Open and closed circulatory systems
In this IBDP topic, you can understand the transport in animals.
- All vertebrates have a closed circulatory system (blood is always enclosed in vessels)
- Insects have an open circulatory system. Their blood is not contained within vessels and just fills their body cavity, which is called a haemocoel.
- The blood in insects does not transport oxygen. This is done by tubes called trachea. It carries air directly to the tissues.
Single and double circulatory systems
- The human circulatory system is a double circulatory system.
- One part serves the lungs and is called the pulmonary circulation while the other serves the rest of the body and is called the systematic circulation.
- Fish have a single circulatory system.
- The blood is pumped out of the heart to the gills, where it picks up oxygen. But instead of going back to the heart, it travels around the rest of the body.
The structure of a human heart
The Cardiac Cycle
- Atriole systole is when the heart is filled with blood and the muscle in the atrial walls contract.
- Blood is forced from the atria down through the atrio-ventricular valve into the ventricles.
- Ventricular systole is when the ventricle contracts and pushes the blood out of the heart.
- As soon as the pressure in the ventricles becomes greater that that in the atria the atrio-ventricular valve shuts. This means the blood can’t go backwards.
- Ventricular diastole then begins. This is when the ventricle relaxes and the pressure drops. The blood can’t flow back though because of the semi-lunar valves.
- Blood from the veins floods into the two atria, and the whole cycle starts again.
The valves shut because the pressure of the blood pushes up against the cusps of the valve.
Contraction of the papillary muscles, attached to the valve by tendons, prevents the valve from being opened the wrong way.
Control of the heart beat
- It is a myogenic muscle- it contracts and relaxes automatically.
- The cardiac cycle is initiated in a small patch of muscle in the wall of the right atrium, called the sino-atrial node (SAN)
- It is often called the pacemaker.
- Each time the SAN muscles contract they set up a wave of electrical activity. The cardiac muscles in the atria walls respond by contracting in the same rhythm as the SAN.
- However the muscles in the ventricles do not contract till after the atrium.
- This delay is caused by a band of fibres between the atria and the ventricles which don’t conduct the excitation wave.
- The excitation must pass through the atrio-ventricular node (AVN)
- The excitation wave is passed on to a bunch of conducting fibres called the purkyne tissue.
- This runs down the septum between the ventricle walls.
- This causes the cardiac muscles in the ventricles to contract from the bottom up. This squeezes blood up into the arteries.
- A graph of voltage over time.
- P= when the SAN first contracts and so the atria contracts also.
- QRS= when the impulse reaches the purkyne tissue and makes the ventricles contract.
- The ventricle muscles relax
•When blood first leaves the heart it is travelling in the arteries.
•Arteries divide to form arterioles.
•These divide to make capillaries
•These join up to form venules.
•And these join to form veins
•Veins carry blood back to the heart.
Blood plasma, tissue fluid and lymph
- Blood is composed of cells floating in a pale yellow liquid called plasma.
- Blood plasma is mostly water. It has nutrients (glucose), and waste products (urea) in it also. They also contain plasma proteins.
How tissue is formed?
- Some plasma leaks out through capillary walls and into spaces between the cells and tissues.
- The leaked plasma is called tissue fluid.
- it is similar to blood plasma but contains much fewer protein molecules. There are some white blood cells.There are two opposing pressures that affect the amount of fluid which leaves the capillaries.
At the venous end, there is a water potential gradient (due to proteins in the plasma) this means that water moves back in.
At the arteriole end of a capillary bed, the blood pressure is enough to push fluid out into the tissue.
- About 90% of fluid that leaks from capillaries eventually seeps back into them. The remaining 10% is collected and returned to the blood system though a series of tubes called lymph vessels or lymphatics.
- Tissue fluid flows into the lymphatics through tiny valves.
- Allows it to flow in but not out.
- Valves are large enough to allow large protein molecules to pass through. Proteins are too big to get into blood capillaries so cannot be taken away by the blood.
- A build up of tissue fluid is called oedema.
- The fluid inside lymphatics is called lymph.
- It is virtually identical to tissue fluid.
- Lymphatics join up to make larger lymph vessels. These gradually transport lymph back up to the large veins just beneath the collarbone, the subclavian veins.
- There are valves in lymphatics to prevent backflow.
- The contraction of surrounding muscles provides pressure.
Carbon dioxide transport
Carbon dioxide diffuses into the blood plasma then one of three things can happen to it...
- Some of it remains as CO2 molecules dissolved in the plasma.
- Some of the CO2 diffuses into the erythrocytes
- Some CO2 diffuses into the erythrocytes and combines with hemoglobin making carbaminohaemoglobin.
- In the cytoplasm of the erythrocytes there is an enzyme called carbonic anhydrase.
- Carbon dioxide and water makes carbonic acid
- This then dissociates (splits)
- Carbonic acid turns to a hydrogen ion and a hydrogencarbonate ion.
- The hydrogen ion combines with haemoglobin to make haemoglobin acid. This makes the haemoglobin release the oxygen its carrying.
- The hydrogencarbonate ions diffuse into the plasma.
This is the end of the topic
Drafted by Eva (Biology)