Respiratory System

IV. Respiration

A. Sites of Gas Exchange w/ the Environment

1. Integument

- Gases diffuse directly across the body covering (skin); no specialize organs for exchange
- Only good for small and/or thin organisms (flatworms, earthworms) --> need high surface-to-volume ratio

2. Tracheal respiration [Fig. 41.5]

- To reduce water loss, insects, spiders "bring the outside in" by means of a system of hollow tubes = trachea; gas exchange occurs at the fluid-filled tip of the finer branches of the tracheal system

3. Gills [Fig. 41.6]
Functional charateristics

- Highly branched to achieve large surface area
- Epidermal surface is moist, thin
- High degree of vascularization

Internal gills

- found in adult fishes (some insects, fish larvae, amphibians have "external" gills)
- countercurrent flow in gill filaments [Fig 41.6] : blood from body flows one way, water carrying oxygen flows in the opposite direction --> highly efficient gas exchange system (the most efficient heat exhange systems, like large-building air conditioning and heating systems, employ the same principle)

4. Lungs

- internal, respiratory surfaces shaped like sac or cavity
- reptiles, birds, mammals generally have paired lungs
- birds have a specialized system in which the lungs are non-elastic; air sacs attached to lungs act as bellows to keep air continually flowing across respiratory exchange surface in one direction regardless of whether the bird is inhaling or exhaling [Fig. 41.8]

C. Human Respiratory System

1. Air passages, thoracic cavity, sites of gas exchange [Fig. 41.9]

- trachea --> bronchi --> (branching) --> respiratory bronchioles = end branches of "bronchial tree"
- lungs surrounded by pleural membrane (double membrane w/ lubricating fluid in between), situated inside rib cage and above diaphragm
- alveoli (sites of gas exchange) = outpouchings of wall of respiratory bronchioles (alveolar sac looks like a bunch of grapes)
- large total surface area (about the size of a tennis court); each alveolus surrounded by a web of capillaries

2. How we breathe [Fig. 42.12]

- increase size of thoracic (chest) cavity --> negative pressure so that air is "pulled" into the lungs when we inhale (air pressure inside lungs is lower than atmospheric pressure)
- to increase chest cavity size: (a) use "intercostal" (rib) muscles to pull ribs up and out (expanding rib cage; mainly occurs only during deep breathing), (b) contract diaphragm muscle to "flatten" dome (this is the major means of inhaling when at rest)
- to decrease chest cavity size: relax diaphragm and rib muscles
- inhaling involves muscle contraction; exhaling is normally passive (muscles relax)
- total lung capacity = vital capacity + residual volume
- tidal volume = amount of air you breathe in and out (approx. 500 ml per breath; of which about 30% remains in passageways)

3. How our breathing is controlled

- nervous system controls rate, depth of breathing; respiratory center located in medulla sends periodic signals to diaphragm causing it to contract (10 -14 times per minute at rest)
- breathing (respiratory) center monitors blood CO2: when blood carbon dioxide goes up, rate and depth of breathing increase [Why can't you hold your breath indefinitely?]
- when you breath faster and deeper, your lung get rid of more carbon dioxide, and breathing slows (= negative feedback)
- actually, breathing center monitors blood pH:

H2O + CO2 <--> H2CO3 <--> HCO3 + H+

(higher CO2 --> higher H+ (=lower pH); reaction speeded up by carbonic anhydrase in red blood cells)
[Why do we breath faster when we run up the stairs than when we walk up slowly?]

4. Gas exchange and transport [Fig. 41.16]

- diffusion is rapid between alveoli and lung capillaries; gases diffuse down their concentration gradients
- Note: "concentration" refers to the amount of gas that is actually dissolved in the blood = "partial pressure." This does not include gases complexed with Hb or carried in the form of carbonic acid.
- partial pressure of oxygen high in alveoli, low in pulmonary arteries --> oxygen diffuses into blood; partial pressure in body capillaries higher than in tissue fluid --> oxygen diffuses out of body capillaries
- our blood would not be able to transport enough oxygen for cellular respiration if it were carried only in dissolved form; hemoglobin (Hb) is a pigment in red blood cells that increases the carrying capacity of blood.
- Hb can pickup four molecules of oxygen = oxyhemoglobin (HbO2); strength of binding fairly weak, depends on partial pressure of oxygen. Hb releases oxygen rapidly where oxygen pressure is low -- in the tissues
- 70% of carbon dioxide carried in blood as carbonic acid; 23% carried bound to Hb (carbaminohemoglobin)

5. Heartrate and breathing rate

- gas exchange is most efficient when the rate of air flow matches the rate of blood flow (read book section on "Local Chemical Contol")
- heartrate and breathing rate coordinated by nervous system
- exercise --> increase in heartrate to increase blood supply to muscles --> build-up of CO2 --> more rapid (and deeper) breathing

D. Respiration in Special Conditions

1. High altitude

- humans born at high altitudes have more alveoli and lung capillaries as adults
- the rest of us make more red blood cells to adapt to high altitudes
- some animals living at high altitudes have evolved a form of Hb that has a higher affinity for oxygen

2. Fetal circulation

- fetal Hb higher affinity for oxygen than adult Hb [Why?]

3. Deep diving mammals (and turtles)

- muscles have large supplies of myoglobin (an oxygen-binding protein in skeletal muscle)
- more blood per unit weight (Weddell seal has about twice the blood volume per kilogram body weight as humans)
- lungs are small and collapse during the dive --> less nitrogen around to get into blood and cause bends during decompression