Respiration and Circulation

Every living cell needs energy to carry out its activities — to grow, to repair itself, to move materials, and to stay alive. This energy is released from food, mainly glucose, inside the cells by a process called cellular respiration. Cellular respiration is the controlled breakdown of glucose to release energy, and this energy is stored in a special energy-carrying molecule called ATP (adenosine triphosphate). ATP is often called the "energy currency" of the cell, because cells "spend" it whenever they need energy. It is important not to confuse respiration with breathing: breathing is just the taking in and giving out of air, while respiration is the chemical release of energy inside cells.

The most common and efficient form is aerobic respiration, which takes place in the presence of oxygen. In aerobic respiration, glucose is completely broken down using oxygen to release a large amount of energy, along with carbon dioxide and water as waste products. The overall reaction can be written as:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (about 38)

This means one molecule of glucose, combined with six molecules of oxygen, produces six molecules of carbon dioxide, six molecules of water, and about 38 molecules of ATP (energy). Aerobic respiration takes place mostly inside tiny cell structures called mitochondria, which are therefore known as the "powerhouses of the cell."

The reason aerobic respiration is so useful is its high energy yield. Because oxygen allows glucose to be broken down completely, the cell can extract a large amount of energy — about 38 ATP molecules from each glucose molecule. This is why organisms that are active and need lots of energy, such as humans and other animals, rely mainly on aerobic respiration. The carbon dioxide produced is removed from the body (in humans, breathed out through the lungs), and the water becomes part of the body's water.

It helps to see respiration as almost the reverse of photosynthesis. In photosynthesis, plants use carbon dioxide and water, with the energy of sunlight, to build glucose and release oxygen. In aerobic respiration, cells use glucose and oxygen to release energy, giving back carbon dioxide and water. So the two processes balance each other in nature. In short, cellular respiration is how cells release the energy stored in food, and aerobic respiration — using oxygen to break glucose down completely — gives the cell its large supply of ATP.

Sometimes cells must release energy from glucose without enough oxygen. The breakdown of glucose to release energy in the absence of oxygen is called anaerobic respiration. Because there is no oxygen to break glucose down completely, the glucose is only partly broken down, so much less energy is released — only about 2 molecules of ATP per glucose, compared with about 38 in aerobic respiration. Anaerobic respiration also produces different waste products depending on the organism, and these products still contain a lot of unused energy. The general theme is: less oxygen → incomplete breakdown → little energy (2 ATP) → energy-rich waste products.

One common type is lactic acid fermentation, which happens in our muscles during hard exercise. When we run fast or lift heavy weights, our muscles use energy quickly, and the blood may not supply oxygen fast enough. The muscle cells then respire anaerobically, breaking glucose down to lactic acid and a small amount of energy. The build-up of lactic acid in the muscles is what causes the aching, tired, cramping feeling during and after intense exercise. When we rest and breathe deeply afterwards, oxygen breaks down the lactic acid, and the ache fades.

Another important type is alcoholic fermentation, carried out by yeast (a single-celled fungus) and some microorganisms. In the absence of oxygen, yeast breaks glucose down into ethanol (alcohol) and carbon dioxide, releasing a little energy. This process is extremely useful to humans: the carbon dioxide produced makes bread rise (the gas bubbles make the dough light and spongy), and the alcohol produced is used in making products in the brewing and fermentation industries. So a process that yeast uses simply to survive without oxygen has become very valuable in food-making.

Comparing the two forms of respiration makes the differences clear. Aerobic respiration needs oxygen, breaks glucose down completely into carbon dioxide and water, and releases a large amount of energy (≈38 ATP). Anaerobic respiration needs no oxygen, breaks glucose down incompletely (into lactic acid, or into alcohol and carbon dioxide), and releases only a small amount of energy (about 2 ATP). Living things use anaerobic respiration as a useful back-up when oxygen runs short, even though it gives far less energy. So anaerobic respiration lets cells keep going without oxygen — in our tired muscles and in yeast — at the cost of releasing only a little energy.

To respire aerobically, our cells need a steady supply of oxygen, and they must get rid of the carbon dioxide they produce. The human respiratory system is the set of organs that takes in oxygen-rich air and removes carbon-dioxide-rich air — the process of breathing. Air enters and travels through a series of passages: the nasal cavity (nose), the trachea (windpipe), the bronchi, and finally into the lungs, where the actual exchange of gases happens in tiny air sacs called alveoli.

Air first enters through the nasal cavity, where it is warmed, moistened, and filtered — tiny hairs and sticky mucus trap dust and germs so that clean air goes down. From the nose, the air passes into the trachea (windpipe), a tube kept open by C-shaped rings of cartilage so it does not collapse. The trachea then divides into two tubes called bronchi (singular: bronchus), one going into each lung. Inside the lungs, each bronchus branches again and again into finer and finer tubes called bronchioles, like the branches of a tree.

At the end of the smallest bronchioles are clusters of tiny, balloon-like air sacs called alveoli (singular: alveolus). The alveoli are where gas exchange takes place — oxygen passes from the air into the blood, and carbon dioxide passes from the blood into the air. The lungs contain millions of alveoli, which together provide a very large surface area for this exchange. Each alveolus has a very thin, moist wall and is surrounded by tiny blood vessels, making it ideal for gases to pass through quickly.

Breathing itself is brought about by the movement of two main structures: the diaphragm (a dome-shaped sheet of muscle below the lungs) and the rib muscles. During inhalation (breathing in), the diaphragm flattens and the ribs move up and out, increasing the space in the chest; air rushes into the lungs to fill this larger space. During exhalation (breathing out), the diaphragm and ribs relax, the chest space shrinks, and air is pushed out of the lungs. So the respiratory system — nasal cavity, trachea, bronchi, lungs, and alveoli — works with the diaphragm and ribs to keep fresh oxygen coming in and carbon dioxide going out.

Once fresh air reaches the alveoli, the actual swapping of gases between the air and the blood takes place. This swapping is called gas exchange: oxygen moves from the air in the alveoli into the blood, while carbon dioxide moves from the blood into the air, ready to be breathed out. Gas exchange happens by a simple process called diffusion — the movement of a substance from a region where it is more concentrated to a region where it is less concentrated. No energy is needed; the gases simply move "downhill" from high to low concentration until balanced.

The alveoli are perfectly built for diffusion. Each alveolus has an extremely thin, moist wall (just one cell thick) and is wrapped in a network of tiny blood vessels called capillaries, whose walls are also one cell thick. So oxygen has only a very thin barrier to cross to get from the air into the blood, and carbon dioxide a very short distance to go the other way. Scientists describe the direction of diffusion using partial pressure, which is simply a measure of how much of a particular gas is present. Oxygen has a higher partial pressure in the alveoli than in the blood arriving there, so oxygen diffuses into the blood. Carbon dioxide has a higher partial pressure in that blood than in the alveolar air, so carbon dioxide diffuses out into the alveoli.

Once oxygen has entered the blood, it must be carried to all the body's cells. This transport is the job of the red blood cells, which contain a special red pigment called haemoglobin. Haemoglobin readily combines with oxygen in the lungs, where oxygen is plentiful, forming a bright-red substance called oxyhaemoglobin. The blood then carries this oxygen-rich haemoglobin to the body's tissues. Because each red blood cell is packed with haemoglobin, the blood can carry far more oxygen than it could otherwise.

When the oxygen-rich blood reaches the body tissues, where oxygen is being used up (and so its partial pressure is low), the haemoglobin releases its oxygen, which diffuses into the cells for respiration. At the same time, the carbon dioxide made by the cells diffuses into the blood and is carried back to the lungs (mostly dissolved in the blood). At the alveoli, this carbon dioxide diffuses out and is breathed away. So gas exchange and transport form a smooth cycle: diffusion loads oxygen into the blood at the alveoli, haemoglobin carries it to the tissues and releases it there, and carbon dioxide is carried back and breathed out.

The circulatory system is the body's transport system. It carries oxygen and digested food to every cell, and carries away wastes such as carbon dioxide. At its centre is the heart, a muscular pump about the size of your fist, which pushes blood through a network of tubes called blood vessels. Together the heart, blood, and blood vessels keep blood circulating around the body continuously, day and night, so the cells are always supplied and cleaned.

The human heart is divided into four chambers. The two upper chambers are the atria (singular: atrium) — the right atrium and left atrium — which receive blood coming into the heart. The two lower chambers are the ventricles — the right ventricle and left ventricle — which pump blood out of the heart. The chambers are separated so that oxygen-rich blood and oxygen-poor blood do not mix. The right side of the heart handles oxygen-poor (deoxygenated) blood, while the left side handles oxygen-rich (oxygenated) blood. Valves between the chambers make sure blood flows in one direction only and does not flow backwards.

Humans have what is called double circulation, which means the blood passes through the heart twice for each complete journey around the body. This is because the blood travels in two separate circuits. In the pulmonary circuit, oxygen-poor blood is pumped from the heart to the lungs, where it picks up oxygen and gives off carbon dioxide, and the now oxygen-rich blood returns to the heart. In the systemic circuit, this oxygen-rich blood is pumped from the heart to the rest of the body, delivers its oxygen, and returns as oxygen-poor blood to the heart.

Putting it together: oxygen-poor blood enters the right atrium, drops into the right ventricle, and is pumped to the lungs (pulmonary circuit). Oxygen-rich blood returns from the lungs into the left atrium, drops into the left ventricle, and is pumped out to the whole body (systemic circuit) — before returning, oxygen-poor, to start again. The advantage of double circulation is that the blood is kept at a high pressure in the systemic circuit and the two kinds of blood never mix, so oxygen is delivered to the body quickly and efficiently. This is why active animals like humans have such an effective transport system.

Blood is the fluid connective tissue that flows through our blood vessels, carrying materials to and from every part of the body. An adult has about 5 litres of blood. Although it looks like a simple red liquid, blood is actually made of a liquid part and several kinds of cells. The liquid part is called plasma, and floating in it are three kinds of blood cells: red blood cells (RBCs), white blood cells (WBCs), and tiny fragments called platelets. Each part has its own important job.

Plasma is a pale-yellow liquid that makes up over half of the blood. It is mostly water, but it carries many dissolved substances — digested food (such as glucose and amino acids), wastes (such as carbon dioxide and urea), hormones, and other materials. Plasma acts as the river in which everything is transported. Red blood cells (RBCs) are the most numerous blood cells; they contain the red pigment haemoglobin and their main job is to carry oxygen (and some carbon dioxide). RBCs give blood its red colour. White blood cells (WBCs) are fewer but very important — they are the body's soldiers, defending it against germs and disease by fighting infections. Platelets are tiny cell fragments that help the blood to clot and seal a wound when we are cut, stopping bleeding.

Human blood also comes in different blood groups. The most important grouping is the ABO system, which sorts blood into four groups: A, B, AB, and O. The group depends on certain markers present on the red blood cells. Blood groups matter a great deal during a blood transfusion (giving blood from one person to another): the blood given must be compatible with the receiver's blood, or the blood may clump dangerously. For example, group O can usually be given to many others (often called the "universal donor"), while group AB can usually receive from many (the "universal recipient").

A second important grouping is the Rh factor, named after the rhesus monkey in which it was discovered. People who have a certain marker on their red blood cells are Rh-positive (Rh⁺), and those who lack it are Rh-negative (Rh⁻). So a person's full blood type combines the ABO group and the Rh factor — for example, "A positive" or "O negative." Knowing both is essential before any blood transfusion, so that only matching, safe blood is given. In summary, blood is made of plasma, RBCs, WBCs, and platelets, each with a special role, and it is classified by the ABO groups and the Rh factor.

Blood is carried all around the body through a network of tubes called blood vessels. There are three main types of blood vessels, each with a different structure suited to a different job: arteries, veins, and capillaries. Together they form a continuous system — blood leaves the heart in arteries, passes through capillaries in the tissues, and returns to the heart in veins. Understanding how the three differ helps explain how blood is delivered to and collected from every part of the body.

Arteries are the vessels that carry blood away from the heart. Because the heart pumps blood into them under high pressure, arteries have thick, strong, muscular and elastic walls that can withstand and smooth out this pressure. Arteries usually carry oxygen-rich blood (the main exception is the artery to the lungs). They do not have valves along their length, because the high pressure keeps the blood moving forward. When you feel your pulse, you are feeling the surge of blood pushed through an artery with each heartbeat.

Veins are the vessels that carry blood back to the heart. By the time blood reaches the veins, it is at low pressure, so veins have thinner walls with less muscle than arteries, and a wider space (lumen) inside. Veins usually carry oxygen-poor blood (the main exception is the vein from the lungs). Because the blood pressure in veins is low, veins contain valves along their length to stop the blood from flowing backwards and to keep it moving towards the heart.

Capillaries are the smallest, thinnest blood vessels, so narrow that blood cells pass through almost in single file. Their walls are only one cell thick, which makes them perfect for exchange: oxygen, food, and other materials pass out of the blood to the cells, while carbon dioxide and wastes pass in from the cells. Capillaries form fine networks within every tissue, connecting the smallest arteries to the smallest veins. So the three vessels work as a team: arteries deliver blood at high pressure from the heart, capillaries allow exchange with the cells, and veins return the blood at low pressure to the heart.

Besides the blood, the body has a second transport-and-defence network called the lymphatic system. As blood flows through the capillaries, some of its watery fluid, carrying dissolved food and oxygen, leaks out into the spaces around the cells to bathe and nourish them. This fluid, once it has left the blood and surrounds the tissues, is called tissue fluid, and when it drains into special vessels it is known as lymph. The lymphatic system collects this lymph and returns it to the blood, and at the same time plays a major role in defending the body against disease.

Lymph is a pale, watery fluid, rather like plasma but containing fewer proteins and many white blood cells (especially the kind called lymphocytes). It carries digested fats absorbed from the intestine, returns leaked fluid to the blood, and carries white blood cells around the body. Lymph flows through a system of lymph vessels (similar to thin veins) that run alongside the blood vessels. Unlike blood, lymph is not pumped by the heart; instead it is moved slowly by the squeezing of nearby muscles, and valves in the lymph vessels keep it flowing in one direction — back toward the blood.

Along the lymph vessels lie small bean-shaped structures called lymph nodes (sometimes called lymph glands). These nodes act as filters and defence stations: as lymph passes through them, they trap germs, dead cells, and other harmful particles, and they contain large numbers of white blood cells that destroy these invaders. Lymph nodes are found in groups in places such as the neck, armpits, and groin. When the body is fighting an infection, the lymph nodes often swell as the white blood cells multiply — this is why a doctor may feel the "glands" in your neck when you are ill.

The lymphatic system's biggest contribution is to immunity — the body's ability to resist and fight disease. The white blood cells made and stored in the lymph nodes and other lymph organs (such as the spleen and tonsils) recognise and destroy germs that enter the body, and help the body "remember" them so it can fight them off faster next time. So the lymphatic system works hand in hand with the blood: it returns leaked fluid to the circulation, absorbs digested fats, and — above all — defends the body through its lymph nodes and white blood cells, forming a key part of our immune defences.