Why big animals need a heart

As an organism gets bigger its surface area : volume ratio falls, and its diffusion distances rise. Diffusion alone is far too slow to supply the metabolic demand of a mammal, so a mass transport system is required: a muscular pump (the heart) moving a fluid (blood) through vessels.

Mammals have a closed double circulation: blood passes through the heart twice per circuit. The pulmonary circuit is low-pressure (protecting the delicate alveolar capillaries); the systemic circuit is high-pressure, so blood reaches every tissue quickly.

Water as the transport medium (1.2): water is a polar molecule and forms hydrogen bonds, which makes it an excellent solvent for ions, glucose, amino acids and urea. Its high specific heat capacity means blood can absorb metabolic heat without a large temperature change, and its cohesion gives it the properties needed to flow through vessels as a continuous column.

Arteries, veins and capillaries

• Artery — thick wall, narrow lumen, a lot of elastic tissue and smooth muscle. Elastic recoil during diastole smooths the pulse and maintains flow.
• Vein — thin wall, wide lumen, very little muscle or elastic tissue; blood is at low pressure, so semilunar valves and the squeezing action of skeletal muscle prevent backflow.
• Capillary — one cell thick (squamous endothelium), giving a very short diffusion distance; huge total surface area; narrow lumen slows the red blood cells so exchange has time to happen.

Exam framing: never say "the artery has thick walls to carry blood" — say why: the thick, elastic wall withstands and smooths the high pressure surges produced by ventricular systole.

Which vessel?

Tap a feature, then tap the vessel it belongs to.

The cardiac cycle

One cycle has three stages, and every valve movement is caused by a pressure difference:

• Atrial systole — atria contract, pushing the last ~20 % of blood through the open atrioventricular (AV) valves into the ventricles.
• Ventricular systole — ventricles contract. Ventricular pressure rises above atrial pressure, so the AV valves shut (first heart sound). When ventricular pressure exceeds aortic/pulmonary artery pressure the semilunar valves open and blood is ejected.
• Diastole — all chambers relax. Ventricular pressure falls below arterial pressure, so the semilunar valves shut (second heart sound); pressure then falls below atrial pressure and the AV valves reopen.

Your turn — heart rate from the cycle

Why does the AV valve close?

How an atheroma forms

Atherosclerosis is a sequence, and the exam wants the sequence in order:

• Endothelial damage — from high blood pressure, toxins in cigarette smoke, or high blood glucose.
• Inflammatory response — white blood cells (chiefly macrophages) move into the artery wall.
• They accumulate cholesterol from LDLs, becoming foam cells; together with lipids and connective tissue this builds an atheroma.
• Calcium salts and fibrous tissue harden it into a plaque. The lumen narrows, so blood pressure rises — which causes more endothelial damage. A positive feedback loop.
• A ruptured plaque triggers a thrombus (clot). In a coronary artery this causes a myocardial infarction; in the brain, a stroke.

The clotting cascade

Clotting is a cascade — each step activates the next, which amplifies the response:

Ca²⁺ ions and vitamin K are required. The fibrin mesh traps red blood cells to form the clot. Fibrin is insoluble; fibrinogen is soluble — that one word is worth a mark.

Clotting and atheroma — match them up

Tap a description on the left, then its term on the right.

Risk factors and the maths of risk

Risk factors for CVD include genetics, age, sex, diet high in saturated fat and salt, high blood pressure, smoking and inactivity. Some are modifiable; age, sex and genotype are not.

Evaluating studies (1.9): ask about sample size, whether the groups were matched for confounding variables, whether it was a cohort (forward-looking) or case–control study, how long it ran, and who funded it.

Correlation ≠ causation. A correlation only becomes evidence of cause when there is a plausible mechanism, a dose–response relationship, the cause precedes the effect, and the result is reproducible. People also misperceive risk: they overestimate risks that are unfamiliar, dramatic or outside their control.

Your turn — relative risk

Reading a study

Carbohydrates and lipids

Carbohydrates. Monosaccharides (glucose, fructose, galactose) join by a condensation reaction, forming a glycosidic bond and releasing water. Two make a disaccharide: glucose + glucose → maltose; glucose + fructose → sucrose; glucose + galactose → lactose. Many make a polysaccharide.

Lipids. A triglyceride is one glycerol + three fatty acids, joined by three condensation reactions forming three ester bonds.

• Saturated fatty acids have no C=C double bonds: straight chains that pack closely, so they are solid at room temperature (mostly animal fats).
• Unsaturated fatty acids have one or more C=C double bonds, producing kinks that stop close packing, so they are liquid oils (mostly plant).

Cholesterol, HDL and LDL: LDLs deliver cholesterol to tissues and, in excess, are taken up by the artery wall — a high LDL:HDL ratio is associated with atherosclerosis. HDLs carry cholesterol back to the liver.

Your turn — energy budget

Your turn — BMI

Bond check

Treating cardiovascular disease

• Antihypertensives — lower blood pressure, reducing endothelial damage. Side effects: dizziness, fainting.
• Statins — inhibit the enzyme that makes cholesterol in the liver, lowering LDL. Side effects: muscle pain, liver damage in a minority.
• Anticoagulants (e.g. warfarin) — reduce clot formation, but risk uncontrolled bleeding.
• Platelet inhibitors (e.g. low-dose aspirin) — reduce platelet aggregation; risk of stomach ulcers.

Core practical 1 investigates the vitamin C content of fruit juice by titration with DCPIP. Core practical 2 investigates the effect of caffeine on the heart rate of Daphnia — which raises the ethical question of whether invertebrates may be used in this way.

Choosing a treatment

The big ideas to know

You have covered the whole of SNAB Topic 1. Press Finish to see your score.