean-shaped retroperitoneal organs, each about 12-cm long and located on the posterior abdominal wall. Figure 6-1 shows the structures that pass through the renal hilus, including the renal artery, the renal vein, and the renal pelvis (the renal nerves and lymphatic vessels are associated with the renal blood vessels). The cut surface of the kidney has a pale outer region, the cortex, and a darker inner region, the medulla. The medulla is further divided into several conical areas called renal pyramids. The apex of each pyramid tapers toward the renal pelvis, forming the renal papillae. The renal pelvis has a funnel-shaped upper end and is continuous with the ureter below it, which conveys urine to the bladder.

###### Figure 6-1

Gross anatomy of the kidney.

Renal colic is pain in the flank that radiates toward the groin. Colicky pain is pain that comes and goes and is often accompanied by nausea, vomiting, diaphoresis, and hematuria. Renal colic occurs with sudden acute onset and is often severely painful. The most common cause of renal colic is renal calculi (“kidney stones”), which make their way from the renal pelvis down the ureter to the bladder. The three common sites of obstruction leading to the clinical presentation of renal colic are:

1. At the junction of the renal pelvis and ureter.

2. At the site where the ureter passes over the pelvic brim.

3. At the uterovesicular junction.

Papillary necrosis is another condition that can result in renal colic. Necrosis of the renal papillae is associated with the use of analgesics (e.g., nonsteroidal anti-inflammatory drugs). The necrotic papillae can slough off, resulting in obstruction and symptoms of renal colic.

#### Structure of the Nephron

The functional unit of the kidney is a nephron, a tube that consists of different transporting epithelia in sequence. Each kidney has between 500,000 and 800,000 nephrons. A nephron has several functionally and histologically distinct segments (Figure 6-2). The glomerulus is the site of primary urine formation where blood plasma is filtered into the nephron. The glomerular capillaries are enveloped by the blind-ended upper part of the nephron, known as Bowman's capsule. Filtrate flows from Bowman's space into the proximal tubule, which ...

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