ZEIT ONLINE: Why do we suffer when it's 35 degrees outside and the sun is burning?

Erika Baum: Our body always produces heat: It arises during all chemical metabolic processes, but also during muscle work. Since we are not cold-blooded creatures that adapt to the outside temperature, but rather need a roughly stable body temperature to survive, we have to cool down when the heat is too great. The hotter and more humid it is, the more difficult it is for the body. The result: we have problems sleeping, feel weak, get headaches or cannot concentrate.

Erika Baum is the President of the German Society for General Medicine and Family Medicine (DEGAM) and professor emeritus at the Philipps University of Marburg. She worked for 36 years as a family doctor in the Hessian Biebertal. © Philipps University of Marburg

ZEIT ONLINE: How exactly does the body cool down?

Tree: The simplest mechanism is to put the vessels wide. The veins protrude on the arms, legs and head. The body sends the blood to the surface. There it is cooled and then goes back to the heart.

ZEIT ONLINE: But that also brings problems.

Baum: Yes, when the vessels expand, the vascular resistance drops and the blood pressure falls. The heart compensates for this effect by beating a little faster. In addition, the other vessels must now be filled. Fluid from the blood is pressed into the tissue in the periphery. To make up for this, we need to drink more. Nevertheless there are distribution problems. Especially when we stand, the blood sinks in our legs. Then sometimes not enough arrives in the head and thus in the brain and we get dizzy. In extreme cases, we can even lose consciousness.

ZEIT ONLINE: And what about sweating?

Tree: Sweating also serves to cool down. When sweat evaporates, it removes heat from the environment. This is called evaporative cooling. Of course, this is only possible if the air is not completely saturated with water. So the drier it is, the more effective the sweating is. That is why many people suffer particularly badly shortly before a thunderstorm. The temperatures are still high, but the humidity is increasing. In weather like this, it is difficult to lower body temperature through evaporation. On the other hand, we perceive a draft or a head wind as pleasant because more sweat can evaporate.

ZEIT ONLINE: Now we not only sweat out water, but also electrolytes, especially salt. Can that be dangerous?

Baum: Yes, especially for older people. If you also take water tablets for age-related high blood pressure, you may develop a sodium deficiency. And in the worst case it leads to confusion. In general, the more people sweat, the more salt they need. Actually, you shouldn't eat too much salt. But if you currently have circulatory problems, you can consciously eat a little more salty. If you are hungry for salt, you can listen to it.