Edexcel A-Level Biology

Why large animals need a transport system

Every cell needs a supply of oxygen and nutrients and a way to get rid of carbon dioxide and other waste. In a very small organism these substances simply diffuse in and out across the body surface. Diffusion is quick over short distances but its time rises steeply with distance, so it cannot supply the middle of anything more than a fraction of a millimetre thick.

A second problem is the ratio of surface area to volume. When an organism gets bigger its volume grows faster than its surface area, so the surface available for exchange is small compared with the number of cells that depend on it. A cube with sides of $\pu{1 cm}$ has a surface area of $\pu{6 cm2}$ and a volume of $\pu{1 cm3}$, a ratio of $6:1$. A cube with sides of $\pu{2 cm}$ has a surface area of $\pu{24 cm2}$ and a volume of $\pu{8 cm3}$, a ratio of $3:1$. The larger cube has half the surface for each unit of volume.

Large, active animals therefore have specialised exchange surfaces, such as lungs, and a mass transport system. Mass transport is the bulk movement of substances over long distances, in a fluid carried by a pump. In mammals the fluid is blood, the pump is the heart and the pipes are blood vessels. Because the blood is moved by pressure and not by diffusion, substances arrive at every tissue quickly, and diffusion is needed only over the final short distance from a capillary to a cell.

Mammals have a double circulation: blood passes through the heart twice on each complete circuit of the body. The right side pumps deoxygenated blood to the lungs at fairly low pressure, which suits the delicate capillaries there. The left side pumps oxygenated blood to the rest of the body at much higher pressure. Returning blood to the heart between the lungs and the body means it can be sent out again at high pressure, and this keeps a steep diffusion gradient at the tissues. A closed circulatory system, in which blood stays inside vessels, allows the pressure and flow to a tissue to be adjusted, for example by narrowing the arterioles that supply it.

Water as a transport medium

Blood plasma is mostly water, and water is a good solvent for transport because its molecules are dipoles. The oxygen atom in a water molecule pulls the shared electrons towards itself, so it carries a small negative charge and each hydrogen atom carries a small positive charge. Ions and polar molecules, such as sodium chloride, glucose, amino acids and urea, are attracted to these charges and are surrounded by water molecules, so they dissolve and can be carried in the blood. Non-polar substances such as lipids do not dissolve, which is why they travel in blood packaged with proteins as lipoproteins.

Water also has a high specific heat capacity, so the blood can carry heat around the body with only small changes in its own temperature, and it is a liquid over the whole range of temperatures at which mammals live.

The structure of the blood vessels

There are three main types of vessel, each adapted to its job. The wall of an artery or vein has three layers: an inner lining of flattened endothelial cells, a middle layer of smooth muscle and elastic fibres, and an outer layer of collagen. A capillary has only the endothelium.

Arteries

Arteries carry blood away from the heart, at high pressure. The wall is thick, so it does not burst under the pressure, and it contains a great deal of elastic tissue. Each time the ventricles contract, the wall is stretched by the surge of blood, and when the ventricles relax the wall recoils. The recoil pushes the blood onwards and evens out the flow, so blood in the smaller vessels leaving the artery moves in a steadier stream rather than in pulses. There is smooth muscle in the wall, which can contract to narrow the lumen, but in the large arteries it is the elastic tissue that matters most. The lumen is narrow compared with the thickness of the wall, which helps to maintain the pressure.

Arterioles, the smaller vessels branching from arteries, have proportionally more smooth muscle and less elastic tissue. When the muscle contracts (vasoconstriction) it reduces the flow to the capillaries it feeds, and when it relaxes (vasodilation) the flow increases. This is how blood is directed to the tissues that need it.

Veins

Veins carry blood back to the heart, at low pressure. The wall is thinner, with much less elastic tissue and muscle, because it does not have to withstand high pressure and does not need to recoil. The lumen is wide, so there is little resistance to flow. Because the pressure is so low, veins have valves, which are flaps of endothelium that close if blood begins to flow backwards. Blood in the veins of the limbs is also pushed along when skeletal muscles contract and squeeze the vessels, and the valves make this squeezing move blood towards the heart and not away from it.

Capillaries

Capillaries are the vessels in which exchange between blood and tissues takes place. The wall is a single layer of endothelial cells, which are very thin, so the diffusion distance is short. The lumen is only just wide enough for a red blood cell, so the cells are squeezed against the wall and pass through slowly, which gives more time for exchange. The capillaries form a dense network in each tissue, which gives a large surface area. There are small gaps between the endothelial cells, through which some plasma leaks out. This plasma becomes tissue fluid, which bathes the cells.

Tissue fluid

Where the blood enters a capillary bed the pressure inside the capillary is high enough to force water and small dissolved molecules out through the gaps in the wall. Cells and large proteins stay in the blood. The fluid that leaves is tissue fluid, which supplies cells with oxygen, glucose and other substances and takes up their waste. Much of it returns to the capillary at the venous end, where the pressure is lower and the proteins that remain in the blood keep water moving back in by osmosis. The rest drains into the lymphatic system and is returned to the blood near the heart.

Investigating vessel structure

The link between structure and function can be studied on slides. In a section of an artery the wall looks thick and the elastic fibres are visible as wavy dark lines, while in a vein the lumen is larger, the wall thinner, and the shape of the section is often collapsed. The elastic recoil of a length of artery and a length of vein can also be compared by hanging a small mass from each and measuring the extension and how far each returns when the mass is removed. The artery returns closer to its original length than the vein does, which fits its need to recoil after each surge of blood.

Structure of the mammalian heart

The heart is a muscular pump made of two halves that do not connect directly. The right side receives deoxygenated blood from the body through the vena cava and sends it to the lungs through the pulmonary artery. The left side receives oxygenated blood from the lungs through the pulmonary vein and sends it to the body through the aorta. Each side has a thin-walled upper chamber, the atrium, which receives blood, and a thicker-walled lower chamber, the ventricle, which pumps it out.

The walls of the chambers differ because each does a different amount of work. The atria only push blood a short distance down into the ventricles, so their walls are thin. The right ventricle pumps blood to the nearby lungs, and its wall is moderately thick. The left ventricle must push blood round the whole body against the resistance of the systemic circulation, so its wall is much thicker and produces a higher pressure.

Four sets of valves keep the blood flowing in one direction:

• the atrioventricular valves lie between each atrium and its ventricle. The right one has three flaps (the tricuspid valve) and the left has two (the bicuspid or mitral valve). They are held from opening backwards into the atria by tough tendinous cords, which are attached to small papillary muscles in the ventricle wall;
• the semilunar valves lie at the exit of each ventricle, in the aorta and the pulmonary artery. They have three pocket-shaped cusps that fill with blood if it tries to flow back and so close the exit.

The heart muscle needs its own blood supply, which is carried by the coronary arteries branching from the base of the aorta. If one is blocked, the part of the muscle it supplies is starved of oxygen.

The cardiac cycle

The cardiac cycle is the sequence of events in one heartbeat. It is described in three stages, in which the chambers contract and relax in turn. Contraction is called systole and relaxation diastole. The valves open and close passively, in response to the differences in pressure on each side of them.

Atrial systole

The atria contract, which decreases their volume and raises the pressure in them above the pressure in the ventricles. The atrioventricular valves are pushed open and blood flows into the ventricles. Most of the ventricle filling has already occurred passively during diastole, so the atria top up the ventricles rather than fill them. The semilunar valves are closed, because the pressure in the arteries is higher than in the ventricles.

Ventricular systole

Once the atria have finished, the ventricles contract, starting at the apex. The pressure in them rises above that in the atria and the atrioventricular valves slam shut, which makes the first heart sound. There is a short time when all four valves are closed and the ventricles contract without changing volume, so the pressure climbs steeply. When it exceeds the pressure in the aorta and pulmonary artery, the semilunar valves are forced open and blood is expelled into the arteries. The atria are relaxed during this time and fill with blood from the veins.

Cardiac diastole

The ventricles relax and their pressure falls below that in the arteries. Blood in the arteries tries to flow back and closes the semilunar valves, which makes the second heart sound. The elastic recoil of the arteries maintains the pressure in them at this time. When the pressure in the ventricles falls below the pressure in the atria, the atrioventricular valves open and blood that has collected in the atria flows into the ventricles. Both the atria and the ventricles are relaxed, and the whole heart fills passively, until the next atrial systole.

Reading pressure and volume graphs

A graph of pressure against time for the left atrium, left ventricle and aorta shows the cycle clearly. During ventricular systole the ventricle pressure rises above the atrial pressure, and the point where the two lines cross is where the valve closes of the atrioventricular valve. Where ventricle pressure crosses above aorta pressure the semilunar valve opens, and where it drops below the valve closes. Pressure in the aorta falls gradually during diastole because of the elastic recoil of the wall. A graph of ventricle volume falls steeply when blood is ejected, and its lowest value is the volume left after systole.

The time for one complete cycle is found by measuring the time between two identical points, such as two successive peaks. If one cycle takes $\pu{0.8 s}$ then the heart rate is

Investigating heart structure and function

The link between structure and function can be investigated by dissecting a mammalian heart, such as a pig or sheep heart, obtained from a butcher. Cutting across both ventricles shows the thicker wall of the left ventricle, and cutting open an atrium and ventricle on one side shows the atrioventricular valve and the tendinous cords. Filling a ventricle with water while holding the artery shows how the semilunar valve fills and closes. The dissection should be done with a sharp scalpel on a tray, with gloves worn, and the surfaces washed and disinfected after use. The findings can be recorded as a labelled scientific drawing, and the thickness of the walls can be measured with a ruler and compared.

Why the pressure in the two sides differs

Blood at high pressure in the lungs would force fluid out of the capillaries into the alveoli, which slows gas exchange. The right ventricle therefore generates a lower pressure. The left ventricle must overcome the resistance of the long systemic circuit and needs the higher pressure to deliver blood to the brain and the limbs quickly, so its muscle is thicker.