The portal vein or hepatic portal vein is a blood vessel that carries blood from the gastrointestinal tract, gallbladder and spleen to the liver. This blood contains toxins extracted from digested contents. 75% of total liver blood flow is through the portal vein, with the remainder coming from the hepatic artery proper. The blood leaves the liver to the heart in the hepatic veins; the portal vein is not a true vein, because it conducts blood to capillary beds in the liver and not directly to the heart. It is a major component of the hepatic portal system, one of only two portal venous systems in the body – with the hypophyseal portal system being the other; the portal vein is formed by the confluence of the superior mesenteric and splenic veins and receives blood from the inferior mesenteric and right gastric veins, cystic veins. Conditions involving the portal vein cause considerable death. An important example of such a condition is elevated blood pressure in the portal vein; this condition, called portal hypertension, is a major complication of cirrhosis.
Measuring 8 cm in adults, the portal vein is located in the right upper quadrant of the abdomen, originating behind the neck of the pancreas. In most individuals, the portal vein is formed by the union of the superior mesenteric vein and the splenic vein. For this reason, the portal vein is called the splenic-mesenteric confluence; the portal vein directly communicates with the inferior mesenteric vein, although this is variable. Other tributaries of the portal vein include the left and right gastric veins. Before reaching the liver, the portal vein divides into right and left, it ramifies further, forming smaller venous branches and portal venules. Each portal venule courses alongside a hepatic arteriole and the two vessels form the vascular components of the portal triad; these vessels empty into the hepatic sinusoids to supply blood to the liver. The portal venous system has several anastomoses with the systemic venous system. In cases of portal hypertension these anastamoses may become engorged, dilated, or varicosed and subsequently rupture.
Accessory hepatic portal veins are those veins that drain directly into the liver without joining the hepatic portal vein. These include the paraumbilical veins as well as veins of the lesser omentum, falciform ligament, those draining the gallbladder wall; the portal vein and hepatic arteries form the liver's dual blood supply. 75% of hepatic blood flow is derived from the portal vein, while the remainder is from the hepatic arteries. Unlike most veins, the portal vein does not drain into the heart. Rather, it is part of a portal venous system that delivers venous blood into another capillary system, the hepatic sinusoids of the liver. In carrying venous blood from the gastrointestinal tract to the liver, the portal vein accomplishes two tasks: it supplies the liver with metabolic substrates and it ensures that substances ingested are first processed by the liver before reaching the systemic circulation; this accomplishes two things. First, possible toxins that may be ingested can be detoxified by the hepatocytes before they are released into the systemic circulation.
Second, the liver is the first organ to absorb nutrients just taken in by the intestines. After draining into the liver sinusoids, blood from the liver is drained by the hepatic vein. Increased blood pressure in the portal vein, called portal hypertension, is a major complication of liver disease, most cirrhosis. A dilated portal vein is a sign of portal hypertension, with a sensitivity estimated at 12.5% or 40%. On Doppler ultrasonography, the main portal vein peak systolic velocity ranges between 20 cm/s and 40 cm/s. A slow velocity of <16 cm/s in addition to dilatation in the MPV are diagnostic of portal hypertension. Clinical signs of portal hypertension include those of chronic liver disease: ascites, esophageal varices, spider nevi, caput medusae, palmar erythema. Portal vein pulsatility can be measured by doppler ultrasonography. An increased pulsatility may be caused by cirrhosis, as well as increased right atrial pressure. Portal vein pulsatility can be quantified by pulsatility indices, where an index above a certain cutoff indicates pathology: Pylephlebitis is infection of the portal vein arising from an infectious intra-abdominal process such as diverticulosis.
Anatomy photo:38:12-0109 at the SUNY Downstate Medical Center - "Stomach and Liver: The Visceral Surface of the Liver" Anatomy image:7959 at the SUNY Downstate Medical Center Anatomy image:8565 at the SUNY Downstate Medical Center Anatomy image:8697 at the SUNY Downstate Medical Center Cross section image: pembody/body8a—Plastination Laboratory at the Medical University of Vienna figures/chapter_30/30-2. HTM: Basic Human Anatomy at Dartmouth Medical School
The atrium is the upper chamber through which blood enters the heart. There are two atria in the human heart – the left atrium connected to the lungs, the right atrium connected to the venous circulation; the atria receive blood, when the heart muscle contracts they pump blood to the ventricles. All animals with a closed circulatory system have at least one atrium; the atrium used to be called the "auricle", that term is still used to describe this chamber in, for example, the Mollusca, but in humans that name is now used for an appendage of the atrium. Humans have a four-chambered heart consisting of the right atrium, left atrium, right ventricle, left ventricle; the atria are the two upper chambers. The right atrium receives and holds deoxygenated blood from the superior vena cava, inferior vena cava, anterior cardiac veins and smallest cardiac veins and the coronary sinus, which it sends down to the right ventricle which in turn sends it to the pulmonary artery for pulmonary circulation; the left atrium receives the oxygenated blood from the left and right pulmonary veins, which it pumps to the left ventricle for pumping out through the aorta for systemic circulation.
The right atrium and right ventricle are referred to as the right heart and the left atrium and left ventricle are referred to as the left heart. The atria do not have valves at their inlets and as a result, a venous pulsation is normal and can be detected in the jugular vein as the jugular venous pressure. Internally, there are the rough pectinate muscles and crista terminalis of His, which act as a boundary inside the atrium and the smooth walled part of the right atrium, the sinus venarum derived from the sinus venosus; the sinus venarum is the adult remnant of the sinus venous and it surrounds the openings of the venae cavae and the coronary sinus. Attached to the right atrium is the right atrial appendage – a pouch-like extension of the pectinate muscles; the interatrial septum separates the right atrium from the left atrium and this is marked by a depression in the right atrium –the fossa ovalis. The atria are depolarised by calcium. High in the upper part of the left atrium is a muscular ear-shaped pouch – the left atrial appendage.
This appears to "function as a decompression chamber during left ventricular systole and during other periods when left atrial pressure is high". The sinoatrial node is located in posterior aspect of the right atrium, next to the superior vena cava; this is a group of pacemaker cells. The cardiac action potential spreads across both atria causing them to contract, forcing the blood they hold into their corresponding ventricles; the atrioventricular node is another node in the cardiac electrical conduction system. This is located between the ventricles; the left atrium is supplied by the left circumflex coronary artery, its small branches. The oblique vein of the left atrium is responsible for venous drainage. During embryogenesis at about two weeks, a primitive atrium begins to be formed, it begins as one chamber which over the following two weeks becomes divided by the septum primum into the left atrium and the right atrium. The interatrial septum has an opening in the right atrium, the foramen ovale which provides access to the left atrium.
At birth, when the first breath is taken fetal blood flow is reversed to travel through the lungs. The foramen ovale is no longer needed and it closes to leave a depression in the atrial wall. In some cases, the foramen ovale fails to close; this abnormality is present in 25% of the general population. This is known as an atrial septal defect, it is unproblematic, although it can be associated with paradoxical embolization and stroke. Within the fetal right atrium, blood from the inferior vena cava and the superior vena cava flow in separate streams to different locations in the heart, this has been reported to occur through the Coandă effect. In human physiology, the atria facilitate circulation by allowing uninterrupted venous flow to the heart during ventricular systole. By being empty and distensible, atria prevent the interruption of venous flow to the heart that would occur during ventricular systole if the veins ended at the inlet valves of the heart. In normal physiologic states, the output of the heart is pulsatile, the venous inflow to the heart is continuous and non-pulsatile.
But without functioning atria, venous flow becomes pulsatile, the overall circulation rate decreases significantly. Atria have four essential characteristics. There are no atrial inlet valves to interrupt blood flow during atrial systole; the atrial systole contractions are incomplete and thus do not contract to the extent that would block flow from the veins through the atria into the ventricles. During atrial systole, blood not only empties from the atria to the ventricles, but blood continues to flow uninterrupted from the veins right through the atria into the ventricles; the atrial contractions must be gentle enough so that the force of contraction does not exert significant back pressure that would impede venous flow. The "let go" of the atria must be timed so that they relax before the start of ventricular contraction, to be able to accept venous flow without interruption. By preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole, atria allow 75% more cardiac output
Development of the reproductive system
The development of the reproductive system is a part of prenatal development, concerns the sex organs. It is a part of the stages of sexual differentiation; because its location, to a large extent, overlaps the urinary system, the development of them can be described together as the development of the urinary and reproductive organs. The reproductive organs are developed from the intermediate mesoderm; the permanent organs of the adult are preceded by a set of structures which are purely embryonic, which with the exception of the ducts disappear entirely before the end of fetal life. These embryonic structures are the paramesonephric ducts; the mesonephric duct remains as the duct in males, the paramesonephric duct as that of the female. The mesonephric duct originates from a part of the pronephric duct. In the outer part of the intermediate mesoderm under the ectoderm, in the region from the fifth cervical segment to the third thoracic segment, a series of short evaginations from each segment grows dorsally and extends caudally, fusing successively from before backward to form the pronephric duct.
This continues to grow caudally. Thus, the mesonephric duct remains after the atrophy of the pronephros duct. In the male the duct persists, forms the tube of the epididymis, the vas deferens and the ejaculatory duct, while the seminal vesicle arises during the third month as a lateral diverticulum from its hinder end. A large part of the head end of the mesonephros disappears. In the female the mesonephric bodies and ducts atrophy; the nonfunctional remains of the mesonephric tubules are represented by the epoophoron, the paroöphoron, two small collections of rudimentary blind tubules which are situated in the mesosalpinx. The lower part of the mesonephric duct disappears, while the upper part persists as the longitudinal duct of the epoöphoron, called Gartner's duct. There are developments of other tissues from the mesonephric duct that persist, e.g. the development of the suspensory ligament of the ovary. Shortly after the formation of the mesonephric ducts a second pair of ducts is developed.
Each arises on the lateral aspect of the corresponding mesonephric duct as a tubular invagination of the cells lining the abdominal cavity. The orifice of the invagination remains open, undergoes enlargement and modification to form the abdominal ostium of the fallopian tube; the ducts pass backward lateral to the mesonephric ducts, but toward the posterior end of the embryo they cross to the medial side of these ducts, thus come to lie side by side between and behind the latter—the four ducts forming what is termed the common genital cord, to distinguish it from the genital cords of the germinal epithelium seen in this article. The mesonephric ducts end in an epithelial elevation, the sinus tubercle, on the ventral part of the cloaca between the orifices of the mesonephric ducts. At a stage the sinus tubercle opens in the middle, connecting the paramesonephric ducts with the cloaca. In the male the paramesonephric ducts atrophy, but traces of their anterior ends are represented by the appendix of testis of the male), while their terminal fused portions form the prostatic utricle in the floor of the prostatic urethra.
This is due to the production of Anti-Müllerian hormone by the Sertoli cells of the testes. In the female the paramesonephric ducts undergo further development; the portions which lie in the genital cord fuse to form the vagina. This fusion of the paramesonephric ducts begins in the third month, the septum formed by their fused medial walls disappears from below upward; the parts outside this cord remain separate, each forms the corresponding Fallopian tube. The ostium of the fallopian tube remains from the anterior extremity of the original tubular invagination from the abdominal cavity. About the fifth month a ring-like constriction marks the position of the cervix of the uterus, after the sixth month the walls of the uterus begin to thicken. For a time the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornix. At about the fifth or sixth month the lumen of the vagina is produced by the breaking down of the central cells of the epithelium.
The hymen represents the remains of the sinus tubercle. The gonads are the precursors of the testes in ovaries in females, they develop from the mesothelial layer of the peritoneum. The ovary is differentiated into a central part, the medulla of ovary, covered by a surface layer, the germinal epithelium; the immature ova originate from cells from the dorsal endoderm of the yolk sac. Once they have reached the gonadal ridge they are called oogonia. Development proceeds and the oogonia become surrounded by a layer of connective tissue cells. In this way, the rudiments of the ovarian follicles are formed; the embryological origin of granulosa cells, on the other hand, remains controversial. Just as in the male, there is a gubernaculum in the female, which pulls it downward, albeit not as much as in males; the gubernaculum becomes the proper ovarian ligament and the round ligament of the uterus. The periphery of the testes are converted into the tunica albuginea. Cords of the
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood volume and for administration of glucose and drugs; the blood pressure inside the umbilical vein is 20 mmHg. The unpaired umbilical vein carries oxygen and nutrient rich blood derived from fetal-maternal blood exchange at the chorionic villi. More than two-thirds of fetal hepatic circulation is via the main portal vein, while the remainder is shunted from the left portal vein via the ductus venosus to the inferior vena cava being delivered to the fetal right atrium. Closure of the umbilical vein occurs after the umbilical arteries have closed; this prolongs the communication between the placenta and fetal heart, allowing for a sort of autotransfusion of remaining blood from the placenta to the fetus. Within a week of birth, the neonate's umbilical vein is obliterated and is replaced by a fibrous cord called the round ligament of the liver.
It extends from the umbilicus to the transverse fissure, where it joins with the falciform ligament of the liver to separate segment 4 from segments 2 and 3 of the left hepatic lobe. Under extreme pressure, the round ligament may reopen to allow the passage of blood; such recanalization may be evident in patients with cirrhosis and portal hypertension. Patients with cirrhosis experience rapid growth of scar tissue in and around the liver functionally obstructing nearby vessels. Vessel occlusion increases therefore leads to hypertension. In portal hypertension, the vessels surrounding the liver are subjected to abnormally high blood pressure—so high, in fact, that the force of the blood pressing against the round ligament is sufficient to recanalize the structure; this leads to a condition called Caput medusae. A newborn baby has a patent umbilical vein for at least a week after birth; this umbilical vein may be catheterised for ready intravenous access. It may be used as a site for regular transfusion in cases of hemolytic disease.
It provides a route for measuring central venous pressure. Human umbilical vein graft Ductus venosus Gray's s139 - "Peculiarities in the vascular system of the fetus" Embryology at Temple Heart98/heart97a/sld020
A ventricle is one of two large chambers toward the bottom of the heart that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The atrium primes the pump. Interventricular means between the ventricles. In a four-chambered heart, such as that in humans, there are two ventricles that operate in a double circulatory system: the right ventricle pumps blood into the pulmonary circulation to the lungs, the left ventricle pumps blood into the systemic circulation through the aorta. Ventricles generate higher blood pressures; the physiological load on the ventricles requiring pumping of blood throughout the body and lungs is much greater than the pressure generated by the atria to fill the ventricles. Further, the left ventricle has thicker walls than the right because it needs to pump blood to most of the body while the right ventricle fills only the lungs. On the inner walls of the ventricles are irregular muscular columns called trabeculae carneae which cover all of the inner ventricular surfaces except that of the conus arteriosus, in the right ventricle.
There are three types of these muscles. The third type, the papillary muscles give origin at their apices to the chordae tendinae which attach to the cusps of the tricuspid valve and to the mitral valve; the mass of the left ventricle, as estimated by magnetic resonance imaging, averages 143 g ± 38.4 g, with a range of 87–224 g. The right ventricle is equal in size to that of the left ventricle and contains 85 millilitres in the adult, its upper front surface is circled and convex, forms much of the sternocostal surface of the heart. Its under surface is flattened, forming part of the diaphragmatic surface of the heart that rests upon the diaphragm, its posterior wall is formed by the ventricular septum, which bulges into the right ventricle, so that a transverse section of the cavity presents a semilunar outline. Its upper and left angle forms a conical pouch, the conus arteriosus, from which the pulmonary artery arises. A tendinous band, called the tendon of the conus arteriosus, extends upward from the right atrioventricular fibrous ring and connects the posterior surface of the conus arteriosus to the aorta.
The left ventricle is longer and more conical in shape than the right, on transverse section its concavity presents an oval or nearly circular outline. It forms a small part of the sternocostal surface and a considerable part of the diaphragmatic surface of the heart; the left ventricle is thicker and more muscular than the right ventricle because it pumps blood at a higher pressure. The right ventricle is triangular in shape and extends from the tricuspid valve in the right atrium to near the apex of the heart, its wall is thickest at the apex and thins towards its base at the atrium. By early maturity, the walls of the left ventricle have thickened from three to six times greater than that of the right ventricle; this reflects the typical five times greater pressure workload this chamber performs while accepting blood returning from the pulmonary veins at ~80mmHg pressure and pushing it forward to the typical ~120mmHg pressure in the aorta during each heartbeat. During systole, the ventricles contract.
During diastole, the ventricles fill with blood again. The left ventricle receives oxygenated blood from the left atrium via the mitral valve and pumps it through the aorta via the aortic valve, into the systemic circulation; the left ventricular muscle must relax and contract and be able to increase or lower its pumping capacity under the control of the nervous system. In the diastolic phase, it has to relax quickly after each contraction so as to fill with the oxygenated blood flowing from the pulmonary veins. In the systolic phase, the left ventricle must contract and forcibly to pump this blood into the aorta, overcoming the much higher aortic pressure; the extra pressure exerted is needed to stretch the aorta and other arteries to accommodate the increase in blood volume. The right ventricle receives deoxygenated blood from the right atrium via the tricuspid valve and pumps it into the pulmonary artery via the pulmonary valve, into the pulmonary circulation; the typical healthy adult heart pumping volume is ~5 liters/min, resting.
Maximum capacity pumping volume extends from ~25 liters/min for non-athletes to as high as ~45 liters/min for Olympic level athletes. In cardiology, the performance of the ventricles are measured with several volumetric parameters, including end-diastolic volume, end-systolic volume, stroke volume and ejection fraction. Ventricular pressure is a measure of blood pressure within the ventricles of the heart. During most of the cardiac cycle, ventricular pressure is less than the pressure in the aorta, but during systole, the ventricular pressure increases, the two pressures become equal to each other, the aortic valve opens, blood is pumped to the body. Elevated left ventricular end-diastolic pressure has been described as a risk factor in cardiac surgery. Noninvasive approximations have been described. An elevated pressure difference between the aortic pressure and the left ventricular pressure may be indicative of aortic stenosis. Right
Livestock is defined as domesticated animals raised in an agricultural setting to produce labor and commodities such as meat, milk, fur and wool. The term is sometimes used to refer to those that are bred for consumption, while other times it refers only to farmed ruminants, such as cattle and goats. Horses are considered livestock in the United States; the USDA uses livestock to some uses of the term “red meat”, in which it refers to all the mammal animals kept in this setting to be used as commodities. The USDA mentions pork, veal and lamb are all classified as livestock and all livestock is considered to be red meats. Poultry and fish are not included in the category; the breeding and slaughter of livestock, known as animal husbandry, is a component of modern agriculture, practiced in many cultures since humanity's transition to farming from hunter-gatherer lifestyles. Animal husbandry practices have varied across cultures and time periods. Livestock were not confined by fences or enclosures, but these practices have shifted to intensive animal farming, sometimes referred to as "factory farming".
Now, over 99% of livestock are raised on factory farms. These practices increase yield of the various commercial outputs, but have led to negative impacts on animal welfare and the environment. Livestock production continues to play a major economic and cultural role in numerous rural communities. Livestock as a word was first used between 1650 and 1660, as a merger between the words "live" and "stock". In some periods, "cattle" and "livestock" have been used interchangeably. Today, the modern meaning of cattle is domesticated bovines. United States federal legislation defines the term to make specified agricultural commodities eligible or ineligible for a program or activity. For example, the Livestock Mandatory Reporting Act of 1999 defines livestock only as cattle and sheep, while the 1988 disaster assistance legislation defined the term as "cattle, goats, poultry, equine animals used for food or in the production of food, fish used for food, other animals designated by the Secretary."Deadstock is defined in contradistinction to livestock as "animals that have died before slaughter, sometimes from illness".
It is illegal in many countries, such as Canada, to sell or process meat from dead animals for human consumption. Animal-rearing originated during the cultural transition to settled farming communities from hunter-gatherer lifestyles. Animals are domesticated when their living conditions are controlled by humans. Over time, the collective behaviour and physiology of livestock have changed radically. Many modern farm animals are unsuited to life in the wild; the dog was domesticated early. Goats and sheep were domesticated in multiple events sometime between 11,000 and 5,000 years ago in Southwest Asia. Pigs were domesticated by 8,500 BC in the Near 6,000 BC in China. Domestication of the horse dates to around 4000 BC. Cattle have been domesticated since 10,500 years ago. Chickens and other poultry may have been domesticated around 7000 BC; the term "livestock" is may be defined narrowly or broadly. Broadly, livestock refers to any breed or population of animal kept by humans for a useful, commercial purpose.
This can mean semidomestic animals, or captive wild animals. Semidomesticated refers to animals which are only domesticated or of disputed status; these populations may be in the process of domestication. Traditionally, animal husbandry was part of the subsistence farmer's way of life, producing not only the food needed by the family but the fuel, clothing and draught power. Killing the animal for food was a secondary consideration, wherever possible its products, such as wool, eggs and blood were harvested while the animal was still alive. In the traditional system of transhumance and livestock moved seasonally between fixed summer and winter pastures. Animals can be kept intensively. Extensive systems involve animals roaming at will, or under the supervision of a herdsman for their protection from predators. Ranching in the Western United States involves large herds of cattle grazing over public and private lands. Similar cattle stations are found in South America and other places with large areas of land and low rainfall.
Ranching systems have been used for sheep, ostrich, emu and alpaca. In the uplands of the United Kingdom, sheep are turned out on the fells in spring and graze the abundant mountain grasses untended, being brought to lower altitudes late in the year, with supplementary feeding being provided in winter. In rural locations and poultry can obtain much of their nutrition from scavenging, in African communities, hens may live for months without being fed, still produce one or two eggs a week. At the other extreme, in the more developed parts of the world, animals are intensively managed. In between these two extremes are semi-intensive family run farms where livestock graze outside for much of the year, silage or hay is made to cove