Renal Anatomy and Physiology

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Renal Anatomy and Physiology

I Gross Anatomy (Figure 13-1)

FIG. 13-1 Gross organization of the kidneys and the urinary system.

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

B Renal vessels and nerves enter on the medial border.

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

D A single urethra leaves the bladder.

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

1. The outer border of the renal pelvis is divided into major calices.

2. Each major calyx is further subdivided into minor calices.

3. Each minor calyx is formed about a renal pyramid.

4. Nephrons, the functional aspect of the kidney, are located within each renal pyramid (Figure 13-2).

FIG. 13-2 Section of the human kidney showing the major vessels that supply the blood flow to the kidney and schematic of the microcirculation of each nephron.

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

1. The outer region, called the renal cortex

2. The inner region, called the renal medulla

II The Nephron (Figure 13-3)

FIG. 13-3 Components of nephron showing direction of fluid flow; schematic of Bowman’s capsule showing parietal and visceral walls and relationship of glomerulus to Bowman’s capsule and adjacent structures.

A The nephron is the functional unit of the kidney.

B Each kidney is composed of approximately 1.3 million nephrons.

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

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

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

1. Proximal convoluted tubule

2. Loop of Henle

a. Descending limb

(1) Thick section

(2) Thin section

b. Ascending limb

(1) Thin section

(2) Thick section

3. Distal convoluted tubule

4. Collecting duct

F The arcuate artery provides the circulatory supply of the nephron.

1. This artery becomes the interlobular artery, which leads to an afferent arteriole leading to the glomerulus.

2. Blood exits the glomerulus via an efferent arteriole.

3. The efferent arteriole forms the peritubular capillaries, which intertwine about the distal and proximal convoluted tubules.

4. The efferent arterioles also form the vasa recta, a long looping capillary that forms about the loop of Henle.

5. Blood leaves the peritubular capillaries and the vasa recta via the interlobular veins and then the arcuate veins.

III Major Functions of the Kidney

A The primary function of the kidney is twofold:

1. Excretion of end products of bodily metabolism

2. Control of the concentration of constituents of the body fluids

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

1. Filtration: Formation of the glomerular filtrate (see Section IV, Glomerular Filtration)

2. Tubular reabsorption (see Section V, Tubular Reabsorption)

3. Tubular secretion (see Section VI, Tubular Secretion)

4. Renin secretion (see Section XI, Renin-Angiotensin)

5. Erythropoietic factor secretion: In the presence of hypoxemia, the kidney secretes erythropoietic factor, which stimulates red blood cell production.

6. Activation of vitamin D: Vitamin D is necessary for appropriate absorption of calcium via the gastrointestinal tract.

7. Gluconeogenesis: The formation of glucose from fats and protein during periods of significant physiologic stress.

8. Excretion of urine.

IV Glomerular Filtration

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

B 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

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

D 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)

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