0610 · 14.3Hormones

Hormones and target organs

0:00You are walking home in the dark, and suddenly a dog barks right behind a fence next to you. Your heart pounds. You are breathing fast. Your eyes are wide open. And here is the strange part: even after you realise it was only a dog, your heart keeps racing for several minutes. Something is still telling your body to stay ready. That something is not a nerve impulse. It is a chemical, travelling in your blood.

0:23By the end of this video you will be able to describe what a hormone is, identify the main hormone glands in the body and the hormones they make, describe what adrenaline does in a fight or flight situation, and compare how nerves and hormones control the body. In the last two videos, we saw that the nervous system controls the body with electrical impulses travelling along neurones. But the body has a second way of sending messages.

0:46It uses chemicals called hormones, and these make up the endocrine system. The syllabus describes a hormone as a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. There are four parts to that definition, so let's take them one at a time. First, a hormone is a chemical substance. It is not an electrical impulse. Second, it is produced by a gland.

1:11The glands that make hormones are called endocrine glands, and they release their hormones straight into the blood. Third, it is carried by the blood. The hormone is dissolved in the plasma, so it travels all around the body in the bloodstream. And fourth, it alters the activity of one or more specific target organs. A target organ is an organ that the hormone acts on. That last part raises a good question.

1:34If the hormone travels all around the body in the blood, why does it only affect its target organs? Because only the cells of the target organs are able to respond to that particular hormone. The hormone reaches almost every organ, but only its targets change what they are doing. It is like a radio broadcast that goes out to the whole city, but only the radios tuned to that station play it.

Major endocrine glands and hormones

1:55The syllabus asks you to identify four endocrine glands in diagrams, and to state the hormone each one secretes. Secretes simply means produces and releases. The adrenal glands are two small glands, one sitting on top of each kidney. They secrete adrenaline. Be careful: the adrenal glands are on the kidneys, but they are not part of the kidneys. The pancreas is a long, thin gland just below the stomach.

2:18It secretes insulin. Insulin decreases the concentration of glucose in the blood, and we will look at exactly how it does that in the next video, on homeostasis. The pancreas also secretes a second hormone, called glucagon. Glucagon does the opposite job to insulin, so the pancreas makes two hormones that work as a pair. The testes are found in males, in a bag of skin outside the body called the scrotum.

2:41They secrete testosterone. And the ovaries are found in females, low down in the abdomen, one on each side of the uterus. They secrete oestrogen. Testosterone and oestrogen are the sex hormones. They control the changes to the body during puberty, which you will study in the topic on reproduction. So the four pairs to learn are: adrenal glands and adrenaline, pancreas and insulin, testes and testosterone, and ovaries and oestrogen.

3:08And for the extended syllabus, add one more fact: glucagon is secreted by the pancreas.

Adrenaline and fight or flight

3:14Now let's look closely at adrenaline, the hormone from the dog story. Adrenaline is the hormone secreted in fight or flight situations. These are moments of danger, fear or stress, when your body has to be ready either to fight or to run away. When the brain detects danger, the adrenal glands release adrenaline into the blood. The syllabus lists three effects of adrenaline that you need to know: increased breathing rate, increased heart rate and increased pupil diameter.

3:40The point of all three is to prepare the body for sudden, intense activity. An increased breathing rate takes more oxygen into the lungs, and removes carbon dioxide faster. An increased heart rate pumps blood around the body faster, so more oxygen reaches the muscles. And an increased pupil diameter, wider pupils, lets more light into the eyes, so you can see the danger more clearly. For the extended syllabus, you need to go one step further, and describe the role of adrenaline in the control of metabolic activity.

4:10Metabolic activity means the chemical reactions going on in the body's cells, and the most important one here is respiration. Respiration is the reaction that releases energy from glucose, and it needs oxygen. The syllabus names two effects. First, adrenaline increases the blood glucose concentration. It causes the liver to release more glucose into the blood. Second, adrenaline increases the heart rate, as we saw. Now put those two together.

4:36There is more glucose in the blood, and the blood is being pumped faster. So more glucose and more oxygen are delivered to the muscles every second. That means the muscle cells can respire faster and release more energy, which is exactly what they need to fight or to run. That is how adrenaline raises metabolic activity.

Nervous versus hormonal control

4:53We now have two ways the body sends messages: nerves and hormones. The syllabus asks you to compare nervous and hormonal control, limited to two things: speed of action and duration of effect. Speed of action means how quickly the response happens. Nervous control is very fast. Impulses travel along neurones in a fraction of a second, which is why your hand leaves the hot pan almost instantly. Hormonal control is slower.

5:18The hormone has to be released into the blood and carried around the body to its target organs, and that takes seconds to minutes, or even longer. Duration of effect means how long the response lasts. Nervous control usually has a short-lived effect. When the impulses stop, the response stops. Hormonal control usually has a longer-lasting effect. The hormone stays in the blood for a while, so its target organs keep responding until it is broken down.

5:44That is why your heart kept racing after the dog stopped barking. The adrenaline was still in your blood. So the comparison is simple: nerves are fast and short-lived, hormones are slower and longer-lasting. Some hormones, like the sex hormones, act over months and even years. Let's see how this can be tested. A typical question shows an outline of the body with a gland labelled, and asks you to name the gland and the hormone it makes.

6:09Suppose the gland is sitting on top of a kidney. It is an adrenal gland, and it secretes adrenaline. Another question might say: a student is about to run a race and feels nervous. Explain how adrenaline helps them run. A strong answer names the gland, the route and the effects. Adrenaline is secreted by the adrenal glands into the blood. It increases the heart rate and the breathing rate, and it increases the blood glucose concentration.

6:34So more oxygen and more glucose are delivered to the muscles, for faster respiration, which releases more energy for running. Now some common mistakes. First, saying hormones travel along nerves. Hormones are carried by the blood. Only impulses travel along neurones. Second, saying the kidneys make adrenaline. The adrenal glands sit on top of the kidneys, but they are separate glands. Third, mixing up glucagon and glycogen. The words look alike, but glucagon is a hormone made by the pancreas, and glycogen is a store of carbohydrate in the liver and muscles.

7:09We will see how they are linked in the next video. Fourth, saying a hormone affects every organ it reaches. It travels everywhere in the blood, but it only alters the activity of its specific target organs. And fifth, in a comparison question, only describing one side. If you say nervous control is fast, also say that hormonal control is slower. Examiners want the comparison made in both directions. Let's recap.

7:35A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. The adrenal glands secrete adrenaline, the pancreas secretes insulin, the testes secrete testosterone, and the ovaries secrete oestrogen. The pancreas also secretes glucagon. Adrenaline is the fight or flight hormone. It increases the breathing rate, the heart rate and the pupil diameter. It also increases the blood glucose concentration, so that, with the faster heart rate, more glucose and oxygen reach the muscles for respiration.

8:09And compared with nervous control, hormonal control is slower to act, but its effects last longer. So the next time a sudden shock leaves your heart racing long after the danger has gone, you will know that it is the adrenaline still in your blood, doing exactly the job it was made for.