General description and location
In both sexes, but especially in females, this layer appears to be a variety of erectile tissue. Journal of the Geological Society. Zygodactyl tracks have been found dating to — Ma early Cretaceous , 50 million years before the first identified zygodactyl fossils. Monocot Stems and Roots Concept A theory accounting for the upward movement of water in plants. A reduction in the adductor chambers has also occurred  These are all conditions seen in the juvenile form of their ancestors. Birds have high-pressure cardiovascular systems like mammals, but have nucleated thrombocytes in their blood rather than platelets.
Some major veins in the avian circulatory system: The jugular veins drain the head and neck. The brachial veins drain the wings. The pectoral veins drain the pectoral muscles and anterior thorax. The superior vena cavae or precavae drain the anterior regions of the body. The inferior vena cava or postcava drains the posterior portion of the body.
The hepatic vein drains the liver. The hepatic portal vein drains the digestive system. The femoral veins drain the legs. The sciatic veins drain the hip or thigh regions. In most species, red blood cells are about 6 x 12 microns in size mammalian RBC's are typically 5.
Typical concentrations are 2. Avian red blood cells have a lifespan of days shorter than mammals, e. Red blood cells contain hemoglobin, the molecule responsible for transporting oxygen throughout the body, and are produced in the bone marrow. However, many bird bones are pneumatic penetrated by air sacs and do not contain marrow. Hemopoietic bone marrow red-blood-cell-producing marrow is located in the radius, ulna, femur, tibiotarsus, scapula, furcula clavicles , pubis,and caudal vertebrae.
Skeleton of a Rock Pigeon Columba livia showing the bones shaded that contain red-blood-cell-producing marrow, including the radius and ulna of the wing, femur and tibiotarsus of the leg, furcula and scapula of the pectoral girdle, pubis of the pelvic girdle, and caudal vertebrae.
Most other bones except for very small ones are pneumatized Schepelmann Under these conditions, natural selection favored the loss of nuclei in the red blood cells of mammals making the cells smaller and allowing capilaries to become even smaller in diameter and change to a biconcave shape increasing the amount of surface area and enhancing diffusion into and out of the red blood cells.
Birds, with their efficient respiratory system, evolved during the Jurassic when the oxygen content in the Earth atmosphere approached the present level, so there was no selective pressure to eliminate nuclei from their red blood cells or change in shape Gavrilov Thrombocytes are important in hemostasis blood clotting.
White blood cells play an important role in protecting birds from infectious agents such as viruses and bacteria. Birds have several types of white blood cells: Avian White Blood Cells The lymphocyte is the most numerous white blood cell. Lymphocytes are either T-lymphocytes formed in the thymus or B-lymphocytes formed in the bursa of Fabricius. B-lymphocytes produce antibodies; T-lymphocytes attack infected or abnormal cells.
The heterophil is the second most numerous WBC in most birds. Heterophils are phagocytic and use their enzyme-containing granules to lyse ingested materials. Heterophils are motile and can leave blood vessels to engulf foreign materials. Monocytes are motile cells that can migrate using ameboid movements. Monocytes are also phagocytic. The function is these cells is unclear.
Scanning electron microscope view of bird thrombocytes adhering to a collagen-lined plate exposure to collagen causes bird thrombocytes, and mammalian platelets, to release chemicals that make them 'sticky'; the chemicals released by mammalian platelets are different from those released by bird thrombocytes and make platelets 'stickier' than thrombocytes.
Avian thrombocytes are larger than mammalian platelets, have a nucleus, and, unlike mammalian platelets, do not form 3-dimensional aggregates. Can birds have heart attacks and strokes? Platelet thrombi that form in the coronary and carotid arteries of humans can also cause common vascular diseases such as myocardial infarction 'heart attacks' and stroke and are the target of drugs used to treat these diseases. Birds have high-pressure cardiovascular systems like mammals, but have nucleated thrombocytes in their blood rather than platelets.
Avian thrombocytes are larger than mammalian platelets and are less 'sticky' because they release different chemicals than mammalian platelets when exposed to collagen connective tissue to which thrombocytes and platelets are exposed when there's a break in a blood vessel. When carotid arteries of mice are damaged, platelets form thrombi that can block blood flow check this video showing the response of human platelets when exposed to a plate covered with collagen ; similar damage to the carotid arteries of Budgerigars similar in size and speed and pressure of blood flow to the carotid arteries of mice did not cause the formation of thrombi check this video showing the response of chicken thrombocytes when exposed to a plate covered with collagen.
These results indicate that mammalian platelets, in contrast to avian thrombocytes, will form thrombi even in arteries where blood flow is rapid and under high pressure, an essential element in human cardiovascular diseases. Heart disease linked to evolutionary changes that may have protected early mammals from trauma. Covariation between relative brain mass of juvenile birds and relative mass of bursa of Fabricius in different bird species.
Relative mass was calculated as residuals from a phylogenetically corrected regression of logtransformed organ mass on logtransformed body mass.
The lines are the linear regression lines for males and females, respectively From: B-lymphocytes, the cells that produce antibodies, are initially produced in the embryonic liver, yolk sac and bone marrow, then move through the blood to the bursa of Fabricius BF. Within the BF, B-lymphocytes mature then migrate to other body tissues. The bursa is a blind sac that extends from the dorsal side of the cloaca, the common portal of the reproductive, urinary, and digestive systems.
Within the bursas of young birds are extensive leaf-like folds composed of simple, columnar epidermis and a loose connective tissue with lots of blood vessels and lymph nodules. Atrophy of the BF typically occurs around the time of sexual maturation. In birds, most of the Ig diversification occurs by gene conversion in the bursa of Fabricius.
However, further Ig diversification is achieved by somatic hypermutation in secondary lymphoid organs From: B-lymphocytes produce three classes of antibodies after exposure to a disease organism: Ig M appears after days following exposure to a disease organism and then disappears by days. Ig A appears after 5 days following exposure. This antibody is found primarily in the mucus secretions of the eyes, gut, and respiratory tract and provides "local" protection to these tissues.
Antibodies do not have the capability to kill viruses or bacteria directly. Antibodies especially IgY perform their function by attaching to disease organisms like bacteria and blocking their receptors. The disease organisms are then prevented from attaching to their target cells. The attached antibodies can also facilitate the destruction of pathogens by phagocytes.
The T-lymphocytes include a more heterogeneous population than the B-cells. Some T-cells act by producing lymphokines over 90 different ones have been identified ; others directly destroy disease organisms. Some T-cells act to enhance the response of B-cells, macrophages, or other T-cells helpers ; others inhibit the activity of these cells suppressors.
Charles Darwin first noted that the choosy peahen plays a crucial role in the evolution of this extravagant sexual display. If this be admitted, there is not much difficulty in understanding how male birds have gradually acquired their ornamental characters," Darwin wrote. Hamilton and Zuk first suggested that 'showy' males were signaling to females that they were, if not parasite-free, then parasite "lite. Moller believes it is because people have been looking at the wrong parasites.
It would be practically impossible, so we decided to focus on the immune system. They discovered that the condition and length of the peacock's tail was related to the production of B-cells, and the size of the eye spots to T-cell production. Males, in effect, are walking billboards advertising their health and status. And these things matter. Previous research has shown that in chickens and quail, at least, the immune system is under genetic control so offspring will inherit their parents' ability to fight parasites.
Thus, it pays for females to be choosy because their chicks, in turn, will survive better and mate with other, equally picky females. However, the physiological mechanisms underlying these demographic patterns of senescence are poorly understood. Immunosenescence, the age-related deterioration of immune function, is well documented in humans and in laboratory models, and often leads to increased morbidity and mortality due to disease.
However, little is known about immunosenescence in free-living organisms. Immune function in female Tree Swallows showed a complex pattern with age; acquired T-cell mediated immunity declined with age, but neither acquired nor innate humoral immunity did.
The bases of these pyramids are irregular, with slender striations extending toward the external kidney surface. The paler, more granular tissue external to the medulla is the cortex. It arches over the bases of the pyramids and fills gaps between the pyramids. Each group of pyramids that projects into a papilla, together with the portion of cortex that arches over the group, is called a renal lobe. The renal sinus includes the renal pelvis , a funnel-shaped expansion of the upper end of the ureter, and, reaching into the kidney substances from the wide end of the funnel, two or three extensions of the cavity called the major calyxes.
The major calyxes are divided in turn into four to 12 smaller cuplike cavities, the minor calyxes , into which the renal papillae project. The renal pelvis serves as the initial reservoir for urine, which flows into the sinus through the urinary collecting tubules, small tubes that open into the sinus at the papillae. The structural units of the kidneys that actually produce urine are the nephrons , of which there are approximately 1,, in each kidney. Each nephron is a long tubule or extremely fine tube that is closed, expanded, and folded into a double-walled cuplike structure at one end.
The capsule and glomerulus together constitute a renal corpuscle , also called a malpighian body. Blood flows into and away from the glomerulus through small arteries arterioles that enter and exit the glomerulus through the open end of the capsule.
This opening is called the vascular pole of the corpuscle. The tubules of the nephrons are 30—55 millimetres 1. The corpuscle and the initial portion of each tubule, called the proximal convoluted tubule , lie in the renal cortex. The tubule descends into a renal pyramid , makes a U-shaped turn, and returns to the cortex at a point near its point of entry into the medulla.
This section of the tubule, consisting of the two parallel lengths and the bend between them, is called the loop of Henle or the nephronic loop. After its reentrance into the cortex, the tubule returns to the vascular pole the opening in the cuplike structure of the capsule of its own nephron. The final portion of the tubule, the distal convoluted tubule , leads from the vascular pole of the corpuscle to a collecting tubule , by way of a short junctional tubule.
Several of the collecting tubules join together to form a somewhat wider tubule, which carries the urine to a renal papilla and the renal pelvis.
Although all nephrons in the kidney have the same general disposition , there are regional differences, particularly in the length of the loops of Henle. Glomeruli that lie deep in the renal cortex near the medulla juxtamedullary glomeruli possess long loops of Henle that pass deeply into the medulla, whereas more superficial cortical glomeruli have much shorter loops.
Among different animal species the length of the loops varies considerably and affects the ability of the species to concentrate urine above the osmotic concentration of plasma. The successive sections of the nephron tubule vary in shape and calibre , and these differences, together with differences in the cells that line the sections, are associated with specific functions in the production of urine.
The intrarenal network of blood vessels forms part of the blood-processing apparatus of the kidneys. The anterior and posterior divisions of each renal artery , mentioned earlier, divide into lobar arteries, each of which enters the kidney substance through or near a renal papilla.
Each lobar artery gives off two or three branches, called interlobar arteries, which run outward between adjacent renal pyramids. When these reach the boundary between the cortex and the medulla they split almost at right angles into branches called arcuate arteries that curve along between the cortex and the medulla parallel to the surface of the kidney. Many arteries, called interlobular arteries , branch off from the arcuate arteries and radiate out through the cortex to end in networks of capillaries in the region just inside the capsule.
En route they give off short branches called the afferent arterioles , which carry blood to the glomeruli where they divide into four to eight loops of capillaries in each glomerulus. Near and before the point where the afferent arteriole enters the glomerulus, its lining layer becomes enlarged and contains secretory granules. This composite structure is called the juxtaglomerular apparatus JGA and is believed to be involved in the secretion of renin see below The role of hormones in renal function.
They are then reconstituted near the point of entry of the afferent arteriole to become the efferent arterioles carrying blood away from the glomeruli. The afferent arterioles are almost twice as thick as the efferent arterioles because they have thicker muscular coats, but the sizes of their channels are almost the same. Throughout most of the cortex the efferent arterioles redivide into a second set of capillaries, which supply blood to the proximal and distal renal tubules.
The efferent glomerular arterioles of juxtaglomerular glomeruli divide into vessels that supply the contiguous tubules and vessels that enter the bases of the renal pyramids. Known as vasa recta, these vessels run toward the apexes of the pyramids in close contact with the loops of Henle. Like the tubules they make hairpin bends, retrace their path, and empty into arcuate veins that parallel the arcuate arteries. Normally the blood circulating in the cortex is more abundant than that in the medulla amounting to over 90 percent of the total , but in certain conditions, such as those associated with severe trauma or blood loss, cortical vessels may become constricted while the juxtamedullary circulation is preserved.
Because the cortical glomeruli and tubules are deprived of blood, the flow of urine is diminished, and in extreme cases may cease. The renal venules small veins and veins accompany the arterioles and arteries and are referred to by similar names.
The venules that lie just beneath the renal capsule , called stellate venules because of their radial arrangement, drain into interlobular venules. In turn these combine to form the tributaries of the arcuate, interlobar, and lobar veins. Blood from the renal pyramids passes into vessels, called venae rectae, which join the arcuate veins. In the renal sinus the lobar veins unite to form veins corresponding to the main divisions of the renal arteries, and they normally fuse to constitute a single renal vein in or near the renal hilus.
Lymphatic capillaries form a network just inside the renal capsule and another, deeper network between and around the renal blood vessels. Few lymphatic capillaries appear in the actual renal substance, and those present are evidently associated with the connective tissue framework, while the glomeruli contain no lymphatics. The lymphatic networks inside the capsule and around the renal blood vessels drain into lymphatic channels accompanying the interlobular and arcuate blood vessels. The main lymph channels run alongside the main renal arteries and veins to end in lymph nodes beside the aorta and near the sites of origin of the renal arteries.
The ureters are narrow, thick-walled ducts, about 25—30 centimetres 9. Throughout their course they lie behind the peritoneum, the lining of the abdomen and pelvis, and are attached to it by connective tissue. In both sexes the ureters enter the bladder wall about five centimetres apart, although this distance is increased when the bladder is distended with urine. The ureters run obliquely through the muscular wall of the bladder for nearly two centimetres before opening into the bladder cavity through narrow apertures.
This oblique course provides a kind of valvular mechanism; when the bladder becomes distended it presses against the part of each ureter that is in the muscular wall of the bladder, and this helps to prevent the flow of urine back into the ureters from the bladder.
The wall of the ureter has three layers, the adventitia, or outer layer; the intermediate, muscular layer; and the lining, made up of mucous membrane.
The adventitia consists of fibroelastic connective tissue that merges with the connective tissue behind the peritoneum. The muscular coat is composed of smooth involuntary muscle fibres and, in the upper two-thirds of the ureter, has two layers—an inner layer of fibres arranged longitudinally and an outer layer disposed circularly. In the lower third of the ureter an additional longitudinal layer appears on the outside of the vessel.
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.