Health Guide

Circulation

Circulation is the process by which nutrients, respiratory gases, and metabolic products are transported throughout the human organs, permitting integration among the various tissues.

The process includes the intake of metabolic materials, the conveyance of these materials throughout the tissues and organs, and the return of harmful by-products to the environment. The human body has a great variety of liquids, cells, and modes of circulation, though what is called an open system, in which fluid passes more or less freely throughout the tissues or defined areas of tissue.

The human body, has a closed system that is, our circulatory system transmits fluid through an intricate network of 1000 km (60,000 mi) of blood vessels, veins and arteries. This system contains two fluids, blood and lymph, and functions by means of two interacting modes of circulation, the cardiovascular system and the lymphatic system; both the fluid components and the vessels through which they flow reach their greatest elaboration and specialization in the in the human body. Main features of circulatory systems The human body takes in molecules from the environments, use them to support the metabolism of their own substance, and release by-products back into the environment. The internal environment differs more or less greatly from the external environment.

It is normally maintained at constant conditions by the mind-body so that it is subject to relatively minor fluctuations. In individual cells, the tissues of our multicellular system, molecules are taken in either by their direct diffusion through the cell wall or by the formation by the surface membrane of vacuoles that carry some of the environmental fluid containing dissolved molecules. Within the cell, cyclosis (streaming of the fluid cytoplasm) distributes the metabolic products. Molecules are normally conveyed between cells and throughout the body multicellular organs in a circulatory blood stream through special channels, blood vessels, by heart. In our mind-body, blood and lymph (the circulating fluids) have an essential role in maintaining homeostasis (the constancy of the internal environment) by distributing substances to parts of the body when required and by removing others from areas in which their accumulation would be harmful. The human members have three layers of cells), with the third cellular layer, called the mesoderm, developing between the endoderm and ectoderm.

At its simplest, the mesoderm provides a network of packing cells around our organs… Circulating fluid is not confined to distinct vessels, and it more or less freely bathes the tissues and organs directly. The functions of both circulating and tissue fluid are thus combined in the fluid, often known as hemolymph. The possession of blood supply and coelom, however, does not exclude the circulation of environmental water through the mind-body. An internal circulatory system transports essential gases and nutrients around the body and removes unwanted products of metabolism from the tissues, and carries these products to specialized excretory organs. Nutrient and oxygen circulate throughout our internal fluid (blood) to feed our cells, tissues and organs. There may also be external circulation that sets up currents in the environmental fluid to carry it over respiratory surfaces. Additionally, the circulatory system may assist the organism in movement… The fluid compartments of the human body are the intracellular and extracellular components.

The intracellular component includes the body cells and blood cells, while the extracellular component includes the tissue fluid, coelomic fluid, and blood plasma. In all cases the major constituent is water derived from our environment. The composition of the fluid varies markedly depending on its source and is regulated more or less precisely by homeostasis. Blood and coelomic fluid are often physically separated by the blood-vessel walls; where a hemocoel (a blood-containing body cavity) exists, however, blood rather than coelomic fluid occupies the cavity. The composition of blood may vary from the environmental water containing small amounts of dissolved nutrients and gases to the highly complex tissue containing many cells. Lymph essentially consists of blood plasma that has left the blood vessels and has passed through the tissues. It is generally considered to have a separate identity when it is returned to the bloodstream through a series of vessels independent of the blood vessels and the coelomic space.

Coelomic fluid itself may circulate in the body cavity. In most cases this circulation has an apparently random nature, mainly because of movements of the body and organs… The heart is a specialized, chambered, muscular pump that receives blood under low pressure and returns it under higher pressure to the circulation. Where the flow of blood is in one direction, as is normally the case, valves in the form of flaps of tissue prevent backflow. A characteristic feature of hearts is that they pulsate throughout life and any prolonged cessation of heartbeat is fatal. Contractions of the heart muscle may be initiated in one of two ways. In the first, the heart muscle may have an intrinsic contractile property that is independent of the nervous system, is the myogenic contraction. In the second, the heart is stimulated by nerve impulses from outside the heart muscle. Chambered hearts consist of a series of interconnected muscular compartments separated by valves.

The first chamber, the auricle, acts as a reservoir to receive the blood that then passes to the second and main pumping chamber, the ventricle. Expansion of a chamber is known as diastole and contraction as systole. As one chamber undergoes systole the other undergoes diastole, thus forcing the blood forward. The series of events during which blood is passed through the heart is known as the cardiac cycle. Contraction of the ventricle forces the blood into the vessels under pressure, known as the blood pressure. As contraction continues in the ventricle, the rising pressure is sufficient to open the valves that had been closed because of attempted reverse blood flow during the previous cycle. At this point the ventricular pressure transmits a high-speed wave, the pulse, through the blood of the arterial system. The volume of blood pumped at each contraction of the ventricle is known as the stroke volume, and the output is usually dependent on the human activity. After leaving the heart, the blood passes through a series of branching vessels of steadily decreasing diameter.

The smallest branches, only a few micrometres (there are about 25,000 micrometres in one inch) in diameter, are the capillaries, which have thin walls through which the fluid part of the blood may pass to bathe the tissue cells. The capillaries also pick up metabolic end products and carry them into larger collecting vessels that eventually return the blood to the heart. There are structural differences between the muscularly walled arteries, which carry the blood under high pressure from the heart, and the thinner walled veins, which return it at much reduced pressure. Although such structural differences are less apparent; the terms artery and vein are used for vessels that carry blood from and to the heart, respectively. To maintain optimum metabolism, all living cells require a suitable environment, which must be maintained within relatively narrow limits. An appropriate gas phase (i.e., suitable levels of oxygen and other gases), an adequate and suitable nutrient supply, and a means of disposal of unwanted products are all essential. For most human cells, the supply of oxygen is largely independent of our action and therefore is a limiting factor in its metabolism and ultimately in its structure and distribution.

The nutrient supply to the tissues, however, is controlled by the individual’s intake and, because both major catabolic end products of metabolism—ammonia (NH3) and carbon dioxide (CO2)—are more soluble than oxygen (O2) in water and the aqueous phase of the body fluids, they tend not to limit metabolic rates. The diffusion rate of CO2 is less than that of O2, but its solubility is 30 times that of oxygen. This means that the amount of CO2 diffusing is 26 times as high as for oxygen at the same temperature and pressure. The oxygen available to a cell depends on the concentration of oxygen in the external environment and the efficiency with which it is transported to the tissues. Dry air at atmospheric pressure contains about 21 percent oxygen, the percentage of which decreases with increasing altitude. Well-aerated water has the same percentage of oxygen as the surrounding air; however, the amount of dissolved oxygen is governed by temperature and the presence of other solutes. For example, seawater contains 20 percent less oxygen than fresh water under the same conditions. The rate of diffusion depends on the shape and size of the diffusing molecule, the medium through which it diffuses the concentration gradient, and the temperature. These physicochemical constraints imposed by gaseous diffusion have a relationship with respiration.

The level of organization of sponges is that of a coordinated aggregation of largely independent cells with poorly defined tissues and no organ systems. The whole animal has a relatively massive surface area for gaseous exchange, and all cells are in direct contact with the passing water current. Blood consists of aqueous plasma containing sodium, potassium, calcium, magnesium, chloride, and sulfate ions; some trace elements; a number of amino acids; and possibly a protein known as a respiratory pigment. Respiratory pigments are normally dissolved in the plasma and are not enclosed in blood cells. The constancy of the ionic constituents of blood and their similarity to seawater has been used by some scientists as evidence of a common origin for life in the sea. Human’s ability to control gross blood concentration (i.e., the overall ionic concentration of the blood) largely governs its ability to tolerate environmental changes. In addition to maintaining the overall stability of the internal environment, blood has a range of other functions. It is the major means of transport of nutrients, metabolites, excretory products, hormones, and gases.

The oxygen carriers in blood take the form of metal-containing protein molecules that frequently are coloured and thus commonly known as respiratory pigments. The most widely distributed respiratory pigments are the red hemoglobins. Hemoglobins consist of a variable number of subunits, each containing an iron–porphyrin group attached to a protein. The green chlorocruorins are also iron–porphyrin pigments. The third iron-containing pigments, the hemerythrins, are violet. They differ structurally from both hemoglobin and chlorocruorin in having no porphyrin groups and containing three times as much iron, which is attached directly to the protein. The presence of a respiratory pigment greatly increases the oxygen-carrying capacity of blood. All respiratory pigments become almost completely saturated with oxygen even at oxygen levels, or pressures, below those normally found in air or water. The oxygen pressures at which the various pigments become saturated depend on their individual chemical characteristics and on such conditions as temperature, pH, and the presence of carbon dioxide. In addition to their direct transport role, respiratory pigments may temporarily store oxygen for use during periods of respiratory suspension or decreased oxygen availability (hypoxia).

They may also act as buffers to prevent large blood pH fluctuations, and they may have an osmotic function that helps to reduce fluid loss from aquatic organisms whose internal hydrostatic pressure tends to force water out of the body. All systems involving the consistent movement of circulating fluid require repeating pumping system; and if flow is to be in one direction, usually some arrangement of valves to prevent backflow. The direction of flow is controlled by valves between the chambers. The filling and emptying of the heart are controlled by regular rhythmical contractions of the muscular walls of the heart. Control of heart rhythm may be either myogenic (originating within the heart muscle itself) or neurogenic (originating in nerve ganglia). Although under experimental conditions acetylcholine (a substance that transmits nerve impulses across a synapse) inhibits molluscan heartbeat, indicating some stimulation of the heart muscle by the nervous system, cardiac muscle contraction will continue in excised hearts with no connection to the central nervous system.

The rate increases with internal pressure often reaches a plateau at optimal pressures. Normally, increasing the body temperature 10° C (50° F) causes an increase in heart rate of two to three times. Oxygen availability and the presence of carbon dioxide affect the heart rate, and during periods of hypoxia the heart rate may decrease to almost a standstill to conserve oxygen stores. The time it takes for blood to complete a single circulatory cycle is also highly variable. The heart lies below the alimentary canal in the front and centre of the chest, housed in its own section of the body cavity. During the development of an embryo, the heart first appears below the pharynx. The heart is basically a tube made of special muscle (cardiac muscle) that is not found anywhere else in the body. This cardiac muscle beats throughout life with its own automatic rhythm. Deoxygenated blood from the body is brought by veins into the most posterior part of the heart tube, the sinus venosus. From there it passes forward into the atrium, the ventricle, and the conus arteriosus (called the bulbus cordis in embryos), and eventually to the arterial system. The blood is pushed through the heart because the various parts of the tube contract in sequence.

As the heart develops from embryo to adult, each part of the tube becomes a chamber, separated from the others by valves, so that blood can neither flow backward in the system nor reenter the heart from the arteries. As the heart grows, it bends into an “S” shape, so that the sinus venosus and atrium lie above the ventricle and conus arteriosus. Deoxygenated blood collects in capillaries and then drains into larger and larger veins, which take it from various parts of the body to the heart. Of these, the anterior and posterior cardinal veins, each with left and right components, take blood to the heart from the front and rear of the body, respectively. They lie dorsal to the alimentary canal, while the heart lies ventral to it. There is a common cardinal vein on each side, often called the duct of Cuvier, which carries blood ventrally into the sinus venosus. Various other veins join the cardinal veins from all over the body. The ventral jugular veins drain the lower part of the head and take blood directly into the common cardinal veins.

Lower vertebrates have two so-called portal systems, areas of the venous system that begin in capillaries in tissues and join to form veins, which divide to produce another capillary network en route to the heart. They are called the hepatic (liver) and renal (kidneys) portal systems. The hepatic system is important because it collects blood from the intestine and passes it to the liver, the centre for many chemical reactions concerned with the absorption of food into the body and the control of substances entering the general circulation. The function of the renal portal system is less clear, but it involves two veins that pass from the caudal vein to the kidneys, where they break up into capillaries. The coronary circulation is that which supplies the heart muscle itself. It is of crucial importance, for the heart must never stop beating. Cardiac muscle needs oxygen from early in embryonic development until death. Coronary blood supply comes from the aorta, close to the heart. The heart obtains its oxygen from blood passing through it. In the lamprey heart the atrium and ventricle are side by side, with the sinus venosus entering the atrium laterally. Nonmuscular valves prevent backflow of blood, and the conus arteriosus contains no cardiac muscle. There is no separate coronary blood supply, and the heart must obtain its oxygen from the blood as it goes through.

The venous system does not include a renal portal section, and there is asymmetry of the common cardinal veins, which take blood from the dorsal anterior and posterior cardinal veins down to the ventral heart. In embryos there are two of these, one on each side of the body. Our lungs exchange carbon dioxide for oxygen from the air. Blood passes in sequence through the sinus venosus, atrium, ventricle, and conus arteriosus. The ventricle is the main pumping chamber in the hearts. Modern amphibians are characterized by the flexibility of their gaseous exchange mechanisms. Amphibian skin is moistened by mucous secretions and is well supplied with blood vessels. It is used for respiration to varying degrees. When lungs are present, carbon dioxide may pass out of the body across the skin, but in some salamanders there are no lungs and all respiratory exchanges occur via the skin. Even in such animals as frogs, it seems that oxygen can be taken up at times by the skin, under water for example. Therefore, regulation of respiration occurs within a single species, and the relative contribution of skin and lungs varies during the life of the animal. The amphibian heart is generally of a tripartite structure, with a divided atrium but a single ventricle. The lungless salamanders, however, have no atrial septum, and one small and unfamiliar group, the caecilians, has signs of a septum in the ventricle.

It is not known whether the original amphibians had septa in both atrium and ventricle. They may have, and the absence of septa in many modern forms may simply be a sign of a flexible approach to the use of skin or lung, or both, as the site of oxygen exchange. In addition, the ventricle is subdivided by muscular columns into many compartments that tend to prevent the free mixing of blood. The conus arteriosus is muscular and contains a spiral valve. Again, as in lungfishes, this has an important role in directing blood into the correct arterial arches. In the frog, Rana, venous blood is driven into the right atrium of the heart by contraction of the sinus venosus, and it flows into the left atrium from the lungs. A wave of contraction then spreads over the whole atrium and drives blood into the ventricle, where blood from the two sources tends to remain separate. Separation is maintained in the spiral valve, and the result is similar to the situation in lungfishes. Blood from the body, entering the right atrium, tends to pass to the lungs and skin for oxygenation; that from the lungs, entering the left atrium, tends to go to the head. Some mixing does occur, and this blood tends to be directed by the spiral valve into the arterial arch leading to the body. Blood returning from the skin does not enter the circulation at the same point as blood from the lungs.

Thus, oxygenated blood arrives at the heart from two different directions—from the sinus venosus, to which the cutaneous (skin) vein connects, and from the pulmonary vein. Both right and left atria receive oxygenated blood, which must be directed primarily to the carotid arteries supplying the head and brain. It is likely that variable shunting of blood in the ventricle is important in ensuring this. A ventricular septum would inhibit shunting; it is at least possible. There is a renal portal system, and an alternative route back to the heart from the legs is provided by an anterior abdominal vein that enters the hepatic portal vein to the liver. An embryo develops only with an adequate supply of oxygen and metabolites. In its early stages these may be provided by diffusion. Because the rate of diffusion becomes limiting beyond a certain size, however, the circulatory system becomes functional early in development, often before other organs and systems are obvious. The heart develops from the middle embryonic tissue layer, the mesoderm, just below the anterior part of the gut. It begins as a tube that joins with blood vessels also forming in the mesoderm. Other mesodermal cells form a coat around the heart tube and become the muscular wall, or myocardium. The heart lies in its own section of body cavity, called the pericardial coelom, formed by partitions that cut it off from the main body cavity.

From an original tube shape, the heart bends back on itself as it grows within the pericardial cavity. The sinus venosus and atrium lie above the ventricle and bulbus cordis (embryonic equivalent of the conus arteriosus). Septa gradually partition the heart into chambers. The circulation has various modifications for diverting oxygenated blood from sources outside the embryo to the body of the embryo. Blood from the placenta travels to the right auricle via the umbilical vein and posterior vena cava; it passes through an opening, the foramen ovale, into the left auricle, and then to the left ventricle and around the body. Deoxygenated blood entering the anterior vena cava fills the right ventricle; however, instead of passing to the lungs, it is shunted through the ductus arteriosus, between the pulmonary and systemic arches, and into the dorsal aorta. From the dorsal aorta the deoxygenated blood travels to the placenta, bypassing the lungs completely. At birth the foramen ovale closes, as does the ductus arteriosus, and the lungs become functional. The circulatory system generally follows a sequence of seven main events. Initially, a tubular heart bends into an “S” shape. Blood then flows from behind forward through the sinus venosus, atrium, ventricle, and bulbus cordis. There is then subdivision of the atrium and ventricle and of the opening between them.

The sinus venosus is incorporated into the right atrium. The pulmonary veins are segregated to open into the left atrium. The bulbus cordis is subdivided into a pulmonary trunk from the right ventricle and a systemic trunk from the left ventricle. Finally, an embryonic set of six arterial arches is reduced to three in adults, and their relationships are further complicated by asymmetrical loss of some parts and development of others. The pressure that develops within the closed vertebrate circulatory system is highest at the heart and decreases with distance away from the heart because of friction within the blood vessels. Because the blood vessels can change their diameter, blood pressure can be affected by both the action of the heart and changes in the size of the peripheral blood vessels.

Blood is a living fluid—it is viscous and contains cells (45 percent of its volume in human beings)—and yet the effects of the cells on its flow patterns are small. Blood enters the atrium by positive pressure from the venous system or by negative pressure drawing it in by suction. Both mechanisms operate in vertebrates. Muscular movements of the limbs and body, and gravity forces propelling blood to the heart. The ventricle is the main pumping chamber, but one of the features of double circulation is that the two circuits require different pressure levels. Although the shorter pulmonary circulation requires less pressure than the much longer systemic circuit, the two are connected to each other and must transport the same volume of fluid per unit time. The right and left ventricles function as a volume and a pressure pump, respectively.

The thick muscular wall of the left ventricle ensures that it develops a higher pressure during contraction in order to force blood through the body. It follows that pressures in the aorta and pulmonary artery may be very different. Valves throughout the system are crucial to maintain pressure. They prevent backflow at all levels; for example, they prevent flow from the arteries back into the heart as ventricular pressure drops at the end of a contraction cycle. Valves are important in veins, where the pressure is lower than in arteries. Another impetus to blood flow is contraction of the muscles in the walls of vessels. This also prevents backflow of arterial blood toward the heart at the end of each contraction cycle. Input from nerves, sensory receptors in the vessels themselves, and hormones all influence blood vessel diameter, but responses differ according to position in the body. Normally, the pressures that develop in a circulatory system vary widely in different.

Body size can be an important factor. The closed circulation systems operate at higher pressures. The heart is myogenic (rhythmic contractions are an intrinsic property of the cardiac muscle cells themselves). Pulse rate varies widely in different in individuals. Each chamber of the heart has its own contraction rate. In humans where the sinus venosus is incorporated into the right atrium at the sinoauricular node, (pacemaker) the heartbeat is initiated at that point. In lower vertebrates, the cardiac muscle cells themselves conduct the wave of excitation. In human however, special conducting fibres (arising from modified muscle cells) transmit the wave of excitation from the sinoauricular node to the septum between the auricles, and then, after a slight delay, down between and around the ventricles.

The electrical activity of the heart can be recorded; the resulting pattern is called an electrocardiogram. Many factors, such as temperature, oxygen supply, or nervous excitement, affect heartbeat and circulation. Blood circulation is controlled mainly via nerve connections, sensory receptors, and hormones. These act primarily by varying the heart’s pulse rate, amplitude, or stroke volume and by altering the degree of dilation or constriction of the peripheral blood vessels (i.e., those blood vessels near the surface of the body).