# 44 The circulatory system - blood vessels

The mammalian circulatory system is a closed double circulation, consisting of a heart, blood vessels and blood.

The heart produces high pressure --> blood moves through the vessels by mass flow.









The mammalian circulatory system is
  closed: blood travels inside vessels
  double circulatory:      
                                 pulmonary system:  heart -->       lungs                       --> heart
                                 systemic system     : heart --> around the rest of body --> heart

Blood vessels

Arteries

• Carry blood away from the heart.
• Blood that flows through arteries is pulsing and at a high pressure. 
• Have thick, elastic walls which can expand and recoil as the blood pulses through.
• The artery wall contains variable amounts of smooth muscle. This muscle does not help to push the blood through them.

Arterioles 

• Arteries branch into smaller vessels called arterioles. 
• They contain smooth muscle in their walls, which can contract and make the lumen (space inside) smaller. 
• Helps to control the flow of blood to different parts of the body. 

Capillaries 

• Tiny vessels with just enough space for red blood cells to squeeze through.
• Their walls are only 1 cell thick, and there are often gaps in the walls through which plasma (the liquid component of blood) can leak out. 
• Deliver nutrients, hormones and other requirements to body cells, and take away their waste products. 
• Small size and thin walls minimise diffusion distance, enabling exchange to take place rapidly between the blood and the body cells.

Venules 

• Small blood vessels that connect the capillary beds to the veins.

Veins 

• Carry low-pressure blood back to the heart. 
• Their walls do not need to be as tough or as elastic as those of arteries as the blood is not at high pressure and is not pulsing. 
• The lumen is larger than in arteries, reducing friction which would otherwise slow down blood movement. 
• Contain valves, to ensure that the blood does not flow back the wrong way. 
• Blood is kept moving through many veins, for example those in the legs, by the squeezing effect produced by contraction of the body muscles close to them, which are used when walking.

Pressure changes in the circulatory system

The pressure of the blood changes as it moves through the circulatory system.

• In the arteries, blood is at high pressure because it has just been pumped out of the heart. The pressure oscillates (goes up and down) in time with the heart beat. The stretching and recoil of the artery walls helps to smooth the oscillations, so the pressure becomes gradually steadier the further the blood moves along the arteries. The mean pressure also gradually decreases.

• The total cross-sectional area of the capillaries is greater than that of the arteries that supply them, so blood pressure is less inside the capillaries than inside arteries.

• In the veins, blood is at a very low pressure, as it is now a long way from thepumping effect of the heart.

 

Syllabus 2015 (v) describe the mammalian circulatory system as a closed double circulation; (m) [PA] describe the structures of arteries, veins and capillaries and be able to recognise these vessels using the light microscope; (n) explain the relationship between the structure and function of arteries, veins and capillaries;

Syllabus 2016  - 2018 As animals  become larger, more  complex and more  active,  transport systems become essential to supply nutrients to, and remove waste from, individual cells. Mammals are far more  active than plants  and require  much  greater supplies of oxygen.  This is transported by haemoglobin inside red blood cells.  Learning outcomes Candidates should  be able to: 8.1    The circulatory system The mammalian circulatory system consists of a pump, many  blood vessels and blood, which is a suspension of red blood cells and white  blood cells in plasma. a)   state that  the mammalian circulatory system is a closed double circulation consisting of a heart,  blood vessels and blood b)   observe and make  plan diagrams of the structure of arteries, veins and capillaries using prepared slides  and be able to recognise these vessels using the light microscope c)   explain the relationship between the structure and function  of arteries, veins and capillaries