6b. Animal Transport & Exchange Systems

What you need to know...

    • In mammals, nutrients, oxygen and carbon dioxide are transported in the blood.

Circulatory System

    • Pathway of blood through heart, lungs and body.

      • Heart structure to include right and left atria and ventricles.

      • Blood vessels to include: aorta, vena cava, pulmonary arteries and veins, and coronary arteries.

    • Arteries have thick, muscular walls, a narrow central channel and carry blood under high pressure.

    • Veins carry blood under low pressure; have thinner walls and a wide channel.

    • Veins contain valves to prevent backflow of blood.

    • Capillaries form networks at organs and tissues, and are thin walled and have a large surface area, allowing exchange of materials.

    • Red blood cells contain haemoglobin and are specialised to carry oxygen.

Respiratory System

    • Rings of cartilage keep airways open.

    • Oxygen and carbon dioxide are exchanged in the alveoli.

    • Alveoli have a large surface area, a good blood supply and thin walls to allow diffusion of gases.

    • Mucus traps dirt and microorganisms and cilia moves this up and out of the lungs.

Digestive System

    • Food is moved through the digestive system by peristalsis.

    • Villi in the small intestine are thin walled, have a large surface area and a good blood supply to aid absorption of glucose and amino acids.

    • The lacteals transport the products of fat digestion.

Source: SQA


As you can see from the list above, this is a big topic. Although it's a big topic, it centres around two relatively straightforward concepts which you should by now have grasped. Firstly, all cells need to respire to release energy from glucose which generates ATP. This process requires glucose and oxygen and produces carbon dioxide as a waste. You know all that, you learnt it long ago in the first unit in some depth. The second concept is what this unit has been all about: the problems of being multicellular. In other words, the cells in your tissues need a constant supply of oxygen and glucose for respiration and they need the toxic carbon dioxide removed and yet they have no direct access to the environment. Complex multicellular organisms, such as yourself, have therefore evolved complex transport and exchange systems to overcome this problem. In this topic you learn about three such systems found in animals, but they all can be viewed from the perspective of ensuring that all the cells in animal can respire in that the respiratory and circulatory systems both ensure the cells have a continuous supply of oxygen and that the carbon dioxide is removed. And the digestive and circulatory systems are providing cells with the glucose required for respiration as well as other raw materials such as the amino acids required for protein synthesis.

Circulatory System

You could debate which of the three systems is the best one to start with. I think it's best to start with the circulatory system as it is crucial for the functioning of both of the other two. The respiratory and digestive systems are the exchange organs which interact with the external environment, whereas the circulatory system is the link between all the cells of the body and these exchange systems. The circulatory system of animals consists of three main structures which you need to know about: the heart, the blood vessels and the blood.

Human circulatory system showing the heart and major blood vessels. Blood vessels coloured red carry oxygenated blood, blood vessels coloured blue carry deoxygenated blood.

Image from WikiMedia Commons

As you will no doubt already know, the heart is the pump of our circulatory system. It consists primarily of cardiac muscle but is divided into a number of chambers. You need to be able name the structures of the heart and the pathway blood takes through it. Whilst the heart is obviously a single organ, in actual fact it can be thought of as two pumps joined together. Although both sides of the heart contract simultaneously, the blood leaving the heart is going to different destinations depending on which side of the heart it is leaving. The right side of your heart pumps blood to the lungs to absorb oxygen and release the waste carbon dioxide. The left side of your heart has to work much harder by pumping the now oxygenated blood throughout the entirety of your body. As a result, the muscle of the left side is much thicker than the right.

Image from WikiMedia Commons

As you can see from the diagram above, there is more complexity to the heart than simply the left and right side. The heart also consists of the upper two chambers, the atria, and the lower two chambers, or ventricles. Blood arriving at the heart arrive at the atria from a vein. The vein which brings the blood to the atria depends on what side of the heart it is. Blood arriving in the right atrium is coming from the body and is deoxygenated. This arrives in the vena cava. Blood arriving in the left atrium is coming from the lungs in the pulmonary veins and is oxygenated. From the atria, blood passes through valves into the ventricles. The valves prevent the backflow of blood, or prevent it from going in the wrong direction in other words. You don't need to know the names of the valves for this course. The ventricles make up the bulk of the structure of the heart and contract to push the blood out of the heart. The right ventricle pushes blood into the pulmonary artery to the lungs, whereas blood leaving the left ventricle enters the large artery known as the aorta.

Obviously, the heart muscle cells require a lot of energy themselves in order to contract and therefore carry out respiration too. The heart muscle therefore has its own blood supply. Blood vessels which bring oxygenated blood to the heart are known as coronary arteries and coronary veins bring the deoxygenated blood away. These are the blood vessels which can seen on the outside of the heart and will most likely be discussed in more depth when you study the next topic as these are the blood vessels which are affected in people with heart disease.

Image from WikiMedia Commons

As you may already know, or will by now have noticed, there are a number of different blood vessels involved in carrying blood around the animal body. The blood vessels in your body can be categorised into one of three main types: arteries, veins and capillaries. Arteries carry blood away from the heart, veins carry blood back to the heart and capillaries connect the two allowing the exchange of substances between the blood and the surrounding tissues. Each of the type of blood vessel has adaptations related to its function.

Image adapted from WikiMedia Commons

As shown in the diagram above, each of the three types of blood vessel is quite different. Arteries carry blood away from the heart and therefore have thick, muscular walls and a narrow central channel (lumen) as they carry blood under high pressure. Veins carry blood back to the heart under low pressure and have thinner walls and a wide channel. Veins also contain valves to prevent the backflow of blood. Capillaries form networks at organs and tissues. They are thin walled and have a large surface area, allowing the exchange of materials such as oxygen, carbon dioxide, glucose and amino acids. Most of the substances which are carried in the blood just dissolve in the fluid, but oxygen is different. Oxygen is carried by red blood cells which have specialised adaptations for this function.

Image adapted from Wikipedia

Red blood cells contain the chemical haemoglobin which binds reversibly to oxygen. Red blood cells absorb oxygen in the lungs and then release it again in the tissues. As you can see from the diagram above they have what we describe as a biconcave shape which increases the surface area for the absorption of oxygen and allows them to squeeze through the narrow capillaries which can be so narrow that only one red blood cell can pass through at a time.

There are LOADS of resources for the circulatory system online. Have a search yourself!

Respiratory System

Now we know what the blood does and how it is transported around the body, it is time to learn about two exchange systems which the blood interacts with. Firstly, the respiratory system. As you will have already learned, the lungs are the organs where oxygen and carbon dioxide are exchanged between our blood and the air. The blood therefore has to come into close contact with the air in the lungs, this happens in the alveoli. Firstly though, the air has to get to the alveoli. The air we breathe in pass through a series of structures after our mouth and nose on their way to the alveoli as shown in the following diagram.

Image adapted from WikiMedia Commons

Air passes through the trachea in your neck, which leads to two bronchi. Each bronchus carries air to one of the lungs. After the bronchi, the air passes into a series of ever smaller branching tubes called the bronchioles and at the end of these are the alveoli. As you can see from the diagram above, these airways are surrounded by a rings of stiff cartilage to keep them open under the changing air pressures as you breathe in and out. This isn't dissimilar the hard rings on your vacuum cleaner's hose pipe!

Something your vacuum cleaner doesn't have however, is cilia and mucus. All of the above airways are lined with ciliated cells which produce mucus. The sticky mucus traps dirt and microorganisms carried into the lungs in the air. Cilia are little hairs on the cells lining the airways and these beat to move this mucus up out of the lungs and into your stomach instead! The image below shows a scanning electron micrograph of these cilia on cells in the trachea.

Image from WikiMedia Commons

As already mentioned, the actual exchange of gases occurs in the alveoli which are found at the end of the bronchioles. These alveoli have adaptations to increase the efficiency of the exchange of oxygen and carbon dioxide between the air and the blood. These include a very large surface area, walls that are one cell thin and a good blood supply. As you can see from the diagram below, the alveoli are surrounded by complex networks of blood capillaries which bring the red blood cells in close proximity to the air. Oxygen diffuses from the air across the alveolar and capillary walls and into the passing red blood cells. At the same time, carbon dioxide is moving in the opposite direction out of the blood and into the air in the lungs to be exhaled. Once the blood has passed through the lungs it returns to the left atrium of the heart in the pulmonary vein.

Image from WikiMedia Commons

Digestive System

The other exchange system you need to know about is the digestive system. Blood interacts with the digestive system in order to absorb the nutrients required by the cells in the body, such as glucose and amino acids. But first, the food we eat must be broken down and digested from large insoluble molecules, to the small soluble molecules which can diffuse and dissolve in the blood. Our food therefore passes through a series of organs which carry out this digestion and absorption. These organs are also surrounded by a series of other organs which play a role in producing the enzymes involved in the digestion process. This system of organs is shown in the following diagram.

Image adapted from WikiMedia Commons

Food is moved through the digestive system by peristalsis. A wave of circular muscle contraction occurs behind the food bolus which pushes it along. If you click on the diagram below it will open an animated gif to show this in action. There's also a more detailed animation here.

Image from WikiMedia Commons

Once the food has been digested into glucose and amino acids it needs to be exchanged into the blood to be transported to all the cells in the body. This occurs in the small intestine. The wall of the small intestine consist of many finger-like projections called villi. These can be seen in the image below.

Villi by Wellcome Images

Although these are quite different in appearance to the alveoli in the lungs, they do in fact have a number of similar features as they are carrying out a similar function. Whereas the alveoli were exchanging oxygen and carbon dioxide into and out of the blood, the villi are exchanging substances such as glucose and amino acids into the blood. Villi therefore also have a large surface area, thin walls and a good blood supply. Notice the dense network of blood capillaries in each of the villi in the image above. These absorb the products of digestion such as glucose and amino acids and carry them to the rest of the cells in the body. What you can't see in the image above is the lacteals. Each villus also has a branch of the lymphatic system at its centre which absorbs the products of fat digestion. This is shown in the diagram below.