as work continues, secondary metabolic effects arise, particularly in the chemical composition of the body fluids. There is an accumulation of metabolic waste products, notably lactic acid, and the kidneys have more waste products to excrete. Muscular activity generates additional heat in the body and, to restore the balance, more heat must be dissipated through the skin by sweating. Some of the heat is also transported in the bloodstream to the lungs where it is released into the air to be exhaled, together with some of the water liberated in the metabolic process. Obviously, the circulatory system is intensively involved in all of this: in the transport of nutrients and oxygen to the working muscles, and in the transport of heat, water and carbon dioxide to the lungs and of heat and water to the skin.
ventilation of the lungs. Within certain limits, the changes sweating, body temperature and especially heart rate show a consumption, or the work per- relationship with energy formed. Since these changes can be measured while a person is at work, they can be used to assess the physical effort involved Table 6.6 shows reactions measured at various workloads.