Body Fluids and Circulation
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Blood is made up of fluid matrix, plasma and formed elements.
Plasma:
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Erythrocytes (RBC), leucocytes (WBC) and platelets are collectively called formed elements and constitute about 45% of blood.
Comparison table for RBC, WBC and platelets are shown in the table below:
There are two types of WBC: granulocytes and agranulocytes.
- Granulocytes consists of neutrophils (60-65%), eosinophils (2-3%) and basophils (.5-1%)
- Agranulocytes consist of lymphocytes (20-25%) and monocytes (6-8%).
Functions of WBCs:
- Neotrophils and monocytes (phagocytic cells) destroy foreign organisms in the body.
- Basophils secrete histamine, heparin and serotonin which are involved in inflamatory reactions.
- lymphocytes (B and T) are responsible for immune response of our body.
- esnophils resist infections.
Coagulation of Blood:
Blood clotting occurs in two stages: (i) primary homeostasis and (ii) secondary homeostasis.
(i) Primary homeostasis:
- Involves platelets.
- When there is a damage in the lining of blood vessels, platelets will immediately form a plug at the damaged site to reduce blood loss. This is the first stage and is called the primary homeostasis.
(ii) Secondary homeostasis:
- Involves coagulation factors which undergoes a set of complex reaction and produces an enzyme complex called thrombokinase.
- Thrombokinase activates inactive enzyme prothrombin to thrombin, which in turn converts inactive fibrinogen into fibrin.
- Fibrin will form a mesh at the site of injury which clumps platelets and blood cells forming a clot that stops the bleeding.
Summary of blood coagulation:
cascade process -------------> thrombokinase
thrombokinase
prothrombin --------------------------> thrombin
thrombin
fibrinogen ---------------------------> fibrin
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Lymph (Tissue fluid):
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Human Blood Groups (ABO grouping and Rh grouping):
- In ABO blood grouping, blood is divided into 4 types: A, B, AB and O based on the presence or absence of two surface antigens (A and B), and two natural antibodies in the plasma.
- Antigens are chemical, toxic or foreign substance that induces immune response in the human body.
- Antibodies are blood protein produced in response to antigens in the blood.
Blood group and donor compatibility table is shown below:
- Rh blood grouping is based on the presence or absence of another antigen (Rh) on the surface of RBC. Similar antigen is also found in Rhesus monkey.
- Individuals with Rh antigen on RBC, are called Rh positive individuals, while individuals without Rh antigen on RBC are called Rh negative individuals.
- When Rh positive and Rh negative bloods are mixed, Rh negative blood will produce specific antibodies against the Rh positive blood. Therefore, Rh groups should also be matched before transfusion of blood.
Erythroblastosis foetalis: During the delivery of the first child (Rh+ve) by a mother (Rh-ve), some amount of Rh+ve blood from the child may be exposed to Rh-ve blood of the mother. If this occurs, the mother will start preparing antibodies against Rh+ve blood. During second pregnancy, if the antibodies in the Rh-ve blood of the mother leak into the Rh+ve blood of the foetus, antibodies from the maternal blood can destroy the RBCs of the foetus (Rh+ve) leading severe anemia and jaundice. This condition is called Erythroblastosis fortalis.
This condition can be avoided by administering anti-Rh antibodies after the first delivery.
Human Circulatory System:
Heart is mesodermal in origin and is protected by a double walled membrane called pericardium, which encloses pericardial fluid.
Human heart consists of 4 chambers: 2 upper chambers called atria and 2 lower chambers called ventricles. Muscular wall that separates the right and left atrium is called inter-atrial septum while the muscular wall that separates the right and left ventricles is called inter-ventricular septum. The atrium and ventricle on the same side are also separated by a fibrous tissue called atrio-venticular septum with an opening. The opening between the right atrium and right ventricle is guarded by a valve called tricuspid valve while the valve at the opening between the left atrium and left ventricle is called bicuspid valve or mitral valve. The valves present at the opening of right ventricle into pulmonary artery and left ventricle into aorta is called semilunar valves. The function of valves is to prevent backward flow of blood.
Nodal tissue is a specialised cardiac musculature present in the heart. Patch of nodal tissue present at upper right corner of right atrium is called sino-atrial node (SAN) and another mass of nodal tissue present at lower left corner of right atrium is called
atrio-venticular node (AVN). Atrio-venticular bundle (AV bundle) from artio-venticular node (AVN) passes through the
inter-ventricular septum and divides into right and left bundle which further divides into minute fibers called purkinje fiber. Purkenji fiber along with right and left bundle is called the bundle of HIS.
The nodal musculature is autoexcitable (generates action potential without external stimuli).
The sino-atrial node (SAN) generates the maximum number action potential and is therefore called the pacemaker.
- Note: Aorta is the largest artery, Vana cava is the largest vein, pulmonary artery artery is one of the only arteries (other than umbilical arteries i foetus) that carry deoxygenated blood, and the pulmonary vein is the only vein that carries oxygenated blood.
Cardiac Cycle:
We’ll begin with the atria and ventricles in their relaxed state (Joint diastole).
- As the tricuspid and bicuspid valves open, the blood in right and left atria will flow into right and left ventricle.
- Now the sino-atrial node (SAN) will generate an action potential that stimulates the right and left atria to contract simultaneously (atrial systole) increasing the flow of blood into the ventricles.
- This action potential is conducted to the ventricles through atrio-venticular node (AVN), atrio-vetricular bundle (AV bundle) and bundle of HIS.
- The action potential will stimulate ventricular stimulation (ventricular systole) which causes the increase in ventricular pressure. Ventricular systole and atrial diastole occurs simultaneously.
- Increase in ventricular pressure causes the closure of tricuspid and bicuspid valves, and opening of semilunar valves causing the flow of blood from ventricles into the circulatory pathways.
- After that, ventricles will relax (ventricular diastole) and ventricular pressure will decline causing the closure of semilunar valves and opening of tricuspid and bicuspid valves due to pressure of blood in the atria.
- The atria and ventricles are again in their relaxed state (joint diastole). The above event repeats cyclically and is therefore called cardiac cycle.
- Heart will produce two different sounds during a cardiac cycle. Closure of tricuspid and bicuspid valves will produce (lub) sound and the closure of semilunar valves will produce (dup) sound.
- Cardiac output = strokes volume (total volume of blood pumped by each ventricle in one cardiac cycle - 70 ml) multiplied by heart rate (number of beats per minute). [Cardiac output = Strokes volume * Heart rate]
Electrocardiograph (ECG):
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Circulatory Pathways:
Open circulatory pathway: is the circulatory pathway in which the blood pumped by the heart passes directly into the body cavities called sinuses. Found in arthropods and molluscs.
Closed circulatory pathway: is the circulatory pathway in which blood pumped by heart is circulated through closed system of blood vessels.
Closed system consists of (i) single circulation, (ii) incomplete double circulation and (iii) double circulation.
- Single circulation is found in fish in which the heart has only two chambers.
- Incomplete double circulation is found in amphibians and reptiles (except crocodile) in which the heart has three chambers (2 atria and 1 ventricle in which the oxygenated and the deoxygenated blood get mixed up).
- Double circulation is found in mammals, bird and crocodile in which the heart has four chambers (2 atria and 2 ventricle) where the deoxygenated and oxygenated blood do not get mixed up.
Double circulation:
Double circulation consists of (a) pulmonary circulation and (b) systematic circulation.
(a) Pulmonary circulation: is the circulation of deoxygenated blood from the right ventricle through pulmonary artery to the lung (where the blood is oxygenated), and then to the left atrium through the pulmonary vein. (b) Systematic circulation: is the circulation oxygenated blood from the left ventricle through aorta to tissues, where deoxygenated blood is collected and carried to the right atrium through vana cava.
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Regulation of Cardiac Activity:
- Normal heart activity is auto-regulated by the nodal tissues present in the heart, therefore the heart is called myogenic.
- Neural center in medulla oblongata also regulates the heart activity through autonomous nervous system (ANS).
Disorders of Circulatory System:
- High Blood Pressure (Hypertension): The normal blood pressure is 120/80. The 120 represents systolic measurement (peak pressure in the arteries and 80 represents diastolic measurement (minimum pressure in the arteries).If the blood pressure is between 120/80-139/89, it is called prehypertension. The blood pressure above 140/90 is called hypertension.
- Coronary Artery Disease (CAD) or Atherosclerosis: is disorder caused by deposition of calcium, fat and fibrous tissues on the lumen of artery supplying blood to heart muscle.
- Agnina or Agnina pectoris: is a disorder that causes acute chest pain when there is no enough oxygen reaching the heart muscle.
- Heart failure or Congestive Heart failure: is a disorder caused when the heart is not pumping the blood efficiently enough to meet the needs of the body.
- Heart failure is different than cardiac arrest (caused when heart stops beating) and heart attack (caused when heart muscle is damaged by inadequate blood supply).
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