Transportation in Organisms

Every cell of a living body needs a constant supply of food (digested nutrients) and oxygen to carry out respiration and stay alive, and every cell produces waste substances such as carbon dioxide that must be removed. In a very tiny organism like Amoeba, these materials can simply pass in and out across its surface. But in a large, complex organism like a human, cells lie deep inside the body, far from the surface, so there must be a special system to carry materials to and from every cell. This is called transportation.

In animals, transportation is carried out by the circulatory system, which acts like the body's delivery and disposal network. Its job is to carry digested food and oxygen to all the cells, and to carry carbon dioxide and other wastes away from the cells to where they can be removed. Without such a system, the inner cells of a large body could not receive nutrients and oxygen or get rid of their waste, and the body could not survive.

The human circulatory system is made up of three main parts working together. The first is the blood, the fluid that actually carries the materials. The second is the heart, a muscular pump that keeps the blood moving. The third is a system of tubes called blood vessels — the arteries, veins, and capillaries — through which the blood flows to reach every part of the body. Together, the heart, blood, and blood vessels keep materials circulating continuously around the body.

Plants also need transport, but they have a different system, which we will study later in this chapter. The key idea is the same for all large organisms: because cells lie far apart and deep inside the body, materials must be actively transported to and from them. In the next topics we look at the blood, the heart, and the blood vessels in turn, and then at how water and food are transported in plants.

Blood is the red fluid that flows through our blood vessels and is the main transport medium of the body. It is often called a fluid connective tissue because, although it is liquid, it connects and serves all parts of the body. Blood carries digested food, oxygen, carbon dioxide, wastes, and many other substances around the body. If we examine blood closely, we find it is made of a liquid part and several kinds of cells floating in it.

The liquid part of blood is called plasma. Plasma is a pale yellow fluid that is mostly water, and it makes up a little more than half of the blood. Its job is to carry the blood cells and dissolved substances — such as digested food, wastes, and carbon dioxide — from one part of the body to another. The blood cells float and travel in this plasma.

There are three main kinds of cells (or cell-like bodies) in blood. The red blood cells (RBCs) are the most numerous and give blood its red colour. They contain a red pigment called haemoglobin, which binds to oxygen and carries it from the lungs to all the cells of the body — so RBCs are the oxygen-carriers of the blood. The white blood cells (WBCs) are fewer in number and act as the body's soldiers: they fight germs and protect the body from infection and disease. The platelets are tiny cell fragments that help the blood to clot at a wound, stopping the bleeding and sealing the cut.

Each component of blood thus has a special role: plasma transports cells and dissolved materials, red blood cells carry oxygen, white blood cells fight infection, and platelets help blood clot. Together they make blood a remarkable, multi-purpose transport and protection fluid. Understanding these components helps explain how blood keeps the body supplied, defended, and able to heal.

The heart is the muscular pump that keeps blood flowing throughout the body. It is roughly the size of a closed fist and lies in the chest, slightly to the left, protected by the rib cage. The heart is made of a special muscle that never tires, contracting and relaxing rhythmically all through life to push blood around the body, day and night, without rest. This pumping is what keeps the entire circulatory system working.

The human heart is divided into four chambers. The two upper chambers are called the atria (singular: atrium), and the two lower chambers are called the ventricles. So there is a right atrium and right ventricle, and a left atrium and left ventricle. The atria receive blood coming into the heart, while the ventricles pump blood out of the heart; because the ventricles have to push blood farther, their walls are more muscular than those of the atria. Valves between the chambers make sure that blood flows in only one direction and does not flow backward.

The heart keeps two kinds of blood separate. Oxygen-rich (oxygenated) blood, which has picked up oxygen in the lungs, enters the left side of the heart and is pumped out to the whole body. Oxygen-poor (deoxygenated) blood, carrying carbon dioxide collected from the body, returns to the right side of the heart and is pumped to the lungs to release carbon dioxide and pick up fresh oxygen. In this way blood keeps circulating: body → right heart → lungs → left heart → body, over and over.

The rhythmic contraction and relaxation of the heart is called the heartbeat. Each heartbeat pushes blood out into the vessels, and a healthy adult heart beats about 70 to 72 times per minute at rest. We can hear the heartbeat with an instrument called a stethoscope. Because the heart works tirelessly to keep blood — and therefore oxygen and food — moving to every cell, it is one of the most vital organs of the body, and keeping it healthy through exercise and a good diet is very important.

Blood travels around the body through a system of tubes called blood vessels. There are three kinds of blood vessels, each with a different structure and job: arteries, veins, and capillaries. Together they form a vast network reaching every part of the body, so that blood pumped by the heart can deliver oxygen and food and collect wastes from every cell.

Arteries are the vessels that carry blood away from the heart to the different parts of the body. Because the heart pumps blood into them with great force, arteries carry blood at high pressure and have thick, muscular, elastic walls to withstand it. Most arteries carry oxygen-rich blood (the main exception being the artery to the lungs). Veins are the vessels that carry blood back to the heart from the body. They carry blood at lower pressure, so their walls are thinner, and they have valves inside them that prevent the blood from flowing backward, helping it move toward the heart.

Capillaries are the smallest and thinnest blood vessels, so fine that their walls are only one cell thick. They form a network connecting the smallest arteries to the smallest veins. Because their walls are so thin, capillaries are the place where the actual exchange of materials happens: oxygen and food pass from the blood in the capillaries into the body cells, and carbon dioxide and wastes pass from the cells into the blood. So capillaries are where the blood finally does its delivery and collection work.

We can feel the flow of blood through an artery as the pulse. Each time the heart beats, it pushes a surge of blood into the arteries, and this surge can be felt as a throb where an artery lies close to the skin, such as at the wrist or the neck. The number of pulse beats per minute equals the number of heartbeats per minute, so counting the pulse is a simple way to measure how fast the heart is beating. Like the heartbeat, the pulse rate increases during exercise, when the body needs blood to flow faster. Understanding arteries, veins, capillaries, and the pulse completes our picture of how blood is carried and felt in the body.

Plants also need to transport materials over the long distances between their roots, stems, and leaves, but they do it without a heart or blood. Instead, plants have two special transport tissues that run through the roots, stem, and leaves like pipelines. One tissue, the xylem, carries water and minerals, and the other, the phloem, carries food. Together they make up the plant's transport system.

The xylem transports water and dissolved minerals absorbed by the roots upward to the stem and leaves, where the water is needed for photosynthesis and other processes. The water is taken in from the soil by the roots, especially through tiny root hairs that increase the surface for absorption, and then moves up through the xylem in the stem to all parts of the plant. This is a one-way (upward) flow, from roots to leaves.

The upward movement of water is helped by a process called transpiration, which is the loss of water vapour from the plant, mainly through the stomata in the leaves. As water evaporates from the leaves, it creates a kind of suction (pull) that draws more water up through the xylem from the roots, somewhat like sipping a drink through a straw. This transpiration pull is the main force that lifts water to the top of even very tall trees. Transpiration also helps to cool the plant and keeps water moving, though too much water loss in hot weather can be a problem.

The phloem transports the food (glucose) made in the leaves by photosynthesis to all the other parts of the plant — to the growing tips, the storage organs, the roots, and the fruits — wherever food is needed or stored. Unlike the xylem's one-way upward flow, food in the phloem can move in different directions, up or down, to wherever it is required. So in plants, the xylem carries water and minerals upward, the phloem carries food in all directions, and transpiration provides the pull that keeps water rising — a transport system very different from, but just as effective as, the animal circulatory system.