Blood vessels & Tissue Fluid

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Introduction

Blood vessels make up 80 000 miles of transport system in the human body.

 The types of blood vessels we need to consider are:

Arteries, Arterioles, Capillaries, Venules, Veins

ARTERIES. These carry blood away from the heart. Each heartbeat will send a high-pressure pulse of blood through the arteries.

 What generates the high pressure?- Ventricular systole

 They are lined with smooth, epithelial tissue called the endothelium. Why do you think this is? - Smooth so reduces friction of blood

 They have thick muscular and elastic walls with the ability to stretch and recoil. Why do you think this is? - Stretch under high pressure, Recoil under low pressure

The lumen of the arteries is relatively small and the lining is highly folded to allow for the expansion of the vessel with each pulse.

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Cross- Section of an Artery

Tunica externa- Consists of: collagen fibres for strength

Tunica intima- Consists of: a single smooth layer of squamous epithelial cells – Endothelium. Folded to allow expansion.

Tunica Media (thick)- Consists of: Elastic fibres to allow the wall to stretch under high pressure when the ventricles contract and recoil under low pressure when the ventricles relax. Smooth muscle which can contract to reduce the size of the lumen and divert blood.

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Hydrostatic Pressure

 how the wall of an artery is adapted both to withstand and maintain high hydrostatic pressure.

  • withstand pressure wall is thick
  • Thick layer of collagen
  • Wall / collagen provides strength
  • Endothelium is corrugated / folded
  • Ensures no damage to endothelium / artery wall as it stretches)
  • To maintain pressure
  •  thick layer of elastic tissue
  • To cause recoil / return to original size
  • Thick layer of smooth muscle
  • Narrows / constricts lumen of artery
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Arterioles

Smaller versions of the arteries. They do not have to withstand as high a pressure because they are further from the heart.

  • They have the endothelium and a layer of smooth muscle, which is under the control of nerves. 
  •  When the smooth muscle contracts this causes vasoconstriction (lumen reduces in size)
  • When the smooth muscle relaxes this causes vasodilation (lumen INCREASES in size)
  • This allows blood flow into the capillaries to be controlled. 
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Veins

VEINS. These carry blood towards the heart. The blood is under low pressure this is because further from the heart, blood is travelling mostly against gravity, increased friction and slowed blood flow due to capillaries.

The lumen is very large, there is still an endothelium layer but the muscle layer is very thin because blood isn’t under high pressure so a thick wall isn’t required because the pressure in the veins is so low, it is not enough to lift blood against gravity from the lower body back to the heart.  The veins have two features to help overcome this:

  •  When muscles in the legs and body contract they squeeze the veins and increase the pressure of the blood inside them.
  • Blood that flows backwards is prevented from doing so by the presence of semi-lunar valves along the length of the veins.
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Cross- section of the veins

Tunica externa- Consists of: collagen fibres for strength

Tunica intima- Consists of: a single smooth layer of squamous epithelial cells – Endothelium. Folded to allow expansion.

Tunica Media (thin) Consists of: Elastic fibres to allow the wall to stretch under high pressure when the ventricles contract and recoil under low pressure when the ventricles relax. Smooth muscle which can contract to reduce the size of the lumen and divert blood.

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Capillaries

CAPILLARIES. The function of the capillaries is to form TISSUE FLUID. This is essential to allow tissue cells to exchange substances such as O2 and CO2. The walls of these are 1 endothelial (flattened) cell thick (approx 2μm).  This means short diffusion distance.

The walls are also permeable to let smaller molecules through.  This is due to tiny pores between endothelial cells.There are vast numbers of capillaries, giving a very large surface area. This increase causes a decrease in the rate of flow. The small diameter of the lumen also reduces blood flow (the red blood cells pass singly).  This allows more time for exchange

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Formation of tissue fluid

Whole Blood’ is composed of three distinct elements:

  •  Plasma:
  • Red Blood Cells:
  • White Blood Cells:

Arteries/Arterioles bring blood to the tissues and venules/veins take blood away from them. How does the exchange of oxygen, carbon dioxide, water, amino acids, sugars, fats, salts and glucose between the blood and tissues occur in the capillaries? Capillaries are ‘leaky’ due to the gaps between endothelium cells; some things are too big to be squeezed out into the tissues:

  •   Plasma Proteins
  •   Red blood cells

Components of the blood small enough will move out of the capillary depending upon:

  •       Pressure
  • Concentration   
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Lympth system

Not all of the liquid which moves into the tissue at the arterial end returns to the capillary at the venous end. If this is not dealt with the tissue will simply swell up, a little mentioned vessel system comes to the rescue: the LYMPH SYSTEM

 The lymph system is a vital part of the cardiovascular system. Think of it as the gutter system of the body, collecting all the liquid and dissolved nutrients which cannot return to the venule end of the capillary due to being too big or against the concentration gradient. 

The lymph system is composed of a network of Lymph Vessels, ‘blind ending’ tubes throughout all the tissues of the body. The excess tissue fluid drains into the lymph vessels through pressure and is pumped through a series of valves, identical to the veins, to where it is re-united with the blood vessels at the subclavian vein.

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