Avian Renal Physiology

The avian urinary system consists of a pair of kidneys and ureters that transport urine to the urodeum of the cloaca. The urinary bladder is absent in birds. The kidney lies in a cavity formed by the ventral surface of the synsacrum.

In avian species, a dual afferent blood supply is present in the kidney via a ‘high-pressure’ (160/120 mm of Hg) renal artery and a ‘low-pressure’ (25 mm of Hg) supply via a renal portal system (RPS). It is estimated that 1/2 to 2/3rd of blood supplied to avian kidneys is through the renal portal system. The renal artery supplies glomerular areas of the kidney; peritubular areas are partly by efferent glomerular arterioles and also by the venous return from the legs communicating with the RPS. The magnitude of the renal portal supply reaching the peritubular regions appears to be controlled by a smooth muscle valve called the renal portal valve. When the valves close by contraction, the peritubular areas of kidney are perfused with blood. When the valve is opened, the blood is directly shunted to posterior vena cava.

The functional unit of the kidney is the nephron. Avian kidney has two kinds of nephrons. One kind is reptilian type with no loops of Henle, located in the cortex, and another is mammalian type with long- or short-length loops located in the medulla (Fig. 9.11). Only a small percentage of nephrons (15-25%) are mammalian-type nephrons in birds.

Avian kidneys usually alternate between the uses of mam­malian- and reptilian-type nephrons. When birds concentrate urine, they opt for mammalian-type nephrons and completely shut down about 80% of the reptilian-type nephrons.

The role of the kidney in the bird is filtration, absorption and secretion, as in the case of other vertebrates. They filter water and water-soluble substances from the blood, including waste products of metabolism and ions, and are voided out through urine. Kidneys also have an important role in con­serving body water and reabsorption of needed substances, viz. glucose.

Blood enters nephrons via afferent arterioles, as in the case of mammals. In the glomerulus, the blood under high pres­sure gets filtered through the walls of the capillaries and the capsule walls. The filtrate entering the proximal tubules is plasma without protein since protein molecules are generally not filtered due to their large size. In the proximal convoluted tubules, the vital substances in the filtrate, such as vitamins and glucose, are reabsorbed into the blood. The kidney tubules can reabsorb almost 98% of the glucose that filters into the tubule even in a carbohydrate-rich dietary state.

Birds can conserve body water by producing urine of more osmotic concentration than plasma, as in the case of mammals. But the urine concentration capacity is limited in birds compared to mammals. On water deprivation, mammals can concentrate urine 5-10 times more than plasma; some mammals can do it about 20-25 times. But birds in water deprivation can only produce 1.4-2.8 times more concentrated urine than plasma. This ‘concentrating capacity’ is a feature of the medullary cones.

Solutes, like sodium chloride, are actively transported out of the ascending limb of the loop of Henle, and they become concentrated in the medulla (medullary cones). Unlike in mammals, only sodium chloride is responsible for maintaining medullary hypertonicity in birds. When the fil­trate passes through the osmotic gradient in the medulla, water gradually leaves the tubules by osmosis, and the filtrate becomes concentrated.

Usually, more water accompanies the solutes that travel from the kidneys through the ureters to the cloaca due to the

Fig. 9.11 Avian kidney. It has both reptilian- and mammalian­type nephrons, and mammalian­type nephrons can be long-loop or short-loop nephrons. The renal medulla has different medullary cones

reduced capacity of avian kidneys to concentrate urine (com­pared to mammals). A mechanism exists in water-deprived birds for reducing the amount of water leaving the kidneys. When dehydration occurs, the pituitary gland releases a hor­mone called arginine vasotocin (AVT) into the blood. AVT causes a decrease in the glomerular filtration rate in the avian kidneys, reducing the amount of water moving from the blood into the kidney tubules. Besides, AVT aids in the opening of protein water channels called aquaporins and thus increases the permeability of the walls of collecting ducts to water. Due to the increased permeability of the collecting ducts, more water leaves by osmosis out of the collecting ducts to the hypertonic medullary cones. From there, water is reabsorbed by kidney capillaries. Studies sug­gest that the effectiveness of AVT in reducing urine produc­tion or water reabsorption varies among species, but, in general, AVT is considered to be less effective in conserving water than the mammalian equivalent antidiuretic hormone (ADH). Therefore, water-deprived birds tend to lose more water from the kidneys than similar sized water-deprived mammals.

Uric acid is the end product of nitrogen metabolism in terrestrial reptiles and birds. In these species, the embryo’s development takes place in eggs that have shells imperme­able to water.

Salt glands of birds are supposed to be evolved from the nasal glands of reptiles. They lie immediately under the skin in supraorbital depressions of the frontal bone in the skull of Charadriiform birds. Still, in other birds, they may be located above the palate or within the orbit of the eye (Fig. 9.12). The marine birds (and some desert and Falconiform birds) secrete excess sodium chloride via the salt glands using less water than is consumed, thus saving water. So, birds are not physi­ologically affected by the high salt load.

Salt glands have a countercurrent blood flow system to remove and concentrate salt ions from the blood. It is paired and crescent-shaped glands. Each gland contains several longitudinal lobes approximately 1 mm in diameter, and each lobe contains a central duct from which radiated

Fig. 9.12 Salt gland function. (a) Salt gland located in the supraorbital depression of frontal bone. (b) Structure of the longitudinal lobe of a salt gland. (c) The pattern of salt excretion from the capillaries, wherein Na⁺ and Cl- are secreted into the tubular lumen

thousands of tubules are enmeshed in blood capillaries. These tiny capillaries carry blood along the tubules of the gland, which have walls just one cell thick and form a simple barrier between the salty fluid within the tubules and the blood­stream.

Learning Outcomes

• Metabolic waste products are eliminated, and water and electrolytes are retained in the body at optimum concentrations by effective glomerular filtration, tubular reabsorption and tubular secretion occurring inside the kidney’s nephrons.

• The kidney can auto-regulate its functions by inher­ent mechanisms, like myogenic response and tubulo glomerular fluid. The countercurrent multiplier and countercurrent exchange systems are responsible for creating and maintaining graded hypertonicity in the medullary interstitium, which is essential for concentrating urine. Urea recycling also helps to maintain medullary hypertonicity.

• The urine formed is conveyed to the urinary bladder through ureters for temporary storage until it is voided out by the process of micturition.

• Structural and functional modifications are present in the avian renal system to eliminate the major metabolic end product of birds, the uric acid. Salt glands provide a means of eliminating excess salt from the body, especially in marine species of birds.

Exercise

Objective Questions

Q1. Inulin clearance study is used to measure________.

Q2. The plasma concentration of a substance at which it first appears in urine is known as.

Q3. Which type of collecting duct cells is involved in acid secretion?

Q4. Which condition stimulates the kidney to release erythropoietin?

Q5. Which is the major force favouring filtration across the glomerular capillary wall?

Q6. Which water channel is responsible for ADH-induced water reabsorption from the collecting duct?

Q7. Which tubular part of the nephron is completely imper­meable to water?

Q8. Which hormone increases renal reabsorption of calcium?

Q9. What is the major end product of nitrogen metabolism in birds?

Q10. The property of tubules to increase reabsorption fol­lowing the increased tubular load due to increased GFR is known as_______.

Q11. Which mechanism helps glucose reabsorption from PCT?

Q12.

Q13. Which organic compounds are responsible for 50% renal medullary interstitial hypertonicity?

Q14. Where are the urea transporters located?

Q15. The bulk of glomerular filtrate is reabsorbed in which part of the nephron?

Q16. PAH clearance is used to study in _______.

Q17. What amount of mammalian-type nephrons are present in the birds?

Q18. The mode of Na⁺ reabsorption from Henle’s loop is

Q19. Which proteolytic enzyme affects the release of aldo­sterone after secreting from JG cells?

Q20. Which specialised tubular epithelial cells are involved in monitoring sodium ion concentration in the tubular fluid?

Subjective Questions

Q1. What is tubuloglomerular feedback?

Q2. Describe the dynamics of glomerular filtration.

Q3. Explain micturition.

Q4. How does ADH-mediated water reabsorption occur in the kidney?

Q5. How does sodium reabsorption occur from the renal tubules?

Q6. Describe the urine concentration mechanisms.

Q7. What is urea recycling?

Q8. Describe renal clearance.

Q9. Write the speciality of avian excretion.

Q10. Write the dynamics of tubular reabsorption.

Answers to Objective Questions

A1. GFR

A2. Renal threshold

A3. Intercalated cells

A4. Hypoxia

A5. Hydrostatic pressure of capillary blood

A6. Aquaporin-2

A7. Thin portion of the loop of Henle

A8. PTH

A9. Uric acid

A10. Glomerulotubular balance

A11. Co-transport/symport/secondary active transport

A12. Principal cells

A13. Urea

A14. Collecting duct

A15. Proximal tubule

A16. Renal plasma flow

A17. 15-25%

A18. Co-transport with K⁺ and Cl^-

A19. Renin

A20. Macula densa

Keywords for Answer to Subjective Questions

A1. Filtration pressure, macula densa, afferent arteriole, mesangium

A2. Hydrostatic pressure, colloid osmotic pressure, net fil­tration pressure

A3. Micturition reflex, reflex centres of pons, relaxation of external sphincter

A4. Dehydration, ADH, aquaporin-2

A5. Co-transport, counter-transport, chloride-driven sodium transport, transport from DCT

A6. Countercurrent multiplier, countercurrent exchange

A7. Inner medullary collecting duct, urea transporters, interstitial hypertonicity

A8. GFR, inulin clearance, creatinine clearance, renal plasma flow, PAH clearance

A9. Uric acid, reptilian and mammalian nephrons, AVT, renal portal system

A10. Hydrostatic pressure, colloid osmotic pressure, capil­lary blood, interstitial fluid

Further Reading

Hall JE (2011) Urine concentration and dilution: regulation of extracel­lular fluid osmolarity and sodium concentration. In: Hall JE (ed) Guyton and Hall textbook of medical physiology, 12th edn. Elsevier Saunders, Philadelphia, pp 345-360

Magouliotis DE, Tasiopoulou VS, Svokos AA, Svokos KA (2020) Aquaporins in health and disease. Adv Clin Chem 98:149-171

Parrah JD, Moulvi BA, Gazi MA, Makhdoomi DM, Athar H, Din MU, Dar S, Mir AQ (2013) Importance of urinalysis in veterinary practice - a review. Vet World 6(9):640-646. https://doi.org/10. 14202/vetworld.2013.640-646

Reece WO (2009) The urinary system. In: Reece WO (ed) Functional anatomy and physiology of domestic animals, 4th edn. Wiley- Blackwell, pp 344-347

Reece WO (ed) (2015) Duke’s physiology of domestic animals. Wiley