Page 276 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

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Urinary system

(organa urinaria)

The urinary system comprises the organs that produce and the kidney, particularly in species with kidneys lacking
modify urine (kidneys) and the passages through which external fissures (horse, small ruminants, pig, carnivores)
the urine is conveyed and excreted (renal pelvises, ureters, (Figures 12.1 and 12.2). Pathological processes may cause
bladder and urethra). the capsule to adhere more firmly to the parenchyma.
Extension of the loose connective tissue into the paren-
Kidney (ren) chyma gives rise to a loose meshwork of collagen fibres
The kidneys have several functions. They perform a central that surrounds the renal corpuscles and tubules, and forms
role in the excretion of waste products and contribute to the adventitia of blood vessels and nerves (renal interstit-
homeostasis by maintaining the normal composition of ium). In the ox, sheep and dog, there are also bundles of
body fluids. Production of urine by the kidneys involves smooth muscle fibres around collecting tubules.
processes of filtration, secretion, reabsorption and On the medial aspect of the kidney, the indented renal
concentration. hilus (hilus renalis) is continuous with the deep renal sinus
Specific functions of the kidney include: (sinus renalis). The sinus is occupied by the renal pelvis
(pelvis renalis) and/or the renal calyces (calices renales).
· excretion of organic products of metabolism (e.g. The hilus is traversed by the ureter, blood vessels, lymph
bilirubin, urea), inorganic compounds (e.g. trace ele- vessels and nerve fibres. These are usually surrounded by
2+
2+
ments, alkaline earth metals [e.g. Mg , Ca ], alkali fat (Figures 12.1 and 12.3).
metals [e.g. Na , K ]) and non-metabolisable exog- The basic structural units of the kidney of domestic
+
+
enous substances (e.g. pharmacological agents), mammals are the renal lobes (lobi renales). These consist
· maintenance of osmotic pressure and H concentra- of an outer cortex and a pyramidal inner medulla. A papilla
+
tion of body fluids by selective reabsorption and/or (papilla renalis) at the tip of the medulla protrudes into the
excretion of ions and water, calyces (or pelvis, depending on species). The boundaries of
· regulation of acid–base balance, adjacent lobes are marked by interlobar arteries and veins.
· synthesis of hormones for regulation of blood pres-
sure (renin–angiotensin system) and control of Species variation
erythropoiesis (erythropoietin) and During development, the renal lobes undergo fusion to
· production of 1,25-dihydroxycholecalciferol for reg- produce a single, discrete organ. The degree of fusion
ulation of blood calcium. varies with species. In most domestic species, the corti-
ces are completely fused and the surface of the kidney
Macroscopic structure of the kidney is smooth and uninterrupted. In the ox, partial fusion
The kidneys are paired, roughly bean-shaped organs results in deep, externally visible fissures.
(exception: right kidney of the horse). They lie in the retro-
peritoneal space and are typically surrounded by a layer of Horse, goat, sheep, cat and dog: The tips of the
fat (capsula adiposa). In addition to acting as a reservoir, medullary pyramids are fused to form the renal crest
this peri-renal adipose tissue supports and protects the kid- (unilobar kidney).
neys. The surface of the kidney is surrounded by a tough Pig and ox: The apices of the medullary pyramids
connective tissue capsule (capsula fibrosa) composed of remain separate and project into renal calyces (multi-
collagen fibres and occasional elastic fibres. Subjacent to lobar kidneys).
the fibrous capsule is a thin layer of loose connective tissue Birds: Renal lobules may be visible on the surface
containing smooth muscle cells. The connection between of the kidney as small dome-shaped bulges (diameter
this layer and the parenchyma is limited to delicate fibre 1–2 mm). However, not all renal lobules reach the sur-
bundles, allowing the capsule to be easily separated from face of the kidney. In functional terms, the avian kidney

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