Human Respiratory System Practice Questions

Correct answer: Larynx

The larynx is a cartilaginous structure that connects the pharynx to the trachea and contains the vocal cords. When air passes over these cords, they vibrate to produce sound. The pharynx serves as a shared pathway for food and air, while the trachea leads directly down to the lungs.

Correct answer: Alveoli

Alveoli are tiny, balloon-like air sacs bundled at the ends of the bronchioles. Their extremely thin walls are surrounded by a dense network of capillaries, allowing oxygen and carbon dioxide to diffuse rapidly. Bronchioles are simply the smaller branching tubes that deliver air to these sacs.

Correct answer: Diaphragm

The diaphragm is a dome-shaped sheet of skeletal muscle that flattens out when it contracts, expanding the thoracic cavity vertically. This expansion drops the internal pressure of the lungs, drawing outside air inward. The intercostal muscles assist this process by moving the ribs upward and outward.

Correct answer: Volume increases, pressure decreases

According to Boyle's Law, volume and pressure share an inverse relationship. When the respiratory muscles contract, they expand the volume of the thoracic cavity, causing the intrapulmonary pressure to drop below atmospheric pressure. This pressure gradient forces ambient air to rush into the lungs.

Correct answer: Simple diffusion

Gas exchange across the thin respiratory membrane occurs purely by simple passive diffusion driven by partial pressure gradients. Oxygen naturally moves from the high partial pressure in the alveoli into the lower partial pressure of the blood. Carbon dioxide moves in the exact opposite direction for the same reason.

Correct answer: As bicarbonate ions in the plasma

The vast majority of carbon dioxide enters red blood cells and is converted into bicarbonate ions by the enzyme carbonic anhydrase. These bicarbonate ions then diffuse out into the blood plasma for safe transport. Smaller fractions remain dissolved in plasma or bind directly to hemoglobin molecules.

Correct answer: Carbonic anhydrase

Carbonic anhydrase is a highly efficient enzyme that facilitates the reaction combining water and carbon dioxide into carbonic acid, which then dissociates into bicarbonate and hydrogen ions. This reaction is entirely reversible, allowing carbon dioxide to be reformed and exhaled when blood reaches the pulmonary capillaries. Without this enzyme, the process would move far too slowly to sustain life.

Correct answer: A rise in blood carbon dioxide and a drop in pH

Central chemoreceptors in the medulla oblongata are exquisitely sensitive to shifts in the pH of cerebrospinal fluid caused by changing carbon dioxide levels. When carbon dioxide builds up, it lowers the pH, signaling the brain to accelerate ventilation. Oxygen levels are only monitored as a primary driver under extreme hypoxic emergencies.

Correct answer: Pleura

The pleura consists of a visceral layer covering the lung surface and a parietal layer lining the inner thoracic wall. The narrow space between these layers contains a lubricating pleural fluid that prevents friction as the lungs inflate and deflate. The pericardium protects the heart, while the peritoneum lines the abdominal cavity.

Correct answer: C-shaped rings of hyaline cartilage

The trachea is reinforced by a series of rigid, C-shaped rings made of hyaline cartilage. These rings keep the main airway open at all times while still allowing the flexible posterior side to compress slightly when food passes down the adjacent esophagus. Without this structural support, the airway would pinch shut during deep inhalations.

Correct answer: Surfactant

Pulmonary surfactant is a specialized mixture of phospholipids and proteins that lowers the surface tension of the fluid film lining the alveoli. By decreasing this tension, surfactant keeps the tiny air sacs from collapsing entirely when air leaves the lungs. Premature infants often struggle to breathe if their lungs have not yet started producing enough surfactant.

Correct answer: Tidal volume

Tidal volume represents the volume of air moved into or out of the lungs during normal, relaxed breathing, typically averaging around 500 mL in a healthy adult. Dead space is the portion of each breath that stays in the conducting airways (like the trachea and bronchi) and never reaches the alveoli to take part in gas exchange. Vital capacity encompasses the maximum amount of air a person can expel after filling their lungs to the absolute maximum, while inspiratory reserve volume is the extra air that can still be forced in after a normal inhalation.

Correct answer: Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Alveoli

Air enters through the nose or mouth, moves into the pharynx (throat), and flows down through the larynx (voice box). From the larynx, it enters the trachea, which divides into the left and right primary bronchi before branching into finer pathways ending at the alveoli. This orderly progression ensures air is cleaned, warmed, and humidified before reaching delicate tissues.

Correct answer: Ciliated pseudostratified columnar epithelium

The trachea is lined with ciliated pseudostratified columnar epithelium containing mucus-secreting goblet cells. The sticky mucus captures dust particles and pathogens, while the coordinated, upward beating motion of the cilia sweeps the dirty mucus up toward the pharynx to be swallowed or coughed out — a defense mechanism often called the mucociliary escalator. Nonciliated pseudostratified columnar epithelium lacks this ciliary sweeping action and instead lines structures such as the epididymis. Simple cuboidal epithelium is a single-layer type found in glands and kidney tubules, while stratified squamous epithelium is built for abrasion resistance and lines surfaces like the esophagus and skin.

Correct answer: Medulla oblongata and pons

The basic rhythm and pattern of involuntary breathing are generated and controlled by respiratory groups within the medulla oblongata and the pons of the brainstem. The medulla sets the baseline pace, while centers in the pons smooth out the transitions between inspiration and expiration. The cerebellum and basal ganglia coordinate fine motor movement rather than breathing rhythm, the cerebrum and limbic system handle conscious thought and emotion, and the thalamus and hypothalamus relay sensory signals and regulate other autonomic functions such as temperature — none of which generate the core respiratory rhythm.

Correct answer: It binds to oxygen to form oxyhemoglobin

In the lungs, where the partial pressure of oxygen is high, oxygen molecules bind readily to the iron ions within hemoglobin, creating bright red oxyhemoglobin. When this blood eventually travels to oxygen-depleted peripheral tissues, the bond weakens, allowing the oxygen to dissociate and enter the cells. Each individual hemoglobin molecule can carry up to four oxygen molecules at once.

Correct answer: Emphysema

Emphysema is a progressive lung disease where the delicate walls separating individual alveoli break down, leaving behind large, inelastic air spaces. This significantly reduces the total surface area available for gas exchange and causes air to become trapped during exhalation. Asthma involves acute airway inflammation, while pneumonia is an infection that fills air sacs with fluid.

Correct answer: 21%

Earth's atmosphere is composed of roughly 21% oxygen and 78% nitrogen, with the remaining 1% consisting of argon, carbon dioxide, water vapor, and trace gases. Carbon dioxide accounts for only about 0.04% of ambient air. Despite nitrogen being the most abundant gas, it is biologically inert during human respiration.

Correct answer: Internal intercostal and abdominal muscles contract actively

Quiet exhalation is completely passive, relying entirely on the natural elastic recoil of lung tissue and the relaxation of inspiratory muscles. Forced exhalation, such as blowing out a candle or coughing, requires active muscle engagement. Internal intercostal muscles and abdominal walls contract to forcibly squeeze the thoracic cavity, expelling air rapidly.

Correct answer: Epiglottis

The epiglottis is a flexible cartilage flap positioned at the entrance of the larynx. When you swallow, the larynx elevates and the epiglottis folds downward to seal off the airway, forcing food and liquids down the esophagus instead. The glottis refers to the actual opening between the vocal cords themselves.