your life biology4

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Module 3: Circulation and Defense Mechanisms of the Body

Topics

1. The Blood

2. The Circulatory System

3. The Lymphatic System

4. The Immune System

1. The Blood

Whole blood is composed of formed elements (cellular components) and plasma, a watery transport medium containing solutes). Blood plasma with the clotting factors, such as fibrin, removed is known as serum. Simply stated,

· blood = cells + plasma

· plasma = blood – cells

· serum = plasma – clotting factors

Table 3.1 describes formed elements, their lifespan and function.

Table 3.1 Formed Elements of Blood

Formed element	Type	Life span	Function
Red blood cells		120 days	Transport oxygen
White blood cells	Neutrophils	6 hours to days	Phagocytize (ingest and destroy) bacteria
	Esosinophils	8–12 days	Phagocytize antigen-antibody complexes; attack parasites; contribute to allergic reactions
	Basophils	Hours to days	Release histamine (a protein that dilates blood vessels and increases the permeability of their walls) in the inflammation response; contribute to allergic reactions
	Monocytes	Months	Phagocytize bacteria, dead cells, and cellular debris
	Lymphocytes	B	Years	Involved in antigen-mediated immunity
		T	Years	Involved in cell-mediated immunity
Platelets		5–10 days	Participate in blood clotting

 

A. Red Blood Cells

Red blood cells (RBCs or erythrocytes) contain the four-subunit (heme) protein hemoglobin (Hb). Each of the hemes encircles an iron (Fe) atom that binds to oxygen (O2), making hemoglobin responsible for nearly all of the oxygen transported in the blood (Chiras, 2005). Carbon monoxide (CO) has a much higher chemical affinity for iron (it forms a bond with iron more easily) than does oxygen; for this reason, it is lethal at high concentrations.

In low-oxygen conditions, the kidneys secrete erythropoietin, a hormone that increases RBC production in the bone marrow. Secretion of erythropoietin is a homeostatic necessity in high altitudes, where oxygen is scarce ("thin air"). Athletes exploit this mechanism, training in elevated locales to force their bodies to produce more RBCs. Increased numbers of RBCs allow them to compete at higher endurance levels when back in a lower altitude. The liver and spleen destroy old RBCs.

Callout

Focus on the Blood: Fifty-Six Facts

Did you know that

1. RBCs carry oxygen to the body's organs and tissues?

2. RBCs live about 120 days in the circulatory system?

3. one pint of blood can save up to thee lives?

4. plasma, which is 90% water, makes up 55% of blood volume?

Go to: https://fourhearts.org/facts/ to read the other fifty-three facts.

 

B. Plasma

Table 3.2 shows plasma constituents with their types and functions.

Table 3.2 Plasma Constituents

Plasma Constituents	Types and Function
Water	Solvent
Proteins	Albumin—contributes to blood osmotic pressure and serves as a buffer
	Globulin—transport, enzymatic action, clotting, and immunity
	Fibrinogen—clotting
Electrolytes	Na+, K+, Ca2+, Mg2+, Cl–, HCO3– - membrane transport, neurological processes, blood osmolarity
Hormones	amino acid-based and lipid-based hormones—modify processes in target tissues
Nutrients	glucose, amino acids, lipids, cholesterol, vitamins, and trace elements
Dissolved gases	CO2, O2, and N2
Waste products	urea, uric acid, creatinine, and bilirubin

 

C. ABO Blood Type Antigens

The ABO blood antigen group consists of types A, B, AB, and O. Studying the relationships between antibody types in the plasma and antigen types on the RBCs can help us predict which transfusions will work and which will not. We can use table 3.3 to determine blood type compatibility.

Table 3.3 Blood Type Compatibility

Individual's blood type	Antigens on his/her blood cells	Antibodies in his/her blood plasma	He/she can donate blood to a person with blood type	He/she can receive blood from a person with blood type	He/she cannot receive blood from a person with blood type
A	A	anti-B	A, AB, O	A, O	B, AB
B	B	anti-A	B, AB, O	B, O	A, AB
AB	A, B	none	AB	A, B, AB, O	(NA—universal recipient)
O	none	anti-A, anti-B	A, B, AB, O (universal donor)	O	A, B, AB

 

D. Rh Factor Antigen

The Rh factor belongs to a different blood type group. It was first discovered in rhesus monkeys, hence the abbreviation Rh. Two types, Rh positive (Rh+) and Rh negative (Rh–), have important effects, especially on childbirth. A pregnant woman who is Rh– will have no Rh antigens on her RBCs; she will instead have the antibodies to this antigen (the antibodies against Rh+). If she bears the child of an Rh+ man, the child's blood type will be Rh+ (inherited from the father). Because the fetal blood can get into the mother's blood, and because maternal antibodies can cross over to the placenta, the mother's anti-Rh antibodies will recognize the fetal blood as foreign, and will produce more anti-Rh antibodies.

Normally, the first child is unaffected by these developments, but a second child will have problems, because there are now a lot of anti-Rh antibodies circulating in the mother's blood. If the mother receives no medication, her anti-Rh antibodies will attack and possibly kill the Rh+ fetus in a condition called hemolytic disease of the newborn (HDN). Thankfully, physicians can prescribe drugs to suppress the anti-Rh antibodies (Rh immune globulin, or RhoGAM). RhoGAM is administered during pregnancy at 28 weeks and then within 72 hours after delivery.

Generally, if one has Rh+ blood, one can receive transfusions from people with Rh+ and Rh– blood types. If one has Rh– blood, however, one can receive transfusions only from Rh– individuals.

Taken together, the ABO and Rh groups give rise to blood types such as A+, O–, and all of the other combinations of these antigens. Each RBC of a person has one of the ABO types plus one of the Rh types. Medical practitioners take these combinations into consideration in blood transfusions to ensure safety. In assessment 3.1, you can test your understanding of blood types!

 

E. Diseases of the Blood

Upsets in homeostasis can cause diseases or problems in the blood. Blood disorders can be divided into the following groups (Martini, 2000):

· inherited—sickle cell disease, hemophilia, thalassemia

· neoplastic—leukemia

· infectious—bacteremia, septicemia, malaria

· nutritional—iron deficiency anemia, pernicious anemia, vitamin K deficiency

· environmental caused by trauma—hemorrhagic anemia

· immune—hemolytic disease of the newborn

 

1. Anemia

Anemia is defined as a decrease in the body's RBCs. The common findings associated with anemia are a decrease in the RBC count, hemoglobin, or hematocrit (percentage of RBCs in the blood). This condition has several causes, including insufficient vitamin and iron intake, hemorrhage (internal and external blood loss), hemolysis (RBC destruction), and low RBC production. Symptoms include weakness, fatigue, and, in extreme cases, heart failure. In the following subsections, we describe several types of anemia.

a. Sickle Cell Anemia

Sickle cell anemia is acquired through a single mutation in the hemoglobin gene. Normal RBCs are shaped like donuts with flattened centers. When RBCs affected by the variant release oxygen into body tissues, the hemoglobin takes the form of long, linear chains, causing the cell to twist into a sickle shape (C or crescent moon-shape). Sickle-shaped RBCs clump together in the blood vessels, causing spleen and liver disorders, chest pain, shortness of breath, fever, and low oxygen levels in the blood (hypoxemia). Affected people tend to have a shortened life span, although technology has lengthened significantly the lives of patients. It is interesting to note that, in parts of Africa, this mutation is prevalent because it protects against malarial infection. Sickle cell anemia is most common among people of African descent.

b. Iron-Deficiency Anemia

Iron-deficiency anemia is caused by insufficient iron in the body and is most common in young children and women. Iron is necessary for the proper maturation and function of RBCs. The deficiency can be related to an insufficient intake of iron, inability to absorb iron (celiac disease), heavy menstrual bleeding or ongoing slow internal bleeding from polyps, or ulcers. Symptoms are quite general—fatigue and shortness of breath. Severe anemia is accompanied by recurrent infections.

Treatment involves parenteral (injection or infusion) administration of iron supplements. According to the Centers for Disease Control and Prevention, iron deficiency anemia can be prevented by eating a healthful diet that contains a wide variety of fruits, vegetables, whole grains, nonfat (or lowfat) dairy products, lean meats, fish, dry legumes, and nuts. Iron from meat, poultry, or fish is absorbed two–three times more efficiently than the iron from plants. Foods that are good sources of vitamin C will increase the absorption of iron in a meal (Centers for Disease Control, 2007).

c. Pernicious Anemia

Pernicious anemia is caused by the inability of the body to absorb vitamin B12 from food because of the lack of intrinsic factor. This lack can be inherited or caused by autoimmune damage of the cells producing intrinsic factor (a protein necessary for absorption of vitamin B12 in the intestines) in the stomach. Other causes include surgical removal of the stomach, celiac disease, and a diet poor in vitamin B12. The process of aging will also decrease the amount of intrinsic factor available in the stomach and therefore decrease the absorption of vitamin B12. Symptoms of deficiency include fatigue and shortness of breath. Vitamin B12 replacement therapy allows people to live normal lives (National Heart, Lung, and Blood Institute, 2008b).

 

2. Thalassemia

Thalassemias belong to an inherited category of disorders. They can be divided into two types, alpha and beta, based on the location of the defect in the protein chains of hemoglobin. Two chains are called alpha globin, and two are called beta globin. Alpha thalassemia is common in people of Indian, Chinese, and Filipino descent, whereas beta thalassemia is common in people of Italian, Greek, Middle Eastern, and African origin. Anemia resulting from thalassemia can range from mild (patients might not display any signs of disease) to moderate (bone issues related to bone marrow and growth are present) to severe (pale appearance, jaundice, dark urine, stunted growth, and enlarged heart and liver) (National Heart, Lung, and Blood Institute, 2008a). Treatment depends on the severity of the disease and may include iron therapy, folic acid supplements, and transfusions.

 

3. Leukemia

There are several types of leukemia, or cancer of the WBCs, such as acute lymphoblastic leukemia. All, however, have the same cause: the bone marrow produces immature or abnormal WBCs, which impair the production of platelets, RBCs, and healthy WBCs. Patients suffer from anemia, blood-clotting problems caused by the lack of platelets, and infections caused by the lack of WBCs. Lack of oxygen causes many other symptoms and may lead to death. Treatment options include chemotherapy and radiation therapy (Komaroff, 2005, p. 725).

 

4. Septicemia

Septicemia or sepsis (blood poisoning) is the invasion of the blood by bacteria and their toxins and is characterized by chills, fever, and prostration (physical and mental exhaustion or collapse). Septic shock can result if the condition is not treated early. Advanced cases may necessitate the use of antibiotics or more invasive treatments. Among injured soldiers in World War I who did not die of their wounds, septicemia was the most common cause of death. By the 1940s, the use of penicillin had eradicated septicemia as a major threat.

 

5. Hemophilia

Hemophilia is an X-linked inherited disorder. X-linked means that the defect is carried on an X chromosome. An interesting historical fact is that Queen Victoria was a carrier for hemophilia genes. Because none of her predecessors had hemophilia, the mutation in the X-chromosome had to occur in her parents' or her own sex cells. As a result of her mutation, 4 of her 9 children, 8 of her 25 grandchildren, and 6 of her 34 great-grandchildren were affected. Among the affected grandchildren was Alexis the son of Czar Nicholas II who married Alexandra (Queen Victoria's granddaughter, carrier of the gene) (Aronova-Tiuntseva & Herreid, 2003).

The blood disease hemophilia has two main types—A and B. Also known as Christmas disease, hemophilia B is characterized by the lack of factor IX, a key component in blood clotting. Those with hemophilia A lack factor VIII, also crucial for clotting. About 90 percent of hemophiliacs have hemophilia A (National Heart, Lung and Blood Institute, 2006). In both instances of the disease, the body has no mechanism to keep bleeding in check. In extreme cases, a single wound can prove fatal. Injections of genetically engineered clotting factors can help prevent excessive blood loss.

 

6. Porphyria

There are seven steps in the biosynthesis of heme, a component of hemoglobin. Each step can be blocked, causing seven different types of porphyria (Thadani, Deacon, & Peters, 2006). Porphyria is a group of disorders with different symptoms depending on where the flaw occurs in heme production.

King George III, who ruled England during the American Revolution, was said to be prone to fits of insanity ("the madness of King George") stemming from acute intermittent porphyria (AIP). This disease is characterized by attacks of abdominal pain and neurological dysfunction. King George was said to have urine the color of port wine, and he had several descendants diagnosed with this disease (Voet, Voet, & Pratt, 2006). History might have turned out very differently had he not had this disorder!

 

2. The Circulatory System

The circulatory system consists of the heart and an extensive network of blood vessels thousands of miles long (Smith, 2001). It is a homeostatic system that performs or helps perform the following functions (Chiras, 2005):

· maintaining constant levels of nutrients and wastes

· regulating body temperature

· distributing body heat

· protecting against microorganisms

· protecting against blood loss (through clotting)

The circulatory system has two separate circuits of blood flow—the pulmonary circuit, which transports blood to and from the lungs, and the systemic circuit, which delivers blood to the rest of the body and back to the heart. Blood travels through the blood vessel network as shown on Figure 3.1. When the body is at rest, the entire blood volume (5–7 liters, or 10–15 pints) takes approximately one minute to circulate throughout the body (Smith, 2001).

Figure 3.1 Heart and Blood Circulation

 

A. The Heart

The heart is a muscular pump located in the thoracic cavity. During a typical lifetime, it beats more than one billion times! It is a double pump—the atria and ventricles alternate pumping in a tightly regulated loop. The medulla in the brain stem controls heartbeats. Heart rate is largely managed by an internal pacemaker, the sinoatrial (SA) node, located in the right atrium. This node connects with a network of nerves that encircle the inner walls of the heart, keeping cardiac muscle contraction under control and in sync with all the parts of the heart.

1. Heart Valves

There are four valves in the heart, and each allows blood to flow in only one direction. The valves allow for the forward passage of blood, and in a normal, healthy heart, prevent the backflow of blood into the previous structure. The atrioventricular (AV) valves regulate blood flow from the top to the bottom parts of the heart (atrium to ventricle). These consist of the mitral or bicuspid valve and the tricuspid valve. The semilunar (SL) valves regulate blood flow to the outside of the heart. These consist of the aortic semilunar valve and the pulmonary semilunar valve.

Each valve has a set of flaps (also called leaflets or cusps). The bicuspid valve has two flaps, and the others have three. Blood flow occurs when a difference in pressure across a valve causes it to open.

2. Blood Flow through the Heart

The flow of deoxygenated blood (low in O2, high in CO2) from the body to the lungs takes this route:

superior and inferior venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → left and right pulmonary arteries (the only arteries that carry deoxygenated blood) → lungs

The lungs replenish the RBCs with O2 and remove CO2.

Oxygenated blood (high O2, low CO2) takes this route from the lungs to the body:

Lungs→ left and right pulmonary veins (the only veins that carry oxygenated blood) → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → body

Callout

Focus on the Heart: Heart Contraction and Blood Flow

The National Institutes of Health (NIH) have put together an animation showing the steps involved in the heartbeat and the associated flow of blood.

3. The Heart's Coronary Vessels

Contrary to popular belief, the heart is not nourished by the blood flowing into its inner chambers. The heart obtains nourishment from the blood flowing through its outer arteries and veins, a network referred to as the coronary vessels.

4. Blood Flow Back to the Heart

How does blood defy gravity, flowing up to the heart from the lower parts of the body? The answer to this question lies in three features of the circulatory system:

1. the slight hydrostatic pressure within the veins

2. the presence of one-way valves in some veins

3. the contraction of skeletal muscles (Chiras, 2005)

The backflow of blood is prevented by the action of one-way valves in the veins. The contraction or squeezing of skeletal muscles pushes the blood in these valves upward, and the valves then close to prevent the blood from flowing down again. Hydrostatic pressure keeps the blood moving smoothly up the vein. All of these forces enable blood to return back to the heart from the lower parts of the body (e.g., the legs) in opposition to the force of gravity.

5. Heartbeat and Blood Pressure

The cardiac cycle has two phases: the heart's systole (contraction) and diastole (dilation—a useful mnemonic). We measure blood pressure as the systole/diastole ratio in millimeters of mercury (mm Hg). Doctors can plot heartbeats on graphs called electrocardiograms (ECGs or EKGs) and study them to determine any defects or problems in the heart.

 

B. Health and Homeostasis in the Heart

The heart is a powerful and efficient pump, but some disorders can upset homeostasis and cause problems. The following is a small sampling of the known problems involving the heart and the cardiovascular system (Martini, 2000).

· infectious disorders—carditis (inflammation of the heart), endocarditis, myocarditis, pericarditis

· neoplastic disorders—sarcoma

· congenital—ventriculo-septal defects (VSDs), open foramen ovale and ductus arteriosus, tetralogy of Fallot

· hemodynamic disorders—coronary artery disease, shock, thrombosis

· degenerative disorders—cardiomyopathy, aneurysm

· functional disorders—edema, cerebrovascular accident (CVA) or stroke

 

We will briefly discuss these problems in the following subsections.

1. Atherosclerosis and Related Complications

Atherosclerosis is the thickening of the arterial walls with deposits (atheroma) of excess fats and cholesterol in the blood. These deposits form plaques consisting of a fatty core and a fibrous cap, which narrow the arterial passageway. If the plaques rupture, blood clots form. If a blood clot blocks a blood vessel leading to the heart, it causes a heart attack. If it impedes blood flow to the brain, it induces a stroke.

2. Thrombosis

Thrombosis refers to the formation or presence of a blood clot within a blood vessel. Deep vein thrombosis occurs in the deep leg veins after long periods of immobility, for example on long airplane flights, giving it the nickname economy class syndrome (ECS), although it can affect anyone on a plane, regardless of class. The clot can travel and lodge itself elsewhere from its place of origin, in a condition known as embolism. The objects that make up the transported clot, or the embolus, may include blood clot fragments, fat from fractured bone marrow, cholesterol crystals, and even air.

3. Heart Attack

Myocardial infarctions (heart attacks) occur when a blood clot, embolus, or other agent blocks the blood supply to the heart. This blockage results in ischemia (oxygen shortage), which causes the damage and death of heart tissue. Cardiac arrest, the complete stoppage of the heart, results in death.

Callout

Focus on Heart Health: Heart Attack Risk Assessment

Are you at risk for a heart attack?

Answering a few questions provided by the American Heart Association (AHA) can help you estimate your risk of having a heart attack or of dying of coronary heart disease in the next ten years.

Go to  https://www.heart.org/en/health-topics/consumer-healthcare/what-is-cardiovascular-disease/heart-health-risk-assessments-from-the-american-heart-association to assess your heart health.

Note: This quiz is not a substitute for the medical advice of a doctor. It is designed to give a preliminary assessment only. For more information, consult a physician.

4. Stroke

Strokes occur when a blood clot, embolus, or other agent blocks the blood supply to the brain. Strokes induce a sudden loss of consciousness, sensation, and voluntary motion. The blocking agent interrupts the transport of oxygen to the brain, leading to neurological damage or death if the patient is not treated immediately; the brain can survive for only a few minutes without oxygen. The American Heart Association lists the following risk factors for strokes that are preventable or treatable or both (American Heart Association, 2007): 

· high blood pressure · tobacco use · diabetes mellitus · carotid or other artery disease · atrial fibrillation · heart disease · physical inactivity

· high RBC count · sickle cell anemia · high blood cholesterol · overweight or obesity · excessive alcohol intake · history of transient ischemic attacks (TIAs, or "ministrokes") · some illegal drugs

5. Heart Failure

Heart failure (ventricular failure) does not mean that the heart cannot beat or that it has stopped beating—it means that the heart cannot pump blood effectively to the body tissues (Smith, 2001). The left or right side of the heart may have a defect (e.g., an abnormal valve or arrhythmia), with the net result that it cannot keep up with the rest of the heart in pumping at an adequate rate or in pumping an adequate volume. Heart failure occurs when the heart muscle is damaged or overworked.

6. Aneurysm

When arterial walls are weakened or diseased, they balloon and swell. These bulges in the arterial walls, or aneurysms, can occur in the aorta, the brain blood vessels, and anywhere else. If they burst, they can cause dangerous bleeding or death, with a reduced risk if they form in the extremities of the body.

7. Angina

Angina is a disease marked by brief, painful episodes of chest pain caused by deficient oxygenation of the heart muscles. It is often accompanied by a throbbing sensation in the jaw, neck, and left arm. Rest quickly alleviates the pain. Angina is a sign of narrowed arteries or some other heart problem that keeps the heart from receiving enough oxygen.

8. Arrhythmias

An arrhythmia is any disorder affecting heart rate. Bradycardia is an abnormally slow heart rate (less than 50 beats per minute [bpm] for an adult), and tachycardia is a rapid heart rate (more than 100 bpm in an adult). Levels of physical activity affect heart rate, but chronic heart rate abnormalities may indicate some form of heart disorder or disease. Tachycardia followed by chaotic beating (fibrillation) is a prelude to cardiac arrest. Fibrillation occurs when the heart muscles twitch irregularly and lose the controlled synchrony of the heartbeat. A defibrillator is an instrument used to administer an electric shock to restart the heart and stimulate a steady heartbeat.

9. Congenital Heart Defects

Congenital heart defects are abnormalities in the heart's structure that are present at birth because of a genetic defect, abnormal development of the heart in the fetus, viral infection, or some other unknown cause. Congenital heart defects include defects of the atrial septum, ventricular septum, open ductus arteriosus, aortic stenosis, stenosis of the pulmonary artery, and tetralogy of Fallot. Septal defects are located in the upper or lower portion of the septum. Defects of the ventricular septum can be found in its membranous or muscular portion. Small defects may close on their own. Holes in the heart can lead to the mixture of oxygenated and deoxygenated blood, and constrictions in the heart's arteries can cause blood-flow problems. Medication can help, but surgery is sometimes necessary to treat these defects.

 10. Endocarditis

Endocarditis is an infectious disease of the endocardium caused mostly by streptococci, especially Streptococcus viridans, and staphylococci. The danger of the infection lies in the possibility of serious changes in the structure and function of the heart valves. Symptoms include fever, skin rash, weight loss, back pain, and sweating. Endocarditis is treated by antibiotics.

 11. Neoplastic Disorders

Primary tumors of the heart are usually benign, but because they are located in the heart they may interfere with the blood flow. Prognosis after surgical removal of a tumor is generally good. An example of a benign tumor is a myxoma. On the other hand, malignant tumors, sarcomas, grow quickly and obstruct the atria and ventricles. Primary tumors in the breast, lungs, and skin can metastasize into the heart.

 

C. Treatment for Heart Diseases

Heart diseases are mostly treatable with surgery and drugs. Below, we discuss some of the most common methods of treatment.

1. Angioplasty

Angioplasty is a procedure in which a physician inserts a catheter into a leg vein and guides it up to the coronary arteries. At the tip of the catheter is a balloon that, when inflated, pushes against the narrowed arterial walls, opening up the constricted region. In some cases, a laser at the tip blasts away the thickened wall areas. The catheter pushes stents, small mesh wire structures, into place and leaves them there to keep the arteries open.

2. Coronary Bypass Surgery

Coronary bypass surgery is also known as open-heart surgery or cabbage (CABG, or coronary artery bypass graft). The idea is to "bypass" a clogged area of the coronary artery or arteries with a vein or artery graft attached to points on each side of the clog. The grafted vein or artery enables blood to circumvent the clogged part, restoring blood flow to the rest of the heart.

Depending on the number of grafts, bypass surgery is referred to as single, double (two grafts), triple (three grafts), or quadruple (four grafts). Quintuple (five grafts) bypass surgery is also possible. Doctors usually cut the vein graft from the leg. They attach one end to the aorta and the other to the coronary artery behind the afflicted area or to an artery from inside the chest wall diverted to the area just beyond the blockage. During the operation, doctors clamp the aorta and divert blood flow through a heart-lung machine, stopping the normal activities of the heart with an injection.

3. Heart Transplants

Heart transplant surgery replaces a patient's heart with a healthy heart from someone who has recently died. The diseased heart is cut away from the aorta, the front walls of the two atria, and the pulmonary artery. During this procedure, the heart-lung machine takes over in pumping blood to the body and lungs. The physician then connects the patient's pulmonary artery and aorta to the new heart, and after restarting the heart and obtaining a steady heartbeat, disconnects the heart-lung machine (Smith, 2001).