Renal Anatomy and Physiology

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Renal Anatomy and Physiology

Gross Anatomy (Figure 13-1)

The kidneys are located outside the peritoneal cavity on each side of the spinal column within the posterior abdominal wall.

Renal vessels and nerves enter on the medial border.

A single ureter that conducts urine to the bladder exits each kidney from the medial border.

A single urethra leaves the bladder.

The renal pelvis is a continuation of the ureter and forms the urine-collecting area of each kidney.

A dissection of the kidney from top to bottom demonstrates two major regions:

II The Nephron (Figure 13-3)

The nephron is the functional unit of the kidney.

Each kidney is composed of approximately 1.3 million nephrons.

Each nephron is composed of a kidney tubule and its corresponding blood supply.

The site of initial formation of urine is the glomerulus. The glomerulus filters blood into Bowman’s capsule, forming the glomerular filtrate.

The kidney tubule itself begins with Bowman’s capsule and continues sequentially with the following structures:

1. Proximal convoluted tubule

2. Loop of Henle

The arcuate artery provides the circulatory supply of the nephron.

III Major Functions of the Kidney

The primary function of the kidney is twofold:

These primary functions are performed by a number of interrelated processes.

IV Glomerular Filtration

Filtration of fluid and electrolytes at the glomerulus follows Starling’s law of fluid exchange (see Chapter 14).

However, because protein is poorly filterable across the glomerulus, except under pathologic conditions, only three forces normally control fluid exchange.

1. Forces moving fluid out of the glomerulus  
  Glomerular hydrostatic pressure 60 mm Hg
  Total outward force 60 mm Hg
2. Forces maintaining fluid in the glomerulus  
  Glomerular colloid osmotic pressure 32 mm Hg
  Bowman’s capsule hydrostatic pressure +18 mm Hg
  Total inward force 50 mm Hg
3. Net filtration pressure  
  Total outward force 60 mm Hg
  Total inward force −50 mm Hg
  Filtration pressure 10 mm Hg

In the average adult approximately 125 ml/min of fluid is filtered across the glomerulus.

This filtrate is essentially protein free and has concentrations of dissolved crystalloids similar to that of plasma (Table 13-1).

TABLE 13-1

Approximate Concentrations of Substances in the Glomerular Filtrate and in the Urine

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Substance Urine Glomerular Filtrate
Glucose (mg%) 100 0
Creatinine (mEq/L) 196 1.1
Uric acid (mEq/L) 3 42
Urea (mEq/L) 26 1820
SO4−2 (mEq/L) 0.7 33
H2PO4−1/HPO4−2 (mEq/L) 2 50
HCO3 (mEq/L) 28 14
Cl (mEq/L)