Systemic Circulation

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The heart ejects oxygen-rich blood under high pressure out of the heart’s main pumping chamber, the left ventricle, through the largest artery, the aorta. Smaller arteries branch off from the aorta, leading to various parts of the body. These smaller arteries in turn branch out into even smaller arteries, called arterioles. Branches of arterioles become progressively smaller in diameter, eventually forming the capillaries. Once blood reaches the capillary level, blood pressure is greatly reduced.

Capillaries have extremely thin walls that permit dissolved oxygen and nutrients from the blood to diffuse across to a fluid, known as interstitial fluid, that fills the gaps between the cells of tissues or organs. The dissolved oxygen and nutrients then enter the cells from the interstitial fluid by diffusion across the cell membranes. Meanwhile, carbon dioxide and other wastes leave the cell, diffuse through the interstitial fluid, cross the capillary walls, and enter the blood. In this way, the blood delivers nutrients and removes wastes without leaving the capillary tube.

After delivering oxygen to tissues and absorbing wastes, the deoxygenated blood in the capillaries then starts the return trip to the heart. The capillaries merge to form tiny veins, called venules. These veins in turn join together to form progressively larger veins. Ultimately, the veins converge into two large veins: the inferior vena cava, bringing blood from the lower half of the body; and the superior vena cava, bringing blood from the upper half. Both of these two large veins join at the right atrium of the heart.

Because the pressure is dissipated in the arterioles and capillaries, blood in veins flows back to the heart at very low pressure, often running uphill when a person is standing. Flow against gravity is made possible by the one-way valves, located several centimeters apart, in the veins. When surrounding muscles contract, for example in the calf or arm, the muscles squeeze blood back toward the heart. If the one-way valves work properly, blood travels only toward the heart and cannot lapse backward. Veins with defective valves, which allow the blood to flow backward, become enlarged or dilated to form varicose veins.

Circulatory System: Operation and Function

Only in the past 400 years have scientists recognized that blood moves in a cycle through the heart and body. Before the 17th century, scientists believed that the liver creates new blood, and then the blood passes through the heart to gain warmth and finally is soaked up and consumed in the tissues. In 1628 English physician William Harvey first proposed that blood circulates continuously. Using modern methods of observation and experimentation, Harvey noted that veins have one-way valves that lead blood back to the heart from all parts of the body. He noted that the heart works as a pump, and he estimated correctly that the daily output of fresh blood is more than seven tons. He pointed out the absurdity of the old doctrine, which would require the liver to produce this much fresh blood daily. Harvey’s theory was soon proven correct and became the cornerstone of modern medical science.

Sub-Topics:
• Systemic Circulation
• Pulmonary Circulation
• Additional Functions
• Blood Pressure

Components of the Circulatory System

The heart, blood, and blood vessels are the three structural elements that make up the circulatory system. The heart is the engine of the circulatory system. It is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. The walls of these chambers are made of a special muscle called myocardium, which contracts continuously and rhythmically to pump blood. The pumping action of the heart occurs in two stages for each heart beat: diastole, when the heart is at rest; and systole, when the heart contracts to pump deoxygenated blood toward the lungs and oxygenated blood to the body. During each heartbeat, typically about 60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If the heart stops pumping, death usually occurs within four to five minutes.

Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.

Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.

Arteries have thicker walls than veins to withstand the pressure of blood being pumped from the heart. Blood in the veins is at a lower pressure, so veins have one-way valves to prevent blood from flowing backwards away from the heart. Capillaries, the smallest of blood vessels, are only visible by microscope—ten capillaries lying side by side are barely as thick as a human hair. If all the arteries, veins, and capillaries in the human body were placed end to end, the total length would equal more than 100,000 km (more than 60,000 mi)—they could stretch around the earth nearly two and a half times.

The arteries, veins, and capillaries are divided into two systems of circulation: systemic and pulmonary. The systemic circulation carries oxygenated blood from the heart to all the tissues in the body except the lungs and returns deoxygenated blood carrying waste products, such as carbon dioxide, back to the heart. The pulmonary circulation carries this spent blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart before transferring to the systemic circulation.

Nucleus

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Nucleus of a Cell
The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.

Nucleus (biology), membrane-bound structure of a cell that plays two crucial roles. The nucleus carries the cell’s genetic information that determines if the organism will develop, for instance, into a tree or a human; and it directs most cell activities including growth, metabolism, and reproduction by regulating protein synthesis (the manufacture of long chains of amino acids). The presence of a nucleus distinguishes the more complex eukaryotic cells of plants and animals from the simpler prokaryotic cells of bacteria and cyanobacteria that lack a nucleus.

The nucleus is the most prominent structure in the cell. It is typically round and occupies about 10 percent of the cell’s total volume. The nucleus is wrapped in a double-layered membrane called the nuclear envelope. The space between the nuclear envelope layers is called perinuclear space. The nuclear envelope is attached to a network of membrane-enclosed tubules that extends throughout the cell called the endoplasmic reticulum. The nuclear envelope is perforated by many holes, called nuclear pores, that permit the movement of selected molecules between the nucleus and the rest of the cell, while blocking the passage of other molecules.

The nucleus contains the nucleolus, which manufactures protein-producing structures called ribosomes. Genetic information in the form of deoxyribonucleic acid (DNA) is stored in threadlike, tangled structures called chromatin within the nucleus. During the process of cell division known as mitosis, in which the nucleus divides, the chromatin condense into several distinct structures called chromosomes. Each time the cell divides, the heredity information carried in the chromosomes is passed to the two newly formed cells.

The DNA in the nucleus also contains the instructions for regulating the amount and types of proteins made by the cell. These instructions are copied, or transcribed, into a type of ribonucleic acid (RNA) called messenger RNA (mRNA). The mRNA is transported from the nucleus to ribosomes, where proteins are assembled.

related topics:

• cell structure
• algae
• eukaryotes
• genetic disorders

Golgi Apparatus

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Golgi Apparatus, also Golgi body or Golgi complex, network of stacked sacs found within nucleated cells that store, package, and distribute the proteins and lipids made in the endoplasmic reticulum.

The Golgi apparatus was first described by Italian anatomist Camillo Golgi in the late 19th century. Located near the nucleus, each apparatus consists of a stack of six or seven flattened, membrane-bound sacs, or cisternae, each separated by a narrow space. The Golgi apparatus is cup-shaped with the convex end, or cis cisterna, facing the cell nucleus and the concave end, or trans cisterna, facing the cell surface. The number of Golgi apparatus in each cell varies but averages between 10 and 20 in animal cells and up to several hundred in plant cells.

Proteins and lipids manufactured in the endoplasmic reticulum bud off in tiny, hollow structures, or vesicles, and fuse with the cis cisterna of the Golgi apparatus. The proteins and lipids move progressively through the stack of cisternae until they reach the trans cisterna. There they may be modified by the attachment of lipids or carbohydrates. The proteins and lipids are enclosed in a membrane to form a vesicle so that they do not affect the rest of the cell. The vesicles are then sorted and their destination is determined.

Proteins that are meant to return to the endoplasmic reticulum carry a distinctive tag. The Golgi apparatus recognizes the tag and transports the proteins back to the endoplasmic reticulum. Some proteins and lipids are sent to the surface of the cell to be released into the external environment. Others are transferred to the small structures that hold digestive enzymes, called lysosomes.

The Golgi apparatus also manufactures long-chained sugars called polysaccharides that cells secrete into their external environments. Examples include cellulose and pectin used to construct plant cell walls, and the polysaccharides in the mucus of animal cells.

Endoplasmic Reticulum

Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. The ER consists of a continuous membrane in the form of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents outside of the nucleus) and connect to the double membrane that surrounds the nucleus. There are two types of ER: rough and smooth.

The outer surface of rough ER is covered with tiny structures called ribosomes, where protein synthesis occurs. Proteins are created as long polypeptide chains, some of which require modification. These proteins are transported into the rough ER, where enzymes fold and link them into the three dimensional shape that completes their structure. The rough ER also transports proteins either to regions of the cell where they are needed or to the Golgi apparatus, from which they may be exported from the cell. Rough ER is particularly dense in cells that manufacture proteins for export. White blood cells, for example, which produce and secrete antibodies, contain abundant rough ER.

Smooth ER lacks ribosomes and so has a smooth appearance. It is involved in the synthesis of most of the lipids that make up the cell membrane, as well as membranes surrounding other cell structures like mitochondria. It also manufactures carbohydrates, stores carbohydrates and lipids, and detoxifies alcohol and drugs such as morphine and phenobarbitol. Cells that specialize in lipid and carbohydrate metabolism, such as brain and muscle cells, or those that carry out detoxification, such as liver cells , tend to have more smooth ER.

Ribosome

Ribosome, cell structure that uses genetic instructions transported in ribonucleic acid (RNA) to link a specific sequence of amino acids into chains to form proteins. Ribosomes, which measure about 0.00025 mm (0.00001 in), are dispersed in the cytoplasm (the cell contents outside the nucleus) of all prokaryotic cells— archaebacteria and bacteria. They are also found in the cytoplasm of all eukaryotic cells—cells of protists, fungi, plants, and animals—where they either float free in the cytoplasm or are bound to networks of membrane-enclosed tubules in the cytoplasm, called the endoplasmic reticulum. In eukaryotic cells, two types of cell structures called mitochondria and chloroplasts also contain ribosomes.