The Circulatory System
The circulatory system transports blood through out the body, and is composed of the heart, blood vessels, and the blood. The heart is the pumping mechanism that is responsible for the unidirectional transport of the blood. The blood vessels are the means of transport for the blood, and they include arteries, veins and capillaries. Arteries carry oxygenated blood away from the heart while veins carry deoxygenated blood to the heart. Capillaries are very thin vessels that allow the exchange of nutrients, waste, oxygen, and carbon dioxide between the arteries and the veins. The diameter of the capillaries is about the diameter of one red blood cell. The blood consists of red blood cells, white blood cells, and plasma (1). Blood flow through the heart begins at the inferior or superior vena cava where it enters the right atrium of the heart. From there blood travels through the tricuspid valve into the right ventricle, and out of the pulmonary artery to the lungs. In the lungs, blood becomes oxygenated, and returns through the heart through the pulmonary veins. It enters the left atrium, and passes through the mitral valve where it then enters the left ventricle. The left ventricle is responsible for pumping the blood to the aorta and to the rest of the body, and this is why the walls of the left atrium are three times thicker than any other chamber in the heart (2).
(4)
Red Blood Cells
(6)
Red blood cells, or erythrocytes, are the most abundant cell in the blood. On average, males have 4.6-6.1 million cells per microliter, and females have 4.2-5.4 million cells per microliter (5). These cells have a biconcave disk, and measure about 7-8 micrometers in diameter and are about 2 micrometers thick. The shape of these cells allows them to pass through narrow capillaries (1). The main function of erythrocytes is to carry oxygen, and they are able to do so because of hemoglobin. The hemoglobin inside of the erythrocytes accounts for one third of their total volume. Red blood cells also aid in removing carbon dioxide from the body. Erythrocytes do not contain nuclei or mitochondria in order to leave as much room as possible to carry hemoglobin. The average life span of a red blood cell is about 120 days (6).
Hemoglobin
(8)
Hemoglobin is a protein component found in the red blood cells. It gives blood its red color, and is composed of a globin component and an iron component. The iron, or heme, is what gives oxygen the ability to bind to the red blood cells (1). Each hemoglobin consists of four polypeptide chains, and each of these chains has a heme group. The average adult male has a hemoglobin concentration of 14-17 g/dL, while the average adult female has a hemoglobin concentration of 12-16 g/dL (7).
Erythropoiesis
Pluripotent stem cells give rise to erythrocytes and are found inside of the bone marrow. These pluripotent stem cells are partially differentiated, but still have the capacity to become a variety of different cells. They can form blood cells, but cannot form muscle or nerve cells (1). Cytokines are signaling molecules released by one cell and they affect the activity of another cell (9). The cytokine that is responsible for the pluripotent stem cells differentiating into blood cells is erythropoietin. Erythropoietin causes erythropoiesis, or the making of red blood cells, and is a glycoprotein hormone that is produced by the kidneys. Low oxygen levels in the blood will trigger the secretion of this hormone, which will then trigger erythropoiesis in order to increase oxygen levels. This is an example of a negative feedback loop (1). Different factors, such as loss of blood, damage to the bone marrow, exercise, high altitude and certain diseases, can stimulate the production of red blood cells (10).
Regulation of Erythrocyte Destruction
Erythrocytes are continuously being produced and destroyed. The destruction of erythrocytes is controlled by macrophages in the spleen, liver, and bone marrow. These macrophages destroy red blood cells and break them down into their membrane, enzyme, and hemoglobin components. The iron is removed from the hemoglobin while the globulin part of hemoglobin is broken down into the amino acids that make it up. The iron that is recycled from the destroyed erythrocytes is recycled and used again (1).
Liver
(15)
The liver is a large, vital organ in the abdomen responsible for detoxifying the blood, metabolizing drugs and chemicals, making proteins, and secreting bile (11). All of theses functions are carried out by hepatocytes, or liver cells. The liver detoxifies the blood from bilirubin, a product that results from the breakdown of the hemoglobin found in erythrocytes. Bilirubin is excreted from the body as bile or in the urine (12). About 35 mg of bilirubin are produced in the break down of 1 g of hemoglobin. Kupffer cells line the sinuses of the liver and prevent bacteria from passing through the liver into the blood stream (1). The liver also detoxifies the body from harmful toxins or drugs, as well as alcohol. The proteins synthesized by the liver include albumin and thrombopoietin. Albumin is the main protein component of human blood and it is necessary for the regulation of osmotic pressure of blood (13). Thrombopoietin is a glycoprotein hormone that is responsible for platelet production in the bone marrow. The liver is also responsible for the production of bile, a digestive fluid that aids in the absorption of fats and fat soluble vitamins (14). Bile that is made by the liver is stored in the gallbladder and is released into the intestines after food consumption.
Immune System
The function of the immune system is to protect the body from disease, and it does so through a network of cells, organs and tissues. It is the job of the immune system to recognize pathogens and launch defense mechanisms in order to rid the body of diseases. The blood and the lymph are the two fluids responsible for transport in the immune system (16). The lymph transports bacteria, that has gotten into the body, to the lymph nodes where they are destroyed. Leukocytes, or white blood cells, are the cells that help protect the body from infectious agents. Leukocytes can be divided into granular leukocytes and agranular leukocytes. Granular leukocytes include the neutrophils, eosinophils, and the basophils. They form a non specific defense system that is capable of fighting off many different types of bacteria and viruses. Agranular leukocytes include monocytes and lymphocytes. They fight off specific types of bacteria and viruses .
Neutrophils- help defend the body from bacteria and viruses by phagocytosis, make up 50-70% of the leukocytes.
Eosinophils- fight larger parasites and are involved in allergic responses, make up 1-4% of the leukocytes.
Basophils- release histamine as a response to inflammation, make up <1% of the leukocytes.
Monocytes- released from the blood and become macrophages, make up 2-8% of the leukocytes.
Lymphocytes- all derive from the same type of stem cell and can be classified as B-lymphocytes, T-lymphocytes, or natural killer cells, make up 20-40% of the leukocytes.
B-lymphocytes help form antibodies to fight off antigens, and T-lymphocytes are involved in cell mediated immunity. T-cells have a T cell receptor on their surface while B-cells have B cell receptors on their surface. Natural killer cells help defend agains tumors and viruses (1).
Immunity means that a person has the needed defenses in order to fight off a certain infection or disease. There are two types of immunity: active or passive. Active immunity results when exposure to a certain organism makes the body produce antibodies against that specific organism. This can be achieved by the bodies natural defenses or through immunization. Passive immunity develops immediately when the person is given the antibodies needed to fight off a specific organism. This can be achieved from mother passing on the antibodies to their baby, or from an injection of an immune globulin (18).
Neutrophils- help defend the body from bacteria and viruses by phagocytosis, make up 50-70% of the leukocytes.
Eosinophils- fight larger parasites and are involved in allergic responses, make up 1-4% of the leukocytes.
Basophils- release histamine as a response to inflammation, make up <1% of the leukocytes.
Monocytes- released from the blood and become macrophages, make up 2-8% of the leukocytes.
Lymphocytes- all derive from the same type of stem cell and can be classified as B-lymphocytes, T-lymphocytes, or natural killer cells, make up 20-40% of the leukocytes.
B-lymphocytes help form antibodies to fight off antigens, and T-lymphocytes are involved in cell mediated immunity. T-cells have a T cell receptor on their surface while B-cells have B cell receptors on their surface. Natural killer cells help defend agains tumors and viruses (1).
Immunity means that a person has the needed defenses in order to fight off a certain infection or disease. There are two types of immunity: active or passive. Active immunity results when exposure to a certain organism makes the body produce antibodies against that specific organism. This can be achieved by the bodies natural defenses or through immunization. Passive immunity develops immediately when the person is given the antibodies needed to fight off a specific organism. This can be achieved from mother passing on the antibodies to their baby, or from an injection of an immune globulin (18).
References
1. Quantitative Human Physiology- Joseph Feher
2. http://medical-dictionary.thefreedictionary.com/left+ventricle
3. http://www.daviddarling.info/encyclopedia/H/heart_valve_disease.html
4. http://www.youtube.com/watch?v=PgI80Ue-AMo
5. http://www.nlm.nih.gov/medlineplus/ency/article/003644.htm
6. http://www.fi.edu/learn/heart/blood/red.html
7. http://www.nlm.nih.gov/medlineplus/ency/article/003645.htm
8. http://themedicalbiochemistrypage.org/hemoglobin-myoglobin.php
9. http://www.news-medical.net/health/What-are-Cytokines.aspx
10. http://faculty.ucc.edu/biology-potter/life_cycle_of_the_erythrocyte.htm
11. http://www.medicinenet.com/liver/article.htm
12. http://www.nlm.nih.gov/medlineplus/ency/article/003479.htm
13. http://www.medterms.com/script/main/art.asp?articlekey=2189
14. http://www.nlm.nih.gov/medlineplus/ency/article/002237.htm
15. http://www.daviddarling.info/encyclopedia/L/liver.html
16. http://www.niaid.nih.gov/topics/immunesystem/Pages/default.aspx
17. http://medical-dictionary.thefreedictionary.com/leukocyte
18. http://uhaweb.hartford.edu/bugl/immune.htm
2. http://medical-dictionary.thefreedictionary.com/left+ventricle
3. http://www.daviddarling.info/encyclopedia/H/heart_valve_disease.html
4. http://www.youtube.com/watch?v=PgI80Ue-AMo
5. http://www.nlm.nih.gov/medlineplus/ency/article/003644.htm
6. http://www.fi.edu/learn/heart/blood/red.html
7. http://www.nlm.nih.gov/medlineplus/ency/article/003645.htm
8. http://themedicalbiochemistrypage.org/hemoglobin-myoglobin.php
9. http://www.news-medical.net/health/What-are-Cytokines.aspx
10. http://faculty.ucc.edu/biology-potter/life_cycle_of_the_erythrocyte.htm
11. http://www.medicinenet.com/liver/article.htm
12. http://www.nlm.nih.gov/medlineplus/ency/article/003479.htm
13. http://www.medterms.com/script/main/art.asp?articlekey=2189
14. http://www.nlm.nih.gov/medlineplus/ency/article/002237.htm
15. http://www.daviddarling.info/encyclopedia/L/liver.html
16. http://www.niaid.nih.gov/topics/immunesystem/Pages/default.aspx
17. http://medical-dictionary.thefreedictionary.com/leukocyte
18. http://uhaweb.hartford.edu/bugl/immune.htm