Birds Flashcards
Link the temperature and metabolic rate of birds.
- Birds are endothermic and homeothermic so they generate heat to maintain a core body temperature within a narrow range.
- This is in the range of 39-42˚C and generally higher than in comparable mammals. So birds have a higher resting metabolic rate.
Describe the metabolic rates of flying birds.
Most birds that fly, especially small passerines/song birds carry a much smaller energy reserve in the form of adipose tissue than mammals do, because of the energetic costs of the extra weight for flying.
Describe the metabolic rates of waterfowl.
When flying through water rather than air, fat can be advantageous by increasing buoyancy, so many waterfowl and marine birds have greater amounts of fat, which also has important function of insulation in cold environment.
Describe the metabolic rates of bird chicks.
Bird chicks have a much higher growth rate than mammals, with 3-4 times the energy requirement. This has led to biparental care in many bird species to meet the high rate of food provision for their chicks.
What is the role of the feathers in thermoregulation?
Have feathers as an insulating layer to retain heat rather than fur so care must be taken during plucking for surgery to minimize feather loss and think about thermoregulatory requirements.
What is the role of shivering in thermoregulation?
Shivering, like mammals, is an important mechanism to generate heat. But birds do not possess brown adipose tissue for thermogenesis, but they do have a similar capability for non-shivering thermogenesis in their muscle tissue.
How does wind affect metabolic rates in birds?
The same basic principles of thermoregulation apply to birds and mammals, seeking shelter from the cold and especially the wind, which can cause rapid loss of heat by convection. Increasing the insulating layer of trapped air by raising the feathers and decreasing the surface area exposed to the environment by buddling together and for some birds hiding their head under their wing, as the bill a major site for heat loss.
What is the role of the tibiotarsal retes in temperature?
- So birds that live in cold environments such as penguins have a counter current heat exchange arrangement via the tibiotarsal retes that cools arterial blood supplying the lower leg and warming the returning venous blood.
- This establishes a gradient of decreasing temperature down the legs that reduces heat loss from the feet.
What is gular fluttering?
- Birds do not have sweat glands. They will lose some heat by insensible water loss via the skin but this is via thermal panting – known as gular fluttering, which is similar to the mechanism in reptiles and panting in dogs.
- They increase the frequency and decrease the tidal volume of ventilation by rapid movement of muscles in the throat.
- This moves air rapidly in and out of the upper airway, providing evaporative cooling at the expense of water loss.
Why is the avian respiratory system 10 times more efficient than the mammalian respiratory system?
- Air sacs act like bellows, pushing the large volume of air in a unidirectional flow across the lungs.
- As the air sacs are doing the contracting and expanding the lungs can be a fixed volume.
- Without the need for elastic and connective tissue, the diffusion barrier between the air and blood is extremely thin and the total surface area for gas exchange is far greater.
- The expansivity of the air sacs enables a much greater tidal volume in birds, which in turn means that they’ve got a lower respiratory frequency than an equivalent mammal.
Describe avian ventilation in the 2 functional groups of air sacs.
Cranial or anterior air sacs (including the clavicular and cranial thoracic air sacs) have partial pressures of oxygen and carbon dioxide that are similar to those of the expired air.
Caudal or posterior air sacs, (including the abdominal and caudal thoracic) contain air that is more similar to the partial pressures found in inspired air.
The inspired air firstly flows into the caudal air sac, then across the gas exchange surface into the cranial air sacs before being expired.
Describe how a single bolus of air takes 2 complete cycles of inspiration and expiration to flow through the avian system.
- During the first cycle, the movement of the sternum down during inspiration increases the volume of the air sacs, and the air flows from the trachea into the caudal sacs, bypassing the cranial air sacs.
- On expiration, the decreased volume of the air sacs pushes the air from the caudal air sacs into the parabronchi of the lungs where gas exchange occurs.
- During the second cycle, inspiration increases the volume of the air, and the air is drawn from the parabronchi into the cranial air sacs.
- On expiration, this air is then forced out of the cranial air sacs in the expired air.
Is respiration unidirectional or bidirectional in birds?
So air is continually flowing into and out of the both caudal and cranial air sacs, but there is a unidirectional flow in the parabronchi of the lung during each respiratory cycle.
Describe counter current flow in the avian lung.
Air entering the parabronchus has a high PO2 and low PCO2.
- Along the parabronchus, oxygen diffuses into the air capillaries and then diffuse into the blood and CO2 will diffuse in the other direction, from the blood into the air capillaries and then into the parabronchial air.
- So by the time that the air leaves parabronchus it has a low PO2 and a high PCO2.
- Although the PO2 in the air leaving the parabronchus has a low PO2, there is still a small partial pressure gradient allowing the blood in the capillary at this end to still load up with a small amount of oxygen.
- The blood exchanging air capillaries further upstream in the parabronchus is able to load up with more and more oxygen, as the partial pressure increases.
- When the blood from all of these capillaries is mixed, as it leaves the lung, the pooled blood has a higher PO2 than the expired air.
What is the difference between the counter current flow in the fish and bird?
The pattern of blood supply, which split into capillaries that exchange gases with the air capillaries along the length of the parabronchus. The blood flow is effectively flowing across the direction of the airflow in the parabronchus at right angles. The blood from the different capillaries is then mixed together as it leaves the lungs.
How do birds control respiration without a diaphragm?
- Both inspiration and expiration are active processes with no elastic recoil, due to the high compliance of their sacs.
- Inspiration depends on the increase in thoracic volume as the sternum moves down and out.
How is ventilation rate controlled in birds?
Increased partial pressure of CO2 in the blood is the main driver to increase ventilation rate, via both central , hind brain chemoreceptor and peripheral carotid body chemoreceptors. The carotid body chemoreceptors also increase firing in response to low paO2 similar to mammals.
Where are the avian stretch receptors that are involved in control of ventilation?
As avian lungs do not change volume, there are no pulmonary stretch receptors. Instead, there are stretch receptors in the air sacs that help to control the switch from inspiration to expiration.
Describe the role of avian pulmonary stretch receptors.
- Not present in mammals.
- They are tonically active and inhibit the cardiorespiratory control centre in the brain.
- Activity of these is inhibited by increased PCO2 in the parabronchial air, disinhibiting the cardiorespiratory control centre to drive increased ventilation.
What urine does an avian kidney produce?
Avian kidney is able to produce hyperosmotic urine but is not able to produce as concentrated urine as mammals.
Describe the nephron types in the avian kidney.
- Mixture of reptilian type nephrons that lack a loop of Henle, with islands of mammalian type nephrons that have a loop of Henle forming medullary cones.
- The mammalian type nephrons are therefore able to generate high interstitial osmolarity locally in the islands of medullary tissue, enabling water reabsorption form the collecting ducts, as in mammals.
What do the avian collecting ducts have in place of mammalian ADH?
Posterior pituitary peptide hormone arginine vasotocin that causes vasoconstriction of the afferent renal arteriole. This decreases GFR by the reptilian type tubules but doesn’t affect the mammalian type tubules and so effectively increases the concentrating ability of the kidney.
What is the supply of the reptilian like nephrons?
Reptilian like nephrons are associated with the peritubular network that receives blood from the renal portal vein.
Describe the proportion of uric acid that is excreted and secreted in chickens.
Only about 10% of the nitrogenous waste of a chicken is excreted via ultrafiltration at the glomerulus. This means that 90% of the uric acid secretion is via secretion from the peritubular network of capillaries and the blood flow from the renal portal vein accounts for about 60% of the uric excreted by chickens.
How does gout arise in birds?
Reduction of overall GFR by arginine vasotocin can decrease water loss via the kidney without greatly affecting its ability to eliminate nitrogenous waste. But excreting nitrogenous waste as uric acid can lead to problems with crystals or uric acid causing gout.
Particularly common in budgies, parrots, cockatoos and other psittacines, especially if they are dehydrated, have too much protein the diet or kidney damage.
Why do birds need post renal modification?
- If birds and reptiles were to absorb too much water from the collecting ducts as that would lead to precipitate of the uric acid out of solution, potentially blocking the collecting ducts.
- So instead, reptiles and birds predominantly rely on post renal modification of the urine to conserve water and electrolytes.
