Animals - Respiration: I. Comparative anatomy

October 24, 2002

1. Respiration is the process by which animals acquire O₂ from their environment, and at the same time dispose of CO₂, the wasteproduct of oxidative metabolism.

• Animals exchange O₂ and CO₂ with the surrounding respiratory medium (i.e. water or air) by diffusion across a respiratory surface. [Remember that O₂ and CO₂ enter and leave cells by diffusing across the lipid bilayer, as determined by their concentration gradients.]
• Some small animals with limited metabolic needs lack a discrete respiratory system (e.g. jellyfish; flatworms). For such animals, their respiratory surface is simply their skin.
• But most animal species have specific respiratory organs that are specialized for gas exchange.
  1. The rate of gas exchange is proportional to the surface area of the respiratory surface. As a result, respiratory organs are highly convoluted structures with large surface areas.
  2. The rate of gas exchange increases as the distance that the gas must travel decreases. To minimize the distance between respiratory medium and the blood, the respiratory surface is typically composed of extremely thin (squamous) epithelial cells underlaid by a dense network of capillaries.
• Many animals use muscular movements to ventilate their respiratory surface, i.e. keep a fresh supply of O₂-rich respiratory medium constantly flowing past the respiratory surface.

2. Animals that evolved to use water as their respiratory medium (e.g. fish) typically have respiratory organs called gills.

• Gills have evolved many times in different animal groups, and the specific anatomy varies widely. But as a general rule, gills consist of fine sheets or filaments of tissue that extend outward from the body into the water [see Campbell, Figs. 42.19 and 20].
• Some fish ventilate their gills by swimming with mouth and gill slits open [e.g. sharks, which die of asphyxiation if immobilized]. But many fish can respire while stationary, and do so by swallowing water through their mouths and forcibly expelling it through the gills.
• Fish maximize gas exchange in their gills by a 'design principle' called countercurrent exchange. Countercurrent exchange requires that two fluids (in this case, the external water and the blood in the gills) flow past each other in opposite directions.
  1. When a fish swims, water moves over its gills from anterior to posterior.
  2. Blood flow in the gill capillary bed is oriented from posterior to anterior. The blood picks up O₂ from the external water (and loses CO₂) as it flows through the gill capillaries.
  3. This countercurrent arrangement insures that the most O₂-depleted blood (entering the gill) is confronted with the most O₂-depleted water (leaving the gill), and that the most O₂-rich blood (leaving the gill) contacts the most O₂-rich water (entering the gill). This arrangement maximizes O₂ absorption [see Campbell, Fig. 42.21].

3. Some of the animals that evolved to use air as their respiratory medium (e.g. mammals, including species that have returned to the ocean such as whales or seals) have respiratory organs called lungs.

• Lungs are internal organs. Internalization of the respiratory surface is important for air-breathing animals, since tissues that exchange O₂ and CO₂ are also susceptible to water loss by evaporation. We do lose some water vapor with every breath [as can be seen by breathing on a mirror], but the air inside our lungs can remain humid even when the atmosphere outside is dry.
• Anatomy of the human respiratory system:

i. Humans can inhale and exhale air through either nostrils or mouth. This is an adaptation of air-breathing vertebrates, whose nostrils and mouth both open into the pharynx.

[In contrast, the nostrils of fish are deadend pits used only for olfaction (smelling).]

ii. During inhalation, air passes from the pharynx into the larynx, and from there down to the lungs through progressively smaller tubes called the trachea, bronchi, and bronchioles. These organs are supported by rings of cartilage so they will not collapse shut when the air pressure inside the tube is decreased.

iii. Within the lungs the bronchioles terminate in clustered sacs called alveoli, which are the lung's respiratory surface. Their arrangement creates an air:lung interface whose surface area is 500X greater than the outer surface of the lung, and they are composed of extremely thin squamous epithelial cells underlaid by a dense network of capillaries.

• Lungs are ventilated by the muscular activity of breathing, i.e. the cyclical inhalation of O₂-rich air into the lungs and exhalation of O₂-depleted air from the lungs.

4. Humans and most other land animals ventilate their lungs by negative pressure breathing, i.e. drawing air into the lungs by means of a 'negative pressure' or suction.

• The human lungs are located in the thoracic cavity, i.e. surrounded by the ribcage and its associated muscles. They lie on top of the diaphragm - a continuous sheet of skeletal muscle that separates the thoracic cavity from the abdomen.
• Inhalation is brought about by the contraction of skeletal muscles, primarily the diaphragm.
  1. The diaphragm is a dome-shaped muscle. When its muscle cells contract, the dome flattens [see Campbell, Fig. 42.24].
  2. Contraction of the diaphragm increases the volume of the thoracic cavity. Remember that liquids and solids can not change volume, i.e. this increase in thoracic volume is experienced as an increase in the volume of air inside the lungs.
  3. The pressure of any quantity of gas is inversely proportional to its volume (= Boyle's Law, in chemistry). Thus, contraction of the diaphragm increases the volume and simultaneously decreases the pressure of the air contained within the lungs.
  4. Gas flows from high pressure to low pressure. Thus, when the diaphragm contracts - and air pressure in the lungs decreases - fresh air flows into the lungs from the surrounding atmosphere.
• Normal exhalation does not require muscular contraction. The diaphragm relaxes, returning to its original position and decreasing the volume of the lungs. This increases the air pressure in the lungs, pushing air out of the lungs into the lower air pressure of the surrounding atmosphere. [Note: when need be, we can augment exhalation by contracting our abdominal muscles. This occurs during coughing.]
• The breathing of mammals is refered to as tidal breathing because air moves into and out of the lungs via the same anatomical pathway.

i. This mode of breathing is not especially efficient. For instance, only 20% of the air in our lungs is exchanged during a normal breath. This fraction can be increased to 80% by forced heavy breathing, but there is always some stagnant air that remains in the lungs.

ii. Another limiting feature of tidal breathing is that it prohibits counter-current exchange of O₂ between the air and the blood. [A counter-current arrangement would require that the air move in one continuous direction, not reverse direction as occurs with each breath.]

5. Birds have evolved a more elaborate respiratory system that relies on positive-pressure breathing and permits counter-current exchange of the respiratory gases.

• Birds have lungs in which they exchange O₂ and CO₂, but they also have air sacs situated at either end of their lungs [see Campbell, Fig. 42.25].
• During inhalation, a bird draws fresh air into its posterior air sac. The posterior air sac then contracts, creating a positive pressure that forces air through the lungs into the anterior air sac.
• This arrangement insures that the flow of fresh O₂-rich air through the bird's lungs is unidirectional (posterior-to-anterior). Deoxygenated blood flows through a bird's lungs in the opposite direction (anterior-to-posterior), resulting in a counter-current exchange of O₂ [fundamentally the same design as a fish's gill, see above].

Learning Goals

1. Learn the basic features of respiratory surfaces - i.e. large surface area; short distance between respiratory medium and blood.

2. How do fish ventilate their gills? What is countercurrent exchange, and how does it maximize the absorption of O₂ by the fish's gills?

3. What is anatomy of the alveoli in a human lung? How are they specialized to function as a respiratory surface?

4. How does the muscular contraction (and subsequent relaxation) of the diaphragm lead to inhalation (and exhalation) in humans? Why is this process called 'negative-pressure' breathing?

5. How does the 'tidal breathing' of humans differ from the 'countercurrent breathing' of birds? Which is more efficient at absorbing O₂ from the inhaled air?