Kidneys - Veterian Key

Gross Anatomy

The kidneys are paired, bean-shaped structures located in the retroperitoneal space directly beneath the sublumbar muscles. The cranial pole of the right kidney lies in the renal fossa of the caudate liver lobe and is located more cranially than the left kidney. The cranial pole of the left kidney lies lateral to the ipsilateral adrenal gland, which is closely associated with the cranial aspect of the left renal vessels. The left kidney is generally more mobile than the right kidney.

Each kidney has a cranial and caudal pole and a ventral and dorsal aspect (Figure 114-1). The concave surface of the kidney is located along the medial aspect and is called the hilus. The hilus is the location where the renal artery enters the kidney and the renal vein and ureter exit. Nerves and lymphatic vessels enter at the hilus as well. Anatomically, the renal vein is located more ventrally, and the renal artery is more dorsally. In an animal of normal body condition, the kidney is typically surrounded by a substantial amount of fat; this fat is maintained even in lean animals. In obese animals, the surrounding adipose tissue can virtually hide the kidney from view, making gross evaluation difficult.

Figure 114-1 Schematic diagram, kidney, dorsal section, in a dog. (From McGavin MD (ed): Pathologic basis of veterinary disease, ed 4, St Louis, Mosby. In Schaller O, Enke F, Stuttgart V (eds): Illustrated veterinary anatomical nomenclature, Kinderhook, NY, 1992, IBD Ltd.)

On gross dissection, the kidney is divided into several distinct areas. Outermost, there is a thin, fibrous capsule covering the kidney. In a healthy kidney, the capsule is easily separated from the underlying parenchyma. In a diseased kidney, the capsule may be thickened and tightly adhered to the underlying renal tissue. On cross-section, the kidney is divided into the outer cortex and the inner medulla. The renal cortex is composed of glomeruli and adjacent structures. The medulla is striated in appearance and projects into the renal pelvis as the renal crest (Figure 114-2). The renal pelvis is the space into which urine is deposited by the renal collecting ducts and is continuous with the ureter.

Figure 114-2 Semischematic drawing of the canine kidney (A). The papillae have joined in an irregular common crest; the thickenings of the crest alternate with recesses of the renal pelvis as shown by the corrosion cast of the pelvis (B). (From Dyce KM (ed): Textbook of veterinary anatomy, ed 3, Philadelphia, 2002, Saunders/Elsevier).

Vessels

The renal arteries arise directly off of the abdominal aorta (Figure 114-3). Although most kidneys are supplied by a single renal artery, multiple renal arteries are reportedly seen in 13% of canine kidneys and in 10% of feline kidneys.23 The left kidney is more likely to have multiple renal arteries than the right kidney. Some single renal arteries branch immediately after leaving the aorta, making it appear that the kidney has two renal arteries. This is primarily of significance during nephrectomy, when severe hemorrhage can be encountered if one of these renal artery branches is not identified and ligated before transection.

Figure 114-3 Female urogenital system in situ, ventral aspect. (From Evans HE, de Lahunta A: Miller’s anatomy of the dog, ed 4, St Louis, 2013, Saunders/Elsevier.)

The renal arteries split into dorsal and ventral branches as they reach the renal hilus. These arteries further branch into three to seven interlobar arteries and then into arcuate arteries at the corticomedullary junction. The arcuate arteries radiate outward toward the periphery of the kidney and further split into interlobar arteries, from which afferent glomerular arteries arise. Efferent glomerular arteries return blood from the glomerular tufts and drain into the venous system.

Small “capsular” arteries may enter the kidney from the capsular surface. These secondary arteries anastomose to the primary renal arteries and create a renal “arterial circle” that permits some arterial flow to a kidney even when a renal artery is obstructed. Capsular arteries most commonly arise from the phrenicoabdominal artery and adrenal arteries23 and appear to be more prominent in diseased kidneys.

The vasa recta is composed of long, unbranched capillaries that extend alongside the nephron from the cortex into the renal medulla. Through these capillaries, water reabsorbed from the collecting tubules is returned back into the circulatory system, helping to maintain hypertonicity of the renal medulla through a countercurrent exchange system.14,15,23,39

The venous circulation of the kidney arises from deep and superficial veins within the renal parenchyma. Veins in the outer cortex drain toward the capsular surface. These veins converge to form the stellate veins, interlobar veins, arcuate veins, and renal vein. The renal vein returns blood back into the caudal vena cava. The left renal vein also receives blood from the left ovarian or testicular veins. Capsular and parenchymal lymphatics converge to make lymphatic trunks, which exit at the renal hilus.23,27

Innervation

The kidney receives innervation from the sympathetic ganglion and parasympathetic sources. These fibers are myelinated and unmyelinated from sympathetic and parasympathetic (vagus) trunks. There are numerous plexuses around the renal vessels.23,26,27