Cardiovascular System: Structure, Function and Disease Prevention

EDUCATIONAL CENTER SÃO LAZARO

Joseph Conrad, Leonardo Nadjane, Priscilla Tauine, Vicente

BIOLOGY ASSIGNMENT

Candeias

2009

EDUCATIONAL CENTER SÃO LAZARO

Joseph Conrad, Leonardo Nadjane, Priscilla Tauine, Vicente.

BIOLOGY ASSIGNMENT

AS REQUESTED BY THE PROFESSOR FOR THIS ASSIGNMENT

A NOTE FOR BIOLOGY BEHAVIOR.

Candeias

2009

CONTENTS

INTRODUÇÃO ................................................. ............... 02

DESENVOLVIMENTO ........................... ...................... ....................... 04

CONCLUSÃO ................................................. ............................. 06

BIBLIOGRAPHY ................................................. ......................... 08

EDUCATIONAL CENTER SÃO LAZARO

Joseph Conrad, Leonardo Nadjane, Priscilla Tauine, Vicente.

BIOLOGY ASSIGNMENT

Candeias

2009

EDUCATIONAL CENTER SÃO LAZARO

Joseph Conrad, Leonardo Nadjane, Priscilla Tauine, Vicente.

AS REQUESTED BY THE PROFESSOR FOR THIS ASSIGNMENT

A NOTE FOR BIOLOGY BEHAVIOR.

Candeias

2009

CONTENTS

INTRODUÇÃO ............................................ ............ 02

DESENVOLVIMENTO ................................................. ...... 04

APPENDICES ................................................. .... 06

CONCLUSÃO ................................................. .08

BIBLIOGRAPHY ................................................. ... 10

INTRODUÇÃO

THIS ASSIGNMENT IS ABOUT THE CARDIOVASCULAR SYSTEM, ITS DISEASES AND PREVENTION.

IT ALSO SERVES AS A WARNING FOR THOSE WHO HAVE HIGH BLOOD PRESSURE, WHO ARE ALREADY HYPERTENSIVE, AND SO ON...

OUR WORK IS CONCERNED WITH THE MAIN SYMPTOMS AND HOW TO PREVENT THEM.

THOSE WHO WANT TO PREVENT THEM, COME ALONG.

DESENVOLVIMENTO

CARDIOVASCULAR SYSTEM

The circulatory or cardiovascular system is a vast network of tubes of various types and sizes, connecting all parts of the body. Blood circulates within these tubes, propelled by the heart through rhythmic contractions.

Functions of the Cardiovascular System

The system allows the following activities to be carried out with great efficiency:

• Gas transport: responsible for obtaining oxygen from the lungs and eliminating carbon dioxide, communicating with the other tissues of the body through the blood.
• Nutrient transport: nutrients resulting from digestion pass through the thin epithelium of the digestive tract and reach the blood. Through this true "highway," nutrients are carried to the body's tissues, where they diffuse into the interstitial fluid and then into cells.
• Metabolic waste transport: metabolic residues originate from cellular activity, and some must be eliminated from the body. These substances are transported by the blood to the excretory organs.
• Hormone transport: hormones are substances secreted by certain organs, distributed by the blood, and able to modify the functioning of other organs. Cholecystokinin, for example, is produced by the duodenum during the passage of food and released into the blood. One of its effects is to stimulate contraction of the gallbladder and the release of bile into the duodenum.
• Exchange of materials: some substances are produced or stored in one part of the body and used in another. The liver, for example, stores glycogen molecules which, when needed, release glucose that the blood carries to other cells of the body.
• Heat transport: blood is also used to distribute heat evenly among the various parts of the body, helping maintain a suitable working temperature in all regions. Heat is also carried to the body surface, where it can be dissipated.
• Defense mechanisms: antibodies and phagocytic cells circulate through the blood, providing defense against infectious agents.
• Blood coagulation: platelets circulate in the blood. They are fragments of a bone marrow cell type (megakaryocytes) and play a role in blood clotting. Blood also contains clotting factors that block possible leaks when a blood vessel ruptures.

Components of the Cardiovascular System

The main components of the cardiovascular system are the heart, blood vessels, blood, lymphatic vessels and lymph.

HEART

The human heart, like that of other mammals, has four chambers: two upper chambers called atria and two lower chambers called ventricles. The right atrium communicates with the right ventricle through the tricuspid valve. The left atrium communicates with the left ventricle through the bicuspid (mitral) valve. The heart valves ensure that blood flows in only one direction, from the atria to the ventricles.

The cardiac chambers contract and relax about 70 times per minute on average. The contraction of each chamber's myocardium (heart muscle) is called systole. Relaxation, occurring between one systole and the next, is called diastole.

a) The electrical activity of the heart

Image: Avancini & Favaretto. Biologia - Uma abordagem evolutiva e ecológica. Vol. 2. São Paulo: Ed. Moderna, 1997.

Sinoatrial (SA) node or sinus node (pacemaker): a special region of the heart that controls heart rate. It is located near the junction between the right atrium and the superior vena cava, and is formed by an agglomeration of specialized muscle cells. The rhythmic contraction frequency of this muscle tissue is about 72 times per minute, while the atrial muscle contracts about 60 times per minute and the ventricular muscle about 20 times per minute. Because the SA node has a higher rhythmic frequency than the other parts of the heart, its impulses spread through the atria and ventricles, stimulating these areas quickly. Thus the SA node sets the pace of the heart, which is why it is called the pacemaker.

Atrioventricular (AV) node and Purkinje system: although the cardiac impulse can travel through all cardiac muscle fibers, the heart has a special conduction system called the Purkinje system, composed of specialized cardiac muscle fibers (Purkinje fibers, or the bundle of His). These fibers transmit impulses at a speed about 6 times greater than normal heart muscle, about 2 m per second compared with 0.3 m per second in ordinary heart muscle.

b) Nervous control of the heart

Although the heart has its own intrinsic control systems and can keep operating without external influences, its effectiveness can be strongly modified by regulatory impulses from the central nervous system. The nervous system connects to the heart through two different groups of nerves: the parasympathetic and sympathetic systems. The parasympathetic nerves produce the following effects on the heart: (1) decrease in heart rate, (2) decrease in the force of atrial contraction, (3) decrease in the conduction speed of impulses through the AV node, increasing the delay between atrial and ventricular contraction, and (4) decrease in coronary blood flow that nourishes the heart muscle itself.

All these effects can be summarized by saying that parasympathetic stimulation decreases all of the heart's activities. Usually the heart's function is reduced by parasympathetic activity during rest, which helps preserve the heart's resources, since during rest there is considerably less wear on the organ.

Sympathetic nerve stimulation has exactly the opposite effects on the heart: (1) increase in heart rate, (2) increase in the force of contraction, and (3) increased blood flow through the coronary vessels, aiming to improve the nourishment of the heart muscle. These effects can be summarized by saying that sympathetic stimulation increases the heart's pumping capacity, in some cases by up to 100 percent. This is necessary in individuals subjected to stress, such as exercise, illness, excessive heat, or other conditions that demand a faster circulatory flow. Consequently, the effects on the heart constitute a sympathetic emergency mechanism, making the heartbeat stronger when needed.

The postganglionic neurons of the sympathetic nervous system secrete mainly noradrenaline, which is why they are called adrenergic neurons. Sympathetic stimulation also promotes the secretion of adrenaline by the adrenal (suprarenal) glands. Adrenaline is responsible for tachycardia (rapid heartbeat), increased blood pressure and breathing frequency, increased sweating, increased blood glucose, mental activity, and constriction of the blood vessels of the skin.

The neurotransmitter secreted by the postganglionic neurons of the parasympathetic nervous system is acetylcholine, which is why they are called cholinergic neurons. Their effects are generally antagonistic to those of adrenergic neurons. Thus parasympathetic stimulation promotes bradycardia (reduced heart rate), decreased blood pressure and respiratory frequency, relaxation of the muscles, and opposes the other effects of adrenaline.

In general, stimulation of the posterior hypothalamus increases blood pressure and heart rate, while stimulation of the preoptic area and the anterior hypothalamus produces opposite effects, causing a notable decrease in heart rate and blood pressure. These effects are transmitted to the cardiovascular control centers in the brainstem and from there travel through the autonomic nervous system.

Pulmonary and systemic circulation

BLOOD VESSELS

Blood vessels are of three basic types: arteries, capillaries and veins.

a) Arteries: thick-walled vessels that carry blood away from the heart to the organs and tissues of the body. They consist of three layers: the innermost layer, called the endothelium, formed by a single layer of flattened cells; a middle layer made of smooth muscle tissue; and an outer layer formed by connective tissue rich in elastic fibers.

When blood is pumped out of the ventricles and enters the arteries, their elastic walls stretch and expand. When the pressure drops, the arteries recoil. If the arteries did not stretch sufficiently, the pressure of the blood inside would rise, risking rupture of the arterial walls. Thus each ventricular systole generates an expansion wave that propagates through the arteries from the heart to the arterioles. During ventricular diastole, the blood pressure decreases. The arteries then contract, keeping the blood in circulation until the next systole.

b) Capillaries: very small vessels that connect the arterioles to the venules at the extremities of the limbs. Capillary walls consist of a single layer of cells (endothelium), similar to that of arteries and veins.

As blood passes through the capillaries, its liquid part (plasma) crosses the capillary walls and spreads among the nearby cells, supplying them with oxygen and nutrients. The cells, in turn, release carbon dioxide and other excretions into the extravasated fluid, called tissue fluid. Most of the tissue fluid is reabsorbed by the capillaries and returned to the blood. Only 1% to 2% of the extravasated fluid is returned by the lymphatic system, a parallel circulatory system, where it is then called lymph and moves slowly through lymphatic vessels equipped with valves.

At the arterial end of the capillary, the blood pressure is higher than the osmotic pressure of the plasma, causing water containing dissolved substances to leave the capillary. At the venous end of the capillary, the blood pressure is reduced, becoming lower than the osmotic pressure of the plasma, so fluid returns to the inside of the capillary.

c) Veins: vessels that return blood from the organs and tissues of the body to the heart. Like arteries, the walls of veins are also formed by three layers. The difference, however, is that the muscular layer and the connective layer of veins are thinner than those of arteries. In addition, unlike arteries, veins of larger caliber have valves inside them, which prevent blood from flowing backward and ensure that circulation occurs in only one direction.

After passing through the arterioles and capillaries, the blood pressure drops considerably, without reaching very low values inside the veins. The return of blood to the heart depends largely on the contraction of skeletal muscles, which compress the veins and push the blood along inside them. Because of the valves, the blood can only move toward the heart.

BLOOD

Blood is composed of a yellowish liquid, the plasma, and cells and cell fragments, generically called formed elements.

a) Plasma:

b) Formed elements include:

Genesis or production of erythrocytes in the bone marrow: blast cells → basophilic erythroblast → polychromatophilic erythroblast → reticulocyte → normoblast → erythrocytes.

During the early stages, the cells divide many times and change color, due to the progressive formation and increasing quantity of hemoglobin. At the normoblast stage, the cell nucleus degenerates and the cell transforms into a reticulocyte. It is at this stage that the cell usually leaves the bone marrow. The reticulocyte still contains small endoplasmic filaments and produces small amounts of hemoglobin. However, the reticulum degenerates within one or two days and the cell becomes a mature erythrocyte, circulating in the blood for about 120 days before being destroyed.

Destruction of erythrocytes in the spleen: the globin portion (a protein-rich group composed of amino acids) is digested and reused.

The heme portion (a prosthetic group composed of iron atoms) is released when the hemoglobin is phagocytized and fragmented. It is almost immediately digested, releasing iron into the bloodstream, which is carried to the bone marrow (for the production of new red cells) and to the liver (for the production of the bile pigment bilirubin).

A reduction in the number of red blood cells (erythropenia) or in the concentration of hemoglobin is called anemia, characterized by fatigue and shortness of breath. Anemia can have a variety of causes:

☞ Nutritional anemias: arise from the deficiency of certain nutrients in the diet, such as iron, vitamin B12 and folic acid. Anemia caused by a lack of iron is called iron-deficiency anemia; that caused by a lack of vitamin B12 is called pernicious anemia.

☞ Blood-loss anemias: result from loss of blood caused by certain diseases, such as amebiasis, hookworm disease, ulcers and gastritis.

Hereditary anemias ☞ have a genetic basis. In thalassemia, or "Mediterranean anemia," there is an imbalance in the production of the beta chains of hemoglobin, causing the production of abnormal hemoglobin molecules. Sickle cell anemia results from the substitution of a single amino acid (glutamic acid by valine) in the beta chain of hemoglobin. Under low oxygen concentrations, the erythrocytes take on a sickle shape and are destroyed.

☞ Aplastic anemia: diseases that originate in the bone marrow, compromising the production of red and other blood cells. Example: leukemia.

An increase in the number of red blood cells (erythrocytosis) is generally a physiological adaptation to high altitudes, where the air is thin. Since the oxygen saturation of the air is lower than at sea level, but the hemoglobin saturation remains normal, the amount of oxygen delivered to the tissues decreases. As an adaptation, the kidneys secrete erythropoietin, which stimulates the bone marrow to produce more red blood cells and more hemoglobin. The increase in these cells raises the body's capacity to capture oxygen from the air.

b-2) Leukocytes (white blood cells): specialized cells that defend the body by combating viruses, bacteria and other invaders in the body.

Leukocytosis is the phenomenon in which the white blood cell count rises above 10,000/mm³ of blood, while leukopenia is when it falls below 2,000/mm³. In leukemia (cancer of the white blood cells), more than 100,000 white cells/mm³ of blood are found. Leukocytosis usually occurs due to an infection, while leukopenia predisposes the body to infections.

Leukocytes are also produced in the bone marrow, as shown in the table below:

Some blood cells can pass through the walls of the blood vessels by diapedesis (a movement outward to the defense site) and settle in other tissues, giving rise to different cell types that have in common the ability of phagocytosis: connective tissue cells give rise to macrophages; in the liver, Kupffer cells; in nervous tissue, microglial cells.

Hemocytopoiesis (hematopoiesis) is the process of formation and maturation of blood cells, and their release into the bloodstream. The hemocytopoietic connective tissue, or reticular tissue, produces the blood leukocytes and erythrocytes. This tissue is found in the spleen and lymph nodes, where it is called lymphoid tissue. In the inner medulla of the bones, this tissue is called myeloid tissue; it occupies the spaces between the sheets of spongy bone.

Blood cells originally form from the so-called pluripotent stem cells found in active red bone marrow, which can give rise to two lineages: lymphoid and myeloid.

Lymphoid cells give rise to the lymphocytes lineage, while myeloid cells give rise to erythrocytes, leukocytes (other than lymphocytes) and platelets.

b-3) Platelets: small, oval, disc-shaped cell fragments about 2 µm in diameter that participate in the blood clotting process. They are fragments of megakaryocytes, extremely large white cells formed in the bone marrow. Megakaryocytes break apart to form platelets while still in the bone marrow, releasing the platelets into the blood afterward. The normal platelet concentration in blood is around 200,000 to 400,000 per milliliter.

In thrombocytopenia, the number of circulating platelets is reduced, and the patient is predisposed to a large number of tiny bleeding spots on the skin and in deep tissues, since the platelet plug mechanism for stopping small vascular bleeding becomes deficient. People with thrombocytopenia also tend to bleed, as do hemophiliacs. Thrombocytopenia may be genetically determined, but in most cases it results from intoxication (toxins, pesticides) or drugs.

In thrombocytosis, the number of circulating platelets increases, which can lead to the formation of thrombi (clots). Individuals with this condition are predisposed to thrombosis, the solidification of blood inside the heart or blood vessels. It is generally genetically determined.

Blood Coagulation

Mechanism of hemostasis (prevention of blood loss):

(1) Vascular spasm: immediately after a blood vessel is broken or cut, the injured vessel undergoes vasoconstriction (contraction). (2) Formation of the platelet plug: platelets accumulate at the site of injury to form a platelet plug (platelets adhere to the damaged area and stick to each other).

(3) Blood coagulation: substances released by both the traumatized vascular wall and the platelets (such as thromboplastin) start a complex cascade of chemical reactions which, in the presence of calcium ions, culminates in the conversion of the plasma protein prothrombin into the active enzyme thrombin. Thrombin, in turn, converts fibrinogen into fibrin, which forms filaments, a network that traps platelets, blood cells and plasma, forming a blood clot.

The synthesis of some clotting factors (such as prothrombin) occurs in the liver and depends on vitamin K, whose deficiency can cause bleeding. Similarly, the conversion of prothrombin into thrombin requires calcium ions. Consequently, a lack of vitamin K and/or calcium can compromise blood clotting, resulting in bleeding tendencies.

NOTE: The blood clotting factors (more than 12) are, for the most part, inactive forms of proteolytic enzymes. When activated, each enzyme converts the next inactive factor, so that these reactions occur in a cascade during the clotting process.

(4) Regeneration: growth of fibrous tissue into the blood clot to seal the opening in the vessel wall.

Hemophilia is an inherited disease of blood coagulation, caused not by the lack of a single clotting factor production in general, but by the absence of a specific factor. Since clotting is a cascade of reactions, the absence of one component interrupts the entire process.

REFERENCES

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