Common raven physiology
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The common raven (Corvus corax), also known as the northern raven, is a large, all-black passerine bird. Found across the Northern Hemisphere, it is the most widely distributed of all corvids. Their Northern range encompasses Arctic and temperate regions of Eurasia and North America, and they reach as far South as Northern Africa and Central America.[1] The common raven is an incredibly versatile passerine to account for this distribution, and their physiology varies with this versatility. This article discusses its physiology, including its homeostasis, respiration, circulatory system, and osmoregulation.
Physiology
Habitat variation and physiological regulation
Maintaining
Common ravens occupy a widespread geographical range and are found in many different habitats, including
In relation to temperature and precipitation, common ravens are exposed to changing seasons with climate extremes. Within the common raven species, the degree of climatic seasonality is related to the magnitude of fluctuations in basal metabolic rate and total evaporative water loss.[4] For instance, populations living in Alberta are subjected to both extremely cold temperatures in the winter and very hot and dry weather during the summer months. Furthermore, the common raven is not known to migrate long distances to avoid the winter season, so it is required to regulate and cope with the environmental conditions.[2]
Habitat variation often leads to changes in activity levels. Ravens engaged in flight are considered metabolically active. During periods of flight, the cells require more oxygen, and the heat generated must be dissipated to avoid
Respiration
Ravens have a high
The respiratory tract of birds possesses unique air movement properties. Air moves in a unidirectional flow and blood travels in a concurrent direction to air flow. An advantage of this type of system is it minimizes dead space and enables the bird to maintain a highly oxidative, active output. The respiratory system of the common raven is no different.
Flight is a unique feat among birds and provides them with many advantages in terms of food, predation, and movement. It is suggested that cardiovascular variables play a large part in avian flight and were naturally selected over time.[8] Specifically, the avian heart evolved to pump more blood throughout a bird’s body while it is engaged in flight. During rigorous activity, especially when flying, the demand for oxygen is high.
Birds proceed through the four steps of the oxygen cascade: 1. Convection of oxygen to lungs via ventilation 2. The diffusion of oxygen from the lungs into the blood stream 3. Oxygen-rich blood is transported to the peripheral tissues by convection 4. Oxygen diffuses into the mitochondria.[9]
Fick's laws of diffusion can be applied to oxygen cascade events in avian species. There is a proportional relationship between the volume of the pulmonary capillaries and respiratory surface area. Avian respiration follows a system of countercurrent flow in which inspiratory and expiratory processes are dependent on an efficient rate of diffusion to oxygenate blood through the air capillaries. The optimization of the volume of tissue within the capillaries in conjunction with the large respiratory surface area allow for an effective diffusion rate.[10][11] Finally, in the avian respiratory system, the partial pressure of oxygen between the gas, lung, and the vascular capillaries depends upon the ventilation rate and air that is already inhaled.[11]
Through the respiratory pathway of evaporative water loss, the common raven is able to effectively maintain body temperature within their variable range. The mechanism by which
A unique feature of avian respiration involves the usage of turbinates within the nasal cavity during routine breathing. The nasal cavities in avian taxa are the first organ to moderate the inhalation of air humidity during periods of rest. The epithelial-lined turbinates within these cavities act as countercurrent heat exchangers. During this exchange, air inhaled becomes saturated as it brought further down into the respiratory tract. The air being exhaled stays saturated, allowing for a recovery of heat and moisture.[15]
Circulation
Like all avian species, the blood of the common raven transports nutrients, oxygen, carbon dioxide, metabolic waste products,
Like other vertebrates with closed circulatory systems, pumping blood of the common raven can be described by several physiological principles. These principles and laws include
Blood composition
The blood composition of the common raven is similar to that of most avian species. In general, the blood is composed of
Osmoregulation
Environmental challenges on osmoregulation
The Corvus corax lives in a wide variety of habitats, and as such it is well suited to many different environments
These populations have a much higher intake of salt compared to the populations in the more inland regions and therefore produce a more potent hypotonic excrement.[23] A diet with sufficient salt concentrations pushes ravens in marine environments to focus on water intake. This is primarily through the food it eats, but if this is not sufficient it will drink water or consume snow in the winter. Corvus corax is adapted to survive across its large range, and as such exhibits a number of variations in osmoregulation to suit its environmental needs. Because these birds are omnivores, their basal rates reflect the diet they follow along with other factors such as environmental conditions.[27]
The interactions between this species and humans present various challenges for these birds. Trash and carrion scraps, whether anthropogenic in origin or not, present opportunities for feeding consistent with ravens' scavenging behavior. These food placements attract breeding and non-breeding birds, which present competitive challenges for breeding adults collecting food for their broods.[25] Corvus corax also present several problems in their normal ranges for humans, and as such efforts to reduce their numbers through osmoregulatory means factor into the viability of this species in certain regions. Specifically, the use of the toxicant 3-chloro-4-methylanine hydrochloride (DRC-1339) is used to induce renal failure and death in ravens impacting the conservation status of other species of bird and livestock numbers.[28]
Primary osmoregulatory organ or system
Regulation of water and
Like other birds, the common raven is considered a
There are two types of avian nephrons, and nephrons become larger as depth from the kidney surface increases. Reptilian-type nephrons are the smallest nephrons, are found near a kidney’s surface, possess simple glomeruli, and do not have loops of Henle. Conversely, between 10% and 30% of the total nephron population is composed of mammalian-type nephrons, which are located in the innermost area of the kidney, have complex glomeruli, and contain loops of Henle.[29]
Once the kidneys receive blood,
Circulation and respiration
The osmoregulatory system is interconnected with the circulatory system to permit effective regulation of salt and water balance. Circulatory fluids function in renal clearance, which is the blood volume that substances are removed from within the kidneys during a certain time period. In addition to filtration, the circulatory system also plays a role in reabsorption. Furthermore, the role of the renal portal system is to regulate renal hemodynamics during times of decreased arterial blood pressure.[29]
Kidneys of common ravens receive arterial and afferent venous blood and are drained by efferent veins. In terms of the arterial blood supply, the arteries entering the kidneys branch into numerous smaller arteries and eventually form afferent arterioles that supply the glomeruli. The peritubular blood supply is composed of efferent arterioles leaving the glomeruli of reptilian-type nephrons that drain into sinuses of the cortex. On the other hand, the
Research indicates that kidneys of avian species receive approximately 10% to 15% of
The avian respiratory system is not in direct contact with the osmoregulatory system. However, the respiratory tract participates in osmoregulation through evaporative water loss. Since common ravens are endothermic and have high rates of ventilation, respiratory water loss is inevitable.[29]
Cells and mechanisms of osmoregulation
Filtration into Bowman's capsule
The kidneys in aves are divided into units called lobules. Within each
Reabsorption in proximal tubes
Reabsorption of molecules and ions back into the blood from the proximal tube is done via epithelial cells. The epithelial cell create a low Na+ concentration within the cell by actively pumping out Na+ into the blood via a Na+/K+ ATPase pump on the basolateral membrane. The
Regulating water loss
A key function of the
Special adaptations
Since the expansion of the human population and urbanization, there have been numerous extinctions of birds. Extinctions threaten nearly 12% of bird species, but this does not account for an additional 12% of species located in small geographical ranges where human actions rapidly destroy habitats.[34] Due to pressures from humans and the environment, birds have unique features that permit adaptations to changing conditions.
The common raven migrates long distances for food and mating. Since ravens, and birds in general, travel to such extents, they have a unique adaptation for flying in high altitude environments. Specifically, neural mediating reflexes increase breathing. The locomotors system stimulates breathing directly from feed forward stimulation from brainstem centers and feedback stimulation from exercising muscles. In the carotid body, the bird’s chemoreceptors detect low oxygen and stimulate breathing during
Not only is a bird’s respiration adapted to handle high-altitude flight, but so too is the circulatory system. In general, birds have larger heart sizes and higher cardiac output. During flight, birds can sustain their heart rates, and their myosin flight muscles have better oxygen diffusion because of a high degree of branching between the capillaries.[36]
The common raven lives in a wide variety of climates. Due to its habitat and food, the common raven has unique features that allow it to regulate osmotic challenges. Common ravens can be observed in oceans consuming water. However, when birds consume salt loaded prey or drink salt water, the body’s internal osmoregularity increases. The solution produced is considerably more concentrated than seawater.[37] Birds are the only group of vertebrates that have the ability to produce hyposmotic urine. The ability to produce hyposmotic urine is from the medullary cones. Urine is mixed with digestive fluids rather than directly eliminated. Consequently, the avian gut plays an important role in water and salt regulation.[38] In mammals, the osmotic gradient is urea, whereas in birds, sodium chloride is the major solute in the medullary cones.[37] In birds, the kidneys are not solely responsible for osmoregulation. A unique feature in birds is the lower intestine, which absorbs fluids and electrolytes that were not absorbed by the small intestine or the kidneys.[37] These osmoregulatory adaptations allow the common raven to thrive in diverse habitats.
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