chapter 6: respiratory system

Path of air into the lungs

Nares → nasal cavity → pharynx (warmed, humidified, filtered) → larynx → trachea → mainstem bronchi → bronchioles → alveoli

Alveoli

Small sacs that interface with pulmonary capillaries; allow gas exchange by simple diffusion across a one-cell-thick membrane

Surfactant

Substance in the alveoli that reduces surface tension at the liquid-gas interface, preventing alveolar collapse

Visceral pleura

Pleural layer that lies directly adjacent to the lung tissue

Parietal pleura

Pleural layer that lines the chest wall

Intrapleural space

Space between the visceral and parietal pleurae; contains a thin layer of lubricating fluid

Diaphragm

Thin skeletal muscle that creates the pressure differential needed for breathing

Inhalation — active or passive?

Active process — requires muscular effort

Inhalation mechanism

Diaphragm and external intercostal muscles contract → thoracic cavity expands → intrapleural pressure decreases → lungs expand → intrapulmonary pressure drops → air flows in

Negative-pressure breathing

The mechanism by which lung expansion (not a pump) draws air in by creating a pressure lower than atmospheric pressure

Passive exhalation

Muscles of inspiration relax and elastic recoil of the lungs reduces thoracic volume, reversing the pressure differential and pushing air out

Active exhalation

Internal intercostal muscles and abdominal muscles forcibly decrease thoracic volume to push out air

Total lung capacity (TLC)

Maximum volume of air in the lungs after a complete inhalation

Residual volume (RV)

Volume of air remaining in the lungs after a complete exhalation; cannot be exhaled

Vital capacity (VC)

The difference between the minimum and maximum volume of air in the lungs (TLC − RV)

Tidal volume (TV)

Volume of air inhaled or exhaled during a normal, resting breath

Expiratory reserve volume (ERV)

Volume of additional air that can be forcibly exhaled after a normal exhalation

Inspiratory reserve volume (IRV)

Volume of additional air that can be forcibly inhaled after a normal inhalation

Spirometer

Instrument used to measure lung volumes and capacities

Ventilation center

A collection of neurons in the medulla oblongata that regulates breathing rate

Chemoreceptors and CO2

Respond to rising blood CO2 (hypercarbia/hypercapnia) by increasing respiratory rate to blow off CO2

Hypoxemia response

Low blood oxygen concentration triggers the ventilation center to increase breathing rate

Cerebrum and breathing

The cerebrum can consciously control breathing; however, the medulla oblongata overrides it during extended hypo- or hyperventilation

Gas exchange in the lungs

Occurs by simple diffusion across concentration gradients between alveoli and pulmonary capillaries

Pulmonary arteries

Carry deoxygenated, high-CO2 blood from the heart to the lungs

Pulmonary veins

Carry oxygenated, low-CO2 blood from the lungs back to the heart

Thermoregulation by lungs

The large alveolar-capillary surface area assists thermoregulation via vasodilation and vasoconstriction of capillary beds

Vibrissae

Nasal hairs that filter large particles from incoming air

Mucous membranes

Trap particulate matter and pathogens in the airway

Mucociliary escalator

Cilia move mucus (with trapped particles) up and out of the airway to be swallowed or expelled

Lysozyme

Enzyme in the nasal cavity and saliva that attacks peptidoglycan cell walls of gram-positive bacteria

Alveolar macrophages

Engulf and digest pathogens in the alveoli; signal the immune system when an invader is detected

IgA antibodies

Cover mucosal surfaces of the respiratory tract to neutralize pathogens

Mast cells

Carry surface antibodies; when triggered, release inflammatory chemicals; also involved in allergic reactions

Bicarbonate buffer system

The respiratory system regulates blood pH by controlling CO2 levels, which shifts the equilibrium: CO2 + H2O ⇌ H2CO3 ⇌ H⁺ + HCO3⁻

Low blood pH response

Respiration rate increases → more CO2 is exhaled → left shift in buffer equation → H⁺ concentration decreases → pH rises back toward normal

High blood pH response

Respiration rate decreases → CO2 is retained → right shift in buffer equation → H⁺ concentration increases → pH falls back toward norma