7.3 Human Body Systems, Ecology, and Evolution

7.3 Human Body Systems, Ecology, and Evolution

This is the part of senior-high biology that scales from a single kidney tubule to the whole biosphere. The items that come from it reward two habits: knowing which organ or process does which job, and being able to run a short calculation about energy or feedback rather than merely naming things.

1. Levels of biological organisation

Cell → tissue → organ → organ system → organism → population → community → ecosystem → biome → biosphere. A population is one species in one place; a community is all the populations living there together; an ecosystem adds the non-living surroundings to that community.

2. The organ systems at a glance

The heart and its two circuits

The heart has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump it. The left ventricle has the thickest wall of all because it must drive blood around the entire body.

   BODY --(vena cava)--> RIGHT ATRIUM
                              |  tricuspid valve
                         RIGHT VENTRICLE
                              |  pulmonary ARTERY  (deoxygenated)
                            LUNGS  -- gas exchange
                              |  pulmonary VEIN    (oxygenated)
                          LEFT ATRIUM
                              |  bicuspid (mitral) valve
                         LEFT VENTRICLE
                              |  aorta
                            BODY

• The pulmonary circuit runs heart to lungs to heart, and it creates the two classic exceptions: the pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood.
• The systemic circuit runs heart to body to heart.
• Arteries carry blood away from the heart under high pressure; veins return it at low pressure and need valves; capillaries are one cell thick so that exchange can happen by diffusion.

Breathing and gas exchange

Air travels nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Each alveolus is a thin, moist, capillary-wrapped sac. Oxygen diffuses from the alveolar air, where it is concentrated, into the blood, while carbon dioxide diffuses the opposite way. Inhalation is the active step: the diaphragm contracts and flattens, the rib cage lifts, chest volume rises, the pressure inside falls below atmospheric pressure, and air rushes in.

Digestion and its enzymes

Bile is not an enzyme. It emulsifies fat into small droplets so that lipase can work on a much larger surface. The liver makes it and the gall bladder stores it.

Nerves and the reflex arc

A neuron has dendrites that receive, a cell body, and an axon that transmits, often insulated by a fatty myelin sheath that speeds the impulse along. Neurons pass signals across a synapse using neurotransmitters. A reflex arc is the fastest circuit in the body and deliberately bypasses the brain:

receptor → sensory neuron → interneuron in the spinal cord → motor neuron → effector muscle

You snatch your hand off a hot kaldero before you consciously feel any pain; the brain is informed after the movement has already happened.

Filtering the blood

The functional unit of the kidney is the nephron. Blood under high pressure in the glomerulus is filtered into Bowman's capsule; glucose, amino acids, salts and most of the water are reabsorbed along the proximal tubule and the loop of Henle; surplus hydrogen and potassium ions are secreted into the distal tubule; and what remains — urea with excess salt and water — drains through the collecting duct as urine. ADH (antidiuretic hormone) makes the collecting duct more permeable when you are dehydrated, producing a smaller volume of concentrated urine.

Hormones and immunity

Innate immunity is immediate, general and present from birth: skin, mucus, stomach acid, phagocytes and inflammation. Adaptive immunity is slow the first time, highly specific and remembered — B lymphocytes make antibodies against one antigen and memory cells make any second encounter almost instantaneous, which is precisely what a vaccine buys.

Worked example 1 — homeostasis by negative feedback

Homeostasis is the maintenance of a steady internal state, and negative feedback achieves it by triggering the opposite of whatever change occurred.

A student eats a plate of rice at lunch and blood glucose climbs from roughly 90 mg/dL to 150 mg/dL.

1. Stimulus: blood glucose rises above the set point.
2. Receptor and control centre: the beta cells of the pancreas detect the rise.
3. Effector and response: insulin is secreted, and liver and muscle cells absorb glucose and store it as glycogen.
4. Result: glucose falls back toward 90 mg/dL and insulin secretion switches off.

Five hours later the student has skipped merienda and glucose drops to 70 mg/dL. Now the alpha cells release glucagon, the liver converts glycogen back into glucose, and the level climbs to the set point again. Thermoregulation runs on the same logic: too hot brings sweating and vasodilation, too cold brings shivering and vasoconstriction. Contrast all of this with positive feedback, which amplifies a change instead — oxytocin during childbirth intensifies contractions until delivery ends the loop.

3. Ecology: energy and matter

Biotic factors are the living components of an environment, such as a mangrove tree, a mudskipper or decomposing bacteria; abiotic factors are the non-living ones, such as salinity, temperature, light, pH and typhoon frequency.

Trophic levels describe feeding position: producers, then primary consumers (herbivores), then secondary and tertiary consumers, with decomposers recycling nutrients from every level. A food chain is a single linear path; a food web is the realistic network of overlapping chains, which is why removing one species can disturb many others.

Worked example 2 — the 10% rule in a Philippine forest

Only about 10% of the energy held at one trophic level becomes biomass at the next. Roughly 90% is lost as metabolic heat, in undigested material and in parts never eaten.

Forwards: if dipterocarp trees fix 25,000 kJ, then primary consumers such as fruit bats hold 25,000 × 0.10 = 2,500 kJ, and the secondary consumer above them holds 2,500 × 0.10 = 250 kJ.

Backwards is the version examiners prefer. Suppose a Philippine eagle, sitting at the third trophic level, must accumulate 40 kJ.

1. The trophic level below it must supply 40 ÷ 0.10 = 400 kJ.
2. The producers must supply 400 ÷ 0.10 = 4,000 kJ.

Each step multiplies the requirement tenfold, which is why one breeding pair of Philippine eagles needs thousands of hectares of forest and why food chains rarely run beyond four or five links. An energy pyramid is therefore always upright, but a pyramid of numbers can be inverted — one narra tree supports thousands of insects — and so can a pyramid of biomass at sea, where a small standing mass of fast-breeding plankton feeds a much larger mass of fish.

The cycles

• Carbon cycle: photosynthesis withdraws carbon dioxide; respiration, decomposition, the burning of fossil fuels and forest fires return it. Oceans and carbonate rock are the largest reservoirs.
• Nitrogen cycle: atmospheric nitrogen gas is unusable until nitrogen fixation by Rhizobium bacteria in the root nodules of legumes, or by lightning, converts it to ammonia. Nitrification turns ammonia into nitrites and then nitrates that roots absorb, ammonification returns nitrogen from dead matter, and denitrification sends nitrogen gas back to the air.
• Water cycle: evaporation from seas and lakes plus transpiration from leaves, then condensation into clouds, precipitation, and finally runoff and infiltration into groundwater.

Relationships between species

Coral bleaching is what happens when the first of those partnerships collapses: heat-stressed corals expel their zooxanthellae, lose their colour and their sugar supply, and starve.

Populations and biodiversity

A population grows exponentially, tracing a J-shaped curve, only while resources are unlimited. In reality food, space, predators and disease act as limiting factors, growth levels off at the carrying capacity (K), and the curve becomes an S-shaped logistic one. Density-dependent limits such as disease and competition bite harder as crowding rises, while density-independent limits such as a typhoon or a volcanic eruption strike no matter how many individuals there are.

The Philippines is one of the world's 17 megadiverse countries and a recognised biodiversity hotspot: extraordinarily rich in endemic species, meaning species found nowhere else, and extraordinarily threatened. The Philippine eagle, the tamaraw of Mindoro, the Philippine tarsier and several Rafflesia species are all endemic. Coral reefs and mangroves are the two most productive coastal ecosystems, with mangroves trapping sediment, sheltering juvenile fish and blunting storm surge. Deforestation, dynamite and cyanide fishing, the conversion of mangroves into fishponds and warming seas are the main pressures; the responses include protected areas under the expanded NIPAS law, such as Tubbataha Reefs Natural Park, community-managed marine protected areas, and mangrove replanting.

4. Evolution

Natural selection, Charles Darwin's mechanism, needs four conditions:

1. Individuals in a population vary, and much of that variation is heritable.
2. More offspring are produced than the available resources can support.
3. Individuals whose traits suit the environment survive and reproduce more successfully.
4. Those traits become commoner in the population over generations.

The thing that evolves is the population, never the individual. The Lamarckian story — a giraffe stretches for high leaves and passes the longer neck to its calves — is the classic wrong answer. The Darwinian account is that giraffes already varied in neck length and the longer-necked ones fed better and left more offspring. Antibiotic resistance is the same process in real time: resistant bacteria were already present, and the antibiotic simply removed their competitors.

An adaptation is any heritable feature that improves survival or reproduction in a particular environment. Speciation occurs when populations stop interbreeding: allopatric speciation follows geographic separation, which is exactly why an archipelago of more than 7,000 islands generates so many endemic species, while sympatric speciation happens without any physical barrier.

5. Classification

Organisms are ranked Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species — remembered as “Dear King Philip Came Over For Good Soup”. Binomial nomenclature, introduced by Linnaeus, gives every species a two-word Latin name in which the genus is capitalised, the species epithet is lowercase, and the whole name is italicised or underlined: Homo sapiens, Pithecophaga jefferyi for the Philippine eagle, Bubalus mindorensis for the tamaraw.

Common traps

• Saying the pulmonary artery carries oxygenated blood. An artery is defined by direction of flow, not by oxygen content.
• Routing a reflex through the brain. The spinal cord completes the arc; the brain is only informed afterwards.
• Swapping negative and positive feedback. Negative feedback reverses the change; positive feedback pushes it further.
• Applying the 10% rule to numbers of organisms. It applies to energy.
• Treating analogous structures as evidence of common ancestry. That is what homologous structures show.
• Writing that an organism adapts during its lifetime and passes the change on. Populations evolve; individuals do not.
• Confusing mutualism with commensalism. In commensalism the second partner gains nothing at all.