There are , B lymphocytes located around each follicle. Air flows anteriorly caudal to cranial through the parallel parabronchi. The hilus is the point of entry and exit of the renal arteries and veins, lymphatic vessels, nerves, and the enlarged upper extension of the ureters. Fossorial creatures live in subterranean environments. Initiation of Translation Review 1 page Concept McKenna and Susan K.
Human excretory organs
As each ureter extends into the bladder wall its circular fibres disappear, but its longitudinal fibres extend almost as far as the mucous membrane lining the bladder.
The mucous membrane lining increases in thickness from the renal pelvis downward. Thus, in the pelvis and the calyxes of the kidney the lining is two to three cells deep; in the ureter, four to five cells thick; and in the bladder, six to eight cells. The mucous membrane of the ureters is arranged in longitudinal folds, permitting considerable dilation of the channel.
There are no true glands in the mucous membrane of the ureter or of the renal pelvis. The chief propelling force for the passage of urine from the kidney to the bladder is produced by peristaltic wavelike movements in the ureter muscles.
The urinary bladder is a hollow muscular organ forming the main urinary reservoir. It rests on the anterior part of the pelvic floor see below , behind the symphysis pubis and below the peritoneum. The symphysis pubis is the joint in the hip bones in the front midline of the body.
The shape and size of the bladder vary according to the amount of urine that the organ contains. When empty it is tetrahedral and lies within the pelvis; when distended it becomes ovoid and expands into the lower abdomen.
It has a body, with a fundus, or base; a neck; an apex; and a superior upper and two inferolateral below and to the side surfaces, although these features are not clearly evident except when the bladder is empty or only slightly distended.
The neck of the bladder is the area immediately surrounding the urethral opening; it is the lowest and most fixed part of the organ. In the male it is firmly attached to the base of the prostate, a gland that encircles the urethra. The superior surface of the bladder is triangular and is covered with peritoneum.
The bladder is supported on the levator ani muscles, which constitute the major part of the floor of the pelvic cavity. The bladder is covered, and to a certain extent supported, by the visceral layer of the pelvic fascia. This fascial layer is a sheet of connective tissue that sheaths the organs, blood vessels, and nerves of the pelvic cavity.
The fascia forms, in front and to the side, ligaments, called pubovesical ligaments, that act as a kind of hammock under the inferolateral surfaces and neck of the bladder. The blood supply of the bladder is derived from the superior, middle, and inferior vesical bladder arteries. The superior vesical artery supplies the dome of the bladder, and one of its branches in males gives off the artery to the ductus deferens , a part of the passageway for sperm.
The middle vesical artery supplies the base of the bladder. The inferior vesical artery supplies the inferolateral surfaces of the bladder and assists in supplying the base of the bladder, the lower end of the ureter, and other adjacent structures.
The nerves to the urinary bladder belong to the sympathetic and the parasympathetic divisions of the autonomic nervous system. The sympathetic nerve fibres come from the hypogastric plexus of nerves that lie in front of the fifth lumbar vertebra.
Sympathetic nerves carry to the central nervous system the sensations associated with distention of the bladder and are believed to be involved in relaxation of the muscular layer of the vesical wall and with contraction of sphincter mechanism that closes the opening into the urethra. The parasympathetic nerves travel to the bladder with pelvic splanchnic nerves from the second through fifth sacral spinal segment. Parasympathetic nerves are concerned with contraction of the muscular walls of the bladder and with relaxation of its sphincter.
Consequently they are actively involved in urination and are sometimes referred to as the emptying, or detrusor, nerves. The bladder wall has a serous coat over its upper surface.
This covering is a continuation of the peritoneum that lines the abdominal cavity; it is called serous because it exudes a slight amount of lubricating fluid called serum.
The other layers of the bladder wall are the fascial, muscular, submucous, and mucous coats. The fascial coat is a layer of connective tissue, such as that which covers muscles. The muscular coat consists of coarse fascicles, or bundles, of smooth involuntary muscle fibres arranged in three strata, with fibres of the outer and inner layers running lengthwise, and with fibres of the intermediate layer running circularly; there is considerable intermingling of fibres between the layers.
The smooth muscle coat constitutes the powerful detrusor muscle, which causes the bladder to empty. The circular or intermediate muscular stratum of the vesical wall is thicker than the other layers. Its fibres, although running in a generally circular direction, do interlace.
The internal muscular stratum is an indefinite layer of fibres that are mostly directed longitudinally. The submucous coat consists of loose connective tissue containing many elastic fibres. It is absent in the trigone, a triangular area whose angles are at the two openings for the ureters and the single internal urethral opening. Slim bands of muscle run between each ureteric opening and the internal urethral orifice; these are thought to maintain the oblique direction of the ureters during contraction of the bladder.
Another bundle of muscle fibres connects the two ureteric openings and produces a slightly downwardly curved fold of mucous membrane between the openings. The mucous coat, the innermost lining of the bladder, is an elastic layer impervious to urine. Over the trigone it firmly adheres to the muscular coat and is always smooth and pink whether the bladder is contracted or distended.
Elsewhere, if the bladder is contracted, the mucous coat has multiple folds and a red, velvety appearance. When the bladder is distended, the folds are obliterated, but the difference in colour between the paler trigonal area and the other areas of the mucous membrane persists.
The mucous membrane lining the bladder is continuous with that lining the ureters and the urethra. The urethra is the channel that conveys the urine from the bladder to the exterior.
In the male it is about 20 centimetres long and carries not only the urine but also the semen and the secretions of the prostate, bulbourethral, and urethral glands.
During urination and ejaculation it opens up, and its diameter then varies from 0. The male urethra has three distinguishable parts, the prostatic, the membranous, and the spongy, each part being named from the structures through which it passes rather than from any inherent characteristics.
The prostatic section of the male urethra commences at the internal urethral orifice and descends almost vertically through the prostate, from the base of the gland to the apex, describing a slight curve with its concavity forward. It is about 2. The membranous part of the male urethra is in the area between the two layers of a membrane called the urogenital diaphragm.
The urethra is narrower in this area than at any other point except at its external opening and is encircled by a muscle, the sphincter urethrae. The two small bulbourethral glands are on either side of it. The membranous urethra is not firmly attached to the layers of the urogenital diaphragm. The spongy part of the male urethra is that part of the urethra that traverses the penis.
It passes through the corpus spongiosum of the penis. The ducts of the bulbourethral glands enter the spongy urethra about 2. The female urethra is much shorter 3 to 4. It begins at the internal opening of the urethra into the bladder and curves gently downward and forward through the urogenital diaphragm, where it is surrounded, as in the male, by the sphincter urethrae. It lies behind and below the symphysis pubis. Except for its uppermost part, the urethra is embedded in the anterior wall of the vagina.
Kidneys also play an important role in conserving water and reabsorbing needed substances like glucose. The urinary organs of birds consist of paired kidneys and the ureters Figure 2 , which transport urine to the urodeum of the cloaca. Avian kidneys are divided into units called lobules. Each lobule has a cortex outer area and medulla or medullary cone; Figure 3. Urine is carried from the avian kidneys to the cloaca and, specifically, the middle section called the urodeum by the ureters Figure from McSweeney and Stoskopf Image from Sherwood et al.
Lobule of an avian kidney. The medullary cones include the loops of Henle and collecting ducts of nephrons plus a number of capillaries called the vasa recta. The avian renal medulla is cone shaped because the number of loops of Henle decreases toward the apex of the medullary cones. The functional unit of the kidney is the nephron. Avian kidneys have two kinds of nephrons.
A reptilian-type, with no loops of Henle are located in the cortex, and a mammalian-type with long or intermediate length loops, are located in the medulla Figure 4. Nephrons filter the blood plasma to eliminate waste products, but, in doing so, must not lose needed materials like glucose or too much water.
Blood enters nephrons via small arteries called afferent arterioles Figure 5. This blood enters the glomerulus a collection of capillaries; Figure 6 under high pressure and 'filters' through the walls of the capillaries and the walls of a surrounding structure called a capsule.
The filtrate that moves from the glomerular capsule into the proximal tubules is basically plasma without protein the protein molecules are too large.
That filtrate, therefore, contains lots of important substances. In the proximal convoluted tubules, those needed substances such as vitamins and glucose are reabsorbed into the the blood Figure 7. Nephron components mammalian type nephron shown: Plasma is filtered from the glomerular capillaries into the glomerular capsule. Filtrate then travels through the tubules and loop of Henle before entering the collecting duct.
Glomeruli of an Anna's Hummingbird. Reabsorption of materials from the proximal convoluted tubule back into the blood. Other than mammals, birds are the only vertebrates that conserve body water by producing urine osmotically more concentrated than the plasma from which it is derived. However, the ability of birds to concentrate urine is limited compared to mammals. Typically, water-deprived birds produce urine that is 1. This 'concentrating capacity' resides within the medullary cones.
Solutes sodium chloride, or NaCl are actively transported out the ascending limb of the Loop of Henle Figure 8 , where they become concentrated in the medulla medullary cones. When urine passes throughout the osmotic gradient in the medulla, water leaves the tubules by osmosis and the urine become concentrated.
Because only the looped nephrons contribute to the intramedullary osmotic gradient, the presence of loopless nephrons may limit the ability of the kidneys to produce hyperosmotic urine. Thus, the concentrating ability of avian kidneys is more limited than in mammals. This reduced capacity of avian kidneys to concentrate urine compared to mammals means that more water accompanies the solutes that travel from the kidneys through ureters to the cloaca.
Water-deprived birds do have a mechanism for reducing the amount of water leaving the kidneys. In response to dehydration, the pituitary gland releases more of a hormone called arginine vasotocin AVT into the blood. In the kidneys, AVT causes a reduction in the glomerular filtration rate the rate at which plasma filters from the glomeruli into the glomerular capsule; Figure 8 so less water moves from the blood into the kidney tubules.
In addition, AVT increases the permeability of the walls of collected ducts to water by opening protein water channels called aquaporins Figure 8. As the collecting ducts become more permeable, more water moves by osmosis out of the collecting ducts because of the higher solute concentration in the medullary cones and can be reabsorbed by kidney capillaries.
Studies to date suggest that the extent to which AVT can reduce urine production and water loss varies among species, but, in general, AVT is less effective in conserving water than the mammalian equivalent antidiuretic hormone, or ADH; Nishimura and Fan Therefore, water-deprived birds tend to produce more urine, and lose more water from the kidneys, than would a similar-sized water-deprived mammal.
A proposed model for urine concentration in the mammalian-type nephron of birds showing transport of NaCl and water in various nephron segments. Left , NaCl is actively exported out of the thick ascending limb TAL and some re-enters the descending limbs DLs by simple diffusion but, despite the resulting high solute concentration, water does not follow because the membrane is not permeable to water.
Loops of Henle exhibit some variation in length and more NaCl is transported out of the longer loops of Henle so the osmotic gradient is greatest near the end of the medullary. AVT arginine vasotocin is a hormone that helps birds conserve water by reducing the glomerular filtration rate the rate at which plasma moves from glomeruli in the glomerular capsules and by increasing the permeability of collecting ducts to water by opening protein water channels called aquaporins; AQP Figure from Nishimura and Fan Nasal respiratory turbinates are complex, epithelially lined structures in nearly all birds and mammals that act as intermittent countercurrent heat exchangers during respiration.
Respiratory turbinates also allow birds to conserve water by helping to 'dehumidify' air during exhalation. During inhalation top , ambient air passes over the respiratory turbinates and is warmed to body temperature. As a result, air is saturated with water vapor, and the turbinates are cooled by evaporative water loss. During exhalation bottom , warm air from the lungs returns through the nasal passage and is cooled as it passes over the turbinate surfaces.
This results in a substantial reduction in the moisture and heat contained in exhaled air. This graph depicts water vapor added to inhaled air shaded arrow and water condensate recovered from exhaled air open arrow.
Hillenius and Ruben Birds that feed on nectar can consume a huge amount of fluid compared to their body weight. And the more dilute the nectar, the higher the volumes ingested. Although the birds derive nutrients and energy from nectar, they have to get rid of the large amounts of water taken in.
Failing to do so can have devastating consequences. Palestine Sunbirds have somehow overcome the problems of life on liquid diet, so McWhorter et al. In the kidney, water is filtered out of blood by specialized structures called glomeruli, and some of the eliminated water is later reabsorbed in the nephron and collecting duct. The researchers set out to test how these processes respond to water intake in Palestine Sunbirds.
Although following the birds around and measuring their nectar intake is difficult, McWhorter and his colleagues came up with an ingenious solution to the problem. They discovered that the birds adjust the amount they consume according to the concentration of sucrose solutions they are fed: In this way, the team could vary the bird's water intake and measure the rates of renal filtration and reabsorption.
McWhorter explained that when the team began investigating this nectarivorous bird's approach to fluid management, it was thought that renal filtration changes according to water status; decreasing in response to water shortage, but increasing only moderately as the birds take on water.
But this was based on ideas developed for birds that do not regularly cope with a large intake of water. McWhorter and his team also knew that when the birds are on dilute diets, water is shunted through the gut without being absorbed. So, how would Palestine Sunbirds' kidneys cope? The team found that renal filtration is not exceptionally sensitive to water loading in sunbirds; it increased only slightly in response to a dramatic decrease in sucrose concentration.
On the other hand, the fractional water reabsorption - a measure of the proportion of the eliminated water that is reabsorbed by the kidney - dropped significantly when the birds were on the most dilute diet. The sunbirds' kidney responds to the elevated water levels by decreasing reabsorption, rather than by raising the filtration rate. The team also found that the glucose and osmotic concentrations in the final excreted fluids were significantly lower than those in the ureteral fluids released by the kidney.
Because the gut and urinary tracts of birds join at the cloaca, the researchers conjecture that the dietary water that shunts through the gut might have diluted the ureteral fluids. They conclude that Palestine Sunbirds deal with large amounts of water intake by not absorbing it in the first place. From an economical standpoint this makes sense, as eliminating water by increasing renal filtration rate can be energetically costly for birds.
Sugar and other metabolites lost during filtration may only be retained by reabsorption, possibly overwhelming the kidney's ability to prevent solute loss. But how the gut could absorb nutrients without taking in dietary water is still a mystery, as the two processes normally come hand in hand Click on the photo to check out Peter Jones' website. Sunbirds and other birds feeding on nectar from aloe erythrina Erythrina livingstoniana flowers.
An important part of the diet of all birds is protein. Proteins are composed of subunits called amino acids, and those amino acids are sometimes used as a source of energy or are converted into fats or carbohydrates.
When amino acids are used for energy or converted to fats or carbohydrates, the amine NH 2 group must be removed. These amine groups are toxic and must be eliminated. Some organisms excrete these nitrogenous wastes as ammonia e. Birds and reptiles excrete these wastes primarily as uric acid. They meet at the acetabulum hip socket and articulate with the femur, which is the first bone of the hind limb. The upper leg consists of the femur. At the knee joint, the femur connects to the tibiotarsus shin and fibula side of lower leg.
The tarsometatarsus forms the upper part of the foot, digits make up the toes. The leg bones of birds are the heaviest, contributing to a low center of gravity, which aids in flight. They have a greatly elongate tetradiate pelvis , similar to some reptiles. The hind limb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk vertebrae as well as fusion with the pectoral girdle. Birds' feet are classified as anisodactyl , zygodactyl , heterodactyl , syndactyl or pamprodactyl.
This is common in songbirds and other perching birds , as well as hunting birds like eagles , hawks , and falcons. Syndactyly, as it occurs in birds, is like anisodactyly, except that the third and fourth toes the outer and middle forward-pointing toes , or three toes, are fused together, as in the belted kingfisher Ceryle alcyon. This is characteristic of Coraciiformes kingfishers , bee-eaters , rollers , etc. This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage.
Zygodactyly occurs in the parrots , woodpeckers including flickers , cuckoos including roadrunners , and some owls. Zygodactyl tracks have been found dating to — Ma early Cretaceous , 50 million years before the first identified zygodactyl fossils. Heterodactyly is like zygodactyly, except that digits three and four point forward and digits one and two point back.
This is found only in trogons , while pamprodactyl is an arrangement in which all four toes may point forward, or birds may rotate the outer two toes backward. It is a characteristic of swifts Apodidae. Most birds have approximately different muscles, mainly controlling the wings, skin, and legs. They provide the powerful wing stroke essential for flight. The muscle medial to underneath the pectorals is the supracoracoideus.
It raises the wing between wingbeats. Both muscle groups attach to the keel of the sternum. This is remarkable, because other vertebrates have the muscles to raise the upper limbs generally attached to areas on the back of the spine. The skin muscles help a bird in its flight by adjusting the feathers, which are attached to the skin muscle and help the bird in its flight maneuvers.
There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. The pygostyle controls all the movement in the tail and controls the feathers in the tail. This gives the tail a larger surface area which helps keep the bird in the air. The scales of birds are composed of keratin, like beaks, claws, and spurs. They are found mainly on the toes and tarsi lower leg of birds , usually up to the tibio-tarsal joint, but may be found further up the legs in some birds.
In many of the eagles and owls the legs are feathered down to but not including their toes. The scales and scutes of birds were originally thought to be homologous to those of reptiles;  however, more recent research suggests that scales in birds re-evolved after the evolution of feathers. Bird embryos begin development with smooth skin.
On the feet, the corneum , or outermost layer, of this skin may keratinize, thicken and form scales. These scales can be organized into;. The rows of scutes on the anterior of the metatarsus can be called an "acrometatarsium" or "acrotarsium". Reticula are located on the lateral and medial surfaces sides of the foot and were originally thought to be separate scales.
However, histological and evolutionary developmental work in this area revealed that these structures lack beta-keratin a hallmark of reptilian scales and are entirely composed of alpha-keratin. The bills of many waders have Herbst corpuscles which help them find prey hidden under wet sand, by detecting minute pressure differences in the water.
However this is more prominent in some birds and can be readily detected in parrots. The region between the eye and bill on the side of a bird's head is called the lore. This region is sometimes featherless, and the skin may be tinted, as in many species of the cormorant family. The beak, bill, or rostrum is an external anatomical structure of birds which is used for eating and for grooming , manipulating objects, killing prey, fighting, probing for food, courtship and feeding young.
Although beaks vary significantly in size, shape and color, they share a similar underlying structure. Two bony projections—the upper and lower mandibles—covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes known as nares lead to the respiratory system. Due to the high metabolic rate required for flight, birds have a high oxygen demand. Their highly effective respiratory system helps them meet that demand. Although birds have lungs, these are fairly rigid structures, which do not expand and contract as they do in mammals, reptiles and many amphibians.
The structures that act as the bellows which ventilate the lungs, are the air sacs distributed throughout much of the birds' bodies. The walls of these air sacs do not have a good blood supply and so do not play a direct role in gas exchange.
They act like a set of bellows  which move air unidirectionally through the parabronchi of the rigid lungs. Birds lack a diaphragm , and therefore use their intercostal and abdominal muscles to expand and contract their entire thoraco-abdominal cavities, thus rhythmically changing the volumes of all their air sacs in unison illustration on the right.
The active phase of respiration in birds is exhalation, requiring contraction of their muscles of respiration. Three distinct sets of organs perform respiration — the anterior air sacs interclavicular, cervicals, and anterior thoracics , the lungs , and the posterior air sacs posterior thoracics and abdominals.
Typically there are nine air sacs within the system;  however, that number can range between seven and twelve, depending on the species of bird. Passerines possess seven air sacs, as the clavicular air sacs may interconnect or be fused with the anterior thoracic sacs.
During inhalation, environmental air initially enters the bird through the nostrils from where it is heated, humidified, and filtered in the nasal passages and upper parts of the trachea. The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi.
Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries or parabronchi where gas exchange occurs. From the dorsobronchi the air flows through the parabronchi and therefore the gas exchanger to the ventrobronchi from where the air can only escape into the expanding anterior air sacs.
So, during inhalation, both the posterior and anterior air sacs expand,  the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" oxygen-poor air that has just passed through the lungs. During exhalation the intrapulmonary bronchi were believed to be tightly constricted between the region where the ventrobronchi branch off and the region where the dorsobronchi branch off. From there the fresh air from the posterior air sacs flows through the parabronchi in the same direction as occurred during inhalation into ventrobronchi.
The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi open up during exhalation, thus allowing oxygen-poor air from these two organs to escape via the trachea to the exterior.
The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system see illustration on the left. The blood capillaries leaving the exchanger near the entrance of airflow take up more O 2 than do the capillaries leaving near the exit end of the parabronchi.
When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air,   but is nevertheless less than half that of the inhaled air,  thus achieving roughly the same systemic arterial blood partial pressure of oxygen as mammals do with their bellows-type lungs.
The trachea is an area of dead space: In comparison to the mammalian respiratory tract , the dead space volume in a bird is, on average, 4. In some birds e. Air passes unidirectionally through the lungs during both exhalation and inspiration, causing, except for the oxygen-poor dead space air left in the trachea after exhalation and breathed in at the beginning of inhalation, little to no mixing of new oxygen-rich air with spent oxygen-poor air as occurs in mammalian lungs , changing only from oxygen-rich to oxygen-poor as it moves unidirectionally through the parabronchi.
Avian lungs do not have alveoli as mammalian lungs do. Instead they contain millions of narrow passages known as parabronchi, connecting the dorsobronchi to the ventrobronchi at either ends of the lungs. Air flows anteriorly caudal to cranial through the parallel parabronchi. These parabronchi have honeycombed walls.
The cells of the honeycomb are dead-end air vesicles, called atria , which project radially from the parabronchi. The atria are the site of gas exchange by simple diffusion. All species of birds with the exception of the penguin, have a small region of their lungs devoted to "neopulmonic parabronchi". This unorganized network of microscopic tubes branches off from the posterior air sacs, and open haphazardly into both the dorso- and ventrobronchi, as well as directly into the intrapulmonary bronchi.
Unlike the parabronchi, in which the air moves unidirectionally, the air flow in the neopulmonic parabronchi is bidirectional. The syrinx is the sound-producing vocal organ of birds, located at the base of a bird's trachea. As with the mammalian larynx , sound is produced by the vibration of air flowing across the organ. The syrinx enables some species of birds to produce extremely complex vocalizations, even mimicking human speech.
In some songbirds, the syrinx can produce more than one sound at a time. Birds have a four-chambered heart ,  in common with mammals, and some reptiles mainly the crocodilia. This adaptation allows for an efficient nutrient and oxygen transport throughout the body, providing birds with energy to fly and maintain high levels of activity.
A ruby-throated hummingbird 's heart beats up to times per minute about 20 beats per second. Many birds possess a muscular pouch along the esophagus called a crop. The crop functions to both soften food and regulate its flow through the system by storing it temporarily. The size and shape of the crop is quite variable among the birds.