1. Blood is a complex, specialised tissue containing a number of different cell types suspended in a fluid matrix When whole blood is spun in a centrifuge, the blood cells collect towards the lower half of the tube leaving a pale, straw-coloured liquid above There are between 5 and 10 thousand white blood cells in every mm 3 of blood There are between 5 and 6.5 million red blood cells in every mm 3 of blood The straw-coloured liquid is plasma, a fluid that contains 92% water in which are dissolved various molecules such as nutrients, salts, hormones and plasma proteins White blood cells collect at the top of the lower fraction and these include cell types known as granulocytes, lymphocytes and monocytes Red blood cells form the bulk of the lower fraction and these cells occupy about 45% of the total volume of the blood Blood is a Tissue

2. Plasma is the fluid matrix and the cells can be classified as RED CELLS (ERYTHROCYTES), WHITE CELLS (LEUCOCYTES) and PLATELETS (THROMBOCYTES) Lymphocytes, monocytes and granulocytes are the different types of white cell This photomicrograph of a blood smear shows the appearance of blood as viewed with the light microscope Blood is a Tissue

3. This scanning electron micrograph (colour enhanced) shows a red cell and white cell on the surface of an epithelial tissue

4. The major function of white blood cells is to defend the body against disease-causing organisms (pathogens) and other foreign matter White cells are classified as either GRANULOCYTES or AGRANULOCYTES WHITE BLOOD CELLS Granulocytes granular cytoplasm; lobed nucleus Agranulocytes non-granular cytoplasm; kidney-shaped or round nucleus LYMPHOCYTES (spherical nucleus) MONOCYTES (kidney-shaped nucleus) White Blood Cells

5. GRANULOCYTE MONOCYTE LYMPHOCYTE Lobed nucleus Granular cytoplasm Kidney-shaped nucleus Non-granular cytoplasm Spherical nucleus occupying a large volume of the cell Non-granular cytoplasm White Blood Cells

6. Human red blood cells have a life span of only 120 days Mammalian red blood cells have the shape of a biconcave disc They have no nucleus allowing space for large amounts of haemoglobin The major functions of these cells is to transport oxygen from the alveoli of the lungs to the body tissues and carbon dioxide from the tissues to the alveoli Each red cell contains about 270 million haemoglobin molecules each of which has a high affinity for molecular oxygen The biconcave disc shape provides the red cells with a large surface area to volume ratio The large surface area to volume ratio ensures that each haemoglobin molecule is close to the cell surface for rapid gas exchange The large surface area to volume ratio ensures that diffusion of oxygen into and out of the red cells occurs at a rate fast enough to meet the metabolic needs of the organism The biconcave centre also allows the cells to fold within the smallest capillaries (facilitating passage) Mammalian Red Blood Cells

7. Scanning electron micrograph of mammalian red blood cells showing the biconcave disc shape that gives these cells a large surface area to volume ratio Red Blood Cells

8. The blood vessels of the cardiovascular system, with the exception of the capillaries, have walls composed of three basic tissue layers ALL BLOOD VESSELS, EXCEPT CAPILLARIES, ARE THEREFORE CLASSED AS ORGANS Blood vessel walls are composed of the following plan LUMEN (hole) The differences between the various blood vessels are related to their function within the circulatory system Tunica Externa (the outer coat consisting mainly of collagen fibres) Tunica Media (the middle coat consisting of muscle and elastic tissue) Tunica Intima (the inner coat consisting mainly of endothelial cells and some collagen fibres) Blood Vessels

9. Blood is pumped out of the heart into arteries which must be able to withstand the pressure of the blood leaving the heart Arteries possess large amounts of muscle tissue in the middle layer Arteries also possess large amounts of elastic tissue in the middle layer The elastic tissue enables the artery walls to stretch to accommodate the blood ejected from the heart as it contracts As the heart relaxes, the elastic tissue recoils and exerts a pressure on the blood The arteries thus act as ‘subsidiary pumps’ helping to maintain the blood pressure and ensuring the unidirectional flow of blood Tunica media (large amounts of muscle and elastic tissue)Arteries

10. As blood is ejected from the heart into the arteries, they stretch to accommodate the additional blood As the heart relaxes, the artery walls recoil and propel blood onwards whilst the heart is at rest The artery acts as a ‘subsidiary pump’ and is pulsatile Elastic Arteries

11. TRANSVERSE SECTION THROUGH AN ARTERY LUMEN Connective tissue Smooth muscle and elastic tissue Endothelium

12. Arteriole means little artery Major arteries branch into smaller and smaller arteries, eventually giving rise to arterioles Arterioles subdivide into capillaries where exchange of materials between the blood and the tissue cells takes place Nutrients Oxygen Excretory products Carbon dioxide Blood from the capillaries collects into larger vessels, called venules, which empty their contents into the thin-walled veins Arterioles control the volume of blood flow to a particular organArterioles

13. Tunica Externa (the outer coat consisting mainly of collagen fibres) Tunica Media (very large amounts of smooth muscle tissue but very little elastic tissue) Tunica Intima (the inner coat consisting mainly of endothelial cells and some collagen fibres) Arterioles, like arteries, are composed of three basic tissue layers but they are about 100 times smaller As arterioles approach the capillaries, their walls become more muscular and less elastic in composition The diameter of the arterioles largely determines the amount of blood that flows to a particular organ The degree of contraction of the smooth muscle in the arteriole walls determines their diameter and is controlled by the sympathetic nervous systemArterioles

14. The large amount of smooth muscle in the walls of the arterioles is essential for controlling blood flow to different organs During exercise there is an increased demand, by skeletal muscles, for additional oxygen This demand is met by redistributing blood flow from the less essential organs of exercise to the skeletal muscles Blood flow to the skeletal muscles is increased when muscles in the arteriole wall become more relaxed and the arteriole diameter increases – a response called vasodilation Blood flow to organs of less importance during exercise decreases as a result of contraction of the arteriole muscle – a response called vasoconstriction Arterioles and the Control of Blood Flow

15. Veins transport blood from the various body organs to the heart Compared to arteries, veins have virtually no elastic fibres or smooth muscle and are very thin-walled VEIN ARTERY Tunica Externa (the outer coat consisting mainly of collagen fibres) Tunica Media (little elastic tissue and smooth muscle; thin muscular walls) Tunica Intima (the inner coat consisting mainly of endothelial cells and some collagen fibres) One-way valvesVeins

16. This photomicrograph of a small artery and vein shows the difference in thickness of the walls of these two types of blood vessel Vein (large lumen) Artery (smaller lumen)

17. Blood flows from the capillary beds into vessels called venules, which empty their contents into the thin-walled veins for return to the heart Blood pressure within the veins is very low and blood flow is maintained by one-way valves within veins, the action of working skeletal muscles and the pressure that is exerted during breathing Vein Lower valve One-way valve Skeletal muscle Upper valve Valve Action in Veins

18. When skeletal muscles around this section of vein are relaxed, both sets of valves are closed When the skeletal muscles contract, pressure in the section between the valves increases and the upper valve is forced open Blood flows upwards through the open valve Other skeletal muscles, below the level of those shown here, also contract during which the lower valve then functions as the upper valve for that section of vein When the skeletal muscles relax again, the upper valve closes due to the backflow of blood

19. Pressure changes within a body cavity also squeeze the veins and aid venous return to the heart During inspiration the pressure in the thoracic cavity falls and the pressure in the abdominal cavity rises Pressure within the abdomen squeezes on the veins Low pressure in the thorax exerts a sucking action on the veins These two pressures aid the flow of blood from the lower organs back to the heart Pressure in the thorax falls Pressure in the abdomen rises Blood Flow in Veins

20. Blood Vessel Diagram Blood Flow Size of Lumen Pressure Wall Structure Artery Blood flows away form the heart to tissues and organs and to the lungs Relatively small 1 – 2.5 cm High Due to the pumping action of the heart Tunica media contains large amounts of elastic tissue and smooth muscle Arteriole Blood flows from arterioles into capillary beds; the lumen diameter of arterioles controls blood flow to the organs Relatively small 20 – 200  m Wide Range – the greatest drop in pressure occurs as blood travels through the arterioles High at the artery end and relatively low approaching the capillaries Tunica media contains very large amounts of smooth muscle tissue but very little elastic tissue Vein Blood flows from the capillary beds into venules and then into veins. Veins carry blood towards the heart Relatively large 2 - 3 cm Low One-way valves, working skeletal muscles and inspiration aid the flow of blood in veins Tunica media contains little elastic tissue and smooth muscle tissue; thin muscular wall Summary of Arteries, Arterioles and Veins

21. Acknowledgements Copyright © 2003 SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.