Exchange & transport

This section contains information on the exchange of materials, the heart and transport systems.

Organisms need to exchange heat, useful substances and waste with their environment. Larger organisms need to use more oxygen and nutrients (and get rid of more waste and excess heat) than smaller organisms. The size means that simple processes such as diffusion are no longer efficient.

The surface area: volume ratio is a measure of how much of an organism’s surface is exposed to the environment for each unit of volume - the table shows that the smaller an organism is, the larger the ratio.

Multicellular organisms have had to evolve a complex gas exchange system to increase the surface area that is in contact with the environment. In mammals, the lungs perform this function.

The trachea (airway) divides to form the bronchi. These then divide further into the bronchioles before terminating in air sacs called alveoli. It is the alveoli where gas exchange takes place, so they adapted to do this efficiently.

Diffusion takes place quickest over short distances: the walls of the alveoli are made of a single layer of flattened cells. The larger the concentration gradient, the faster diffusion will occur - a constant flow of blood maintains this gradient (high O2 concentration in the lungs, low in the blood; high CO2 concentration in the blood, low in the lungs). Epithelial cells are permeable to dissolved gases and provide a large surface area for diffusion to take place

Ventilation (breathing) uses two sets of muscles: the diaphragm and intercostal muscles. When they contract, the ribs move upwards and outwards and the diaphragm flattens. This increases the volume of the thorax, decreasing the pressure, so air moves into the lungs. The opposite happens when you exhale.

There also needs to be some way of transporting the substances necessary for respiration to individual cells around the body. Mammals have evolved a circulatory system that consists of the heart (a double pump system) and three main types of blood vessel:

Arteries carry blood away from the heart
Veins carry blood back to the heart
Capillaries are small blood vessels linking arteries and veins. They are also the site of exchange of materials.

The heart itself is made up of four chambers: two atria (s. atrium) and two ventricles. The left side of the heart receives blood from the lungs and pumps it around the body, the right side of the heart receives blood from the body and pumps it to the lungs.

The arrangement of the heart means that the atria do not need thick walls - they only pump the blood a short distance. The ventricles have to pump blood further, but still show a noticeable difference in the thickness of the walls. Due to the pressure necessary to pump blood around the entire body, the left ventricle wall is much more muscular (and so thicker) than the right ventricle wall.

It is important to keep the flow of blood within the body moving in one direction. High blood pressure in the arteries (caused by the contraction of the ventricles) ensures that this is the case, but the pressure in the veins is much lower. Valves are found in the heart and in veins to prevent backflow of blood. They open when the pressure is higher below them than above and vice versa. Because the beating of the heart does not have an effect on the pressure of blood in the veins, many of them run close to (or within) muscles. Muscle contraction increases the pressure inside the vein, forcing blood through the valves and back towards the heart.

The cardiac cycle is the sequence of events and movements of the heart during one beat. Both side of the heart follow this sequence at the same time. There are four stages to the cycle:

Blood flows into the atria from the vena cava (R) and pulmonary artery (L)
The atria contract, forcing blood into the ventricles
The ventricles contract, forcing blood into the aorta (L) and pulmonary artery (R)
The heart relaxes and blood flows into the atria

Blood flows through the heart and around the body due to changes in pressure. It always moves from an area of high pressure to an area of low pressure. The valves in the heart maintain the flow of blood in the right direction. These work in a passive way (don’t need energy) and operate entirely due to pressure changes. When the atria contract, the atrioventricular valves open, and when the ventricles contract they close (contraction of the ventricles also causes the pulmonary and aortic valves to open).

Cardiac (heart) muscle is myogenic. This means that it does not need the central nervous system to send any impulses before it will start to contract. Instead, there is a group of cells in the right atrium called the sino-atrial node which generates its own electrical impulses. These impulses spread across the two atria and down into the ventricles, causing contraction.

Because the heart is a muscle, it requires a constant supply of oxygen and glucose for respiration. As in all organs, they is supplied by the blood. The coronary arteries branch off the aorta soon after it leaves the heart, and further divide into capillaries to supply the entire heart muscle.

Cardiovascular diseases (CVD) affect the heart and blood vessels. Most of these start off as atheromas - fatty build ups on the walls of the blood vessels. These restrict blood flow and increase the risk of blood clots (thrombosis) forming. There are four common examples of CVD:

High blood pressure - variety of causes; can lead to other diseases because of an increased risk of blood clots
Stroke - caused by blood clots in the arteries supplying blood to the brain and brain damage occurs due to a lack of oxygen
Coronary heart disease (CHD) - the coronary arteries that supply the heart muscle with oxygen get blocked, increasing the risk of blood clots
Heart attack - the coronary arteries become completely blocked, damaging the muscle because of a lack of respiratory substrates. Also known as a myocardial infarction.

Blood vessels

Arteries carry blood away from the heart. The walls of arteries consist of a thin, folded endothelium (inner layer), a thick layer of elastic tissue & smooth muscle and a fibrous outer layer. The elastic layer helps to maintain the blood pressure when the ventricles relax. Veins carry blood towards the heart, and have thinner walls and a larger lumen than arteries. They also have valves to maintain blood flow in one direction.

Arterioles are narrow arteries, and contain muscle fibre but less elastic tissue than arteries. This means that arterioles can control the flow of blood by contracting the muscle - less blood will flow through a smaller lumen. Capillaries are microscopic blood vessels that are the site of exchange. The capillary wall is made up of a single layer of cells, meaning that any substances diffusing in or out only have a small distance to move. Blood also flows slowly though capillaries. There are many capillaries in the body, giving a massive surface area for diffusion to take place over.

Haemoglobin (Hb)

This is the red pigment in red blood cells. It is a protein that carries oxygen around the body by combining with it to form oxyhaemoglobin. In high O2 concentrations, the Hb molecule will combine with four O2 molecules and be 100% saturated (full). CO2 is produced by respiring tissues, which require a good supply of oxygen. Hb releases oxygen much more easily when there are large amounts of CO2 present, so the tissues which are respiring most will therefore cause oxygen to be released easiest. This is known as the Bohr effect, and is shown on the oxygen dissociation curve (graph) by a movement of the curve to the right.

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