8.2.1 Structure and function of the blood

Blood Has Four Main Components

1. Plasma is the liquid that suspends the blood cells and carries dissolved materials and heat.
2. Red blood cells, also called erythrocytes, transport oxygen.
3. White blood cells defend the body against pathogens.
4. Platelets are cell fragments involved in blood clotting.

Red Blood Cells Carry Oxygen

1. A red blood cell is a biconcave disc, so its curved surfaces give a large surface area relative to its volume for rapid oxygen diffusion.
2. The thin centre gives oxygen a short diffusion distance into and out of the cell.
3. A mature red blood cell has no nucleus, leaving more internal space for haemoglobin.
4. Haemoglobin binds oxygen reversibly in the lungs to form oxyhaemoglobin and releases oxygen where cells are respiring.
5. The cell is small and flexible, allowing it to bend through capillaries that are narrower than its resting diameter.

Phagocytes Engulf Pathogens

1. White blood cells have a nucleus and can change shape as they move through tissues.
2. A phagocyte detects a pathogen, surrounds it with its cell membrane and encloses it inside the cell.
3. Digestive enzymes then break down the engulfed pathogen in a process called phagocytosis.
4. The defence is non-specific because a phagocyte can engulf different types of pathogen.

Lymphocytes Make Specific Antibodies

1. A lymphocyte recognises an antigen, a molecule on the surface of a pathogen.
2. The lymphocyte produces an antibody with a binding site complementary to that antigen.
3. The antibody binds only to its matching antigen, which helps the immune system identify and destroy the pathogen.
4. This response is specific because a different antigen requires a different antibody.

Platelets Form Blood Clots

1. When a blood vessel is damaged, platelets gather at the wound and trigger a series of clotting reactions.
2. A mesh of insoluble fibres forms and traps blood cells, producing a clot.
3. The clot reduces blood loss and blocks pathogens from entering through the break in the skin.
4. The clot later dries into a scab while the tissue underneath repairs.

Plasma Carries Dissolved Materials

1. Plasma carries carbon dioxide from respiring tissues towards the lungs.
2. It transports soluble products of digestion, including glucose and amino acids, from the small intestine to cells.
3. It carries urea from the liver to the kidneys for excretion.
4. It transports hormones from endocrine glands to their target organs.
5. It distributes heat from active organs to other parts of the body.
6. Blood cells and platelets are suspended in plasma and carried through the circulation.

8.2.2 Structure of blood vessels

Arteries Carry Blood at High Pressure

1. Arteries carry blood away from the heart in pulses produced by ventricular contractions.
2. Their walls contain thick layers of smooth muscle that withstand high pressure and can alter the vessel diameter.
3. Their walls contain many elastic fibres that stretch when pressure rises and recoil between heartbeats, helping maintain blood flow.
4. A relatively small lumen helps the vessel maintain high pressure.
5. A tough outer layer prevents the wall from bursting.

Veins Return Blood at Low Pressure

1. Veins carry blood towards the heart after pressure has fallen through the capillary network.
2. Their walls are thinner and contain less muscle and elastic tissue because they do not experience arterial pressure.
3. A large lumen reduces resistance to flow at low pressure.
4. Valves open towards the heart and close if blood begins to move backwards.
5. Contractions of surrounding skeletal muscles squeeze veins, and the valves make this pressure move blood towards the heart.

Capillaries Allow Rapid Exchange

1. Capillaries form extensive networks close to body cells, giving a large total surface area for exchange.
2. Their walls consist of a single layer of flattened cells, so substances cross a very short diffusion distance.
3. Their lumen is narrow, which brings red blood cells close to the wall and slows their passage through the network.
4. Oxygen and glucose diffuse from the blood into tissues, while carbon dioxide diffuses from tissues into the blood.
5. Small gaps between capillary wall cells allow plasma and dissolved substances to move into tissue fluid, but blood cells remain inside.

Pulmonary Vessels Are Exceptions

1. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
2. The pulmonary vein carries oxygenated blood from the lungs to the left atrium.
3. Most systemic arteries carry oxygenated blood and most systemic veins carry deoxygenated blood, but direction remains the reliable definition.

Compare Vessels by Function

1. For an artery, link high pressure to a thick muscular and elastic wall, then explain that recoil maintains blood flow.
2. For a vein, link low pressure to a large lumen and explain that valves prevent backflow.
3. For a capillary, link exchange to a one-cell-thick wall, narrow lumen and large network.

Pressure and Exchange Explain Structure

1. Arteries need strength and elasticity because they receive blood directly from the ventricles.
2. Veins need help preventing reverse flow because their blood pressure is low.
3. Capillaries sacrifice thick walls for rapid exchange because pressure has already fallen before blood enters them.

8.2.3 Structure of the heart and circulatory system

The Heart Is a Double Pump

1. The right side pumps deoxygenated blood to the lungs in the pulmonary circulation.
2. The left side pumps oxygenated blood to the rest of the body in the systemic circulation.
3. Blood returns to the heart between the two circuits, so it passes through the heart twice in one complete journey.
4. Separate pumps allow lower pressure through delicate lung capillaries and higher pressure through the longer systemic circuit.

Chambers Control Blood Flow

1. The right atrium receives deoxygenated blood from the body through the vena cava.
2. The right atrium contracts and moves blood through an atrioventricular valve into the right ventricle.
3. The right ventricle contracts and pumps blood through the pulmonary artery to the lungs.
4. The left atrium receives oxygenated blood from the lungs through the pulmonary vein.
5. The left atrium contracts and moves blood through an atrioventricular valve into the left ventricle.
6. The left ventricle contracts and pumps blood through the aorta to the body.

Valves Prevent Backflow

1. Atrioventricular valves lie between each atrium and ventricle.
2. They open when pressure is greater in an atrium than in its ventricle, allowing blood to move forwards.
3. They close when ventricular pressure rises, preventing blood from returning to the atria.
4. Valves at the exits of the ventricles prevent blood in the pulmonary artery and aorta from flowing back into the heart.
5. Valves open and close because of pressure differences, rather than contracting by themselves.

The Septum Prevents Mixing

1. The septum keeps oxygenated blood on the left side separate from deoxygenated blood on the right side.
2. This separation allows blood sent to the body to retain a high oxygen concentration.
3. A steep oxygen concentration gradient can therefore be maintained between systemic capillaries and respiring cells.

Pressure Determines Wall Thickness

1. Atria have thin walls because they push blood only a short distance into the ventricles.
2. The right ventricle has a thicker wall than an atrium because it pumps blood out of the heart to the lungs.
3. The left ventricle has the thickest muscular wall because it must produce high pressure to drive blood around the whole body.
4. The lower pressure produced by the right ventricle protects the thin exchange surfaces in the lungs.

Contraction Produces One-Way Flow

1. Atrial contraction raises pressure and pushes blood into the ventricles.
2. Ventricular contraction raises pressure, closes the atrioventricular valves and opens the exit valves.
3. Relaxation lowers pressure, allowing the chambers to refill before the next heartbeat.

8.2.4 Cardiac output

Heart Rate Means Beats per Minute

1. Heart rate is measured in $\mathrm{beats}\mathrm{mi}{\mathrm{n}}^{-1}$, often written as $\mathrm{bpm}$.
2. Each pressure wave felt in an artery is a pulse produced by one heartbeat.
3. A pulse oximeter can display heart rate electronically, while a pulse can also be counted at the wrist or neck.

Stroke Volume Is Blood per Beat

1. Stroke volume is usually measured in $\mathrm{c}{\mathrm{m}}^3\mathrm{bea}{\mathrm{t}}^{-1}$.
2. A stronger ventricular contraction ejects a larger stroke volume.
3. During exercise, increased venous return and stronger contractions can raise stroke volume.

Cardiac Output Is Blood per Minute

1. Cardiac output combines how often the heart beats with how much blood leaves a ventricle on each beat.
2. The equation is $cardiac\; output=stroke\; volume\times heart\; rate$.
3. In symbols, $Q=SV\times HR$.
4. Useful rearrangements are $SV=\frac{Q}{HR}$ and $HR=\frac{Q}{SV}$.
5. If stroke volume is in $\mathrm{c}{\mathrm{m}}^3\mathrm{bea}{\mathrm{t}}^{-1}$ and heart rate is in $\mathrm{beats}\mathrm{mi}{\mathrm{n}}^{-1}$, cardiac output is in $\mathrm{c}{\mathrm{m}}^3\mathrm{mi}{\mathrm{n}}^{-1}$.

Exercise Raises Cardiac Output

1. Contracting muscles use more oxygen and glucose for aerobic respiration.
2. A higher heart rate and stroke volume increase cardiac output, delivering these reactants faster.
3. The faster blood flow also removes more carbon dioxide and lactic acid from muscle tissue.
4. The increased rate of aerobic respiration releases more energy for muscle contraction.

Check Units Before Calculating

1. The word per represents division, so $\mathrm{c}{\mathrm{m}}^3\mathrm{bea}{\mathrm{t}}^{-1}$ means cubic centimetres for each beat.
2. Multiplying $\mathrm{c}{\mathrm{m}}^3\mathrm{bea}{\mathrm{t}}^{-1}$ by $\mathrm{beats}\mathrm{mi}{\mathrm{n}}^{-1}$ cancels beats and leaves $\mathrm{c}{\mathrm{m}}^3\mathrm{mi}{\mathrm{n}}^{-1}$.
3. Use $1\mathrm{d}{\mathrm{m}}^3=1000\mathrm{c}{\mathrm{m}}^3$ when converting cardiac output.

8.3.1 Aerobic and anaerobic respiration

Respiration Releases Energy Continuously

1. Living cells respire continuously because they need a constant transfer of energy.
2. Respiration is exothermic because chemical energy from glucose is transferred to the surroundings and to processes in the cell.
3. Cells use the released energy for muscle contraction, active transport and building larger molecules from smaller ones.
4. Energy released by respiration also helps mammals and birds maintain a stable body temperature.
5. Respiration is a series of enzyme-controlled reactions, not a single reaction and not the same process as breathing.

Aerobic Respiration Uses Oxygen

1. Aerobic respiration uses oxygen and completely breaks down glucose.
2. The word equation is $glucose+oxygen\to carbon\; dioxide+water$.
3. The balanced symbol equation is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$.
4. Most aerobic respiration occurs in mitochondria, so cells with high energy demands often contain many mitochondria.
5. Complete breakdown of glucose releases much more energy than anaerobic respiration.

Anaerobic Respiration Uses No Oxygen

1. Anaerobic respiration occurs when oxygen supply cannot meet the demand for aerobic respiration.
2. Glucose is broken down incompletely, so less energy is released from each molecule of glucose.
3. In animal muscle cells, the word equation is $glucose\to lactic\; acid$.
4. Lactic acid can accumulate during vigorous exercise and is associated with muscle fatigue.
5. In yeast and plant cells, the word equation is $glucose\to ethanol+carbon\; dioxide$.
6. Anaerobic respiration occurs in the cytoplasm rather than the mitochondria.

The Two Pathways Have Clear Differences

1. Oxygen: aerobic respiration requires oxygen, while anaerobic respiration does not.
2. Breakdown: aerobic respiration breaks glucose down completely, while anaerobic respiration breaks it down incompletely.
3. Energy: aerobic respiration releases much more energy per molecule of glucose.
4. Animal products: aerobic respiration produces carbon dioxide and water, while anaerobic respiration produces lactic acid.
5. Yeast and plant products: anaerobic respiration produces ethanol and carbon dioxide.
6. Location: most aerobic respiration occurs in mitochondria, while anaerobic respiration occurs in the cytoplasm.

Exercise Can Use Both Pathways

1. At the start of exercise, muscles increase their demand for energy before oxygen delivery has fully increased.
2. Aerobic respiration supplies most energy when enough oxygen reaches the muscles.
3. During vigorous activity, anaerobic respiration adds a faster but smaller energy release when oxygen supply is insufficient.
4. Heart rate and breathing rate rise to deliver more oxygen and glucose and to remove carbon dioxide.