THE KIDNEYS AND ADRENALS

The kidneys lie against the diaphragm and psoas muscles dorsally, each enclosed within a capsule of fat. The right kidney lies ventral to the last two or three ribs and first lumbar transverse process; the left one lies ventral to the last rib and first two or three processes and is thus about half a kidney length caudal to the level of its fellow (Figure 21-18/10,11).

The cranial pole of the right kidney fits into the renal impression of the liver; caudal to this it is ventrally attached to the pancreas and the base of the cecum (Figure 21-14/5,2). The duodenum winds around the lateral margin and adjoining part of the ventral surface, which is the only region sometimes covered with peri­toneum. The short medial border is indented by the hilus and is related to the caudal vena cava and the right adrenal gland (Figure 21-22).

The ventral surface of the left kidney has a more complete covering of peritoneum and is related to coils of small colon and small intestine, generally including the duodenojejunal junction. Cranioventrally it lies against the spleen; it may make contact with a distended stomach (see Figure 21-10). The medial border is related to the aorta and the left adrenal gland (see Figure 21-22).

The kidneys are of a modified unipyramidal type; the numerous constituent pyramids are completely fused,

Figure 21-22 Kidneys and adrenal glands in situ, ventral view. 1, Right kidney; 1', liver; 2, left kidney; 3, ureter; 4, renal artery; 5, renal vein; 6, aorta; 7, caudal vena cava; 8, cranial mesenteric artery; 9, celiac artery; 10, caudal mesenteric and testicular arteries; 11, testicular veins; 12, adrenal glands.

and their former boundaries are revealed only by the arrangement of the interlobar arteries. A clearer indica­tion of the lobation, with some external fissuration, is common in the foal. The structure is best revealed in section (Figure 21-23). The strong external fibrous capsule can normally be easily stripped away, except within the renal sinus, where it merges with the adven­titia of the structures entering and leaving. The division of the parenchyma between cortex and medulla is indi­cated by a color change and by the sectioned arcuate arteries. The cortex is brownish red and granular. The peripheral part of the medulla is dark red, the inner part pale; both show radial striations. The apices of the fused medullary pyramids form a common renal crest that projects into the pelvis. This has a curious form consisting of a central expansion (Figure 21-23/4) at the origin of the ureter and two terminal recesses toward the poles (Figure 21-23/5); most papillary ducts open into the recesses. The pelvic mucosa produces a mucous secretion, and as a result the unfiltered urine normally contains some protein (physiological albuminuria).

Figure 21-23 Dorsal section through a kidney, semische­matic. 1, Renal cortex; 2, renal medulla; 3, ureter; 4, pelvis; 5, terminal recess; 6, papillary ducts; 7, renal artery; 8, interlobar arteries.

The renal vessels are short and wide. The artery often splits before reaching the hilus, and a number of branches may enter the ventral surface independently (Figure 21-23/5).

The ureters are wide at their origins but soon reduce to narrow, more uniform calibers. They bend caudally on emerging from the renal sinus and thereafter pursue a tortuous course over the roof of the abdomen to reach the pelvis. Here they follow the lateral parts of the broad ligaments (genital fold in the male) before inclin­ing medially to pierce the bladder wall close to its neck.

The elongated and irregular adrenal glands lie against the cranial parts of the medial borders of the corre­sponding kidneys (Figure 21-22/72). Each consists of an outer bright yellow cortex and an inner brownish red medulla. The glands are relatively large in juvenile animals.