6.4 Gas Exchange

Understanding: Ventilation maintains concentration gradients of O2 & CO2 between air in alveoli & blood flowing in adjacent capillaries. oxygen & CO2 between air in alveoli & blood flowing in adjacent capillaries. First, let’s define terms: Respiration is the transport of oxygen to cells where energy production takes place, and involves three key processes: Ventilation:  The exchange of air between the lungs and the atmosphere; it is achieved by the physical act of breathing Gas exchange:  The exchange of oxygen and carbon dioxide in the alveoli and the bloodstream; it occurs passively via diffusion (absorbing one gas from the environment & releasing a different one) Cell Respiration:  The release of ATP from organic molecules; it is greatly enhanced by the presence of oxygen (aerobic respiration) Next, let’s learn how to draw & label the human ventilation system: https://www.youtube.com/watch?v=eGfWTbkYOuA

Sample IB Question Distinguish between ventilation, gas exchange and cell respiration Ventilation:  The exchange of air between the lungs and the atmosphere; it is achieved by the physical act of breathing whereas, Gas exchange:  The exchange of oxygen and carbon dioxide in the alveoli and the bloodstream; it occurs passively via diffusion Cell Respiration:  The release of ATP from organic molecules; it is greatly enhanced by the presence of oxygen (aerobic respiration)

Sample IB Question Draw an label a diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli

Gases only diffuse when there’s a concentration gradient.

Alveoli = tiny air sacs in lungs where gas exchange occurs Q: Why do we need to keep breathing fresh air in? A: To maintain a concentration gradient between the alveoli & capillaries!

Sample IB Question Explain the need for a ventilation system. Because gas exchange is a passive process, a ventilation system is needed to maintain a concentration gradient within the alveoli Oxygen is needed by cells to make ATP via aerobic respiration, while carbon dioxide is a waste product of this process and must be removed Therefore, oxygen must diffuse from the lungs into the blood, while carbon dioxide must diffuse from the blood into the lungs This requires a high concentration of oxygen - and a low concentration of carbon dioxide - in the lungs  A ventilation system maintains this concentration gradient by continually cycling the air in the lungs with the atmosphere

Skill: Monitoring of ventilation in humans at rest and after mild & vigorous exercise. (Practical 6) Dependent variable can be either: Ventilation rate = # of times air drawn in or expelled per minute - count the number of times air is exhaled or inhaled in a minute, while breathing naturally as slowly as possible without getting out of breath - measured using a data-logger & an inflatable chest belt to measure pressure variations in the belt Tidal volume = volume of air drawn in & expelled - one normal breath is exhaled through a tube into a vessel & volume is measured - measured using a data-logger and a spirometer to measure flow rate into and out of the lungs

Skill: …Monitoring of ventilation in humans at rest and after mild & vigorous exercise. (Practical 6) Other factors to consider when designing this experiment: Independent variable = levels of activity ranging from inactive to very active: lying down, sitting and standing, walking, jogging, sprinting (qualitative indep var) running at different speeds on a treadmill (quantitative independent var) Dependent variable should be measured several times at all levels of exercise with each person in the trial. As many different people as possible should be tested. Make sure controlled variables are kept constant. What would those be?

Understanding: Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange. Adaptations of the alveoli that increase the rate of gas exchange: Huge numbers of alveoli (~600 million) in lungs with a large total surface area for diffusion (area of a tennis court) Single layer of cells in both alveolus and capillary, so distance gas diffuses is very small (0.5 μm)

Understanding: Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension. Thin film of moisture allows O2 in alveolus to dissolve and diffuse to blood in capillaries Also provides area from which CO2 can evaporate into air

surfactants Surfactants = compounds that decrease the surface tension between two liquids or between a liquid and a solid Surfactant similar structure to phospholipids in cell membranes

Sample IB Question (on your whiteboard!) Describe the features of alveoli that adapt them to gas exchange Thin wall:  Made of a single layer of flattened Type I pneumocytes so that diffusion distance is small Rich capillary network:  Alveoli are covered by a dense network of capillaries that help to maintain a concentration gradient Increased SA:Vol ratio:  High numbers of spherically-shaped alveoli optimise surface area for gas exchange (600 million alveoli = 80 m2)  Moist:  Type II pneumocytes in the lining secrete surfactant to allow gases to dissolve and to prevent alveoli from collapsing (through cohesion)

Understanding: Air is carried to the lungs in the trachea & bronchi & then to the alveoli in bronchioles. Flow of Air sequencing activity Correct Sequence: Mouth or nose Trachea – rings of cartilage to keep it open 2 bronchi – walls of cartilage many bronchioles – narrower, smooth muscle allowing airway width to vary alveoli – where gas exchange occurs

Understanding: Muscle contractions cause the pressure changes inside the thorax that force air in & out of the lungs to ventilate them. Boyle’s Law https://www.youtube.com/watch?v=eR49g3ubTBg https://www.youtube.com/watch?v=N5xft2fIqQU https://www.youtube.com/watch?v=27yqJ9vJ5kQ inspiration = air being drawn into lungs Expiration = air being forced out from the lungs Caused by changes in pressure due to muscle contractions Lungs do no work!

Understanding: Different muscles are required for inspiration & expiration because muscles only do work when they contract. Muscles can only pull. They do not push. Muscles can either contract (shorten) in which they pull on a bone, or they can relax (lengthen). Muscles contract on their own, but do not lengthen themselves. Another opposite muscle must pull muscles into a lengthened state. Antogonistic muscles = muscles that perform opposite movements E.g. biceps brachii and triceps brachii Rectus abdominis and erector spinae

Inspiration Expiration Application: External & internal intercostal muscles, & diaphragm & abdominal muscles as examples of antagonistic muscle action. Inspiration Expiration Diaphragm contracts and moves down Abdominal muscles relax External intercostal muscles contract, pulling ribcage up & out Volume increases inside thorax Pressure decreases inside thorax Air rushes in to equalize pressure (write the opposite statements)

Nature of Science: Obtain evidence for theories: Epidemiological studies have contributed to our understanding of the causes of lung cancer. Epidemiology = study of incidence & causes of disease (through observation, not experiments) A theory about the cause of disease is proposed. (Smoking causes lung cancer.) Evidence is gathered through surveys. (Survey smoking habits of ppl who have lung cancer & smoking habits of ppl who do NOT have lung cancer.) Correlation does not imply causation! Other confounding factors may LOOK like they are a cause when they are NOT. (There is an association between lung cancer and the lean-ness of a person, however leanness does not cause lung cancer. Smoking just decreases appetite, so smokers on average are leaner.) Surveys include confounding factors in order to rule them out. (Age & sex.)

Application: Causes and consequences of lung cancer. Most common cancer in world Causes: Smoking (87% of cases; increase incidence as number cigs smoked per day & number of year increases) Air pollution (5%) - (diesel exhaust fumes, N oxides from all vehicle exhaust fumes, & smoke from burning coal & wood) Radon gas (3%) (radioactive gas that leaks out of granite, badly ventilated buildings http://www.radon.com/radon/radon_facts.html) Second-hand smoke (3%) Asbestos & silica if inhaled (2%) (“World Trade Center Cough” http://www.asbestos.com/world-trade-center/) Consequences: Difficulty with breathing Persistent cough Coughing up blood Chest pain Loss of appetite Weight loss Fatigue 15% survive more than 5 years after diagnosis Treatments: Chemotherapy, radiation, surgery

Application: Causes and consequences of emphysema. Normally, phagocytes in alveoli engulf bacteria & produce elastase to kill them Normally an enzyme inhibitor (alpha 1-antitrypsin or A1AT) prevents elastase from digesting lung tissue In smokers, # of phagocytes increases, increasing the elastase Genetics affects #’s and effectiveness of A1AT Alveolus walls weakened and destroyed Consequences: Less alveoli, but they are larger Less surface area for gas exchange Gas exchange less effective Less elastic lungs Labored breathing; shortness of breath; more rapid breathing Lack of energy due to decreased O2 & increased CO2 in blood