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Blood and circulation
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The transport system of an animal moves substances to where they are needed in the body. Even the smallest animal must have the means of transporting substances around its body. Oxygen and food molecules must move to all the cells, and the waste products must be removed from the cells and expelled into the environment. A variety of fluid systems, called vascular systems, help such transport in most members of the animal kingdom. A circulatory system is a vascular system (i.e. with tubes or vessels) in which the transport fluid moves rhythmically in a particular direction, since it is propelled by a muscular pumping structure.

Unicellular organisms (Protozoa) have no specialized system for circulation of fluid. Sponges and coelenterates also lack a specialized circulatory system. Part of the evolution of multicellular animals was the development of the body fluids (tissue fluids, blood and lymph). These fluids provide all body cells a stable and relatively non-fluctuating environment, and fill in the space between cells and cell layers.

The body fluids are divided into (a) intracellular (inside the cells) and (b) extracellular, (blood plasma and interstitial fluid). The blood plasma is present in blood vessels and the interstitial fluid in the spaces around the cells. Nutrients and gases passing between blood vessels and cells have to cross this fluid.

There are two basic types of transport systems:

(1) In an open (vascular) circulatory system (Fig 18.2) the blood is not completely enclosed within vessels and it flows within blood vessels for only a limited part of its circuit. The blood vessels open into the open fluid spaces (sinuses or hemocoel) so that the circulating fluid (hemolymph) can reach the cells directly. This type occurs in insects, crustaceans and molluscs. Circulation and O2 transport is slower in an open circulatory system. (Smaller or sluggish animals).

(2) In a closed circulatory system (Fig. 18.3) the blood remains within a completely closed (unbroken) system of vessels, and never comes in direct contact with the cells. This occurs in molluscs (squids), annelids (earthworms), echinoderms, and vertebrates (animals and humans). The advantages of a closed circulatory system are : faster transport of oxygen; greater efficiency of blood flow; economy of blood volume and maintenance of sufficient blood pressure for a large body.

A circulatory system, whether open or closed, has several functions such as transportation of dissolved nutrients, hormones, gases, antibodies, control of fluid volume, pH, and regulation of body temperature.


  1. Position and External Features of Heart

The human heart is a muscular, four-chambered organ

about the size of a clenched fist. It is located in the thorax between

the lungs (mediastenum) and above the central depression of

the diaphragm. It is somewhat cone-shaped with the broader

end (base) upwards, and the blunt end (apex) lying on the diaphragm.

It is slightly oriented towards the left. The heart weighs about

300 grams in males, and is usually slightly smaller in females.

The heart is enclosed in a tough, double walled membrane

called the pericardium. The outer tough layer of pericardium is

fibrous and forms a protective sheath. It is attached by

ligaments to the breastbone and spine, anchoring the heart in

position. Inside the fibrous pericardium is a thin but

tough double membrane, the parietal pericardium. The pericardial

fluid between the two membranes prevents friction, protects it from

external shocks and over-distention of the heart with blood in extreme conditions.

The heart consist of four chambers, two atria (auricles) and two ventricles.

The atria are thin walled and receive blood from four pulmonary veins (auricles) and two ventricles. The atria are thin-walled and receive blood from four pulmonary veins and two vena cavae.

The main tasks of the atria are to prime (fill) the ventricles between "pumps", and to help monitor blood pressure in the vascular system. The ventricles are thick walled and muscular. The walls of the left ventricle are at least three times thicker than those of the right, as it pumps blood to the whole body through the aorta.

Two thick-walled muscular vessels arise from the heart -- the aorta from the left side, which pumps blood to the body, and the pulmonary artery from the right side of the heart. The left and right coronary arteries arise from the aorta and supply oxygenated blood to the atria and ventricles. The coronary arteries divide several times to form a capillary network over heart muscles. Blood is collected from the capillaries by the coronary veins and returned to the right atrium. When defects in coronary arteries prevent blood flow to the heart muscles, that is when a heart attack occurs.

(B) Internal Structure of the Heart

Internally, the heart is divided into two sides (right and left) by two muscular partitions, and these two sides work in unison but independently as a double pump.

The auricles. The two atria or auricles are separated by a thin inter atrial septum. The left atrium receives oxygenated blood by four pulmonary veins, which opens into its upper part. Their openings are without valves. The left atrium opens into the left ventricle by atrio-ventricular orifice (opening ), guarded by the bicuspid or mitral valve.

The right atrium receives deoxygenated blood from the body through the superior vena cava (opens into the upper part), and inferior vena cava, which opens into the lower part of the atrium. Its opening is guarded by a rudimentary valve. A small opening of coronary sinus (vein), guarded by a valve, is present between the orifice of the inferior vena cava and atri-ventricular opening. The fossa ovalis is a small oval depression above and to the left of the opening of the inferior vena cava. The right atrium opens into the right ventricle, and this opening is guarded by tricuspid valves to prevent back-flow of the blood from the ventricle to the atrium.

The ventricles. The right and left ventricles are separated by inter ventricular septum. The septum slopes obliquely with convexity towards the right ventricle. The left ventricle is longer and more conical than the right, and forms the apex of the heart. The left ventricle shows the following important features:

  1. The left atrio-ventricular opening guarded by a mitral or bicuspid valve.
  2. A circular opening of the aorta guarded by aortic or semilunar valves. The valves allow the blood to enter the aorta from left ventricle during ventricular contraction and prevents it from flowing back into the ventricle during relaxation. The aortic valve has 3 cups-two posterior (right and left) and one anterior.
  3. The trabeculae carneae (chordae tendinae), are attached to the margins of the bicuspid valve and prevent them from everting into the atrium.
  4. The papillary muscles, two in number, to which one end of the chordae tendinae is attached .

The right ventricle has :

  1. Right atrio-ventricular opening guarded by the tricuspid valve.
  2. A rounded opening of the pulmonary artery, guarded by semilunar valves. The semilunar valves prevent backflow of the blood.
  3. The trabeculae carneae are as in the left ventricle, but they are not as stout and strong.
  4. The papillary muscles are conical in shape with their bases attached to the walls of the ventricle and their apices directed towards the ventricular cavity.

i) The heart beat: The heart is a remarkable organ. The heart beats continuously throughout an animal’s life. The heart contracts rhythmically, pumping a certain volume of blood from each ventricle to different parts of the body. This volume is called cardiac output.

The contraction of the heart is called systole and the relaxation, diastole. The contraction and relaxation of the heart forms a heart beat. The human heart beat can be heard as "lub-dub" sound (heart sound) with a stethoscope. When the body is at rest, the heart beats approximately 70 times per minute, and each cardiac cycle lasts for about 0.8 seconds.

Heart sounds: During ventricular systole, simultaneous closure of atrio-ventricular valves makes the first sound --"lub". The second sound -- "dub"- which is higher-pitched, shorter and sharper, is caused by the simultaneous closure of aortic and pulmonary valves. Extra heart sounds, called murmurs or a "hiss" sound, occurs when blood leaks from the artery back into the ventricle, usually because one of the valves is defective. (In that case the rhythm would be: Lub-hiss-dub or lub-dub-hiss.)

Arterial Blood Pressure

Blood pressure may be defined as the pressure exerted by the blood against the vessel walls which contain it. This pressure is greatest in the arteries during ventricular contraction (systole), and is called systolic pressure. During ventricular relaxation (diastole), blood pressure falls, reaching a minimum pressure just prior to the next systole. The minimum pressure is referred to as diastolic pressure.

Average arterial pressure during systole is about 120 mm Hg in an adult, while the diastolic pressure is about 80 mm Hg. This is normally expressed as blood pressure of 120/80, the upper number indicating systolic pressure and the lower number indicating the diastolic pressure. From these figures it is obvious that there is a fluctuation in blood pressure during each heart beat. The difference between these two pressures, 40 mm Hg, is termed pulse pressure.

Blood pressure is measured by an apparatus called the sphygmomanometer, which measures blood pressure in an artery. When the blood pressure is taken, the cuff is wrapped around the arm just above the elbow over the brachial artery, and air is pumped into the bag. A stethoscope is placed over the artery so that the pulse can be read. Air is pumped untill flow stops in that artery and the sounds cease. Now the bag is slowly deflated until blood starts to flow and the pulse (sound) can just be heard. The manometer reading at this time indicates systolic pressure. Deflation of the bag is continued and a reading just before the last pulse sound is taken. This indicates diastolic pressure i.e. lowest arterial pressure caused by the ventricular diastole

Extreme deviations from the normal blood pressure (120/80) are indications of a heart malfunction, unusual blood volume in the system, arterial inelasticity (arteriosclerosis), kidney disease, etc. A decrease in the blood pressure below the normal level is referred to as hypotension or low blood pressure. This may happen due to low blood volume or sometimes due to a defect of the heart. A persistant increase in blood pressure above the normal level (say 160/95), is referred to as hypertension or high blood pressure. People with high blood pressure are susceptible to stroke, heart diseases, kidney failure, headaches, etc.


(A) Composition of blood

Blood is a viscous fluid of bright red color, heavier than water and slightly alkaline. It consists of two components. The liquid part is called plasma, and the large number of cells is called blood corpuscles suspended in the plasma.

a) Plasma : It is a straw-colored fluid consisting of the following chemicalonents:

(1) 90% of water.

(2) 7% of plasma proteins like serum albumin, serum globulin and fibrinogen.

(3) 3% of remaining substances, such as

(i) Nutrients like glucose, amino acids and fatty acids.

(ii) Inorganic salts like bicarbonates of sodium, potassium, calcium and magnesium.

(iii) Organic compounds like enzymes, hormones, antibodies, antitoxins and heparin, etc.

( iv) Gases like oxygen, carbon dioxide and nitrogen.

(v) Waste materials like urea, uric acid, etc.

The composition of plasma is altered in tissues and controlled by organs like the liver and the kidneys.

Functions of plasma: Plasma is the chief transporting medium supplying water, Oxygen and metabolites to all cells and also collects nitrogenous waste products and Carbon Dioxide from them.

(B) Blood corpuscles: These are blood cells suspended in plasma and are of three types, namely, erythrocytes, leucocytes and blood platelets.

Types of Blood Cells

    1. Red Blood Cells it is also known as Erythrocytes (G. erythos, red; G. kytos, cell).
      1. Function in the transport of respiratory gases (oxygen and carbon dioxide). Almost all of the oxygen in the blood is carried by hemoglobin.
      2. Production is under the control of erythropoietin, a substance formed in the blood by the action of an enzyme release principally by the kidney called Renal Erythropoietin Factor.
      3. Life span is about 80 to 120 days. Aged cells are destroyed principally in the spleen. Their destruction gives rise to the bile pigment bilirubin.
      4. Deficiency in the amount of oxygen transported by red blood cells result in anemia. It is most commonly caused by:
        1. A decrease in the rate of formation of red blood cells.
        2. Insufficient hemoglobin synthesis.
        3. Increased rate of destruction of red blood cells.
      5. Classification of Anemia:
        1. Pernicious Anemia – macrocytic anemia involving a pronounced reduction in red cell production due to Vitamin B12 deficiency based on defect in gastric secretion. The cause of this condition is a failure of the stomach to produce enough intrinsic factor a glycoprotein that facilitates the absorption of Vitamin B12 from the small intestine into the blood stream.
        2. Iron Deficiency Anemia (IDA) – the number of red blood cell is usually normal, but the individual cells are much smaller and pale, owing to a lack of sufficient hemoglobin. When the supply of iron becomes departed because of increase red blood cell formation to compensate for the blood loss, hemoglobin production is diminished and anemia result. This deficiency may also occur when the demand for iron is unusually great, as during infancy, adolescence, or pregnancy.
        3. Sickle Cell Anemia – abnormality in the protein portion of hemoglobin causing misshaping of the red blood cells and premature rupture. When the oxygen concentration in the blood is lowered following the release of oxygen from the red blood cells, the abnormal hemoglobin molecules aggregate and distort the cell into bizarre shapes, including the originally described crescent, or sickle shape. Because the sickle cell is rigid, it causes clogging of the capillaries, which leads to the early destruction of the cells.
        4. Hypoplastic, or, in severe cases, Aplastic Anemia – reduced red blood cell formation caused by damage to the bone marrow. Bone marrow can be destroyed among other things by radiation, infections, and drugs, especially some used in cancer chemotherapy.
        5. Hemolytic Anemia – anemia due to shortened in the body survival of the erythrocytes and inability of the bone marrow to compensate for their decreased life span. A number of hereditary diseases exist in which structural abnormalities in the red blood cells read to their premature removal from the circulation, principally by the spleen. One such thalassemia is characterized by the fragile cell membrane.
    2. White Blood Cells it is also known as Leukocytes (G. leuco, white)
      1. Three general types: granulocytes, lymphocytes, and monocytes.
      2. Neutrophils, the most numerous granulocytes, are phagocytic; the granules are lysosomes.
      3. Lymphocytes function in the immune response.
      4. Monocytes become transformed into macrophages at sites of infection.
      5. Disease involving abnormalities of the white blood cells:
        1. Leukocytosis – an increase in the white cell count, generally indicating an acute infection.
        2. Leukopenia – a reduction in the number of white cells, occur occasionally in viral disease.
        3. Leukemia – is characterized by a rapid and abnormal growth of leukocytes and by the presence of immature leukocytes in the peripheral blood.
        4. Infections mononucleosis - is a beginning disease associated with an increase in mononuclear leukocyte. A usually occurs in children and young adults, and is believed caused by a virus. The patient with infectious mononucleosis evidence a slightly a elevated temperature, enlarged lymph node, fatigue, and a sore throat.
    3. Platelets it is also known as Thrombocytes (G. thrombos, lump)
      1. Cytoplasmic fragments of megakaryocytic; essential for normal blood clotting.
      2. A deficiency in platelets causes tendency to bleed. One such is known as Idiopathic Thrombocytopenia Purpura (ITP). Idiopathic means cause unknown; thrombocytopenia (Latin for purple) is a condition in which hemorrhages occur in the skin and mucous and serious membranes most commonly of pinhead size (petenia). Some individuals with disease apparently have a substance in their blood that destroys platelets.