General description and location
Ammonotely in a passerine nectarivore: Typical concentrations are 2. Water runs between the beak tips as a result of adhesion and capillary action. 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. The size of an animal is also a factor in determining diet type Allen's rule.
Human excretory organs
The process of cytokinesis in animal cells, characterized by pinching of the plasma membrane; specifically, the succession of rapid cell divisions without growth during early embryonic development that converts the zygote into a ball of cells. The first sign of cleavage in an animal cell; a shallow groove in the cell surface near the old metaphase plate. Also, see gene cloning.
An agent used to transfer DNA in genetic engineering, such as a plasmid that moves recombinant DNA from a test tube back into a cell, or a virus that transfers recombinant DNA by infection. Most vitamins function as coenzymes in important metabolic reactions. Any nonprotein molecule or ion that is required for the proper functioning of an enzyme. Cofactors can be permanently bound to the active site or may bind loosely with the substrate during catalysis.
A theory accounting for the upward movement of water in plants. According to this theory, transpiration of a water molecule results in a negative below 1 atmosphere pressure in the leaf cells, inducing the entrance from the vascular tissue of another water molecule, which, because of the cohesive property of water, pulls with it a chain of water molecules extending up from the cells of the root tip.
The process by which plants increase their tolerance to freezing by exposure to low, nonfreezing temperatures. The location in the kidney where filtrate from renal tubules is collected; the filtrate is now called urine. All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.
A type of plant cell that is connected to a sieve-tube member by many plasmodesmata and whose nucleus and ribosomes may serve one or more adjacent sieve-tube members. Interaction between members of the same population or of two or more populations using the same resource, often present in limited supply.
The concept that when the populations of two species compete for the same limited resources, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population. A substance that reduces the activity of an enzyme by entering the active site in place of the substrate whose structure it mimics.
A group of at least 20 blood proteins that cooperate with other defense mechanisms; may amplify the inflammatory response, enhance phagocytosis, or directly lyse pathogens; activated by the onset of the immune response or by surface antigens on microorganisms or other foreign cells.
A digestive tube that runs between a mouth and an anus; also called alimentary canal. An incomplete digestive tract has only one opening. A type of multifaceted eye in insects and crustaceans consisting of up to several thousand light-detecting, focusing ommatidia; especially good at detecting movement. A regular increase of decrease in the intensity or density of a chemical substance.
When a gradient exists, the ions or other chemical substances involved tend to move from where they are more concentrated to where they are less concentrated. A reaction in which two molecules become covalently bonded to each other through the loss of a small molecule, usually water; also called dehydration reaction.
A gymnosperm whose reproductive structure is the cone. Conifers include pines, firs, redwoods, and other large trees.
Animal tissue that functions mainly to bind and support other tissues, having a sparse population of cells scattered through an extracellular matrix.
A goal-oriented science that seeks to counter the biodiversity crisis, the current rapid decrease in Earth's variety of life. A heterotroph that derives its energy from living or freshly killed organisms or parts thereof.
Primary consumers are herbivores; higher-level consumers are carnivores. The gradual movement of the Earth's continents that has occurred over hundreds of millions of years. A gradation of small differences in a particular trait, such as height, within a population; occurs in traits that are controlled by a number of genes.
A steroid hormone, produced by the adrenal cortex, that promotes the formation of glucose from protein and fat; also suppresses the inflammatory and immune responses. The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient. The opposite flow of adjacent fluids that maximizes transfer rates; for example, blood in the gills flows in the opposite direction in which water passes over the gills, maximizing oxygen uptake and carbon dioxide loss.
In cells, the linking of endergonic energy-requiring reactions to exergonic energy-releasing reactions that provide enough energy to drive the endergonic reactions forward. A process by which some species of plants in hot, dry climates take in carbon dioxide during the night, fixing it in organic acids; the carbon dioxide is released during the day and used immediately in the Calvin cycle.
The reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I. Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule second messenger in eukaryotic cells, for example, in vertebrate endocrine cells. It is also a regulator of some bacterial operons. In the vertebrate immune system, protein factors secreted by macrophages and helper T cells as regulators of neighboring cells.
In animal development, substances deposited by the mother in the eggs she produces that regulate the expression of genes affecting the early development of the embryo. A circular flow of cytoplasm, involving myosin and actin filaments, that speeds the distribution of materials within cells. Movement of Molecules in Solution Closer Look: Concentration Gradient Concept 4: Movement of Molecules and Cells Concept 5: Water Potential Concept 7: Calculating Water Potential Concept 8: Water Potential and Potato Cores Exercise 4: Water Potential Exercise 5: Osmosis Analysis of Results Lab Quiz.
Enzyme Structure Concept 2: Binding Specificity Concept 3: Induced Fit Concept 4: The Cell Cycle Closer Look: Nuclear Division Karyokinesis Closer Look: Cytoplasmic Division Cytokinesis Concept 2: Spore Formation in Sordaria Exercise 1: The Process of Respiration Closer Look: Features and Functions of a Respirometer Exercise 2: How the Respirometer Works Exercise 3: How to Read a Pipette Exercise 4: Assembling the Respirometer Exercise 5: More Information on Germinating Peas Exercise 6: Bacterial Colonies Concept 2: Transformation Procedure Closer Look: Step 1 Closer Look: Step 2 Closer Look: Step 3 Closer Look: Step 4 Closer Look: Step 6 Exercise 2: Transformation Procedure Animation Exercise 3: How Do Restriction Enzymes Work?
Preparing the Gels Exercise 2: Loading the Gel Exercise 3: Filling the Wells Exercise 4: Running the Gel Exercise 6: Parental Generation Exercise 2: A Large Breeding Population Concept 2: Random Mating Concept 3: No Immigration or Emigration Concept 5: No Natural Selection Concept 6: Estimating Allelic Frequency Concept 7: The Hardy-Weinberg Equation Concept 8: Sample Problem 1 Concept 9: Sample Problem 2 Concept Sample Problem 3 Concept Hydrogen Bonding Concept 2: How Do Guard Cells Function?
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. The avian stomach is composed of two organs, the proventriculus and the gizzard that work together during digestion. The proventriculus is a rod shaped tube, which is found between the esophagus and the gizzard, that secretes hydrochloric acid and pepsinogen into the digestive tract. The gizzard is composed of four muscular bands that rotate and crush food by shifting the food from one area to the next within the gizzard.
The gizzard of some species of herbivorous birds, like turkey and quails,  contains small pieces of grit or stone called gastroliths that are swallowed by the bird to aid in the grinding process, serving the function of teeth.
The use of gizzard stones is a similarity found between birds and dinosaurs , which left gastroliths as trace fossils. The partially digested and pulverized gizzard contents, now called a bolus, are passed into the intestine , where pancreatic and intestinal enzymes complete the digestion of the digestible food.
The digestion products are then absorbed through the intestinal mucosa into the blood. The intestine ends via the large intestine in the vent or cloaca which serves as the common exit for renal and intestinal excrements as well as for the laying of eggs. There are three general ways in which birds drink: Fluid is also obtained from food. Most birds are unable to swallow by the "sucking" or "pumping" action of peristalsis in their esophagus as humans do , and drink by repeatedly raising their heads after filling their mouths to allow the liquid to flow by gravity, a method usually described as "sipping" or "tipping up".
The only other group, however, which shows the same behavior, the Pteroclidae , is placed near the doves just by this doubtlessly very old characteristic. Although this general rule still stands, since that time, observations have been made of a few exceptions in both directions. In addition, specialized nectar feeders like sunbirds Nectariniidae and hummingbirds Trochilidae drink by using protrusible grooved or trough-like tongues, and parrots Psittacidae lap up water.
Many seabirds have glands near the eyes that allow them to drink seawater. Excess salt is eliminated from the nostrils. Many desert birds get the water that they need entirely from their food. The elimination of nitrogenous wastes as uric acid reduces the physiological demand for water,  as uric acid is not very toxic and thus does not need to be diluted in as much water. Male birds have two testes which become hundreds of times larger during the breeding season to produce sperm.
Some species of birds have two functional ovaries, and the order Apterygiformes always retain both ovaries. Most male birds have no phallus. In the males of species without a phallus, sperm is stored in the seminal glomera within the cloacal protuberance prior to copulation.
During copulation , the female moves her tail to the side and the male either mounts the female from behind or in front as in the stitchbird , or moves very close to her. The cloacae then touch, so that the sperm can enter the female's reproductive tract.
This can happen very fast, sometimes in less than half a second. The sperm is stored in the female's sperm storage tubules for a period varying from a week to more than days,  depending on the species.
Then, eggs will be fertilized individually as they leave the ovaries, before the shell is calcified in the oviduct. After the egg is laid by the female, the embryo continues to develop in the egg outside the female body. Many waterfowl and some other birds, such as the ostrich and turkey , possess a phallus. This appears to be the primitive condition among birds, most birds have lost the phallus.
These vaginal structures may be used to prevent penetration by the male phallus which coils counter-clockwise. In these species, copulation is often violent and female co-operation is not required; the female ability to prevent fertilization may allow the female to choose the father for her offspring. After the eggs hatch, parents provide varying degrees of care in terms of food and protection. Precocial birds can care for themselves independently within minutes of hatching; altricial hatchlings are helpless, blind, and naked, and require extended parental care.
The chicks of many ground-nesting birds such as partridges and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole-nesters though, are often totally incapable of unassisted survival.
The process whereby a chick acquires feathers until it can fly is called "fledging". Some birds, such as pigeons, geese, and red-crowned cranes , remain with their mates for life and may produce offspring on a regular basis.
Avian kidneys function in almost the same way as the more extensively studied mammalian kidney, but with a few important adaptations; while much of the anatomy remains unchanged in design, some important modifications have occurred during their evolution. A bird has paired kidneys which are connected to the lower gastrointestinal tract through the ureters. Blood vessels and other tubes make up the remaining mass.
Unique to birds is the presence of two different types of nephrons the functional unit of the kidney both reptilian-like nephrons located in the cortex and mammalian-like nephrons located in the medulla. Reptilian nephrons are more abundant but lack the distinctive loops of Henle seen in mammals. The urine collected by the kidney is emptied into the cloaca through the ureters and then to the colon by reverse peristalsis.
Birds have acute eyesight—raptors birds of prey have vision eight times sharper than humans—thanks to higher densities of photoreceptors in the retina up to 1,, per square mm in Buteos , compared to , for humans , a high number of neurons in the optic nerves , a second set of eye muscles not found in other animals, and, in some cases, an indented fovea which magnifies the central part of the visual field.
Many species, including hummingbirds and albatrosses , have two foveas in each eye. Many birds can detect polarised light. The avian ear is adapted to pick up on slight and rapid changes of pitch found in bird song. General avian tympanic membrane form is ovular and slightly conical. Morphological differences in the middle ear are observed between species.
Ossicles within green finches, blackbirds, song thrushes, and house sparrows are proportionately shorter to those found in pheasants, Mallard ducks, and sea birds. In song birds, a syrinx allows the respective possessors to create intricate melodies and tones. The middle avian ear is made up of three semicircular canals, each ending in an ampulla and joining to connect with the macula sacculus and lagena, of which the cochlea, a straight short tube to the external ear, branches from.
Birds have a large brain to body mass ratio. This is reflected in the advanced and complex bird intelligence. The immune system of birds resembles that of other animals. Birds have both innate and adaptive immune systems. Birds are susceptible to tumours , immune deficiency and autoimmune diseases. The bursa of fabricius , also known as the cloacal bursa, is a lymphoid organ which aids in the production of B lymphocytes during humoral immunity.
The bursa of fabricius is present during juvenile stages but curls up, and in the sparrow is not visible after the sparrow reaches sexual maturity. The bursa of fabricius is a circular pouch connected to the superior dorsal side of the cloaca.
The bursa is composed of many folds, known as plica, which are lined by more than 10, follicles encompassed by connective tissue and surrounded by mesenchyme.
The large veins carrying blood from the kidneys usually lie in front of the corresponding arteries and join the inferior vena cava almost at right angles. The left vein is longer than the right vein because the inferior vena cava lies closer to the right kidney.
The kidneys are supplied with sympathetic and parasympathetic nerves of the autonomic nervous system , and the renal nerves contain both afferent and efferent fibres afferent fibres carry nerve impulses to the central nervous system; efferent fibres, from it. A cross section of a kidney reveals the renal sinus and two layers of kidney tissue distinguishable by their texture and colour.
The innermost tissue, called the renal medulla , forms comparatively dark cones, called renal pyramids , with bases outward and apexes projecting, either singly or in groups, into the renal sinus.
Each projection of one or more pyramid apexes into the sinus is known as a renal papilla. 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.