Mammalian transport

Transport in Mammals

A recurring theme in biological systems is the surface area to volume ratio. All cells require nutrients and most require oxygen as well. Wastes also need to be removed.

With a small organism this demand can be met by simple diffusion over the body surface but larger or very active organisms need a transport system with a pump to ensure that the supply meets the demand of all cells, even those deep within the body.

In mammals, the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels, with substances being released out of, or into the blood as it flows through certain vessels called capillaries.

Blood Vessels



Blood is carried within a closed transport system that is made up of three types of vessel:
arteries, capillaries and veins.

Arteries carry blood away from the heart.

Capillaries are the site of the exchange of materials between the blood and tissues. Veins take blood back into the heart.

Arteries

Blood pumped out of the heart is at a very high pressure, so the structure of the arteries must be adapted to this. They can withstand high pressure by having very thick walls made up of elastic fibres and smooth muscle. These allow the wall to stretch as blood surges through them so that they don’t burst or rupture.

It also means that as the artery increases in diameter, the pressure is reduced a little. After they ‘give’, the elastic fibres recoil back inwards as the pressure falls. The artery decreases in diameter thus raising the pressure a little.

The lowering of the pressure when it is high, and the raising of it when it is lower produces some smoothing out of the flow of the blood. It should be obvious though when you feel your neck or wrist that it is by no means complete – you can still feel the pulses of the flow some distance from your heart.

A large artery will split into smaller arterioles that then branch further into many tiny capillaries.

Capillaries

To work efficiently, the capillaries need to be small enough to be in close proximity with small groups of cells and their walls need to be thin enough to allow substances to move in and out of the blood.

To enable this there are tiny gaps between the cells making up the wall of the capillary. These allow substances to leave the blood and bathe the cells of the tissues. The fluid made up of plasma and dissolved substances is called tissue fluid.

Tissue Fluid

Tissue fluid is formed because of the high hydrostatic pressure of the blood at the arteriole end of the capillary that pushes fluid out of the blood.

The blood contains plasma proteins giving the blood a relatively high solute potential (and therefore a low water potential), tending to draw water into the blood. Since the hydrostatic pressure has a greater effect than the solute potential at the arteriole end, the net effect is that fluid leaves the capillary. No blood cells or large proteins leave as they are too big to fit through the gaps.

At the venule end of the capillary, since fluid has been lost, the hydrostatic pressure of the blood is lower and the solute potential is higher. Because of this, fluid drains back into the blood. At this stage, the useful materials such as amino acids and glucose will have been taken up by the cells and the tissue fluid will now contain waste substances such as carbon dioxide and urea.

Veins

The capillaries then join to form larger venules which themselves then join to form veins.

Since at this stage, the pressure of the blood is low, blood needs to be ‘encouraged’ to flow back to the heart. To prevent any backflow of the blood (particularly important if blood is flowing against gravity) there are valves in the veins. Also the veins pass through or very close to muscles. When the muscles are active in contracting and relaxing, the squeezing on the veins moves blood along but due to the valves, only ever towards the heart.

As the pressure is so much lower in the veins than in the arteries, there is little need for the elastic fibres and smooth muscle in the walls.

The Heart



The structure of the heart is shown in a diagram below:

The structure is closely related to its function.

Mammals have a double circulation, which means that the right hand side of the heart pumps deoxygenated blood to the lungs in the pulmonary artery to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left hand side of the heart in the pulmonary vein.

From there the blood is pumped to the body in the aorta, eventually returning to the right hand side of the heart in the vena cava to start the cycle again.

Since the right side pumps to the lungs which are situated close to the heart, the walls are much thinner than the left side which has to pump blood out of the heart to the body.

The heart has 4 chambers; 2 on the right hand side (the left as you look at it in the screen) and 2 on the left. The top chamber on each side is called the atrium; the bottom is called the ventricle. The atria receive blood as it enters the heart and pumps it into the ventricles. The ventricles pump blood out of the heart.

Due to this, the walls of the atria are much thinner than the walls of the ventricles.
Inside the heart and at the base of the vessels that leave the heart are valves. These valves only open one way, which ensures that there is no backflow of blood.

The valves are held open or closed by tendons (chordae tendinae or perhaps better known as heart strings), which are attached at the other end to the papillary muscles in the ventricle walls. The valves open to let blood through and then snap shut. This sound of the valves closing is the ‘lub dub’ sound of the heartbeat.

The muscle of the heart is called cardiac muscle and is made of tightly connecting cells. This close contact allows rapid ion transport from cell to cell. This then allows smooth, efficient waves of depolarisation to produce contractions (and repolarisation to bring about relaxation), which pass through the heart.

The tissue is said to be myogenic i.e. it does not need electrical impulses from a nerve to make it contract. If the cardiac muscle is supplied with oxygen and nutrients (a task carried out by the coronary arteries which you can see running over the surface of the heart) it will continue to contract at a steady pace.

Nerves supplying the heart, though they are not needed to start the contractions, can bring about an increase or decrease in the rate of contractions when appropriate.

The cardiac cycle



One cardiac cycle consists of the atria and then the ventricles contracting so that the blood that has entered the heart is pumped out. This occurs about 70 times every minute and is continuous. The periods of contraction are called systole. The periods of relaxation are called diastole.

We shall start when the atria and ventricles are in diastole.

Blood at a low pressure in the veins flows into the atria. This increases the pressure inside the empty atria as they fill. Some of the blood trickles through the open atrioventricular valves into the relaxed ventricles below.

When the atria are full, they go into atrial systole, their walls contract and blood is pushed through the valves into the ventricles. The pressure in the atria is increased due to the contractions and the pressure is increasing in the ventricles as they fill with blood.

When the atria contract, blood cannot flow back into the veins because the pressure of the blood pushes on the valves in the veins to shut them.
After a short delay the ventricles contract from the apex (base) upwards. The pressure inside the ventricles increases due to the ventricular systole. As the pressure increases to a higher level than the pressure in the atria, blood pushes against the atrioventricular valves, shutting them (the first heart sound) and preventing backflow.

The semilunar valves open under the pressure and blood leaves the heart.

The ventricles relax – ventricular diastole – and the semilunar valves snap shut behind the blood (the second heart sound).

To work out from a graph what stage of the cycle the heart is in, it is important to look at the relative pressure of the atria and ventricles.

Regulation of the cardiac cycle by the heart itself



The heartbeat is initiated in a specialised area of muscle in the right atrium called the sinoatrial node (SAN) or the pacemaker. The SAN starts the waves of depolarisation, which results in contraction.

The waves spread out over the 2 atrial walls so that they contract. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles.

There is an area, however, which does conduct in the septum, and the waves can pass from here through the ventricles. This specialised area is called the atrioventricular node (AVN) and will pass on the waves of depolarization after about 0.1s.

It would be disastrous if the ventricles contracted at the same time so that is why there is a short period of delay before the ventricles contract.

The AVN passes them on to the Purkinje (also called Purkyne) fibres in the inter-ventricular septum. The excitation is passed to the apex of the heart and then through the ventricle walls. This causes the ventricles to contract from the base upwards ensuring that the blood is forced up and out in the vessels leaving the heart.

Regulation of the cardiac cycle by the heart by other factors



The total amount of blood pumped by the heart in lone minute = cardiac output.

Cardiac output = stroke volume + number of beats per minute
(Stroke volume is the volume pumped in one beat)

Increasing the stroke volume therefore can increase cardiac output. A larger volume might enter the atria through the veins during exercise because the vessels become dilated to enable more blood to flow to the muscles to supply more oxygen and nutrients. The atria are stretched more than normal; the heart detects this and responds by beating faster and with more force.

Increasing the number of beats per minute can also increase the cardiac output.

The effect of hormones

This occurs when adrenaline is released from the adrenal medulla, flows in the blood and affects the SAN. The SAN is stimulated, works faster, increasing the heart rate.

The effect of nervous stimulation

One nerve, the accelerator nerve, runs from the cardioacceleratory centre in the medulla of the brain to the SAN.

Another, the vagus nerves, runs from the cardioinhibitory centre in the medulla of the brain to the SAN.

These nerves are stimulated in various situations e.g. during exercise, the accelerator nerve is stimulated. It releases noradrenaline at the SAN resulting in the heart rate increasing due to a decreased delay at the AVN and increasing the force of the contractions.

If the vagus nerve is stimulated, acetylcholine is released at the SAN. The delay at the AVN increases and the cardiac output falls.

Blood pressure also affects the cardiac output. Some blood vessels (e.g. the aorta and carotid arteries) have baroreceptors (also called stretch receptors) in their walls. These detect the pressure and send impulses to the cardiac centre in the medulla.

If the pressure is too high: the cardioinhibitory centre is stimulated, impulses are sent down the vagus nerve, the heart rate is slowed and the pressure will fall.

If the pressure is too low: the cardioacceleratory centre is stimulated, impulses are sent down the accelerator nerve, the heart rate is increased and the pressure will rise.

Blood


Just over half of the blood volume is made up of a pale yellow fluid called plasma. The rest of the blood is made up of cells (red blood cells and white blood cells) and platelets.

Blood has several vital functions:

1. Transport - oxygen in the red blood cells, absorbed, digested food by the plasma, excretory products by the plasma, hormones by the plasma etc.

2. Defence – by the white blood cells (a.k.a. leucocytes).

3. Formation of lymph and tissue fluid

4. Homeostasis

Red blood cells

Also known as erythrocytes. These contain a pigment, haemoglobin, which gives them their colour.

Red blood cells are made in the bone marrow (the liver in a foetus) of many bones. They have a life span of about 120 days and are about 7m in diameter (very small).

Being like a biconcave disc in shape, the surface area to volume ratio is very large. Oxygen can therefore diffuse very quickly into the cell and because the cell is so small, quickly bind to a haemoglobin molecule.

They lack organelles meaning that there is more room for haemoglobin. Their size and flexible membrane also means that they can squeeze through capillaries and transport oxygen extremely close to cells.

White blood cells

These cells all have a nucleus, most are much larger than red blood cells and are spherical or irregular in shape.

There are two basic types of white blood cells; the granulocytes (they have granular cytoplasm and lobed nuclei) and agranulocytes (the cytoplasm appears smooth and the nucleus is rounded or horseshoe in shape).

Made in

Function

Agranulocytes

Monocytes

Red bone marrow

Phagocytic against bacteria and antibody-coated viruses

Lymphocytes

Spleen, lymph nodes

Produces antibodies

Granulocytes

Neutrophils

Red bone marrow

Phagocytic and contain lysosomes to break down ingested bacteria

Eosinophils

Red bone marrow

Phagocytic. Works against allergens by making antihistamines

Basophils

Red bone marrow

Make antihistamines, make heparin (prevents unnecessary blood clotting) and make serotonin (makes the capillaries more leaky so that phagocytes can leave the blood and enter the site of infection

Platelets

These are formed in the bone marrow and are fragments of larger cells. They have no nucleus but reactions do take place in the cytoplasm.

They have a variety of role such as blood clotting and the production of prostaglandins that regulate the degree of constriction or dilation in blood vessels.

Blood Clotting: platelets stick to damaged cells on the inner surface of blood vessels forming a plug. Unless the damage is small, platelets are involved in a chain of reactions by releasing particular chemicals. A soluble protein, fibrinogen (present in the plasma), becomes an insoluble protein, fibrin, and this forms layers of fibres across the wound. The mesh that this creates traps red blood cells and platelets and a scab is formed.

This has two useful effects:
1. Blood does not leak out of he vessel.
2. It is less likely that an infectious organism will enter from outside and cause harm.

Blood Groups

The most commonly required blood-grouping system is the ABO system. It concerns two antigens that can occur on the surface of red blood cells. The antigens are called agglutinogens in this case and are: agglutinogen A and agglutinogen B.

Plasma also contains antigens, called agglutinins in this case, and they are agglutinin A and agglutinin B.
I shall call agglutinogens A or B and the agglutinins a or b.
If A and a come into contact, the red cells will clump together.
If B and b come into contact the red cells will clump together.
Therefore, in your blood you will not contain the agglutinogen and the agglutinin of the same type.

Blood Group

Agglutinogen A

Agglutinin
A

Agglutinogen B

Agglutinin
B

A

v

X

X

v

B

X

v

v

X

AB

v

X

v

X

O

X

v

X

v

Blood Transfusions

It is important to match blood correctly so that agglutinins in the recipient don’t clump the red blood cells of the donor.

In transfusions it is important to remember that the volume of blood donated is relatively small compared to the volume of the recipients blood. The agglutinins in the plasma from the donor are so diluted that no harm is done. However the aggluinogens on the red blood cells are not so diluted so harm can be done. These are the possible transfusions (v is safe, X is not):

Recipient

Donor

A

B

AB

O

A

v

X

X

v

B

X

v

X

v

AB

v

v

v

v

O

X

X

X

v

Oxygen Carriage



Oxygen does dissolve in plasma but the solubility is low and decreases further if the temperature increases. The amount that could be carried by the plasma therefore would be completely insufficient to supply all cells.

There is a protein in the blood however that will carry 4 molecules of oxygen. The protein is called haemoglobin (Hb) and is made up of 4 polypeptide chains, each with a haem group. Each haem group can pick up 1 molecule of O2. The protein, being fairly small, could pass out of the blood during ultrafiltration in the kidneys so, to ensure that it is not lost, it is found within red blood cells.

Note – during this topic you will come across the term of partial pressure of oxygen. It does not mean the pressure of the blood itself. Essentially it is a measure of the concentration of oxygen. It is written in shorthand as pO2 and is measured in kilopascals (kPa).

Inhaled air in the alveoli has a pO2 = 14kPa. The pO2 of resting tissue = 5.3kPa (lower pO2 = lower O2 concentration due to respiration) and the pO2 of active tissues = 2.7kPa. In either case, blood arriving at the lungs has a lower pO2 than that in the lungs.

There is therefore a diffusion gradient and oxygen will move from the alveoli into the blood. The O2 is then loaded onto the Hb until the blood is about 96% saturated with oxygen. The Hb is now called oxyhaemoglobin (HbO2).

Hb + 4O2 HbO8

Dissociation Curves

The blood is then taken to tissues where the cells are respiring all the time, using oxygen. The pO2 will be low. As the red blood cell enters this region, the Hb will start to unload the O2, which will diffuse into the tissues and be used for further respiration. Since much of the Hb will have unloaded the O2, a much lower percentage of the blood will be saturated with O2.

A graph of the percentage saturation of blood with O2 i.e. the amount of HbO2 as opposed to Hb at different pO2 is shown below. It is called an Oxygen Dissociation Curve.

It is S-shaped because of the behaviour of the Hb in different pO2.

The first molecule of O2 combines with an Hb and slightly distorts it. The joining of the first is quite slow (the flatter part of the graph at the beginning) but after the Hb has changed shape a little, it becomes easier and easier for the second and third O2 to join. This is shown by the curve becoming steeper. It flattens off at the top because joining the fourth O2 is more difficult.

Overall, it shows that at the higher and lower end of the partial pressures, there isn’t a great deal of change in the saturation of the Hb, but in the middle range, a small change in the pO2 can result in a large change in the percentage saturation of the blood.

The Effect of pH – The Bohr Effect

The amount of O2 carried and released by Hb depends not only on the pO2 but also on pH.

An acidic environment causes HbO2 to dissociate (unload) to release the O2 to the tissues. Just a small decrease in the pH results in a large decrease in the percentage saturation of the blood with O2.

Acidity depends on the concentration of hydrogen ions.

H+ displaces O2 from the HbO2, thus increasing the O2 available to the respiring tissues.

H+ + HbO2 HHb + O2

HHb is called haemoglobinic acid.

This means that the Hb mops up free H+. That way the Hb helps to maintain the almost neutral pH of the blood. Hb acts as a buffer.

This release of O2 when the pH is low (even if the pO2 is relatively high) is called the Bohr effect.

When does the pH decrease because of free H+ in the blood?

During respiration, CO2 is produced. This diffuses into the blood plasma and into the red blood cells. Inside the red blood cells are many molecules of an enzyme called carbonic anhydrase. It catalyses the reaction between CO2 and H2O. The resulting carbonic acid then dissociates into HCO3- + H+.
(Both reactions are reversible)

CO2

+

H2O

H2CO3

carbon dioxide

water

carbonic acid

H2CO3

HCO3-

+

H+

Carbonic acid

hydrogencarbonate ion

hydrogen ion

Therefore, the more CO2, the more the dissociation curve shifts to the right.

Carbon Dioxide Transport



About 85% of the CO2 produced by respiration diffuses into the red blood cells and forms carbonic acid under the control of carbonic anhydrase.

The HCO3- diffuses out of the red blood cell into the plasma. This leaves a shortage of negatively charged ions inside the red blood cells. To compensate for this, chloride ions move from the plasma into the red blood cells. This restoration of the electrical charge inside the red blood cells is called the chloride shift.

About 5% of the CO2 produced simply dissolves in the blood plasma.
Some CO2 diffuses into the red blood cells but instead of forming carbonic acid, attaches directly onto the haemoglobin molecules to form carbaminohaemoglobin. Since the CO2 doesn’t bind to the haem groups the Hb is still able to pick up O2 or H+.

Other Effects



Carbon monoxide

If carbon monoxide is breathed in (for example from car exhaust fumes), it binds irreversibly with haemoglobin to form carboxyhaemoglobin. This means that the Hb cannot load and carry O2.

To make matters worse, Hb combines with CO about 250 times more readily than it does with O2 so that, even if the CO concentration is fairly low, it can cause death due to lack of O2 to the tissues.

Cigarettes produce CO-containing smoke, which means that a small percentage of a smoker’s blood is unable to transport O2.

Foetal Haemoglobin

A foetus developing in the uterus must be able to load O2 from its mother’s blood. Due to the respiration occurring in the foetus’ cells, the pO2 is lower in the foetus blood than in the mother’s blood. Some will unload from the mother’s HbO2 and diffuse across to the foetus. However, because of the relatively small concentration difference, not much O2 is passed across.

To maximise the amount of O2 that the foetus receives, it has different haemoglobin – foetal haemoglobin. This has a higher affinity for O2 than adult Hb (it combines more readily with O2) so the foetus picks up enough O2. The dissociation curve shifts to the left.

Myoglobin

Skeletal muscle contains a pigment called myoglobin. It is very similar to Hb but has a higher affinity for O2.

It will load with O2 as Hb unloads and will store the O2 in the muscle until it is required. It only releases the O2 when the pO2 is very low – when the Hb cannot supply O2 fast enough and the demand is great. The dissociation curve shifts to the left.

Temperature

The higher the temperature, the less saturated the blood is with O2 i.e. the more the HbO2 unloads the O2.

This situation might arise during exercise – heat is produced during metabolic activity and during this time, the O2 supply will need to increase. The dissociation curve shifts to the right.

Environment

Animals living where there is a shortage of oxygen, animals living at high altitude need to be able to pick up O2 when it is at a very low pO2. Their dissociation curves will look like those of foetal haemoglobin and myoglobin – the curve shifts to the left.


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