Circulatory System Flashcards
Describe gas exchange systems of insects, fish and humans
In insects, their gas exchange system has evolved so that gases can diffuse directly to and from body cells
However in fish and humans, gases dissolve in blood which acts as a transport system
The blood then moves around the circulatory system, transferring gases between the cells and the gas exchange system
Blood also transfers other essential molecules such as glucose and amino acids
What is mass transport
When molecules are carried in a transport medium such as blood through a circulatory system
circulatory system is a mass transport system
Describe the features of a single circulatory system
The blood only passes through the heart once as it moves around the circulatory system
Describe how blood is pumped around the body and oxygen delivered to the body’s cells
Deoxygenated blood is pumped under high pressure from the heart to the lungs
In the lungs, the blood passes through narrow capillaries
Oxygen diffuses from the air into the blood
Because the blood has passed through capillaries, it is now moving relatively slowly with lower pressure
However, now the oxygenated blood returns back to the heart which pumps the blood at high pressure around the body
As it passes through the body tissues, the blood passes through capillaries and oxygen diffuses to the body cells
The low pressure, deoxygenated blood now makes its way back to the heart to be pumped again
Describe the features of a double circulatory system
In a double circulatory system, the blood passes through the heart twice
This ensures that the blood moves to the body tissues rapidly and under high pressure
Because of this, a double circulatory system can deliver oxygen more efficiently than a single circulatory system
Mammals have a double circulatory system
Describe the problem of a single circulatory system
Problem with single circulatory system:
When the blood leaves the heart, the pressure of the blood is high and the blood is moving rapidly
However, the blood then passes through two sets of narrow capillaries, firstly in the gills and secondly as it passes the body tissues
When blood passes through capillaries, the blood slows down and loses pressure
This means that once the blood passes through the gills (gas exchange surface), the blood is moving relatively slowly
This limits how rapidly oxygen can be delivered to the body cells
Describe the advantages of a double circulatory system compared to a single system
In a double circulatory system, the blood passes through the heart twice
This ensures that the blood moves to the body tissues rapidly and under high pressure
Because of this, a double circulatory system can deliver oxygen more efficiently than a single circulatory system
Heart has two completely separate sides sides - one with oxygenated blood one with deoxygenated blood
under normal conditions these never mix
Describe the general structure of the human circulatory system
The blood in the right side of the heart is deoxygenated and the blood in the left side of the heart is oxygenated
Blood leaves the heart in arteries
The blood in the arteries is under very high pressure and the pressure increases every time the heart contracts
This is called the pulse
The blood continually moves forward in arteries even between contractions
Blood travels from the heart to the lungs in the pulmonary artery (only artery which carries deoxygenated blood)
Oxygenated blood travels from the heart to the body in a very large artery called the aorta
The aorta divides and these arteries carry oxygenated blood to different organs
(Individual arteries are named based on the organ that they supply)
Before the blood reaches the organs, it passes through blood vessels called arterioles
Once the oxygenated blood reaches the organs, it passes through very narrow blood vessels called capillaries
Capillaries are the sites of gas exchange with oxygen diffusing from the blood to the body cells
CO2 also diffuses from the body cells back into the blood
Once the blood has passed through the capillaries, the blood pressure is much lower and the blood is no longer surging in pulses
Now the blood passes into larger blood vessels called venules and then into veins
Veins carry deoxygenated blood away from the organs towards the heart
Veins are named based on the organs that they come from
In the veins, the blood is under relatively low pressure and the blood is not surging in pulses
All of the veins from the body organs finally connect into one very large vein called the vena cava
The vena cava returns the deoxygenated blood back to the heart
Veins carry deoxygenated blood back to the heart
Exception: Blood travels from the lungs to the heart in the pulmonary vein and the blood in the pulmonary vein is oxygenated
Describe the functions of arteries, veins and capillariest
Name the arteries and veins serving the kidneys
Kidneys are supplied by the renal arteries
The veins from the kidneys are called the renal veins
Describe the structure of arteries
Describe how the structure of arteries is adapted to their function
Arteries carry blood away from the heart to the organs
The artery wall is relatively thick
This allows the artery to withstand the high pressure of the blood
The wall of the artery consists of several layers
The outer layer is rich in the fibrous protein collagen (collagen-rich outer layer)
Collagen plays a structural role strengthening the artery wall against the pressure of the blood
Next we have a layer containing smooth muscle (smooth muscle layer)
When the smooth muscle contracts, the diameter of the artery narrows
This allows the body to control how much blood flows to different organs
Smaller arteries tend to contain a greater proportion of smooth muscle than larger arteries
That is because smaller arteries play a greater role in controlling blood flow
Next we have a layer which is rich in elastic fibres (elastic layer)
Elastic fibres contain the protein elastin which can stretch
When the heart contracts, a surge of high pressure blood passes down the artery. As the surge moves through, the elastic fibres stretch. They then recoil once the surge has passed
This elastic recoil helps to keep the blood moving smoothly forward in between contractions of the heart
Next layer called endothelium
The central cavity of the artery is called the lumen
The lumen is where the blood flows through
The lumen is lined with a thin layer of endothelial cells
This presents a very smooth surface to reduce friction as the blood flows through
Explain why smaller arteries tend to contain a greater proportion of smooth muscle than larger arteries
Smaller arteries tend to contain a greater proportion of smooth muscle than larger arteries
That is because smaller arteries play a greater role in controlling blood flow
Describe the structure of arterioles
Arteries branch into smaller blood vessels called arterioles
The job of the arterioles is to deliver blood to capillaries
The wall of the arterioles contains the same layers as arteries (collagen-rich outer layer, smooth muscle layer, elastic layer), but they differ in relative thickness
In arterioles, the blood pressure is lower than in arteries and the effect of the pulse is weaker
This means that in arteries, the collagen-rich outer layer and the elastic layer are relatively thin compared to arteries
The smooth muscle layer is relatively thicker in arterioles compared to arteries
That’s because arterioles are involved in controlling the amount of blood passing through capillaries
When the smooth muscle in arterioles contracts, blood flow through the capillaries is reduced. - called vasoconstriction
However, when the smooth muscle in arterioles relaxes, blood flow increases through the capillaries - called vasodilation
Vasodilation takes place when an organ requires an increased amount of oxygen
Oxygenated blood is carried from the heart to the organs in blood vessels called arteries
Arteries then branch to form narrower blood vessels called arterioles
Arterioles carry the blood to the capillaries in each organ
In capillaries, molecules diffuse from the blood to the body cells e.g. glucose and oxygen
Other molecules diffuse from the body cells back to the blood e.g. co2 and urea
Blood then passes from capillaries to blood vessels called venules and then to veins which carry the blood back to the heart
Describe the structure of capillaries and how they are adapted for the diffusion of molecules
Capillaries are extensively branches, no body cell is very far from a capillary (at least one cell away)
network of capillaries = capillary bed (these are where substances are exchanged between the blood and the body cells)
The extensive branching of capillaries provides a massive surface area for the exchange of materials
Unlike other blood vessels, capillaries have an extremely thin wall
The wall of capillaries consist of a single layer of endothelial cells
On the outside there is a thin membrane called the basement membrane
Because the capillary wall consists of a single layer of cells. This means that there is a very short diffusion distance between the blood and the cells near the capillary
This short distance increases the rate of diffusion of molecules between the blood and the cells e.g. o2 and co2
The diameter of the capillary lumen is only slightly greater/wider than the diameter of a red blood cell
This means that when red blood cells pass through capillaries, they are pressed against the capillary wall. This reduces the distance for the diffusion of o2 from the red blood cells to the tissue cells.
This also means that red blood ells travel through capillaries in single file. Because of this, (the blood) red blood cells move through capillaries more slowly than in arteries and arterioles.
This relatively slow movement increases the time available for molecules to diffuse in and out of the blood
At the capillary wall, there are small gaps between the endothelial cells
These gaps allow fluid to pass out of the blood. This fluid is called tissue fluid
Tissue fluid bathes the cells, providing essential molecules such as glucose and amino acids
The gaps in the capillary wall also allow white blood cells to leave the blood stream
Where does the blood flow after it has passed through capillaries
Once the blood has passed through the capillary bed, it makes its way into very small veins called venules
Venules then connect into larger veins
Veins carry deoxygenated blood to the vena cava where it passes into the heart
Describe the structure of veins and how their structure is adapted to their function
Unlike arteries, the blood in venules and veins is under low pressure and is not travelling in pulses - means structure of veins is different to that of arteries
Veins tend to have a thinner wall than arteries
That is because the walls of veins do not have to withstand high blood pressure
Veins have a larger lumen and carry a greater volume of blood compared to arteries
The smooth muscle layer and the elastic layer are also thinner in veins compared to arteries
The blood in vein does not travel in pulses so there is no elastic recoil
Like arteries, the lumen of veins has an internal lining of endothelial cells
This smooth surface reduces friction between the blood and the wall of the vein
Describe the role of valves in veins
Veins contain valves
These valves help to keep the blood moving in the forward direction
Blood in veins is moving back to the heart
This means that the blood may be well be moving against gravity (especially if the veins are in the legs or arms)
PROBLEM: However, the blood in veins is travelling slowly and is under low pressure
what makes the blood in veins move back to the heart?
Many veins are found lying between skeletal muscles such as the large muscles of the arms and legs
When these muscles contract e.g. during normal movement, they squeeze the veins lying between them
Veins have a relatively thin wall, so when they are squeezed they change shape
This squeezing forces the blood along
If the blood moves forwards then the valves remain open
However if the blood starts to move backwards then the valves shut
The combined effect of the muscles squeezing the veins, as well as the action of the valves, helps to keep the blood moving towards the heart
Another factor
When we inhale, the pressure of our chest cavity decreases
This decrease in pressure helps the blood in the chest veins to move towards the heart
Describe the function of tissue fluid
In the capillaries, fluid passes out of the blood and bathes the tissue cells. This is called tissue fluid
Tissue fluid leaves the blood at the parts of the capillary which are near the artery
Tissue fluid transfers molecules such as oxygen and glucose to the tissue cells
Waste molecules from the tissue cells such as co2 pass into the tissue fluid
The tissue fluid then returns back to the bloodstream at the parts of the capillary which are near the vein
How does tissue fluid transfer in and out of the blood
Between endothelial cells in capillaries are tiny gaps
Tissue fluid is forced out of the blood at the arterial end of the blood capillary and returns back to the blood at the venous end of the blood capillary
Hydrostatic pressure and oncotic pressure - two factors
Describe how tissue fluid is formed
transfers in and out of the blood
Tissue fluid is formed at the arterial end of a capillary
At the arterial end of the capillary, the blood has just passed through an artery and an arteriole
Because of this, the blood at the arterial end of the capillary is still under relatively high pressure
This is called hydrostatic pressure
Hydrostatic pressure tends to force fluid out of the blood and into the tissue.
In the blood plasma there are plasma proteins such as albumin. Plasma proteins are hydrophilic so they lower the water potential of the blood plasma
Because of the plasma proteins, there is a tendency for water to move back into the blood by osmosis. This is called the oncotic pressure
At the arterial end of the capillary, the hydrostatic pressure is greater than the oncotic pressure
This means that tissue fluid is forced out of the capillary through the gaps between the endothelial cells
This process is called ultrafiltration
Blood cells and plasma proteins are too large to leave, so they remain in the blood plasma
At the venous end of the capillary, the hydrostatic pressure is much lower. This is because a large amount of water has left the blood
However, the oncotic pressure is still high due to the plasma protein in the blood plasma
So because of this, at the venous end, the hydrostatic pressure is less than the oncotic pressure. This causes water to move back into the blood by osmosis.
Around 90% of the tissue fluid is reabsorbed back into the blood
Describe the function of lymph fluid
The remaining 10% of tissue fluid, drains into a series of blind ended vessels called lymph capillaries
Lymph capillaries connect into larger lymph vessels, forming the lymphatic system
Lymph fluid moves along when lymph vessels are squeezed by nearby skeletal muscles
Valves in the lymph vessels help to keep the lymph fluid moving forward
Eventually the lymph fluid returns to the blood stream via the blood vessels under the collar bone
Red blood cell (erythrocytes) adaptations for transporting oxygen
Erythrocytes have a biconcave structure which gives them a large SA/V ratio
This allows oxygen to diffuse in and out rapidly
Each erythrocyte contains around 300 million molecules of the oxygen-carrying protein haemoglobin
Although erythrocytes initially have a nucleus, the nucleus is lost before the erythrocytes enter circulation
The absence of a nucleus means that more of the erythrocyte’s volume is available to carry haemoglobin
Describe the structure of haemoglobin
Describe the role of haemoglobin in transporting oxygen
Haemoglobin molecule has four polypeptide chains.
Each polypeptide chain is bound to a prosthetic group called haem
Haem contains the iron ion Fe2+
Because haemoglobin contains the haem prosthetic group, it is an example of a conjugated protein
Each of the Fe2+ groups in the haem molecules can combine with one molecule of oxygen
Because there are four haem groups in each haemoglobin molecule, one molecule of haemoglobin can combine with four molecules of oxygen
When haemoglobin binds to oxygen, it is called oxyhaemoglobin
This reaction is reversible- so oxyhaemoglobin can also release the oxygen when required
Word and symbol equation to show the binding of oxygen to haemoglobin
Hb + 4O2 ⇌ Hb(O2)4
haemoglobin + oxygen ⇌ oxyhaemoglobin
Describe the oxygen dissociation curve
The amount of oxygen that combines with haemoglobin can be measured on an oxygen dissociation curve
Y-axis - the percentage saturation of haemoglobin with oxygen
x - axis - the partial pressure of oxygen (since oxygen is a gas, we dont say the concentration of oxygen, we say the partial pressure)
The curve has an s shape
This is called a sigmoid curve
What does the oxygen dissociation curve tell us about haemoglobin
E.g. someone has a sample of haemoglobin
These haemoglobin molecules are not bound to any oxygen molecules
As we increase the partial pressure of oxygen, the percentage saturation of haemoglobin increases relatively slowly
At around 4kPa of oxygen, we have achieved 25% saturation
Meaning each haemoglobin molecule is bound to one oxygen molecule on average.
What this means is that at low partial pressures of oxygen, haemoglobin has a low affinity for oxygen
affinity = how strongly the oxygen is bound to the haemoglobin
Once one oxygen molecule is bound, the affinity of haemoglobin for oxygen increases. Therefore, it becomes much easier to bind further oxygen molecules
If we increase the partial pressure of oxygen to around 7kPa then we achieve 75% saturation
Two more oxygen molecules have bound
Each haemoglobin molecule is bound to three o2 molecules on average
Explain the oxygen dissociation curve in terms of the structure of haemoglobin
Describe and explain oxygen dissociation curve when body cells are picking up oxygen at the alveoli
We can explain this by looking at the structure of the haemoglobin molecule -
Haemoglobin has four polypeptides
Each polypeptide contains a haem group which can bind oxygen
If there is no oxygen bound, then the haem groups have a low affinity for oxygen molecules
This means that it takes a relatively larger partial pressure of oxygen for the first oxygen molecule to bind to a haem group
However, when one oxygen binds, the quaternary structure of the haemoglobin molecule changes
This now increases the affinity of the haem groups for oxygen
So binding more oxygen molecules, only requires a relatively small increase in the oxygen partial pressure
This effect is called positive cooperativity
The fourth haem group only binds to oxygen at a fairly high partial pressure
That is because three of the four haem groups have already been filled, so that the chances of an oxygen molecule colliding with the fourth haem group is relatively low
Describe and explain oxygen dissociation curve from the alveoli to body cells
Explain the oxygen dissociation curve in terms of the structure of haemoglobin
Graph in reverse
In the alveoli, the partial pressure of o2 is high and the haemoglobin in red blood cells is around 97% saturated (towards top of curve)
However as red blood cells make their way into the body tissues, the partial pressure of oxygen decreases as the tissues are carrying out aerobic respiration
At a certain point, one oxygen molecule now unloads from the haemoglobin molecule
This unloading changes the quaternary structure of the haemoglobin molecule. The effect of this is to decrease the oxygen affinity of the remaining haem groups
If the red blood cells move into more active tissue, then the oxygen partial pressure will be even lower and two more oxygen molecules will rapidly unload from the haemoglobin molecule
For the final oxygen molecule to unload, the partial pressure of oxygen has to be very low. This is likely to happen under normal conditions but it could take place in very active tissue e.g. in muscle tissue during very intense exercise
Describe the effect of carbon dioxide on the oxygen affinity of haemoglobin (the Bohr effect)
Another factor - affecting the curve - CO2
Aerobic respiration produces the gas CO2
Effect of increased CO2 on the oxygen dissociation curve compared to lower partial pressure of CO2
Higher partial pressure id CO2 in muscle tissue
The effect of increased partial pressures of CO2 is to shift the whole oxygen dissociation curve to the right
What this means is that co2 causes the oxygen affinity of haemoglobin to decrease. This is called the Bohr effect
E.g. with normal curve, if partial pressure of Co2 is low, then the haemoglobin is 75% saturated at around a partial pressure of O2 of around 7kPa
However if the partial pressure of CO2 is high, then the haemoglobin is now only 25% saturated at the same partial pressure of oxygen as before
Effects
Haemoglobin has a higher affinity for oxygen, in conditions where the partial pressure of CO2 is low enough e.g. in the lungs
So in the lungs, haemoglobin has a high level of oxygen saturation
However, the partial pressure of CO2 will be high, in active tissue undergoing aerobic respiration e.g. muscle tissue
Because the haemoglobin now has a lower oxygen affinity, it is more likely to unload it’s bound oxygen in these tissues
Explain the Bohr effect in terms of the structure of haemoglobin
In the blood can form the acidic molecule carbonic acid
Carbonic acid releases the hydrogen ion H+
The H+ combines with haemoglobin and causes the quaternary structure of the haemoglobin molecule to change
As a result of this change in the quaternary structure, the haemoglobin has a lower affinity for oxygen
Which causes the haemoglobin to unload its oxygen more easily
