Nephrology and Urology of Small Animals

Joe Bartges, David Polzin, Joe Bartges, David Polzin

• Publisher: Wiley-Blackwell
• Year: 2011
• Print ISBN:
  9780813817170
• eBook ISBN:
  9780470958889
• Edition: 1
• Topic: Medicine
• Subtopic: Veterinary Medicine

Nephrology and Urology of Small Animals provides veterinarians with the knowledge needed to effectively diagnose and treat urologic diseases in canine, feline, and exotic patients. Serving as an easy-to-use, comprehensive clinical reference, the text takes an evidence-based approach to detailed coverage of specific diseases and disorders, including etiology and prevalence, clinical signs, diagnosis, treatment, prevention, prognosis, controversies, and references. Coverage also includes practical review of anatomy and physiology of the urinary system, fundamentals of diagnostic testing and therapeutic techniques.

Section 1

Anatomy and physiology

1

Anatomy of the kidney and proximal ureter

Christina E. Clarkson and Thomas F. Fletcher

The kidney removes waste via blood filtration followed by tubular reabsorption and secretion. This chapter highlights renal anatomy of the dog and cat, including anatomy of the renal pelvis and proximal ureter.

Gross anatomy

Kidney topography and surface features

The kidneys of the dog and cat are similar in structure and relative size. They are paired, bean-shaped, and located dorsally in the abdominal cavity. Kidneys are retroperitoneal, dorsal to the peritoneal cavity, and covered by parietal peritoneum only. The surface facing laterally is convex; the medial surface is concave, with an indented region called the hilus (hilum) where vessels, nerves, and the ureter enter/exit the kidney. The right kidney is positioned cranial to the left, with its cranial pole situated within a recess of the caudate lobe of the liver.

In the dog, the right kidney is more firmly attached to the dorsal body wall than the left kidney. Thus, the location of the right kidney is more predictable, extending from vertebrae T13 to L2. The left kidney is approximately half a kidney length caudal to the right (Osborne et al. 1972); its looser attachment can result in movement during respiration or body positioning. In the cat, both kidneys are pendulous, moveable, and more caudally located than in the dog. The feline right kidney is positioned at the level of vertebrae L1 to L4, left kidney at the level of L2 to L5 (Nickel et al. 1973).

The kidney develops embryologically from discrete lobes that fuse for the most part in the dog and cat. The carnivore kidney is classified as unilobar. It is devoid of lobe demarcations externally, presenting a smooth surface encased within a fibrous capsule. Sectioning the kidney reveals an outer layer of dark-colored, highly vascular cortex, surrounding a lighter colored medulla (Figure 1.1).

Figure 1.1 Anatomy of the canine kidney, pelvis, and ureter. (a) The intact kidney appears unilobar and enclosed in a fibrous capsule. Vessels and the ureter connect at the hilus of the kidney. (b) When sliced marginally on a sagittal plane, the cut surface reveals a fibrous capsule, a continuous renal cortex, and a medulla segregated into renal pyramids by interlobar arteries. The papilla of each pyramid is within a recess of the renal pelvis. (c) When sliced transversely, inner and outer zones of medulla can be seen surrounded by cortex. Along the midline, renal pyramids have fused into a renal crest located above the renal pelvis. (d) A mid-sagittal slice of kidney reveals renal crest rather than pyramids. Renal pelvis and fat occupy a space designated renal sinus. (e) External view of an isolated renal pelvis and ureter. (f) Side view of an isolated renal pelvis and ureter. The wall of the renal pelvis is scalloped, divided into pelvic recesses by wall projections around interlobar arteries (modified from Evans, 1993).

Renal cortex and medulla

The cut surface of the renal cortex has a relatively rough texture due to collections of capillary tufts (glomeruli) and a labyrinth of tubules (cortical labyrinth). Medullary rays, smooth striations that appear to be radiating out of the medulla toward the periphery of the cortex, are scattered throughout the cortex.

The renal medulla is composed of renal pyramids that fuse to form a central ridge called the renal crest. The pyramids, wedges of medulla separated by interlobar vessels, are apparent in marginal planes of the section. Each pyramid has an apex (papilla) directed toward the renal pelvis. (Renal pyramids are remaining evidence of embryonic lobation.) The medulla contains papillary ducts that open onto the renal crest surface.

Renal pelvis and proximal ureter

Urine collects in the renal pelvis, a dilatation of the proximal end of the ureter. The pelvis is located within the renal sinus, a fat-containing, medial recess situated at the hilus. In the carnivore, the funnel-shaped renal pelvis has irregular margins due to reflection around interlobar vessels. The scalloped outpockets between vessels are referred to as pelvic recesses (Figure 1.1).

The ureter is an extension of the renal pelvis. It runs retroperitoneally along the dorsal wall of the abdominal cavity and through the lateral ligament of the bladder. The pelvis and ureter are lined by transitional epithelium and have smooth muscle walls. Peristaltic contraction waves initiated in the wall of the renal pelvis travel down the ureter, conveying urine into the bladder. (The distal ureter is discussed in chapters on the lower urinary tract.)

Renal vessels

In most cases, a single renal artery divides into dorsal and ventral rami before entering the renal hilus (Marques-Sampaio et al. 2007). Further branching gives rise to interlobar arteries that enter kidney parenchyma. Arcuate arteries are branches of interlobar arteries at the corticomedullary junction. Interlobular arteries (cortical radial arteries) arise from arcuate arteries and run radially through the cortex toward the kidney surface, some extending into the capsule (Nickel et al. 1973). In the cat, additional small branches from renal artery rami travel along the periphery of the kidney to supply the cortical surface and renal capsule (Fuller and Huelke 1973).

Interlobular arteries give off afferent arterioles. Each afferent arteriole gives rise to a ball of capillary loops, called a glomerulus. Glomerular capillaries unite to form an efferent arteriole that feeds a second capillary network surrounding renal tubules. The kidney is unusual in having two capillary beds connected by an arteriole, and thus an arterial portal system. The glomerulus constitutes the first capillary bed.

Efferent arterioles from glomeruli located peripherally in the cortex supply peritubular capillaries around cortical tubules, whereas efferent arterioles from juxtamedullary glomeruli supply peritubular capillaries around medullary tubules (Figure 1.2). In the medulla, the peritubular capillary network forms between descending efferent arterioles and venules that ascend back toward the cortex. These more or less straight vessels and their connections are referred to as vasa recta. They participate in an important countercurrent exchange between vessels and tubules. (Less commonly, peritubular capillaries arise from afferent arterioles or arcuate arterial branches (Christensen 1952; Nickel et al. 1973).)

Figure 1.2 From an interlobular artery, blood flows to the glomerulus via an afferent arteriole (a). The glomerulus constitutes the first capillary network within the kidney. Efferent arterioles (e) convey blood to a second capillary bed that supplies the renal tubules, peritubular capillaries (arrows). Efferent arterioles from peripheral glomeruli (upper corpuscle) form a capillary network around cortical tubules; efferent arterioles from juxtamedullary glomeruli (lower corpuscle) form capillary networks around tubules located in the medulla. Medullary vessels (vasa recta) are closely associated with the loop of Henle. Peritubular capillaries drain via venules primarily into the interlobular veins.

Kidney veins are generally satellites of arteries. Venules arise from capillary beds surrounding cortical and medullary tubules. Veins travel with arterial branches, emptying into interlobular, arcuate, interlobar, and finally renal veins. A subcapsular venous system drains the renal capsule. In the cat, these veins are located within or immediately under the capsule (capsular veins); in the dog, the veins are deeper within the cortex (stellate veins) (Yadava and Calhoun 1958; Nickel et al. 1973). The cat’s prominent capsular veins continue around the kidney periphery to the hilar region where they drain directly into the renal vein (Fuller and Huelke 1973). In the dog, the stellate veins drain into cortex and eventually into the renal vein (Christensen 1952). (The canine renal vein may also receive a direct contribution from stellate veins near the hilus (Fuller and Huelke 1973).)

Microscopic renal anatomy

Nephron

The nephron (renal corpuscle plus renal tubules) is the functional unit of the kidney. Urine produced within the nephron is further modified as it travels through the collecting duct system. (The term “uriniferous tubule” refers to the nephron plus its associated collecting duct.)

Nephron: renal corpuscle

The renal corpuscle is composed of a spherical complex of capillaries (glomerulus) surrounded by a double-wall capsule (Bowman’s capsule). Arterioles enter/exit the corpuscle at its vascular pole, and ultrafiltrate exits at the opposite end of the corpuscle, the urinary pole.

Renal corpuscles are typically scattered throughout the cortex, although there is a small region immediately below the capsule where they are absent in the canine (Sherwood et al. 1969; Bulger et al. 1979). The number of corpuscles per kidney varies with species: 400,000–600,000 in the dog (Horster et al. 1971; Finco and Duncan 1972; Eisenbrandt and Phemister 1979) and approximately 200,000 in the cat (Kunkel 1930).

The glomerulus is formed by an afferent arteriole that gives rise to a ball of capillary loops, which then coalesce, into an efferent arteriole. The diameter of the afferent arteriole is usually larger than that of the efferent. Arteriolar size is autoregulated to maintain a consistent glomerular filtration rate over a wide range of systemic arterial pressures. Intraglomerular mesangial cells and matrix occupy the spaces between capillary loops within the glomerulus (see below).

The blind-ended beginning of the renal tubule, called Bowman’s capsule, surrounds the glomerulus. The capsule is cup-shaped, with inner and outer layers separated by a cavity, the urinary space. The outer layer of the capsule, called the parietal layer, is continuous with the epithelial cells of the proximal tubule at the urinary pole of the renal corpuscl...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Contributors
• Preface
• Section 1: Anatomy and physiology
• Section 2: Diagnostic testing
• Section 3: Therapeutic techniques
• Section 4: Clinical syndromes
• Section 5: Upper urinary tract disorders
• Section 6: Fluid, electrolyte, and acid—base disorders
• Section 7: Systemic arterial hypertension
• Section 8: Upper and lower urinary tract disorders
• Section 9: Lower urinary tract disorders
• Section 10: Urinary disorders of avian and exotic companion animals
• Section 11: Counseling clients
• Appendix: Physiology of the lower urogenital tract
• Index