H1 Gas exchangeAS Level and A Level Biology Chapter 11 Pages 142-149

Learning outcomesCandidates should be able to: (a) [PA] describe the structure of the human gas exchange system, including the microscopic structure of the walls of the trachea, bronchioles and alveoli with their associated blood vessels; (b) [PA] describe the distribution of cartilage, ciliated epithelium, goblet cells and smooth muscle in the trachea, bronchi and bronchioles; (c) describe the functions of cartilage, cilia, goblet cells, smooth muscle and elastic fibres in the gas exchange system; (d) describe the process of gas exchange between air in the alveoli and the blood;

Introduction How are our gaseous exchange and cardiovascular systems linked?

1. The gaseous exchange system Adaptations of the system: cleans and warms the air we breathe, maximises surface area for exchange of oxygen and carbon dioxide, minimises distance for diffusion to occur, maintains adequate diffusion gradient.

Lungs Key words: trachea, bronchi, bronchioles, cartilage, mucus, goblet cells, cilia, alveoli, pleural membrane, thoracic cavity.

2. Breathing rate and heart rate The rate of supply of oxygen to the cells is determined by the rate and depth of breathing and by the pulse rate.

Breathing rate and depth At rest we need to ventilate our lungs with 6dm3 of air per minute. About 0.35dm3 of new air enters the alveoli with each breath. It is impossible to empty the lungs completely, i.e. even during forced exhalation, about 1dm3 of air still remains in the alveoli and the airways (residual volume). The effect of exercise on breathing is measured by calculating the ventilation rate the total volume of air moved into the lungs in one minute, expressed as dm3 min-1.

Ventilation rate = tidal volume x breathing rate A well-trained athlete can achieve adequate ventilation by increasing the tidal volume with only a small increase in the rate of breathing when taking moderate exercise. This is possible because training improves the efficiency of the muscles involved in breathing,

Measuring lung volumes Ventilation brings about changes in lung volume, and these changes can be measured by a spirometer.

The movements of the chamber are recorded on a kymograph trace. Two measurements can be obtained from the traces: tidal volume is the volume of air breathed in and then breathed out during a single breath. The tidal volume at rest is about 0.5dm3, vital capacity is the maximum volume of air that can be breathed in and then breathed out of the lungs by movement of the diaphragm and ribs. In young men the average is about 4.6dm3 and young women 3.1dm3.

Pulse rate The volume of blood pumped out from each ventricle during each contraction is the stroke volume. The total volume pumped out per minute is the cardiac output. The stretch and subsequent recoil of the aorta and arteries travels as a wave along all arteries and is called the pulse.

The pulse rate is identical to the heart rate and the resting pulse rate is an indication of fitness. At rest, the cardiac output is about 5dm3 of blood every minute. If the resting pulse rate is low and the stroke volume is high, during moderate exercise only a small increase in pulse rate is required for adequate blood supply.

The normal range of resting pulse rates is 60 to 100 beats per minute. The average is higher during and after exercise, and also after eating or smoking. It is at its lowest when people are asleep.

Blood pressure Blood pressure is a measure of how hard the heart is working to pump blood around the body. It is measured as systolic pressure/diastolic pressure in mm Hg Systolic pressure is the maximum arterial pressure during ventricular systole and this is the pressure at which blood leaves the heart through the aorta. Diastolic pressure is the minimum pressure in the arteries. Its value reflects the resistance of the small arteries and capillaries to blood flow. Blood pressures are determined using a sphygmomanometer. Typical blood pressures are: systolic 120mm Hg, diastolic 80mm Hg.

Systolic pressure may rise during exercise but diastolic pressure rarely changes much in healthy people.

If systolic and diastolic pressures are high at rest this indicates that the heart is working too hard at pumping blood and is known as hypertension. The risks of cardiovascular diseases such as stroke and coronary heart disease increase considerable with blood pressures in excess of 140/90. In 90% of cases the cause of hypertension is unknown. However, it is closely linked to: excessive alcohol intake, smoking, obesity, too much salt in diet, genetic factors.