Avian Nephrology & Urology Flashcards
How is uric acid excreted in the avian kidney?
90% is secreted by proximal tubules
10% is filtered by glomerulus
Terio Ch 32 - Psittacines
Where are the kidneys in birds located anatomically?
How are they divided?
What are all the organs involved in osmoregulation?
What are teh two different types of nephrons in the avian kidney?
Describe the filtration rate of the avian kidney?
How do birds make up for not being able to concentrate urine as much?
What hormones regulate salt excretion in the salt glands?
Changes in GFR are regulated by what hormone in birds?
The Urinary and Osmoregulatory Systems of Birds
Orosz, Susan E., and M. Scott Echols.
Vet Clin North Am Exot Anim Pract 2020;23(1):1-19
Key Points:
- The kidneys of birds are fixed under the area of the synsacrum and are composed of di-visions not lobes.
- Birds osmoregulate using their kidneys; intestinal tract; and, in some birds, salt glands.
- Avian nephrons are of 2 main types: the cortical, loopless, or reptilian nephrons and the medullary or looped nephrons. Knowing this anatomy, along with that of the renal portal system, is important in understanding the pharmacodynamics of drugs in birds.
- The kidney filters up to 11 times the total body water per day. Although the nephrons do not concentrate urine to the extent of mammals, they use the large intestine and/or the colon for resorption of ureteral urine. In addition, urates are modified by special colonic bacteria and some products produced are recycled through the renal portal system.
- There are some birds, particularly marine species, that are able to remove excess sodium through a countercurrent system in the salt glands. Angiotensin II has been shown to inhibit secretion of sodium chloride, whereas atrial natriuretic peptide enhances secretion.
- Changes in GFR are regulated by the antidiuretic hormone of birds, arginine vasotocin
Compare and contrast the reptilian and mammalian nephrons in the avian kidney.
How do birds compensate for poor renal compensating ability?
Describe the three sections of the avian kidney.
How distinct are these divisions across avian taxa?
What vessels supply the three divisions?
What are a few reasons you shouldn’t consider a nephrectomy in a bird?
Describe the location of spinal nerves at that site? How does kidney disease cause a neuropathy?
Renal Anatomy
- 3 divisions: cranial, middle, caudal
- Chicken and many parrots: distinct divisions
- Passerines: middle division blends
- Puffins, penguins, herons: caudal divisions fused on midline
- Blood Supply
- External Iliac artery: divides cranial and middle
- Ischiatic artery: divides middle and caudal
- Spinal nerves from lumbar and sacral plexuses move through parenchyma
- Swelling or dorsal pressure on kidneys can reduce nerve function: become non-weight bearing, muscle atrophy and sometimes loss of bone mass (esp lateral femur), minimal or absent deep pain. Esp budgies
Describe the renal organization of avian species.
How are cortex and medulla differentiated?
What is the basic unit of the avian kidney? How are they arranged?
Where are the two types of nephrons located within this unit?
Where are the vessels and collecting ducts located?
Basic unit: lobule
- Long axis perpendicular to long axis of kidney, pear shaped, wedged between interlobular veins of renal portal system
- Wide portion: cortical region contains cortical and medullary-type nephrons
- Narrow portion: medullary region, contains collecting tubules and loops of Henle from medullary-type nephrons
- Artery: in center of lobule
- Intralobular vein
- Collecting ducts are on periphery of each lobule: collecting tubules converge to form branches of ureter
Compare and contrast the types of avian nephrons.
Where are they located?
Is a loop of henle present?
What is the product they produce?
Are they capable of concentrating urine?
Which makes up most of the nephrons in the kidney?
How do the kidneys in birds in arid environments differ?
Nephrons
- Medullary: looped, mammalian-type, contain loops of Henle (10-30%)
- Cortical: loopless, reptilian-type, *most of avian nephrons - secrete uric acid
- +/- transitional nephron: elongated looping intermediate segments not in medullary cones or rays
- Arid environment: smaller kidneys, larger medullary volume, smaller cortical volume
Describe the avian renal corpuscle.
How do they differ from mammalian ones?
What are podocytes?
How does the size of the podocyte slits affect the composition of avian urine?
Nephrons
- Renal corpuscle: glomerular capsule and tuft of capillaries, midway between interlobular and intralobular veins
- Simpler capillary tufts than mammals, only 2-3 capillaries with limited interconnections
- RBCs larger, fusiform, and more rigid than mammalian so capillaries are larger to accommodate and capillaries tufts are larger than mammals
- Podocytes: have slits that act as pores for solute movement from blood into Bowman space
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Larger podocyte slits (40-80% larger in chickens) and decreased polyanionic charge on filtration barrier compared to mammals contributes to greater flow and larger particles passing
- Avian ureteral urine contains 100x more protein (5 mg/mL) than mammals
How do the tubules combine to form collecting ducts in the avian kidney?
What is the function of the macula densa?
What is the function of juxtaglomerular cells (JG cells)?
How would this apparatus respond to decreased sodium levels? What about increased sodium levels?
Nephrons
- Proximal and distal portions of collecting ducts
- Perilobular collecting tubules and medullary collecting tubules combine to form collecting duct to secondary branch of ureter
- Juxtaglomerular apparatus
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Macula densa: epithelial thickening at beginning of distal convoluted tubule, detect sodium concentrations in tubules leaving renal corpuscle, signal release of renin from JG cells
- Taller with more prominent nuclei making them appear darker (‘denser’)
- Juxtaglomerular (JG) cells: specialized myocytes in afferent arterioles that secrete renin
- Extraglomerular mesangium: modified connective tissue between macula and specialized cells of afferent arterioles
- Decreased sodium levels: relaxation of afferent arteriole: increases glomerular blood flow: increases glomerular hydrostatic pressure: increases filtration rate
- Macula densa signal JG cells to release renin
- Renin increases blood pressure via renin-angiotensin-aldosterone system
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Macula densa: epithelial thickening at beginning of distal convoluted tubule, detect sodium concentrations in tubules leaving renal corpuscle, signal release of renin from JG cells
Describe the dual blood supply to the avian kidney and which portions of the kidney they supply.
What are the renal arteries? How do they branch?
Describe the venous supply from teh kidney?
How does the renal portal system supply the kidney? Why is this important for renal function?
Describe the anatomy of the renal portal system.
How are the renal portal valves controlled? What stimulates they to open, what stimulates them to close?
Arterial supply
- Cranial, middle, and caudal renal arteries from abdominal aorta
- Branch to form intralobular arteries (midway between interlobular and intralobular veins)
- Intralobular arteries form short afferent glomerular arteries
- Coalesce to form glomerulus
- Continues as efferent glomerular arterioles
- Divide to form peritubular capillary plexus
- surrounds epithelium of convoluted tubules in cortical region
- Form arteriolar recta lying alongside loops of Henle in medullary region before forming venulae recta to drain the area
Venous supply
- Loops of henle drained by venulae recta into intralobular veins
- Drain into efferent renal veins or branches
- Cranial renal veins drain into common iliac vein after renal portal valve or into abdominal vena cava directly
- Caudal renal vein drains into common iliac vein after renal portal valve
- Renal portal system: venous blood to peritubular capillary plexus surrounding proximal convoluted tubules at periphery of the lobule, responsible for urate secretion
- (urates also filtered by glomerulus but rate is insufficient)
- Renal portal system provides about ⅔ of the blood supply to the kidneys that bypasses the glomeruli
- Forms a venous ring with both kidneys
- Right and left cranial renal portal veins connect via internal vertebral venous sinus (drains the vertebral column)
- Right and left caudal renal portal veins anastomose with caudal mesenteric vein
- Afferent renal branches have muscular sphincters at base to control volume of blood entering the kidney
- Afferent renal branches connect with interlobular veins
- Interlobular veins connect to peritubular capillary plexus at periphery of each lobule
- Renal portal valves: found in lumen of common iliac veins between renal and portal veins
- Innervated by adrenergic and acetylcholine receptors
- Valve closure inhibited by norepinephrine and epinephrine
- Sympathetic tone: valves open
- Valves open: blood bypasses the kidney and flows directly into the caudal vena cava (and/or caudal mesenteric vein to the liver, internal vertebral venous plexus within the vertebral canal)
- *often only partially activated, not bypassing kidneys completely
- Acetylcholine stimulates valve closure
- Parasympathetic: valves close
- Valves close: blood flows into parenchyma of kidney
Describe the glomerluar filtration of the avian kidney.
How does GFR of a single nephron compare to those in mammals/
How does GFR change depending on hydration status?
How much can it decrease in periods of water deprivation?
What hormone regulates changes in GFR?
How does it function, where is it secreted, what stimulates its secretion?
Glomerular filtration
- Function of hydrostatic pressure produced by the heart
- Charges on fenestra and size of openings oppose movement of large, negatively charged proteins such as albumen
- GFR of a single nephron is lower in avian than mammals but total kidney GFR is the same, offset by larger number of nephrons
- Whole-kidney GFR most dependent on hydration status
- Hummingbirds alter GFRs diurnally
- Water deprivation: GFR can decrease up to 65%
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Changes in GFR regulated by arginine vasotocin (antidiuretic hormone of birds)
- Arginine vasotocin alters tone of renal vasculature and tubular epithelium to conserve water
- Peptide hormone released by neurohypophysis, most commonly stimulated by increased extracellular fluid osmolality
- Dehydration: increased circulating arginine vasotocin levels: decreases GFR and enhances tubular reabsorption: reduces urine flow
- Reptilian/cortical/loopless nephrons most sensitive to arginine vasotocin
- Water overload: increased GFR but mechanism is unknown
- Birds capable of autoregulation of renal blood flow over wide range of systemic blood pressures (chickens systemic BP as low as 50 mmHg with maintenance of GFR)
Describe the nitrogen excretion in the avian kidney.
What is the major end product? What are the minor end products?
Describe the metabolic cost of synthesizing uric acid.
How does the kidney prevent urates from crystalizing?
Nitrogen excretion
- Major end product of nitrogen catabolism in birds: uric acid (70-80%)
- Minor amounts: creatinine, amino acids, urea
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Uric acid: low solubility, removes 4 nitrogen atoms/molecule
- Metabolic cost of synthesis is much higher than ammonia or urea
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Urates pass freely through fenestrae of glomerulus
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Albumen takes urates out of solution and prevents crystal formation
- Small spherical structures begin to form in proximal tubule and grow as they travel toward ureter, most are 65% urates
- Urate does not contribute to osmolality of urine when taken out of solution by albumen
- As concentration of urates secreted into lumen increases past solubility limit: potential is created for crystal formation in the proximal tubule
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Albumen takes urates out of solution and prevents crystal formation
- Dietary vitamin A helps keep reptilian/cortical/loopless nephrons healthy by reducing squamous metaplasia
Describe water regulation within the avian kidney.
What dietary types meet their water needs from food items?
What physiologic stimuli lead a bird to drink?
How much volume does the avian kidney filter?
How is water reclaimed?
How does concentration of urine occur? Is it very effective?
Water absorption in the tubules is dependent on active absorption of what solutes?
Water/solute Intake
- Carnivores, frugivores: many species meet water needs from food items
- Est. 100g bird drinks ~5% BW daily
- Lower BW increases water requirement (10-20g, drinking rate increases to 50%)
- Birds with salt glands typically need an increased volume of water compared to those lacking glands
- Physiologic stimuli to drink primarily include cellular dehydration, extracellular dehydration, angiotensin II
Water excretion
- Kidney filters large volume of fluid daily (up to 11x total body water in 100g bird)
- Most (~95% filtered fluid) reclaimed by tubular reabsorption
- Able to concentrate urine by varying degree of tubular reabsorption (range <70->99%)
- Concentrating ability generally varies inversely with body mass (small birds 10-25g concentrate to 1000 mmol/kg vs birds >500g concentrate to 600-700 mmol/kg)
- Tubular reabsorption depends on active sodium reabsorption but not bicarbonate
- Osmolality gradient in medullary region produces countercurrent multiplier system in loop of Henle which allows for reabsorption
- Further concentration of urine occurs by retroperistalsis into coprodeum and large intestine: single layer of columnar epithelium can reabsorb water
Describe how the lower GI tract of avian species functions to concentrate urine.
What happens to the protein contents of the urine?
How is the movement of ureteral urine and urates controlled within the GI tract?
Water absorption in the coprodeum and colon follows absorption of what ion?
Lower GI Tract Absorption of Water from Urine
- Retroperistalis moves urates and urine from urodeum into large intestine or colon and cecum
- Moves around a central fecal core, contacting brush border of colonic epithelium and its resident bacterial population
- Large amounts of albumen in ureteral urine is degraded into amino acids, dipeptides, and tripeptides, used by the bacteria to produce volatile short-chain fatty acids
- Glutamic acid (a major product formed): glutamine transported into cells - deaminated - recycled to ammonia, transported into caudal mesenteric vein to enter renal portal system
- Recycling of nitrogen may be particularly important in species with low nitrogen diets (nectarivores, frugivores)
- Movement of ureteral urine and urates is controlled by tonicity of fluid in GI tract
- Tonicity 200 mOsm/kg H2O higher than plasma: retrograde peristalsis is stopped or significantly slowed.
- Villanoid receptor type in the cloaca responds to changes in shape of cells that occurs from changes in their osmotic environment
- Receptors allow local feedback effect to help maintain osmolality
- Sodium transport in coprodeum and colon results in recovery of urinary water
- low sodium diet increases plasma aldosterone resulting in increased sodium absorption in coprodeum and colon
Describe the physiology behind avian salt glands.
Where are they typically located?
What hormones regulate excretion in these glands?
Which ions are actively transported? Which ions move freely?
Salt gland
- Located close to nasal cavity or orbit
- Series of blind-ended tubules that branch out
- Epithelium is strongly secretory, with basolateral infoldings and large numbers of mitochondria
- Tubules coalesce to form 2 secretory ducts that drain into nasal cavity
- Blood flow is countercurrent to flow in tubules
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Hyperosmolar fluid produced, consisting of predominantly sodium
- Chloride is likely actively transported across epithelium vs sodium moves passively through intercellular spaces
- Increase in plasma sodium level causes enhanced secretory activity of tubules and increased blood flow to gland
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Hormones modulate rate of secretion but do not seem to initiate secretion
- Angiotensin II shown to inhibit secretion
- Atrial natriuretic peptide enhances secretion
