Transport in Animals
Transport – the movement of substances such as oxygen, nutrients, hormones, waste and heat around the body
Three factors influence the need for transport systems:
- Surface area/volume ratio
- Level of metabolic activity
Features of a good transport system:
- A fluid or medium to carry substances around the body e.g. blood, lymph
- A pump to create pressure to push fluid round the body
- Exchange surfaces that enable substances to enter the fluid (blood) e.g. capillaries
- Tubes or vessels to carry the fluid by mass flow
- Two circuits, one for collection of oxygen and another for the delivery of oxygen
Single circulatory system – one in which the blood flows through the heart only once for each circuit of the body e.g. heart -> gills -> body -> heart
Double circulatory system – one in which the blood flows through the heart twice for each circuit in the body e.g. heart -> body -> heart -> lungs -> heart
- The blood pressure must not be too high in the pulmonary circuit so as not to damage the delicate capillaries
- The heart can increase the pressure after it returns to the lungs so that the blood circulates through the body quickly
- The systemic circulation can carry higher pressured blood than the pulmonary circulation
- Artery walls are thick to withstand high pressure
- The lumen is small to maintain the high pressure
- The inner layer consists of a thin layer of elastic tissue that allows the walls to stretch and recoil
- The middle layer consists of a thick layer of smooth muscle
- The outer layer is a thick layer of collagen and elastic tissue for strength and to support the recoil for maintaining pressure.
- Small blood vessels that distribute the blood from arteries to the capillaries
- They have a layer of smooth muscle which contracts to increase resistance to flow and reduces the rate of blood flow
- Constriction of arteriole walls is used to divert blood to regions of the body that are demanding oxygen
- Very thin walls consisting of a single layer of endothelium
- Narrow lumen to squeeze red blood cells against the wall to aid in the transfer of oxygen and reduce the diffusion distance
- Leaky walls to allow blood plasma and dissolved substances to leave the blood (e.g. in lymph)
- These collect blood from the capillary bed and lead into the veins
- Consists of thin layers of muscle and elastic tissue as the pressure is relatively low
- Carry blood back to the heart
- Large lumen in order to ease the flow of blood
- Walls are thin as they do not need to stretch and recoil and are not actively constricted in order to reduce the blood flow
- They contain valves to prevent back flow of blood and help the blood flow into the heart properly
Blood plasma and tissue fluid
Plasma is the fluid portion of the blood which may move into the body, containing dissolved substances such as O2 and CO2. Tissue fluid is formed by blood plasma leaking from the capillaries into the tissues.
- t the arteriole end of the capillary bed the blood is at high hydrostatic pressure (the pressure a fluid exerts when pushing against the sides of a vessel or container
- This high hydrostatic pressure pushes the plasma out of the capillaries between the tiny gaps between the cells in the walls
- The fluid contains dissolved nutrients and oxygen but the cells and the platelets are too large to leave the blood system
- Not all the fluid returns to the blood, some is directed into the lymphatic system, which drains the excess tissue fluid out of the tissues and returns it to the blood system in the subclavian vein
- The lymphatic system contains many more lymphocytes which are produces in the lymph nodes
Oncotic pressure – the pressure created by the osmotic effects of the solutes and causes the movement of tissue fluid into the blood (it has a negative figure and is also measured in kPa)
Structure of the Heart
Atria – These chambers have relatively thin walls as they do not need to create much pressure
Right ventricle – thicker walls than the atria but still not as thick as the left ventricle as the blood only needs to be pumped as far as the lungs which lie next to the heart in the chest cavity
Left ventricle – two or three times thicker than the right ventricle as an inordinate amount of pressure must be created here to overcome the resistance of the systemic circulation
Cardiac Muscle Structure
- They are divided into contractile units called sarcomeres
- There are numerous mitochondria between the myofibrils (muscle fibres)
- The intercalated discs facilitate synchronised contraction
The Cardiac Cycle
Blood enters the aorta and pulmonary artery in a rapid spurt but must be delivered in an even flow to prevent damage so the structure of the artery walls come into play. The smooth muscle and elastic fibre layers allow stretch and recoil with each beat of the heart which them lowers the pressure. The further along the blood flows into the arterioles the less obvious the fluctuations tend to be. It is important to maintain blood pressure so that the blood can travel all the way around the body and deliver nutrients to all the tissues.
Coordination of the cardiac cycle
The heart is myogenic as it can initiate its own contraction.
If the contractions of the atria and ventricles are not synchronised it is known as fibrillation.
The sino-atrial node generates a wave of excitation at regular intervals (55-80x per min).
The wave spreads over the walls of both atria causing the cardiac muscle cells to contract during atrial systole.
At the top of the atrioventricular septum, the atrioventricular node delays the signal before conducing it down the purkyne tissue and spreads out through the walls of both ventricles.
This means that they contract from the apex upwards, forcing the blood in the correct direction.
- Wave P shows the excitation of the atria
- QRS indicates the excitation o the ventricles
- T shows diastole
Sinus rhythm – normal
Bradycardia – slow heart rate
Tachycardia – fast heart rate
Atrial fibrillation – atria beating faster than the ventricles
Ectopic heartbeat – irregular heart beat
Transport of Oxygen
Haemoglobin + oxygen à oxyhaemoglobin
Haemoglobin has a high affinity for oxygen and each of the four haem groups binds with one oxygen molecule
Oxygen is absorbed in the blood as it passes the alveoli in the lungs. The oxygen binds reversibly to the haemoglobin.
In the body tissue dissociation takes place and the oxyhaemoglobin releases the oxygen for the cells to use in aerobic respiration
The ability of haemoglobin to take up and release oxygen depends on the oxygen partial pressure. Association of oxygen and haemoglobin takes place when the partial pressure is high whereas the dissociation of oxygen and haemoglobin takes place when the partial pressure of oxygen is low.
Partial pressure symbol = pO2
Haemoglobin dissociation curve:
After the first oxygen binds to the haemoglobin the haemoglobin undergoes a slight conformational change which allows other oxygen molecules to more easily bond to its remaining haem groups.
Fetal haemoglobin lies to the left of the normal haemoglobin dissociation curve as fetal Haemoglobin has a higher oxygen affinity that normal haemoglobin as the fetal haemoglobin must cause the dissociation of the mother’s haemoglobin in the placenta and absorb oxygen from the surrounding fluid as well.
Transport of CO2
Carbon dioxide is transported in three ways:
- 5% is dissolved directly in the plasma
- 10% is combined directly with haemoglobin to form carbaminohaemoglobin
- 85% is transported in the form of hydrogencarbonate ions
- Carbon dioxide diffuses into the red blood cell and combines with water to form carbonic acid (catalysed by carbonic anhydrase) CO2 + H2O à H2CO3
- The carbonic acid releases H+ ions and hydrogencarbonate ions.
- The hydrogen carbonate ions diffuse out of the cell into the plasma.
- Charge of the RBC is maintained by the chloride shift, where negative chloride ions move into the cell to adjust the charge.
- The hydrogen ions have the potential to make the RBC very acidic so the haemoglobin acts as a buffer and binds with the H+ ions to make haemoglobinic acid.
The Bohr Effect
Carbon dioxide concentration increases
The following increase in cell cytoplasm acidity causes changes in the tertiary structure of the haemoglobin and reduced its affinity for oxygen
This causes the dissociation of the oxygen to the tissues in order to provide them with it for respiration
This ensures that tissues that are respiring more such as contracting muscles get more oxygen in comparison to those not respiring as quickly/actively
The Bohr shift refers to the change in the haemoglobin dissociation curve which moves down and right when more CO2 is present.