Circulatory system

• Fluids in our body

  • Blood
  • Tissue fluid
  • Lymph
• Blood

  • Contained in the heart and in the blood vessels (arteries, veins and capillaries) of the circulatory system
• Tissue fluid

  • Occupying spaces between cells in the organs
• Lymph

  • Contained within lymph vessels and lymphatic organs such as the spleen and the tonsils
• Blood, tissue fluid and lymph are the three principal fluids in our body
• Blood circulates in a closed manner through blood vessels, called a closed blood circulatory system
• In certain animals such as insects, the blood flows through open spaces in the body, called an open blood circulatory system
• Veins

  Carry deoxygenated and CO2 transporting blood (dark red)
• Non-circulating fluids include synovial fluid and vitreous humour
• Blood

  • Always in motion from the heart to the arteries and back through the veins
  • Somewhat thick fluid
  • Bright red when taken from an artery
  • Dark red when taken from a vein
  • Average adult human has 5 to 6 litres of blood
  • Slightly alkaline with a pH of 7.3 to 7.45
• Functions of blood

  • Transport
  • Protection
• Transport by blood

  • Transport of digested food
  • Transport of oxygen
  • Transport of carbon dioxide
  • Transport of excretory substances
  • Distribution of hormones
  • Distribution of heat
• Transport of digested food

  1. From the alimentary canal to the tissues

  2. Substances transported: simple sugars like glucose, amino acids, vitamins, mineral salts
• Transport of oxygen

  1. From the lungs to the tissues

  2. Occurs by means of red blood cells in combination with haemoglobin in the form of oxyhaemoglobin

  3. Oxyhaemoglobin breaks up to deliver oxygen at the tissues
• Transport of carbon dioxide

  1. From the tissues to the lungs

  2. Occurs partly in combination with haemoglobin and partly as solution in blood plasma
• Transport of excretory material

  From the tissues to the liver, kidney or the skin for elimination or to render them harmless
• Distribution of hormones

  Secreted by endocrine glands directly into the blood
• Distribution of heat

  Blood helps in keeping the temperature of the body uniform by distributing heat
• Protection by blood

  • Blood clot formation to prevent further blood loss and entry of disease-causing germs
  • White blood cells protect the body from diseases by engulfing bacteria
  • Production of antitoxins and antibodies to neutralise poisonous substances and kill germs
• Plasma


  Fluid part of blood, constitutes 55-60% of blood
• Composition of plasma

  • Water (90-92%)
  • Proteins (2-8%)
  • Inorganic salts (1%)
  • Other substances (traces)
• Serum


  Plasma from which the protein fibrinogen has been removed
• Cellular elements of blood

  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Blood platelets (thrombocytes)
• Red blood cells

  • Minute biconcave disc-like structures
  • Flat in the centre and thick and rounded at the periphery
  • Small size (about 7 micron in diameter) enables travel through fine capillaries
  • Adult male has about 5 million RBCs per cubic mm, adult female has slightly less
• Haemoglobin

  • Effective chemical constituent of RBCs
  • Formed of an iron-containing part (haemin) and a protein (globin)
  • Able to combine readily with oxygen to form oxyhaemoglobin, an unstable compound which readily gives up oxygen to tissues
  • Can carry a small quantity of carbon dioxide in the form of carbamimohaemoglobin
  • Has strong affinity for carbon monoxide forming a stable compound carboxyhaemoglobin, reducing oxygen transport capacity
• Veins carry oxygen, just less than arteries
• Life and death of RBCs

  • Produced in bone marrow of long bones, liver and spleen in embryo
  • Mature RBCs have no nuclei
  • Average life of an RBC is about 120 days
  • Old and weak RBCs are destroyed in spleen, liver and bone marrow
• Mammalian red blood cells

  • Deficient of nucleus, mitochondria and endoplasmic reticulum
  • More efficient in oxygen transport due to increased surface area, flexibility and ability to transport all oxygen without consuming it
• New born infants have 6-7 million RBCs per cubic mm, higher than adults
• RBC count is lowered by 5% during sleep, increased during physical activity, pregnancy and emotional upsets
• Polycythaemia


  Abnormally increased number of RBCs
• Erythropenia


  Abnormally decreased number of RBCs
• White blood cells

  • Have a nucleus and do not contain haemoglobin
  • Number is much less than RBCs, usually 4,000-8,000 per mm3
  • Most are amoeboid and can squeeze through capillary walls into tissues (diapedesis)
• Types of white blood cells

  • Neutrophils
  • Eosinophils
  • Basophils
  • Lymphocytes
  • Monocytes
• Leukemia


  Cancer of tissue forming white blood cells, with manifold increase in their number at the cost of RBCs
• Leukopenia

  Abnormal decrease in the number of white blood cells
• Functions of leukocytes (white blood cells)

  • Phagocytosis - engulfing and destroying bacteria and germs
  • Inflammation - migration to site of infection and fighting disease-causing germs
  • Formation of antibodies - killing or neutralising germs and their toxins
• Antibodies

  Chemical substances produced by lymphocytes that circulate freely in blood plasma and neutralise toxins produced by disease-causing germs
• Antibodies may persist long after the disease has been overcome, making the person immune to the disease
• Formation of antibodies

  1. WBCs (specially lymphocytes) produce antibodies

  2. Antibodies kill or neutralise germs or their poisons

  3. Introducing weakened germs or germ substances (vaccines) stimulates formation of particular antibodies