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  Section: General Zoology » Activity of Life
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Plan of Vertebrate Circulatory Systems

Internal Fluids and Respiration
Internal Fluid Environment 
Composition of the Body Fluids
Composition of Blood 
  - Hemostasis: Prevention of Blood Loss
  - Open and Closed Circulations
  - Plan of Vertebrate Circulatory Systems
  - Arteries
  - Capillaries
  - Veins
  - Lymphatic System
  - Problems of Aquatic and Aerial Breathing 
  - Respiratory Organs
  - Gas Exchange Through Tubes: Tracheal Systems
  - Lungs
  - Structure and Function of the Mammalian Respiratory System

Plan of Vertebrate Circulatory Systems
In vertebrates the principal differences in the blood vascular system involve the gradual separation of the heart into two separate pumps as vertebrates evolved from aquatic life with gill breathing to fully terrestrial life with lung breathing. These changes are shown in Figure 33-10 which compares the circulation of fish, amphibians, and mammals.
Internal Fluids and Respiration, General Zoology. Biocyclopedia
Figure 33-10 Circulatory systems of fish, amphibian, and mammal, showing evolution of separate
systemic and pulmonary circuits in lung-breathing vertebrates.

A fish heart contains two main chambers in series, an atrium and a ventricle. The atrium is preceded by an enlarged chamber, the sinus venosus, which collects blood from the venous system to assure a smooth delivery of blood to the heart. Blood makes a single circuit through a fish’s vascular system; it is pumped from the heart to the gills, where it is oxygenated, then flows into the dorsal aorta to be distributed to body organs, and finally returns by veins to the heart. In this circuit the heart must provide sufficient pressure to push the blood through two sequential capillary systems, first that of the gills, and then that of the remainder of the body. The principal disadvantage of the singlecircuit system is that the gill capillaries offer so much resistance to blood flow that blood pressures to the body tissues are greatly reduced.
Internal Fluids and Respiration, General Zoology. Biocyclopedia
Figure 33-11 Route of blood through a frog heart. Atria are completely separated, and the
spiral valve helps to route blood to lungs and systemic circulation.

With evolution of lung breathing and elimination of gills between the heart and aorta, vertebrates developed a high-pressure double circulation: a systemic circuit that provides oxygenated blood to the capillary beds of the body organs; and a pulmonary circuit that serves the lungs. The beginning of this major evolutionary change probably resembled the condition seen in lungfishes and amphibians. In modern amphibians (frogs, toads, salamanders) the atrium is completely separated by a partition into two atria (Figure 33-11). The right atrium receives venous blood from the body while the left atrium receives oxygenated blood from the lungs. The ventricle is undivided, but venous and arterial blood remain mostly separate by the arrangement of vessels leaving the heart. Separation of the ventricles is nearly complete in some reptiles (crocodilians) and is completely separate in birds and mammals (Figure 33-12). Systemic and pulmonary circuits are now separate circulations, each served by one half of a dual heart (Figure 33-12).
Internal Fluids and Respiration, General Zoology. Biocyclopedia
Figure 33-12 Human heart. Deoxygenated blood enters right side of heart and is pumped
to the lungs. Oxygenated blood returning from the lungs enters left side of the heart and is pumped
to the body. The left ventricular wall is thicker than that of the right ventricle, which needs less
muscular force to pump blood into the nearby lungs.

Mammalian Heart
The four-chambered mammalian heart (Figure 33-12) is a muscular organ located in the thorax and covered by a tough, fibrous sac, the pericardium. Blood returning from the lungs collects in the left atrium, passes into the left ventricle, and is pumped into the body (systemic) circulation. Blood returning from the body flows into the right atrium, and passes into the right ventricle, which pumps it into the lungs. Backflow of blood is prevented by two sets of valves that open and close passively in response to pressure differences between the heart chambers. The bicuspid (between left atrium and ventricle) and tricuspid (between right atrium and ventricle) valves separate the cavities of the atrium and ventricle in each half of the heart. Where the great arteries, the pulmonary from the right ventricle and the aorta from the left ventricle, leave the heart, semilunar valves prevent backflow into the ventricles.
Internal Fluids and Respiration, General Zoology. Biocyclopedia
Figure 33-13 Human heart in systole and diastole.

Contraction is called systole (sisto-lee), and relaxation, diastole (dy-asto-lee) (Figure 33-13). When the atria contract (atrial systole), the ventricles relax (ventricular diastole), and ventricular systole is accompanied by atrial diastole. Rate of the heartbeat depends on age, sex, and especially exercise. Exercise may increase cardiac output (volume of blood forced from either ventricle each minute) more than fivefold. Both heart rate and stroke volume increase. Heart rates among vertebrates vary with general level of metabolism and body size. Ectothermic codfish have a heart rate of approximately 30 beats per minute; endothermic rabbits of about the same weight have a rate of 200 beats per minute. Small animals have higher heart rates than do large animals. The heart rate in an elephant is 25 beats per minute, in a human 70 per minute, in a cat 125 per minute, in a mouse 400 per minute, and in the tiny 4 g shrew, the smallest mammal, the heart rate approaches a prodigious 800 beats per minute. We must marvel that the shrew’s heart can sustain such a frantic pace throughout this animal’s life, brief as it is.

Excitation and Control of the Heart
The vertebrate heart is a muscular pump composed of cardiac muscle. Cardiac muscle resembles skeletal muscle—both are types of striated muscle—but the cells are branched and joined end-to-end by junctional complexes to form a complex branching network (see Figure 9-7,). Unlike skeletal muscle, vertebrate cardiac muscle does not depend on nerve activity to initiate a contraction. Instead, regular contractions are established by specialized cardiac muscle cells, called pacemaker cells. In a tetrapod heart the pacemaker is in the sinus node, a remnant of the sinus venosus in the fishlike ancestor. Electrical activity initiated in the pacemaker spreads over the muscle of the two atria and then, after a slight delay, to the muscle of the ventricles. At this point electrical activity is conducted rapidly through the atrioventricular bundle to the apex of the ventricle and then continues through specialized fibers (Purkinje fibers) up the walls of the ventricles (Figure 33-14). This arrangement allows the contraction to begin at the apex or “tip” of the ventricles and spread upward to squeeze out the blood in the most efficient way; it also ensures that both ventricles contract simultaneously. Structural specializations in Purkinje fibers, such as well-developed intercalated discs (see Figure 9-7,) and numerous gap junctions, facilitate rapid conduction through these fibers.

Internal Fluids and Respiration, General Zoology. Biocyclopedia
Figure 33-14 Neuromuscular mechanisms controlling heartbeat. Arrows indicate spread of
excitation from the sinus node, across the atria, to the atrioventricular node. Wave of excitation is
then conducted very rapidly to ventricular muscle over the specialized conducting
bundles and Purkinje fiber system.

The control (cardiac) center in the brain is located in the medulla and connects to two sets of nerves. Impulses sent along one set, the vagus nerves, apply a braking action to the heart rate, and impulses sent along the other set, the accelerator nerves, speed it up. Both sets of nerves terminate in the sinus node, thus guiding the activity of the pacemaker.

The cardiac center in turn receives sensory information about a variety of stimuli. Pressure receptors (sensitive to blood pressure) and chemical receptors (sensitive to carbon dioxide and pH) are located at strategic points in the vascular system. The cardiac center uses this information to increase or reduce heart rate and cardiac output in response to activity or changes in body position. Feedback mechanisms thus control the heart and keep its activity constantly attuned to needs of the body.

Because the heartbeat is initiated in specialized muscle cells, vertebrate hearts, together with the hearts of molluscs and several other invertebrates are called myogenic (“muscle origin”) hearts. Although the nervous system does alter pacemaker activity to slow down or speed up heart rate, a myogenic heart will beat spontaneously and involuntarily even if completely removed from the body. An isolated turtle or frog heart beats for hours if placed in a balanced salt solution. Some invertebrates, for example decapod crustaceans, have neurogenic (“nerve origin”) hearts. In these hearts a cardiac ganglion located on the heart serves as pacemaker. If this ganglion is separated from the heart, the heart stops beating, even though the ganglion itself remains rhythmically active.

Coronary Circulation
It is no surprise that an organ as active as the heart needs a generous blood supply of its own. The heart muscle of frogs and other amphibians is so thoroughly channeled with spaces between muscle fibers that the heart’s own pumping action squeezes through sufficient oxygenated blood. In birds and mammals, however, the thickness of the heart muscle and its high rate of metabolism require that the heart have its own vascular supply, the coronary circulation. Coronary arteries divide to form an extensive capillary network surrounding the muscle fibers and provide them with oxygen and nutrients. Heart muscle has an extremely high oxygen demand. Even at rest the heart removes 70% of oxygen from the blood, in contrast to most other body tissues, which remove only about 25%. Therefore, an increase in the work of the heart must be met by a massive increase in coronary blood flow—up to nine times the resting level during strenuous exercise. Any reduction in coronary circulation due to partial or complete blockage (coronary artery disease) may lead to a heart attack (myocardial infarction) in which heart cells die from lack of oxygen.

Thickening and loss of elasticity in arteries is known as arteriosclerosis. When arteriosclerosis is caused by fatty deposits of cholesterol in artery walls, the condition is atherosclerosis. Such irregularities in the walls of blood vessels often cause blood to clot around them, forming a thrombus. When a bit of the thrombus breaks off and is carried by the blood to lodge elsewhere, it is an embolus. If the embolus blocks one of the coronary arteries, the person has a heart attack (a “coronary”).The portion of the heart muscle served by the branch of the coronary artery that is blocked is starved for oxygen. It may be replaced by scar tissue if the person survives.


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