BIO 224 - Midterm #2

Brachiation

Arm-based, arboreal (through-the-trees) locomotion seen widely in primates; gibbons can brachiate up to ~35 mph.

Why does brachiation stress the shoulder girdle?

Swinging from a single limb puts a lot of force/stress on the limb and especially on its attachment point to the trunk (the girdle).

Scapula position in brachiators

Shifts from laterally placed (as in quadrupeds like dogs) to more dorsally placed, allowing a much wider range of motion.

Rounded humeral head (brachiators)

Converts the shoulder into a ball-and-socket joint (vs. a hinge), allowing ~180 degrees of rotation — same trend seen in bird flight.

Hand/wrist adaptations for brachiation

Increased wrist dexterity, shorter nails, and hook-like curved phalanges for a strong, accurate grip.

Hominoid

The group including bipedal apes; used to compare bipedal body plans against non-bipedal close relatives.

Definition of true bipedalism (spine orientation)

The spine's long axis is perpendicular to the plane of movement (vs. parallel, as in a quadrupedal chimpanzee).

Weight distribution: chimpanzee vs. human

Chimpanzee distributes body mass across 4 limbs (4 pillars); human funnels mass through the pelvis into 2 lower limbs (2 pillars).

Chimpanzee pelvis shape

Elongated ilium, narrow pelvis.

Human pelvis shape

Short ilium, broad, bowl/basin-shaped pelvis — better distributes weight across two legs.

Chimpanzee ribcage shape

Flared, funnel-shaped; narrow at top, flares out at bottom; very short gap between ribs and pelvis (no real waist).

Human ribcage shape

Barrel-shaped, tapered at both ends, widest at the middle — creates a functional waist.

Functional benefit of having a waist

Allows lateral flexibility/side-bending so the upper body can recenter over the pelvis (e.g., recovering balance when bumped while walking).

Fetal spinal curvature

A single continuous curvature, similar in shape to a quadruped's spine.

Primary spinal curvatures

Curvatures that resemble the original fetal condition (thoracic and sacral regions).

Secondary spinal curvatures

Curvatures that differ from the fetal condition, produced by intervertebral disc development (cervical and lumbar regions).

Function of alternating spinal curvatures

Acts like a spring, absorbing/dispersing compressive forces generated during bipedal locomotion.

Angle of inclination (femur/hip)

Angle between femoral shaft and neck at the acetabulum; ~135 degrees in a 3-year-old, becomes more acute with age due to mechanical loading — used forensically to estimate age.

Pubic symphysis angle sex difference

More obtuse in females, which helps widen the birth canal.

Genu valgum ("knock-kneed" stance)

Natural human stance where knees sit medial to their pelvic attachment; more dramatic in females due to a wider pelvis, linked to higher rates of certain athletic knee injuries.

Foot differences: arboreal vs. terrestrial primates

Arboreal primates (gibbons, orangutans) retain long digits and strongly opposable hallux for grasping; terrestrial walkers (chimp, gorilla, human) show reduced hallux opposability and more cushioning.

Longitudinal (medial) arch of the foot

Shock-absorbing arch visible from the side; tendons/ligaments let the foot flex under load and spring back.

Transverse arch of the foot

Arch visible from the front; midfoot is the high point, helps flatten/distribute force on impact.

Predator avoidance (bipedalism selective pressure)

Standing up allows seeing over obstacles/grasses; seen in meerkats (sentinels) and grazing gazelles as an analogous behavior.

Carrying objects (bipedalism selective pressure)

Bipedalism frees the forelimbs to carry resources while the lower limbs handle locomotion (e.g., a gorilla walking bipedally while carrying something).

Temporary vs. obligate bipedalism

Some primates (e.g., gorillas) can walk bipedally temporarily but their skeleton doesn't support it as a default mode, unlike obligate bipeds like humans.

What makes bipedal running effective (vs. just walking)?

Relatively longer legs, elastic energy storage via Achilles tendon/gastrocnemius, and a basin-like pelvis for robust gluteal attachment.

Role of the Achilles tendon and gastrocnemius in running

Provide elastic "spring/launch" energy, storing and returning energy to propel the body forward.

Nuchal ligament function

A flat, stretchy connective tissue band running along the back of the neck in humans; buffers/dissipates shock transmitted up from running (like pulling a rubber band instead of a rope, as chimps do via direct muscle attachment to the skull).

Two core goals of respiration

Get oxygen into the body from the environment, and dump CO2 out of the body into the environment.

Boyle's Law

At constant temperature, gas pressure and volume are inversely related (part of PV = nRT).

Dalton's Law of Partial Pressures

Total pressure of a gas mixture equals the sum of the partial pressures of each individual gas in the mixture.

Henry's Law

Gas molecules in contact with a liquid surface partially dissolve into it, with the amount increasing at higher pressure; solubility differs greatly between gases.

Which dissolves in water better, O2 or CO2?

CO2 dissolves into water much more readily than O2 (O2 has poor solubility in water).

Fick's First Law of Diffusion

Rate of diffusion is proportional to (Area x Concentration difference) / Distance.

How does increasing surface area affect diffusion rate?

Increases it (area is in the numerator of Fick's Law).

How does increasing diffusion distance affect diffusion rate?

Decreases it (distance is in the denominator of Fick's Law).

Gills (structural origin)

Evaginated (outward-folding) tissue, richly capillarized, used with a liquid (water) medium; the ancestral exchange structure.

Lungs (structural origin)

Invaginated (inward-folding) tissue creating an internal pocket, used with a gaseous (air) medium.

Open flow gas exchange model

Seen in amphibian skin: thin capillaries sit directly against the open external environment.

Uniform pool gas exchange model

Seen in mammalian alveoli: a bag wrapped in a capillary mesh containing a mixed, homogeneous gas pool.

Cross-current gas exchange model

Seen in bird lungs: air flows unidirectionally through a tube while blood vessels cross perpendicularly (like ladder rungs).

Counter-current gas exchange model

Seen in fish gills: water and blood flow in opposite directions, maintaining a positive diffusion gradient across the entire exchange length; the most efficient model.

Why is counter-current exchange so efficient?

Because water O2 concentration stays higher than blood O2 concentration at every point along the exchange surface, diffusion occurs across the entire length, not just part of it.

Approximate PO2 of ambient atmospheric air

About 170 mmHg.

Where does actual molecular gas exchange occur in the body?

At capillaries.

Why does alveolar PO2 drop sharply compared to ambient air?

Because alveolar air is a mixed pool that has already exchanged O2 for CO2 with the blood.

How do surface area requirements come into play for human respiration?

The lungs maximize the "area" term of Fick's Law by packing millions of tiny alveolar sacs into a sponge-like structure, creating an enormous total gas-exchange surface within a small thoracic volume, enough to meet the body's O2 demand.

Why can't gas just pass through the skin and tissues of most vertebrates?

Per Fick's Law, diffusion distance is too large (skin is many cell layers thick, capped with keratin in mammals) and body size gives a low surface-area-to-volume ratio, so cutaneous exchange alone can't meet metabolic O2 demand (amphibians are the exception because their skin is thin enough).

Respiratory cascade: relative PO2 across the first 5 compartments

Ambient air (highest, ~170 mmHg) > alveolar gas (drops sharply, mixed pool) > arterial blood (high again after leaving lungs) > capillary blood (a declining range as O2 diffuses to tissue) > venous blood (lowest, O2 already delivered).

Why does PO2 only drop at certain points in the respiratory cascade?

PO2 drops specifically where molecular gas exchange with tissue actually occurs (alveoli, capillaries) — not simply from blood moving through a vessel.

Why is water ventilation energetically costly for fish?

Water is dense/viscous, so pushing it across the gills costs significant ATP, meaning the O2 payoff must exceed that cost.

Unidirectional flow in fish gill ventilation

Water flows mouth to buccal cavity to gills to out the operculum, never recycled/backtracked.

Positive pressure ventilation in fish

Fish use negative pressure briefly to fill the buccal cavity, then squeeze/compress it to force water positively across the gills.

Afferent vs. efferent vessels in fish gills

Afferent vessels carry deoxygenated blood to the gill lamellae; efferent vessels carry newly oxygenated blood away to the body.

Ram ventilation

A strategy in fast, pelagic fish where forward swimming with an open mouth forces water across the gills passively.

Typical function of a fish swim bladder

Locomotor/buoyancy control (adjusting gas content changes density); normally NOT respiratory, and lined with impermeable collagen.

Conditions favoring derived (respiratory) swim bladders

Shallow, stagnant, warm water — stagnant water isn't aerated, and warm water holds less dissolved O2.

Air-breathing fish process (swim bladder)

Fish gulps air, it passes to the anterior swim bladder, crosses a thin capillarized respiratory gas bladder region for gas exchange, then is burped out posteriorly.

Lungfish habitat and adaptation

Africa, Australia, South America; lobe-finned; undergo estivation (long dormancy with reduced metabolism) in seasonally drying habitats.

Lungfish lungs vs. swim bladders (location & homology)

Lungfish lungs sit ventrally and are homologous to tetrapod lungs; swim bladders sit dorsally and are not the same structure.

Frog lung structure

Simple, bag-like, with relatively little internal septation (unlike sponge-like mammalian lungs).

Buccal pumping in frogs

Positive pressure, tidal ventilation: buccal cavity expands (negative pressure draws air in), then compresses (positive pressure pushes air into lungs); air exits the same route.

Why do frogs show intermittent respiration during calling?

Calling uses the same buccal apparatus as breathing (glottis closed, air forced out nares), so during calling bouts the frog doesn't breathe; tolerated due to low O2 demand as an ectotherm.

Cutaneous respiration

Gas exchange across the skin; effective in amphibians because their skin is very thin (short diffusion distance) compared to thick, keratinized mammal skin.

O2 exchange through amphibian skin vs. lungs

Lungs dominate O2 uptake; skin contributes very little (flat line on the O2 exchange graph) because O2 doesn't dissolve well in the wet skin surface.

CO2 exchange through amphibian skin vs. lungs

Lungs still dominate but skin contributes meaningfully more to CO2 exchange than to O2 exchange, since CO2 dissolves in water more readily.

Lungless salamander

An amphibian with no lungs at all; exchanges essentially all gas cutaneously, tolerated due to adequate ambient O2 and low metabolic demand.

Costalia (amniote ribs)

Rib-like structures in amniotes that both define body-cavity space and are pulled by axial muscles to actively change thoracic volume/pressure for negative pressure ventilation.

Sauropsid (reptilian) lung structure

"Faveolar" — one central passage (like an elevator shaft) with small tidal side chambers; overall airflow through the main shaft is largely unidirectional.

Synapsid (mammalian) lung structure

"Alveolar" — air enters, dead-ends, bounces around, and exits via the same pathway (fully tidal).

Locomotion-respiration conflict in lizards

Lateral (axial) body bending during running compresses one lung while expanding the other, pushing stale air between lungs instead of exchanging with fresh air.

Gular pumping

A positive-pressure mechanism in reptiles (e.g., lizards) where the throat (gular) region expands to draw air in, then compresses to force fresh air into the lungs, independent of the locomotor cycle.

How do turtles ventilate without a flexible ribcage?

They move their limbs: pulling limbs in compresses internal volume (exhalation); extending limbs out expands volume (inhalation, negative pressure).

Hepatic piston (crocodilian) ventilation

The liver and diaphragmaticus muscle shift position: contraction drags the liver caudally, increasing lung-area volume (inspiration); relaxation reverses this (expiration).

2008 crocodilian study finding

Diaphragmaticus excitation also occurs during diving, correlated with body inclination angle — showing the liver/diaphragmaticus system also functions in buoyancy control.

2010 crocodilian study finding

Mapped unidirectional airflow in alligator lungs, achieved without the accessory air-sac system that birds use.

Evolutionary O2 context: synapsids vs. archosaurs

Synapsids diversified in the high-O2 Permian; archosaurs (crocodilians, birds) diversified in the low-O2 Triassic, favoring efficient unidirectional gas exchange.

Function of bird air sacs

Store and direct bulk airflow but perform NO molecular gas exchange themselves; exchange occurs only in the lung capillary beds.

Bird air sac airflow path

External air to posterior air sacs to lungs to anterior air sacs to out via the trachea — lung airflow is unidirectional.

Evidence that bird air sacs evolved before flight

Fossil evidence of air-sac-related bony passageways (foramina) in non-flying theropod and sauropod dinosaurs.

Avian two-cycle breathing: Inspiration 1

Air drawn from environment into posterior air sacs.

Avian two-cycle breathing: Expiration 1

Air moves from posterior air sacs into the lungs.

Avian two-cycle breathing: Inspiration 2

Air moves from lungs into anterior air sacs.

Avian two-cycle breathing: Expiration 2

Air pushed out of anterior air sacs and out of the body.

Muscles driving avian ventilation

Back muscles pull the pelvis caudally/dorsally (inspiration); abdominal muscles pull sternum and pelvis together (expiration) — no diaphragm.

Parabronchi cross-current exchange (birds)

Air flows through parallel parabronchi between the dorsobronchus and ventrobronchus, meeting fresh capillaries continuously (cross-current); achieves roughly 50% O2 extraction.

How does gill structure relate to diffusion and metabolic requirements?

Number of lamellae and total gill surface area scale with an organism's activity level — more active, higher-metabolic-demand fish invest in more elaborate gill structure (more area = more diffusion per Fick's Law = more O2 to support higher ATP turnover).

Turtle breathing figure — what is actually being measured?

Lung ventilation events and heart rate plotted together over time in intermittent-breathing turtles/tortoises, showing heart rate rises around/during breathing bouts and settles between them.

Why does turtle heart rate couple to breathing bouts?

It lets the animal maximize circulation exactly when fresh O2 is available from a breath, rather than wasting a flat heart rate during the long apneic gaps between breaths.

Upper respiratory tract structures

Nose/nasal cavity, pharynx, larynx — mainly warm/moisten air and aid water conservation.

Lower respiratory tract structures

Trachea, bronchial tree (bronchi to bronchioles to alveolar ducts to alveolar sacs).

Anatomical dead space

All conducting airway regions where NO molecular gas exchange occurs (everything except the alveolar sacs).

Why is hyperventilation potentially dangerous?

Rapid shallow breaths mostly flush dead-space air without cycling fresh air down to the alveoli, allowing CO2 buildup despite heavy breathing.

Type I alveolar cells

Flattened, squamous epithelial cells forming the bulk of the alveolar wall and the actual gas-exchange (respiratory) membrane.

Type II alveolar cells

Support cells that secrete surfactant.

Surfactant function

Acts as a lubricant that reduces the cohesive force of water lining the alveoli, allowing them to reopen easily after exhaling (also mildly antibacterial).

Why can premature infants have respiratory distress?

Type II alveolar cells (and surfactant production) mature late in gestation, so alveoli can stick together and fail to reopen after birth without medical support.

Myoglobin structure

A monomer (single protein unit) with one heme group, giving one O2 binding site; found largely in muscle.

Hemoglobin structure

A tetramer built from four myoglobin-like subunits, giving four heme groups/four O2 binding sites.