LEC 4 PHYSIOLOGY

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Lec: 4
The major components of the circulatory system are heart, blood vessels and blood. The circulatory system consists of the pulmonary and systemic circulations. The major functions of the circulatory system are:-
1. Transports oxygen and carbon dioxide to and from tissues.
2. Transports nutrients, waste products, and hormones.
3. Regulates body temperature.
4. Plays a role in the immune response.
The term cardiac cycle refers to the repetitive pumping process that begins with the onset of cardiac muscle contraction and ends with the beginning of the next contraction. Pressure changes produced within the heart chambers as a result of cardiac muscle contraction are responsible for blood movement because blood moves from areas of higher pressure to areas of lower pressure.
Atrial systole is contraction of the atria, and ventricular systole is contraction of the ventricles. Atrial diastole is relaxation of the atria, and ventricular diastole is relaxation of the ventricles.
The right side of the heart forces blood to flow through vessels to the lung and back to the heart through the pulmonary circulation. The left side of the heart forces blood to flow through vessels to the tissues of the body and back to the heart through the systemic circulation.
Conduction system of the heart
Contraction of the atria and ventricles is coordinated by specialized cardiac muscle cells in the wall of the heart. The sinoatrial node (SA node), which functions as the pacemaker of the heart, is located in the upper wall of the right atrium and initiates the contraction of the heart. Action potentials originate in the SA node and spread over the right and left atria, causing them to contract.
A second area of the heart, called the atrioventricular node (AV node), is located in the lower portion of the right atrium. When Action potentials reach the AV node, they spread slowly through it and then into a bundle of specialized cardiac muscle called the bundle of His, which projects through the fibrous connective tissue plate that separates the atria from the ventricles.
The Cardiovascular System
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The bundle of His then divides into two branches called the left and right bundle branches. At the tips of these branches, the conducting tissue forms many small bundles of Purkinje fibers. These Purkinje fibers extend to the cardiac muscle of the ventricle walls. The coordinated contraction of the ventricles depends on the rapid conduction of action potentials by the conduction system. Following their contraction, the ventricles begin to relax. After the ventricles have completely relaxed, another action potential originates in the SA node to begin the next cycle of contractions.
Conduction system of the heart
Electrocardiogram
Action potentials conducted through the heart during the cardiac cycle produce electric currents that can be measured at the surface of the body. Electrodes placed on the surface of the body and attached to a recording device can detect the small electric changes resulting from the action potentials in all of the cardiac muscle cells. The record of these electrical events within the heart is an electrocardiogram (ECG).
The electrocardiograph is an extremely important diagnostic tool. Analysis of an ECG can be used to identify abnormal heart rates or rhythms, abnormal conduction pathways, hypertrophy or atrophy of portions of the heart, and the approximate location of damaged cardiac muscle.
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The normal ECG consists of a P wave, a QRS complex, and a T wave. The P wave results from depolarization of the atrium, and precedes the onset of atrial contraction. The QRS complex results from depolarization of the ventricles, and precedes ventricular contraction. The T wave represents repolarization of the ventricles, and precedes ventricular relaxation.
Electrocardiogram
Blood pressure
Blood pressure is a measure of the force blood exerts against the blood vessel walls. During ventricular systole, the heart pushes blood into the arteries, and the pressure reaches a maximum called the systolic pressure. When the ventricles relax, blood pressure in the arteries falls to a minimum value called the diastolic pressure.
The auscultatory method of determining blood pressure is used under most clinical conditions. Sphygmomanometer is an instrument for measuring blood pressure. The standard measure for blood pressure is in terms of millimeters of mercury (mm Hg). A standard blood pressure for a resting young adult male is 120 Hg for the systolic pressure and 80 Hg for the diastolic pressure, which is reported as 120/80. Generally, if the blood pressure exceeds 140/90, it is considered too high.
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Lymphatic system
The lymphatic system includes lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, the spleen, and the thymus gland. The lymphatic system is part of the body’s defense system against microorganisms and other harmful substances. In addition, it helps to maintain fluid balance in tissues and to absorb fats from the digestive tract.
Lymphatic vessels
The lymphatic system removes fluid from tissues. The removal of fluid begins in lymphatic capillaries, which are tiny, closed-ended vessels consisting of simple squamous epithelium. Fluid moves from blood into tissue spaces, and some of the fluid enters lymphatic capillaries to become lymph. The lymphatic capillaries empty lymph into lymphatic vessels, which converge into larger ducts, that return the lymph to the blood.
Three factors assist in the transport of lymph through the lymphatic vessels: (1) contraction of surrounding skeletal muscles during activity, (2) contraction of smooth muscles in the lymphatic vessel wall, and (3) pressure changes in the thorax during respiration.
The lymphatic vessels converge and eventually empty into the blood at two locations in the body. Lymph vessels from the upper right limb and the right half of the head, neck, and chest form the right lymphatic duct, which empties into the right subclavian vein. Lymphatic vessels from the rest of the body enter the thoracic duct, which empties into the left subclavian vein.
Lymphatic organs
Lymphatic organs include the tonsils, lymph nodes; the spleen, and the thymus gland. Lymphatic tissue, which consists of many lymphocytes and other cells, is found within lymphatic organs. The lymphocytes originate from red bone marrow and are carried by the blood to lymphatic organs. When the body is exposed to microorganisms or foreign substances the lymphocytes divide and increase in number. The increased number of lymphocytes is part of the immune system response that causes the destruction of microorganisms and foreign substances.
Tonsils
There are three groups of tonsils. The palatine tonsils usually are referred to as the "tonsils," and they are located on each side of the posterior opening of the oral cavity. The pharyngea1 tonsils are located near the internal opening of the nasal cavity. The lingual tonsils are on the posterior face of the tongue.
The tonsils form a protective ring of lymphatic tissue around the openings between the nasal and oral cavities and the pharynx. They provide protection against pathogens and other potentially harmful material entering from the nose and mouth. Sometimes the tonsils become chronically infected and must be removed.
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Lymph nodes
Lymph nodes are rounded structures, varying in size. Lymph nodes are distributed along the various lymphatic vessels. Although lymph nodes are found throughout the body, there are three superficial aggregations of lymph nodes on each side of the body: inguinal nodes in the groin, axillary nodes in axilla, and cervical nodes in the neck.
Lymph nodes are divided into compartments contain lymphatic tissue and lymph sinuses. The lymphatic tissue consists of lymphocytes and other cells that can form dense aggregations of tissue called lymph nodules. Lymph sinuses are spaces between lymphatic tissues which contain macrophages on a network of fibers. Lymph enters the lymph node through afferent vessels, passes through the lymphatic tissue and sinuses, and exits through efferent vessels.
As lymph moves through the lymph nodes two functions are performed. One function is activation of the immune system. Microorganisms or other foreign substances in the lymph can stimulate lymphocytes in the lymphatic tissue to start dividing. Another function of lymph nodes is the removal of microorganisms and foreign substances from the lymph by macrophages.
Spleen
The spleen is located in the left, superior corner of the abdominal cavity. The spleen filters blood instead of lymph, and contains two specialized types of lymphoid tissue. White pulp surrounds the arteries within the spleen, and red pulp is associated with the veins.
Lymphocytes in the white pulp can be stimulated in the same manner as in lymph nodes. Before blood leaves the spleen through veins, it passes through the red pulp. Macrophages in the red pulp remove foreign substances and worn out red blood cells through phagocytosis.
The spleen also functions as a blood reservoir, holding a small volume of blood, in emergency situations such as hemorrhage, smooth muscles in splenic blood vessels and in the splenic capsule can contract. The result is the movement of a small amount of blood out of the spleen into the general circulation.
Thymus
The thymus is a bilobed gland. It is located in the superior mediastinum, the partition dividing thoracic cavity into left and right parts. The thymus continues to grow until puberty. After puberty, the thymus decreases in size, and become (0) in adults, the thymus may be so small that it is difficult to find during dissection.
The thymus functions as a site for the production and maturation of lymphocytes. After thymic lymphocytes have matured, they enter the blood and travel to other lymphatic tissues, where they help to protect against microorganisms and other foreign substances.
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