What you'll learn
- Why large mammals need a mass transport system rather than relying on diffusion alone.
- How blood flows through the double circulatory system and the four chambers of the heart.
- How arteries, veins and capillaries are adapted to their functions.
- How pressure changes drive the cardiac cycle, including valve opening and closing.
Why large animals need a transport system
Every cell needs substances such as oxygen and glucose, and it must remove waste products such as carbon dioxide and urea. In very small organisms, diffusion can be enough because the diffusion distance is short.
Humans are large, multicellular organisms. Many cells are far from the external environment, so diffusion alone would be too slow to meet metabolic demand.
Mass transport
Mass transport is the bulk movement of substances, such as oxygen, carbon dioxide, glucose and urea, around an organism in a transport medium such as blood.
Blood is moved by the heart through a network of blood vessels. This maintains steep concentration gradients at exchange surfaces, such as the lungs and body tissues, so diffusion can still happen quickly over short distances.
Why the heart matters
The heart does not directly “give” cells oxygen; it keeps blood moving so exchange surfaces can continually supply oxygen and remove waste.
The mammalian double circulatory system
Mammals have a double circulatory system, meaning blood passes through the heart twice during one complete circuit of the body.
Double circulatory system
A double circulatory system has two linked circuits: the pulmonary circulation, between the heart and lungs, and the systemic circulation, between the heart and the rest of the body.
The right side of the heart pumps deoxygenated blood to the lungs. The left side pumps oxygenated blood to the body. The two sides are separated by the septum, a muscular wall that prevents oxygenated and deoxygenated blood mixing.
The diagram shows the route of blood through the heart. Remember that “right” and “left” are named from the person’s point of view, so they may look reversed on a diagram.
The basic route is:
- Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs
- Lungs → pulmonary vein → left atrium → left ventricle → aorta → body
Artery does not always mean oxygenated
An artery carries blood away from the heart, while a vein carries blood towards the heart. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood.
Heart chambers, valves and one-way flow
The heart has four chambers. The atria are the upper chambers that receive blood. The ventricles are the lower chambers that pump blood out of the heart.
The left ventricle has a much thicker muscular wall than the right ventricle because it must generate higher pressure to pump blood around the whole body. The right ventricle only pumps blood to the lungs, which are nearby and delicate.
A valve is a structure that prevents backflow. Heart valves open and close because of pressure differences; they do not open by muscular contraction.
The main valves are:
- Tricuspid valve: between right atrium and right ventricle.
- Bicuspid valve or mitral valve: between left atrium and left ventricle.
- Pulmonary semilunar valve: between right ventricle and pulmonary artery.
- Aortic semilunar valve: between left ventricle and aorta.
The heart wall is made of cardiac muscle. The myocardium is the muscular wall of the heart, and it needs its own supply of oxygen and glucose from the coronary arteries, which branch from the aorta.
Tracing blood through the heart
A red blood cell enters the heart in the vena cava. Trace its route until it leaves through the aorta.
- It enters the right atrium from the vena cava, then passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary semilunar valve into the pulmonary artery, which carries it to the lungs.
- In the lungs, the red blood cell becomes oxygenated as oxygen diffuses into the blood.
- It returns through the pulmonary vein to the left atrium, passes through the bicuspid valve into the left ventricle, and is pumped through the aortic semilunar valve into the aorta.
Blood vessels: structure matches function
Blood travels through three main types of vessel: arteries, veins and capillaries. The lumen is the space inside a blood vessel where blood flows. The endothelium is the thin inner lining of a blood vessel.
This comparison shows how vessel structure links to pressure and exchange.
Arteries
Arteries carry blood away from the heart at high pressure. They have thick walls containing elastic fibres and smooth muscle. Elastic fibres allow the artery wall to stretch during ventricular systole and recoil during diastole, helping to maintain blood pressure.
Arteries usually have a relatively narrow lumen compared with veins. This helps maintain high pressure.
Veins
Veins carry blood back towards the heart at lower pressure. They have thinner walls and a wider lumen than arteries. Many veins contain valves, which prevent blood flowing backwards.
Blood flow in veins is helped by nearby skeletal muscles contracting and squeezing the veins. This is especially important in the legs, where blood must return to the heart against gravity.
Capillaries
Capillaries are tiny vessels that connect arterioles to venules and allow exchange between blood and tissues. Their walls are one cell thick, giving a very short diffusion distance.
Capillaries also have a narrow lumen, so red blood cells pass through slowly and close to the capillary wall. This increases time for diffusion of substances such as oxygen, carbon dioxide and glucose.
Structure follows pressure
Arteries are adapted to withstand and maintain high pressure, veins are adapted for low-pressure return, and capillaries are adapted for rapid exchange.
The cardiac cycle
The cardiac cycle is the sequence of events in one heartbeat. It includes contraction and relaxation of the atria and ventricles.
Systole and diastole
Systole means contraction of heart muscle. Diastole means relaxation of heart muscle.
Pressure is crucial. Blood moves from higher pressure to lower pressure, and valves open only when the pressure behind them is greater than the pressure in front of them.
The cardiac cycle has three main stages.
1. Atrial systole
The atria contract. This increases pressure in the atria and pushes blood into the ventricles through the atrioventricular valves.
At this point, the ventricles are relaxed, so their pressure is low. The semilunar valves remain closed because pressure in the arteries is higher than pressure in the ventricles.
2. Ventricular systole
The ventricles contract. Ventricular pressure rises sharply.
When ventricular pressure becomes greater than atrial pressure, the atrioventricular valves close. This prevents blood being forced back into the atria.
When ventricular pressure becomes greater than pressure in the aorta and pulmonary artery, the semilunar valves open. Blood is ejected from the ventricles into the arteries.
3. Diastole
The ventricles relax, so ventricular pressure falls. When pressure in the arteries becomes greater than pressure in the ventricles, the semilunar valves close. This prevents blood flowing back from the arteries into the ventricles.
As ventricular pressure falls below atrial pressure, the atrioventricular valves open again and the ventricles begin to fill passively.
On a pressure graph, you can work out valve movements by comparing pressures in neighbouring chambers or vessels.
Valves follow pressure
A valve opens when pressure behind it is higher than pressure in front of it. It closes when the pressure difference would cause backflow.
Using pressures to predict valve state
At one moment, left atrial pressure is 1.0 kPa, left ventricular pressure is 14 kPa, and aortic pressure is 12 kPa. Decide which left-side valves are open.
- Compare the left ventricle with the left atrium: 14 kPa>1.0 kPa14\,\text{kPa} > 1.0\,\text{kPa}14kPa>1.0kPa, so blood would tend to move back towards the atrium unless the bicuspid valve closes.
- Compare the left ventricle with the aorta: 14 kPa>12 kPa14\,\text{kPa} > 12\,\text{kPa}14kPa>12kPa, so blood can be forced from the ventricle into the aorta.
- Therefore, the bicuspid valve is closed and the aortic semilunar valve is open. This is during ventricular systole.
Electrical coordination of the heartbeat
Cardiac muscle is myogenic, meaning it can generate its own electrical excitation without needing a nerve impulse from the brain.
The sinoatrial node or SAN is the heart’s natural pacemaker. It is a small region of tissue in the right atrium that starts each heartbeat. The wave of excitation spreads across the atria, causing atrial systole.
The excitation then reaches the atrioventricular node or AVN, which delays the impulse slightly. This delay allows the ventricles to fill before they contract.
The impulse travels down the bundle of His in the septum and then through Purkyne fibres in the ventricle walls, causing the ventricles to contract from the apex upwards. This helps push blood out through the arteries.
Electrical impulses do not open valves
The electrical system coordinates heart muscle contraction. Valves still open and close because of pressure differences, not because nerves tell them to move.
Cardiac output
Stroke volume is the volume of blood pumped out of one ventricle in one beat. Heart rate is the number of heartbeats per second. Cardiac output is the volume of blood pumped out of one ventricle per second.
Stroke volume can be calculated from ventricular volumes:
SV=EDV−ESVSV = EDV - ESVSV=EDV−ESVwhere EDVEDVEDV is end-diastolic volume and ESVESVESV is end-systolic volume.
Cardiac output is calculated using:
CO=HR×SVCO = HR \times SVCO=HR×SVwhere COCOCO is cardiac output, HRHRHR is heart rate, and SVSVSV is stroke volume.
Unit check
If heart rate is given in beats per minute, divide by 60 to convert it to per second before calculating cardiac output in m3 s−1\text{m}^3\,\text{s}^{-1}m3s−1.
Calculating cardiac output
A student has a heart rate of 1.20 s−11.20\,\text{s}^{-1}1.20s−1 and a stroke volume of 7.0×10−5 m37.0 \times 10^{-5}\,\text{m}^37.0×10−5m3. Calculate cardiac output.
- Choose the equation: CO=HR×SVCO = HR \times SVCO=HR×SV.
- Substitute the values with units: CO=1.20 s−1×7.0×10−5 m3CO = 1.20\,\text{s}^{-1} \times 7.0 \times 10^{-5}\,\text{m}^3CO=1.20s−1×7.0×10−5m3.
- Multiply the numbers and carry the units through: CO=8.4×10−5 m3 s−1CO = 8.4 \times 10^{-5}\,\text{m}^3\,\text{s}^{-1}CO=8.4×10−5m3s−1.
In the exam
- For heart pathway questions, track blood by direction of flow, not by whether the vessel is called an artery or vein.
- For cardiac cycle graphs, compare pressures on either side of a valve to decide whether it is open or closed.
- For vessel structure questions, link each feature to a function: pressure resistance, pressure maintenance, preventing backflow, or short diffusion distance.
- For cardiac output calculations, write the formula, substitute values with units, and check that the final unit matches volume per time.
Check yourself
- Why does the left ventricle need a thicker muscular wall than the right ventricle?
- What happens when ventricular pressure falls below aortic pressure, and why?
- How does capillary structure increase the rate of exchange between blood and tissues?
