The Blood & Blood Vessels - Gidemy Class Notes

Blood micrograph

Composition of human blood

Components of the Blood Table

Red blood cells are specialised cells which carry oxygen to respiring cells
They are adapted for this function in 3 key ways
- They are full of haemoglobin, a protein that binds to oxygen to form oxyhaemoglobin
- They have no nucleus which allows more space for haemoglobin to be packed in
- The shape of a red blood cell is described as being a ‘biconcave disk’ this shape gives them a large surface area to volume ratio to maximise diffusion of oxygen in and out

Red blood cells

White blood cells are part of the body’s immune system, defending against infection by pathogenic microorganisms
There are two main types, lymphocytes and phagocytes
Lymphocytes:
- Produce antibodies to destroy pathogenic cells and antitoxins to neutralise toxins released by pathogens
- They can easily be recognised under the microscope by their large round nucleus which takes up nearly the whole cell and their clear, non-granular cytoplasm
Phagocytes:
- Carry out phagocytosis by engulfing and digesting pathogens
- Phagocytes have a sensitive cell surface membrane that can detect chemicals produced by pathogenic cells
- Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
- They can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm

Phagocytosis

Platelets are involved in helping the blood clot
Platelets are fragments of cells that are involved in blood clotting and forming scabs where the skin has been cut or punctured
- When the skin is broken (i.e. there is a wound) platelets arrive to stop the bleeding
- A series of reactions occur within the blood plasma
- Platelets release chemicals that cause soluble fibrinogen proteins to convert into insoluble fibrin and form an insoluble mesh across the wound, trapping red blood cells and therefore forming a clot
- The clot eventually dries and develops into a scab to protect the wound from bacteria entering
Blood clotting is important because:
- Blood clotting prevents continued / significant blood loss from wounds
- Scab formation seals the wound with an insoluble patch that prevents entry of microorganisms that could cause infection
- It remains in place until new skin has grown underneath it, sealing the skin again

How the blood clots

Plasma is a straw coloured liquid which the other components of the blood are suspended within
Plasma is important for the transport of many substances including:
- Carbon dioxide – the waste product of respiration, dissolved in the plasma as hydrogencarbonate ions and transported from respiring cells to the lungs
- Digested food and mineral ions – dissolved particles absorbed from the small intestine and delivered to requiring cells around the body
- Urea – the waste substance produced in the breakdown of proteins by the liver. Urea is dissolved in the plasma and transported to the kidneys
- Hormones – chemical messengers released into the blood from the endocrine organs (glands) and delivered to target tissues/organs of the body
- Heat energy – created in respiration (an exothermic reaction), heat energy is transferred to cooler parts of the body or to the skin where heat can be lost

There are three main types of blood vessel:
- Arteries
- Veins
- Capillaries
Smaller vessels that branch off from arteries are called arterioles (small arteries) and those that branch into veins are called venules (small veins)
Each vessel has a particular function and is specifically adapted to carry out that function efficiently

Key features:
- Carry blood at high pressure away from the heart
- Carry oxygenated blood (except the pulmonary artery)
- Have thick muscular walls containing elastic fibres
- Have a narrow lumen
- Blood flows through at a fast speed
The structure of an artery is adapted to its function in the following ways
- Thick muscular walls containing elastic fibres withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through
- A narrow lumen also helps to maintain high pressure

Key features:
- Carry blood at low pressure towards the heart
- Carry deoxygenated blood (other than the pulmonary vein)
- Have thin walls
- Have a large lumen
- Contain valves
- Blood flows through at a slow speed
The structure of a vein is adapted to its function in the following ways:
- A large lumen reduces resistance to blood flow under low pressure
- Valves prevent the backflow of blood as it is under low pressure

Comparing the structure of arteries and veins

Key features:
- Carry blood at low pressure within tissues
- Carry both oxygenated and deoxygenated blood
- Have walls that are one cell thick
- Have ‘leaky’ walls
- Speed of blood flow is slow
The structure of a capillary is adapted to its function in the following ways:
- Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them
- The ‘leaky’ walls allow blood plasma to leak out and form tissue fluid surrounding cells

Structure of a capillary

As arteries get further away from the heart, they divide more and get narrower
The narrow vessels that connect arteries to capillaries are called arterioles
Veins also get narrower the further away they are from the heart
The narrow vessels that connect capillaries to veins are called venules

The blood vessel network

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