Transport in Humans

White blood cells (leucocytes)

White blood cells belong to the immune system. Their job is to defend the body against pathogens. Two main types exist:

• Phagocytes carry out non-specific immunity. They engulf and digest any pathogen they meet.
• Lymphocytes carry out specific immunity. They produce antibodies, proteins that recognise one particular pathogen.

A typical white blood cell is much larger than a red blood cell and has a prominent nucleus when viewed under a microscope. Both types are covered in detail in section 6.

Platelets

Platelets are not whole cells but tiny fragments broken off from much larger cells in the bone marrow. They have no nucleus. Their job is to make the blood clot at a wound, which seals the cut, prevents excessive blood loss, and stops bacteria getting in. Covered in section 8.

3. Blood vessels

Humans use three kinds of blood vessel, each shaped for one job:

A handy memory trick: Arteries carry blood Away from the heart.

Why each vessel has the structure it does

Arteries have to cope with the high pressure of blood freshly pumped from the heart:

• Thick muscular wall withstands the pressure without bursting
• Elastic fibres in the wall stretch when the heart beats and recoil between beats, evening out the surges of pressure
• Narrow lumen keeps the pressure high so blood is delivered quickly to distant tissues

Veins carry blood back to the heart under much lower pressure:

• Thinner wall is enough for the low pressure
• Wider lumen offers less resistance to the slow blood flow
• Valves stop the blood from flowing backwards. When you tense the muscles around a vein (e.g. by walking), the squeezing pushes blood forwards; the valves stop it sliding back when the muscle relaxes
• Many large veins run between skeletal muscles, so movement of the limbs helps pump the blood back to the heart

Capillaries are where the actual exchange between blood and tissues happens. They form a dense network branching out from arteries and joining back up into veins inside every tissue:

• One cell thick: the wall is a single layer of cells, so the diffusion distance for oxygen, glucose, CO₂ and urea is as short as possible
• Very narrow lumen forces red blood cells to pass through one at a time, slowing the flow and giving plenty of time for diffusion
• "Leaky" walls allow plasma to seep out into the surrounding tissue as tissue fluid, which bathes the cells and delivers their supplies. Most tissue fluid is reabsorbed; the rest is collected by a separate network of lymphatic vessels and returned to the bloodstream

4. The heart

The heart is a muscular pump about the size of a closed fist, sitting just left of centre in the chest. It is really two pumps stuck together, working side by side: a right side that pumps deoxygenated blood to the lungs, and a left side that pumps oxygenated blood to the rest of the body.

Four chambers

Each side of the heart has two chambers:

The atria are the small upper chambers that receive blood from the veins. The ventricles are the larger lower chambers that pump blood out into the arteries.

The two sides are completely separated by a thick muscle wall called the septum. This separation keeps oxygenated and deoxygenated blood from mixing.

Why the left ventricle has the thickest wall

The left ventricle is built with a much chunkier muscle wall than its right counterpart. The reason is workload: the left ventricle has to drive blood at high pressure all the way around the whole body (the systemic circuit), while the right ventricle only sends blood a short way to the lungs (the pulmonary circuit). A thicker, stronger muscle generates the higher pressure.

The right ventricle deliberately pumps at lower pressure: the delicate capillaries in the alveoli of the lungs would be damaged by high-pressure blood.

Four valves

The heart has four one-way valves that stop blood from flowing backwards:

The pathway of blood through the heart

Starting from where blood arrives back from the body:

1. Deoxygenated blood from the body enters the right atrium through the vena cava.
2. The right atrium contracts, pushing the blood through the right atrioventricular (tricuspid) valve into the right ventricle.
3. The right ventricle contracts, pushing the blood through the pulmonary semilunar valve into the pulmonary artery.
4. The pulmonary artery carries the deoxygenated blood to the lungs, where it picks up oxygen and releases carbon dioxide in the capillaries surrounding the alveoli.
5. Oxygenated blood returns from the lungs through the pulmonary vein into the left atrium.
6. The left atrium contracts, pushing the blood through the left atrioventricular (bicuspid / mitral) valve into the left ventricle.
7. The left ventricle contracts strongly, pushing the blood through the aortic semilunar valve into the aorta.
8. The aorta carries the oxygenated blood out to the whole body.

In reality both atria contract together, then both ventricles contract together; the diagram describes one side at a time only for clarity.

The coronary arteries

The heart muscle itself needs a constant supply of oxygen and glucose for its own respiration. These are delivered by the coronary arteries, which branch off the aorta as soon as it leaves the heart and run across the outside of the heart muscle. If a coronary artery becomes blocked, the patch of heart muscle it feeds runs out of oxygen and dies, causing a heart attack (section 7).

5. The double circulation

The mammalian circulation is a double circulation: blood passes through the heart twice for every complete journey around the body. The two halves of the loop are called the pulmonary circuit and the systemic circuit:

• Pulmonary circuit (heart → lungs → heart): the right side of the heart pumps deoxygenated blood out to the lungs through the pulmonary artery. Blood picks up oxygen and drops off CO₂ in the lungs, then returns to the left side of the heart through the pulmonary vein.
• Systemic circuit (heart → body → heart): the left side of the heart pumps oxygenated blood out to every other organ through the aorta. Cells take the oxygen and add their waste CO₂; the now-deoxygenated blood returns to the right side of the heart through the vena cava.

Why a double circulation is so useful

In a single circulation (like in a fish), blood passes through the heart only once and slows right down in the gills' capillaries before being pumped on to the rest of the body. By the time it reaches the muscles it is moving sluggishly. In a double circulation, the blood is re-pressurised between the lungs and the rest of the body, so it can be delivered fast to every tissue. This allows mammals to be much more active than fish of comparable size.

The main blood vessels of the major organs

6. White blood cells and the immune response

Two types, two strategies

Phagocytes are the rapid-response part of the immune system. They patrol the blood and tissues looking for anything that does not belong (bacteria, viruses, fungi). When they find a pathogen they engulf it whole (a process called phagocytosis) and digest it inside themselves using digestive enzymes. This is a non-specific response: a phagocyte will engulf any pathogen it meets, not just one type.

Lymphocytes are the targeted-response part of the immune system. They produce antibodies: proteins that exactly fit a specific feature on the surface of one type of pathogen. This is a specific response, since each lymphocyte makes antibodies against only one type of pathogen.

Antigens and antibodies

• An antigen is a molecule (usually a protein) found on the surface of a cell or pathogen. The body recognises antigens that belong to itself as "self", and any antigen that doesn't match as "foreign".
• An antibody is a Y-shaped protein, made by a lymphocyte, that has a complementary shape to one specific antigen. The antibody locks on to its matching antigen like a key fits its lock.

When antibodies bind to pathogens, they:

• Clump pathogens together (agglutination), making them easier for phagocytes to engulf in batches
• Mark pathogens for destruction, by giving the phagocytes a clear signal of what to engulf
• Neutralise toxins released by some bacteria, by binding to the toxin so it can no longer harm body cells

How the immune system fights an infection

1. A pathogen enters the body and starts to multiply, releasing toxins and damaging cells.
2. Phagocytes are the first to respond. They engulf and digest pathogens they encounter (non-specific phase).
3. Eventually a lymphocyte meets a pathogen whose antigens match its receptors.
4. That lymphocyte begins making specific antibodies against that antigen and divides rapidly, producing many identical lymphocytes that all churn out the same antibody.
5. The antibodies bind to the antigens on the pathogens, clumping them and tagging them for destruction.
6. Phagocytes engulf the tagged pathogens in bulk and digest them.
7. Some of the lymphocytes remain in the body afterwards as memory cells, ready to mount a faster response if the same pathogen ever attacks again.

Immunity and memory cells

The first time you meet a new pathogen, the immune response is slow (it takes the lymphocytes time to find the right one and multiply), and you feel ill. By the time the pathogen is cleared, the body has a stock of memory cells specific to that pathogen.

If the same pathogen invades a second time, the memory cells recognise it immediately and produce more antibodies, faster, and in greater quantities than the first time around. The pathogen is destroyed before it has a chance to cause symptoms. You are now immune to that pathogen.

Vaccination

Vaccination is a way of getting the memory-cell benefit without ever needing to suffer the disease. A vaccine contains a harmless version of a pathogen, prepared in one of three ways:

• Killed pathogens (e.g. the older type of flu vaccine)
• Weakened (attenuated) live pathogens (e.g. the MMR vaccine for measles, mumps, rubella)
• Pieces of pathogen such as surface antigens (e.g. the hepatitis B vaccine)

The vaccine carries the pathogen's antigens without being able to cause the disease. The body responds as if it has been infected:

1. Lymphocytes recognise the antigens.
2. They produce specific antibodies and divide.
3. Memory cells are formed and remain in the bloodstream.

If the real pathogen ever attacks later, the memory cells trigger a fast, strong antibody response and the person remains symptom-free. Vaccination has dramatically reduced or eradicated diseases including smallpox (eradicated worldwide), polio (almost eradicated), measles, rubella, tetanus and many others.

7. Coronary heart disease

Coronary heart disease (CHD) is the leading cause of death in many developed countries. It develops when the coronary arteries (the small arteries that supply the heart muscle itself) become narrowed by fatty deposits.

How it develops

1. Cholesterol and other lipids in the blood get deposited on the inside walls of the coronary arteries, forming patches called atheromas (sometimes called fatty plaques).
2. The atheromas gradually narrow the lumen of the artery, restricting the flow of blood to the heart muscle.
3. The rough surface of an atheroma can trigger a blood clot to form. The clot blocks the artery further, or even completely.

Two outcomes

• Partial blockage: the heart muscle still gets some blood but not enough during exercise. This causes a sharp, gripping chest pain called angina, especially during physical exertion.
• Complete blockage: the patch of heart muscle fed by that artery is suddenly cut off from oxygen. Cells in that region cannot respire aerobically and quickly die. This is a heart attack (medically, a myocardial infarction). If a large enough area dies, or if the dying area includes the heart's electrical pacemaker, the heart may stop beating altogether.

Risk factors

Coronary heart disease has several risk factors that increase a person's chance of developing it. Most can be managed, given the right lifestyle choices:

The first six are within the patient's control. Stopping smoking, eating less saturated fat, exercising more, losing weight and keeping diabetes under control all reduce CHD risk substantially.

8. Platelets and blood clotting

When a blood vessel is cut, the body has to act fast to stop blood escaping and to seal off the wound before bacteria can enter. The system that does this involves platelets and a chain of plasma proteins.

The steps of blood clotting:

1. Platelets at the site of the wound release chemicals that trigger the clotting cascade.
2. A soluble plasma protein called fibrinogen is converted into long insoluble fibres of fibrin.
3. The fibrin fibres form a tangled mesh across the wound.
4. Red blood cells get trapped in the mesh, forming a blood clot.
5. The clot dries out at the surface to form a scab, which stays in place while new skin grows underneath.

The clot does two things: it stops further blood loss, and it provides a physical barrier that keeps bacteria out of the wound while it heals.

A condition called haemophilia is caused by inheriting a defective version of one of the clotting proteins. People with haemophilia bleed for much longer than normal after even minor cuts, because their clots form very slowly.

9. Heart rate and exercise

A typical adult's resting heart rate is around 60 to 80 beats per minute (bpm). The heart rate is set by a special patch of cells in the right atrium called the pacemaker, which sends out regular electrical signals that trigger each heartbeat.

Why heart rate changes with exercise

When you exercise, the muscles need extra energy for every contraction, so the muscle cells step up the rate of respiration. Faster respiration means:

• More oxygen and glucose needed at the muscles
• More carbon dioxide produced that needs removing
• More heat produced that needs distributing or losing

The nervous system responds by speeding up the heart rate, which:

• Pumps more oxygen-rich blood to the muscles per minute
• Removes carbon dioxide from the muscles faster

Each individual heartbeat also pumps more strongly (technically the stroke volume, meaning how much blood is ejected per beat). The combined effect is that cardiac output can rise to roughly five times its resting value during hard exercise.

Adrenaline

The hormone adrenaline, released by the adrenal glands during exercise or fear, also makes the heart beat faster and harder. This is part of the body's fight-or-flight response, preparing the muscles for sudden action.

Heart rate after exercise

The heart rate does not drop straight back to normal when you stop. It stays elevated for several minutes because the body is still paying back the oxygen debt built up during anaerobic respiration in the muscles. The faster the heart rate returns to normal, the fitter the person is, giving a simple way to estimate fitness from heart-rate recovery time.