The heartbeat is regulated by the cardiac conduction system

26 Jun The heartbeat is regulated by the cardiac conduction system

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Lecture – The Circulatory System

Blood

The functions of blood fall into three categories: transport, defense, and regulation. Blood moves from the heart to all of the different organs and tissue where exchange of gases and other materials takes place. Blood picks up oxygen from the lungs and nutrients from the digestive tract and carries these to the tissues. It also picks up and transports cellular wastes away from the tissues. The blood defends the body against invasion by pathogens. Blood also removes dead and dying cells as well as destroying mutated cells. Certain white blood cells engulf and destroy pathogens or cancer cells; others produce and secrete antibodies. These antibodies incapacitate pathogens, making it easier for other white blood cells to destroy them. When an injury occurs, blood forms a clot which prevents blood loss. Blood helps regulate body temperature by picking up heat, mostly from active muscles, and transporting it throughout the body. If the blood is too warm, the heat dissipates to the environment from dilated blood vessels in the skin. Blood also contains buffers which help regulate body pH keeping it relatively constant.

The adult human body contains about 5 liters of blood with just over 50% being plasma. Although blood appears to be homogeneous, it is actually made up of formed elements—blood cells and platelets—suspended in plasma, the liquid part of the blood. Blood is the only fluid tissue in the body but is classified as connective tissue. It is heavier and more viscous—thicker—than water, and slightly alkaline with a pH of 7.35-7.45.

Plasma is about 92% water with the remaining 8% being various salts and organic molecules. There are three major types of plasma proteins: albumins, globulins, and fibrinogen. Albumin is important in regulation of osmotic pressure. Globulins generally are involved in the immune system, transport, and clotting. Fibrinogen is important in clotting with the production of fibrin, a threadlike protein that forms blood clots. .0/msohtmlclip1/01/clip_image002.jpg”>.0/msohtmlclip1/01/clip_image003.gif” alt=”erythrocyte”>

The formed elements consist of red blood cells, white blood cells and platelets. The largest number of formed elements are the red blood cells— erythrocytes. These cells are quite unique in that they are biconcave and as mature cells, they lack a nucleus. (See Figure 1) The erythrocytes are formed in the red bone marrow (typically found in long bones). As the cell matures, it gains the hemoglobin and loses the nucleus and most of the organelles. The primary function of the red blood cells is to carry oxygen to all of the other cells. Besides carrying oxygen to cells of the body, RBCs help to remove CO 2.

About one-third of the RBC is hemoglobin.The hemoglobin combines readily with oxygen which it then carries from the lungs to the remaining cells of the body. When the hemoglobin drops off the oxygen at the cells, it picks up carbon dioxide which it carries back to the lungs for expulsion. The life span of RBCs is about 120 days. It is estimated that about 2 million red blood cells are destroyed every second; therefore it is important that an equal number be made to maintain balance.

.0/msohtmlclip1/01/clip_image004.jpg” alt=”lymphocyte”>Leukocytes—white blood cells—retain their nucleus and are present in much smaller numbers than erythrocytes. Leukocytes help provide a defense against disease organisms, and certain cells either promote or decrease inflammatory responses. There are five types of WBCs. All of which must be stained in order to be distinguishable.

Notice how many more of the RBCs than WBCs there are. Also each type of WBC has a different function. Neutrophils are the most abundant of the white blood cells and are the first to respond to an infection. They can surround and destroy bacteria and other foreign particles. Eosinophilsincrease rapidly during allergic reactions neutralizing histamines and also act to destroy parasitic worm infections. Basophils release histamine during allergic reactions as well as heparin which prevents clotting and promotes blood flow. Lymphocytesare the smallest leukocytes and play a vital role in immunity (which we will discuss later). Monocytesare the largest WBCs and are active in phagocytosis trapping and destroying bacteria and cellular debris. Monocytes migrate out of the vessels and take up residence in the tissues.

Platelets (thrombocytes) are actually just fragments of cells that clump together to plug breaks in blood vessels, and they start the clotting process. The clotting process is calledhemostasisand consists of three parts: vascular spasm, platelet plug formation, and coagulation. The immediate response to a broken blood vessel is a vascular spasm (which reduces blood loss). The next event is the platelet plug formation; platelets will stick to the collagen fibers which are exposed. Two plasma proteins, fibrinogen and prothrombin, participate in the actual clotting action. Vitamin K is necessary for prothrombin formation, so your diet even plays a role in blood clotting.

Human Blood Types

We will spend a little more time on blood types since this is such an important topic and one that everyone needs to know about. While several different blood types occur in humans, the most familiar ones involve the ABO blood groups (types A, B, AB, and O) and the Rh factor. There are three different alleles for this blood group.

Blood type alleles

Blood Type

I A

A

I B

B

i

O

Since there are three different alleles, there are a total of six different genotypes at the human ABO genetic locus.

Allele from
Parent 1

Allele from
Parent 2

Genotype of
offspring

Blood types of
offspring

I A

I A

I AI A

A

I A

I B

I AI B

AB

I A

i

I Ai

A

I B

I A

I AI B

AB

I B

I B

I BI B

B

I B

i

I Bi

B

i

i

i i

O

What type you have depends on whether or not there are certain proteins, called antigens, on the red blood cells or if there are antibodies to these substances.

If you havetype A blood, you can only receive types A and O blood. Type A individuals have A antigens and anti-B antibodies.

If you havetype B blood, you can you can only receive types B and O blood. Type B individuals have B antigens and anti-A antibodies.

If you havetype AB blood, you can only receive types A, B, AB, and O blood. This type has both A and B antigens but neither antibodies.

If you havetype O blood, you can you can only receive type O blood. Type O has neither A nor B antigens but both antibodies.

Blood typing also routinely tests for the presence of the Rh antigen. If the Rh antigen is present on the RBCs, the blood is typed as Rh positive (Rh +); if the antigen is absent, the blood is Rh negative (Rh -). Anti-Rh antibodies are formed only when Rh +RBCs are introduced into an individual with Rh -blood. Erythroblastosis fetalisresults from destruction of the fetal erythrocytes by maternal antibodies. This is not really a problem during the first pregnancy and childbirth, but because of the buildup of anti-Rh antibodies in subsequent pregnancies, problems may result.

Heart and Blood Vessels

.0/msohtmlclip1/01/clip_image005.jpg” alt=”cardiovascular system”>The heart and blood vessels are part of the cardiovascular system. The heart pumps blood through a closed system of blood vessels. Arteries carry blood away from the heart to capillaries in body tissues. Veins carry blood from capillaries in body tissues back to the heart.

This system can be divided into two functional systems: the pulmonaryand systemic circuits.The pulmonary includes the vessels that transport blood to and from the lungs while the systemic is for transport of blood to and from all parts of the body except the lungs.

The heartis a four-chambered muscular pump that is located between the lungs in the thoracic cavity just superior to the diaphragm. From the moment it begins beating until the moment it stops, the human heart works tirelessly. In an average lifetime, the heart beats more than two and a half billion times without ever pausing to rest. The apex (pointed end) points down and to the left. The typical heart is 5 inches (12 cm) long, 3.5 inches (8-9 cm) wide and 2.5 inches (6 cm) from front to back, and is roughly the size of your fist. The average weight of a female human heart is 9 ounces and a male’s heart is 10.5 ounces.

The heart has three layers. The smooth, inside lining of the heart is called the endocardium. The middle layer is heart muscle called the.0/msohtmlclip1/01/clip_image006.jpg” alt=”position of heart”>myocardium(by far the thickest). It is surrounded by a two-layered serous membrane called the pericardium.

Each chamber of the heart has a sort of one-way valve at its exits that prevents blood from flowing backwards. When the chamber contracts, the valve at its exit opens. When it is finished contracting, the valve closes so that blood does not flow backwards. (See Figure 5)

The tricuspid valveis at the exit of the right atrium into the right ventricle.
The pulmonary semilunar valveis at the exit of the right ventricle to the pulmonary artery.
The mitral valve(also called the bicuspid) is at the exit of the left atrium to the left ventricle.
The aortic semilunar valveis at the exit of the left ventricle to the aorta.
.0/msohtmlclip1/01/clip_image007.jpg” alt=”heart chambers and valves”>When the heart muscle contracts or beats (called systole), it pumps blood out of the heart. The heart contracts in two stages. In the first stage, the right and left atria contract at the same time, pumping blood to the right and left ventricles; then the ventricles contract together to propel blood out of the heart. Then the heart muscle relaxes (called diastole) before the next heartbeat. This allows blood to fill up the heart again.

The sounds of the heartbeat are usually described as lub-dup(pause) lub-dup. These sounds are made by the closing of the heart valves. The first sound results from the tricuspidand mitral(called atrioventricular valves) closing. The second sound results from the closing of the pulmonary and aortic( semilunar) valves.

The right and left sides of the heart have separate functions. The right side of the heart collects oxygen-poor blood from the body and pumps it to the lungs where it picks up oxygen and releases carbon dioxide. The left side of the heart then collects oxygen-rich blood from the lungs and pumps it to the body so that the cells throughout your body have the oxygen they need to function properly.

Circuit of Vessels
The natural pacemaker of the heart is a special group of cells that have the ability to generate electrical on their own and is called the sinoatrial.0/msohtmlclip1/01/clip_image008.jpg” alt=”heart electrical conduction system”>node(SA node). It is located in the right atrium. The heart also contains specialized fibers that conduct the electrical impulse from the pacemaker (SA node) to the rest of the heart (see Figure 6).

The electrical impulse leaves the SA node(1) and travels to the right and left atria, causing them to contract together. This takes .04 seconds. There is now a natural delay to allow the atria to contract and the ventricles to fill up with blood. The electrical impulse has now traveled to the atrioventricular node (AV node)(2). The electrical impulse now goes to the Bundle of His(3), then it divides into the right and left bundle branches(4) where it rapidly spreads using Purkinje fibers(5) to the.0/msohtmlclip1/01/clip_image009.gif” alt=”electrical impulse animation”>muscles of the right and left ventricle, causing them to contract at the same time.

As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body. This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram(ECG, or EKG). A “typical” ECG tracing is shown below in Figure 7. The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles.

.0/msohtmlclip1/01/clip_image010.jpg” alt=”electrocardiogram”>

BLOOD VESSELS

.0/msohtmlclip1/01/clip_image011.jpg” alt=”comparison of blood vessels”>There are three basic types of blood vessels that form a closed system: arteries, capillaries, and veins. The walls of arteries and veins have the same basic structure. However, arterial walls have more smooth muscle and elastic connective tissues because of the higher blood pressure found in them. Another difference is that large veins have valves that prevent a backflow of blood. Capillaries are the most numerous and smallest blood vessels. A capillary’s diameter is so small that RBCs must pass through them single file. The walls have only one layer of cells which allows the exchange of materials between the blood in them and the body cells.

Lymphatic System

During the exchange of materials between the capillary blood and interstitial fluid, more fluid enters the tissue than is returned to the blood. The lymphatic systemreturns the excess fluid to the blood for reuse. By recycling this fluid, the system helps to maintain homeostasis by conserving water and dissolved substances. The lymphatic capillaries are small, closed-ended vessels that are made of simple squamous epithelium but without a basement membrane. This allows the fluid to easily flow into the capillary. See Figure 9 for the relationship between the cardiovascular and lymphatic system.

Once the fluid enters a lymphatic capillary it is called lymph. The lymphatic vessels carry the lymph back through lymphatic glands and eventually empty into a large vein near the heart. There are several lymphatic organs or glands including lymph nodes, tonsils, spleen, and thymus gland. A major function of lymph nodes is the filtration and cleansing of the lymph. The tonsils, spleen and thymus all function to intercept and destroy pathogens as well as the maturation of T lymphocytes( T cells). Other cells become B lymphocytes( B cells).

All cells have surface recognition molecules called antigens. (Remember that antigens are used in typing blood.) During the specialization process, lymphocytes “learn” to distinguish “self” antigens. They are then able to tell the difference between ‘invaders’ and our own tissue (which can be both good and bad).

The immune response involves one or both of two different mechanisms. Antibody-mediated immunityinvolves both T and B cells. Cell-mediated immunityinvolves only the T cells. The first step in an immune response against a pathogen is recognizing that its antigens are foreign. This involves an antigen-presenting celland a T cell. When a T cell is activated it becomes either a cytotoxic (killer) T cellor a helper T cell. The helper T is what actually gets the immune response started. The helper T cell activates B cells to divide; some become plasmacells which actually produce the antibodies and others become memoryB cells. Memory B cells remain to launch an even stronger immune response if the same pathogen ever reenters the body. This antibody defense is referred to as humoral defense. The killer T cells participate in the cell-mediated defenseby directly attacking the abnormal and foreign cells.