Comparative urinary physiology and function

While we have focused on the mammalian system to describe and illustrate the anatomy and physiology of the urinary system, some appreciation of the diversity in other animals is important.

Avian urinary organs consist of the paired kidneys and ureters, which carry urine to the urodeum of the cloaca. As a reminder, the major segments of the avian digestive tract include esophagus, proventriculus and gizzard, small intestine, cecum, and rectum. The rec­tum (also referred to as the colon in birds) is relatively short and serves to link the ileum of the small intestine with the coprodeal compartment of the cloaca. The cloaca acts as a common pathway for digestive wastes, excretory material, and reproductive activity.

The avian kidney is typically elongated and divided into anterior, middle, and posterior regions. Within each region there are cone-shaped subunits with a cortex and medulla called medullary cones. The med­ullary cone and region of associated cortex that is drained create a kidney lobule. Compared with a typical mammalian kidney, avian nephrons are very heterogeneous. Smaller nephrons, usually positioned near the surface of lobules, have simple (modestly coiled) glomeruli and they lack loops of Henle (LLN, Ioopless nephrons). These were traditionally called reptilian-type (RT) nephrons because they resemble nephrons found in reptiles. Deeper in the lobules the nephrons do exhibit the loop of Henle (LN, looped nephrons). This appearance is typical of mammals, thus the older terminology of mammalian-like (MT) nephrons. There are also variations in structure between the LN and LLN nephrons that are called transitional nephrons (TN). On average, about 20% of nephrons are classified as LN or MT. Regardless, both classes of nephrons empty in a regular pattern into common collecting ducts. The collecting ducts combine and descend to the end of the medullary cone where a single large collecting duct empties into the ureter. Since the tissue of the cloaca, like the large intestine, has substantial resorption capacity, much of the water can ultimately be recovered.

Like the mammalian system, the avian kidney has a countercurrent multiplier arrangement where the loops of Henle parallel the collecting ducts (LN neph­rons). This means that birds are capable of producing urine that is hyperosmotic to blood plasma. For example, water-deprived birds can produce urine that is approximately two times more concentrated than plasma.

Avian nitrogen excretion

Urates, ammonia, urea, and creatinine all contribute to nitrogen excretion in urine of birds. However, based on experiments with domestic chicken, whether fed or fasted or provided low or high protein diets, urates constitute 55-84% of total nitrogen excretion. Indeed, urates are the major waste product of nitrogen­containing metabolites excreted by the urinary system of birds and most reptiles and amphibians. Secretion of urates or Uricotelism is an important adaptation to allow animals with no ability to concentrate (i.e., greater osmolarity than blood plasma) urine (reptiles and am­phibians) or limited capacity (birds) to survive and prosper in arid habitats. Alterations in ammonia excre­tion are also closely related to acid-base homeostasis.

Urate circulating in avian blood is believed to be mostly unbound and therefore available to be filtered by the glomeruli. However, given the plasma concen­tration and rate of filtration, clearance of urate exceeds that of inulin. It is clear that 90% or more of urinary urate comes from tubular excretion. Most urate is derived from liver metabolism of purine. Uric acid is relatively inert and markedly less toxic than ammonia or urea. After secretion or filtration, the fate of uric acid varies. In an acidic environment, uric acid is poorly soluble, but with a typical urinary pH of 6-7, most of the uric acid exists as monobasic urate. Urate also forms salts with sodium or potassium. This is important because of the abundance of these ions in urine. Sodium and potassium urate are also much more soluble (6.8 and 12.1 mmol∕L, respectively) than uric acid (0.38mmol∕L). However, this still does not account for the high concentrations of urate typical in avian urine. Urates can exist in supersaturated solu­tions because of their ability to form colloidal sus­pensions and to combine with mucopolysaccharides and glyoproteins, especially in the distal convoluted tubules.

Avian salt glands

For many terrestrial animals, sodium is not easy to obtain. Most fresh water is low in minerals and most plants are low in sodium. This explains why herbi­vores such as sheep, cows, and deer use natural salt licks or mineral-rich soil. Most of these animals also have high blood levels of aldosterone, the steroid hormone important to enhance renal reabsorption of sodium to minimize losses. For marine animals, excessive sodium intake is a serious issue. Yet many species of marine and shore birds can drink sea water as their only source of water for indefinite periods of time. Given the relative inability of birds to concen­trate their urine compared with most mammals, how is this possible? The answer is that there is a nonrenal pathway.

Suborbital or salt glands exist evidently in nearly all birds but are fully functional primarily in marine birds and in species that depend on hypersaline food and water. These glands are best described in the domestic duck. In fact, when ducklings never before exposed to excessive salt become osmotically stressed, the salt glands quickly begin to secrete a hypertonic sodium chloride solution and within 48 hours cells per gland increase two- to threefold, and the secretory cells become fully differentiated and active.

The glands are paired and located in depressions either in or above the orbit of the eye. The glands are also distinct from lacrimal glands. Structurally, they are composed of lobes that exhibit closed secretory units (tubuloacinar gland structure) that drain into central canals.

Salt gland secretions, as you would guess from the name, are nearly all NaCl with small amounts of K⁺, Ca⁺⁺, HCO₃^-, and Mg⁺⁺. It is important to remem­ber that, just as in mammals, interactions and integra­tion between multiple systems is required for osmotic regulation. Nonetheless, the salt glands represent a critical adaptation in these animals. For example, when the salt glands are stimulated by salt feeding or dehydration, 75% or more of Na⁺ excretion occurs via salt gland secretion, as does more than 30% of K⁺ elimination.