URINARY SYSTEM

This consists only of the kidneys and ureters as, in keeping with weight reduction, birds have no bladder. The ureter terminates in the urodeum of the cloaca.

Kidneys

The kidneys are paired and lie retroperitoneally in the ventral (renal) fossa of the synsacrum.

The kidneys are relatively large and extend from the caudal synsacrum cranially as far as the lungs. In most species they are roughly divided into cranial, middle, and caudal parts by the external iliac and ischiadic branches of the abdominal aorta. In passerines the middle portion often appears to be lacking (Johnson, OW 1979). Some species like penguins and herons have a fused caudal kidney in the midline (King & McLelland 1984).

CLINICAL NOTE

The spinal nerves of the lumbar and sacral plexuses run through the kidney parenchyma. This is why many renal conditions like tumors can first present as a lameness problem.

The anatomy of the avian kidney is confusing in compari­son to the logical lobule structure of the mammalian kidney (Siller 1983). This is because the avian kidney does not have a distinct demarcation between the cortex and medulla and there is no renal pelvis. Instead the ureter runs along the ventral side of the kidney and branches into collecting ducts that each drain a lobule, consisting of a large area of cortical tissue and a small cone of medullary tissue (Shoemaker 1972). The cortical tissue contains both types of nephron while the medullary cone only contains the loops of Henle, collecting ducts, and the vasa recta capillary network (King & McLelland 1984).

Nephrons

The basic unit is the nephron, as in mammals, but birds show a compromise between reptiles and mammals and have two types of nephron.

Cortical nephrons

These are the most numerous (about 90%) and are confined to the cortical region of the kidney. They excrete uric acid by filtering it through the glomeruli and secreting it into the proximal convoluted tubules. They have no loop of Henle.

Medullary nephrons

These nephrons are also located in the cortical region but have their loop of Henle in the medullary region. It is these that are responsible for forming and concentrating the urine. Birds from arid region tend to have more of these nephrons.

CLINICAL NOTE

As birds have fewer loops of Henle they have decreased ability to concentrate their urine much above blood levels. As a consequence, seabirds, which have a high salt intake, have to rely on extra renal structures called salt glands.

Renal blood supply

The kidneys have a dual afferent blood supply. The high- pressure cranial, middle, and caudal renal arteries subdivide into the afferent glomerular arteries which provide the glomerular filtrate (Johnson, OW 1979). This is influenced by the state of hydration and arginine vasotocin produced from the posterior pituitary gland (Shoemaker 1972).

The external iliac vein, which drains the pelvic limb, branches to form the common iliac vein and caudal renal portal vein (Fig. 6.33). This renal portal vein provides two thirds of the renal blood flow to the kidneys and supplies the proximal convoluted tubules that are responsible for the secretion of urates (King & McLelland 1984; Orosz et al. 1997). Glomerular filtration does not clear urates so it is thought that the renal portal vein plays a significant role in the elimination of these (Siller 1971).

The renal portal system has a unique smooth muscle valve lying at the junction of the common iliac and renal veins (Siller 1983). This has a rich nerve supply and controls the amount of venous blood entering the kidneys.

Ureter

The ureters have a branched network throughout the kidney parenchyma, terminating in the collecting ducts of each lobule. These are lined by pseudostratified columnar epithelium, which secretes mucus to help the urates pass down (Siller 1971). It often appears whitish due to the urate content inside. The ureter opens into the dorsal wall of the urodeum as there is no urinary bladder or urethra in birds. Urine and urates reach the cloaca and go by retroperistalsis into the rectum and colon where there is mixing with feces and reabsorption of water. The cloaca therefore plays a role in avian osmoregulation (Johnson, OW 1979; Phalen 2000; Sykes 1971).

Nasal gland

All birds have nasal or salt glands located above the eye that are distinct from Harderian and lacrimal glands. In marine species these are well developed and function as salt glands to enable them to drink seawater and excrete the excess salt. These glands also play a role in water conservation in birds from arid climates (Evans 1996; Maina 1996). They are similar to renal tissue, using a system of countercurrent blood flow to remove salt from the bloodstream and pass it down from the nasal passage to be sneezed out of the nostrils. The ostrich, cormorant, duck, goose, falcon, gull, and penguin all have functioning salt glands. They tend to be absent or vestigial in passerine species (Shoemaker 1972).

CLINICAL NOTE

Nasal glands will atrophy if not exposed to salt so this may need to be provided when seabirds are being rehabilitated back to the wild.

Osmoregulation

Most birds gain water by drinking or eating succulent foods like fruit, nectar or meat. Some birds from arid climates (e.g., the Zebra finch) can produce metabolic water instead and conserve water by producing scant urine (Goldstein 2000). Desert birds produce only 55% water in their excrement whereas birds with plentiful access to water provide 75-90% water (Orosz et al. 1997). They also conserve water by cooling warm air as it passes through the nares.

GENERAL INTEREST

Migratory birds often fly for over 72 hours without drinking. Dehydration is prevented because high altitudes prevent evaporative water loss and metabolic water is produced from fat stores. Flying by night also helps to conserve water when the air is cooler.

Urine

Avian urine is the semi-solid product of the kidneys which, on standing, separates out into a white precipitate (urates) and supernatant fluid (urine). Birds have the ability to con­centrate their urine but it is less developed than in mammals. Urine is produced by the glomerular filtrate and 90% is reabsorbed by the tubules (Phalen 2000). Like in mammals, therefore, the major control of urine output is tubular resorption.

uric acid levels will also cause gout in joints and organs such as the kidneys, spleen, and pericardium (Fig. 6.58).

CLINICAL NOTE

Prior to egg laying, parathyroid hormone (PTH) demineralizes medullary bone to allow calcium to be deposited in the egg yolk and shell. The excess phosphate is excreted via the kidneys causing diuresis. Hence, many hens become polyuric prior to egg laying.

Dehydration in birds is serious as it can rapidly lead to irreversible gout. Urea levels can be monitored to estimate dehydration, but 70% of the kidney needs to be damaged before you get elevated blood uric acid levels. Carnivorous birds like birds of prey must also be fasted for 24 hours to avoid false positives.

Uric acid

Birds, like reptiles, are uricotelic meaning they excrete 60% of their nitrogenous waste in the form of chalky white urates.

Uric acid is synthesized in the liver and excreted in the kidney by glomerular filtration (10%) but mainly tubular secretion (90%) (Goldstein 2000; Phalen 2000). Urates are also produced independently of urine flow, being secreted by the reptilian-style cortical nephrons and so will still be produced even by very dehydrated birds (Phalen 2000).

Gout

Gout is not a disease but a symptom of hyperuricemia or kidney damage. If the proximal renal tubules are damaged uric acid cannot be excreted, so urate levels rise. As the rate of excretion of urates is relatively independent of glomerular filtration, high levels of urates can be produced even in dehy­drated birds. However, if they cannot be flushed out by the kidney they will precipitate causing renal gout. High blood ADAPTATIONS MADE BY BIRDS IN ARID ENVIRONMENTS

• Kidneys have more medullary nephrons to concentrate urine (Johnson, OW 1979)

• Salt glands excrete excess sodium

• Absorption from coprodeum and colon

• Vascular network in nasal cavity (rete mirabili) to cool incoming air

• Behavioral adaptation

KEY POINTS

• No renal pelvis or bladder

• Dual afferent blood supply

• Two forms of nephron - cortical and medullary

• Uricotelic - production of urates excreted by renal portal system

• Nasal salt glands in marine species

• Production of urates means even mild dehydration or renal damage predisposes to gout