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Previous chapters discussed the theory of evolution. They looked at abiogenensis and human ancestry. This topic will to discuss the human body and its major organ systems. It will look at the respiratory and circulatory systems of the human body.

What do we need oxygen for?

The respiratory system works with circulatory system. It supplies the body with oxygen and removes metabolic waste products. Oxygen is drawn into our body by breathing. Breathing is an involuntary action of the body which is controlled by the lower part of our brain called the medulla oblongata.

So, what do we need oxygen for?

All our cells have organelles called mitochondria. They are also known as the energy producer of the cell. Mitochondria create energy for our body in the form of adenosine triphosphate (ATP) molecules. These highly reactive ATP molecules are formed in a process of a series of chemical reactions between oxygen and body nutrients, called the Krebbs cycle. Oxygen is delivered to our body's cells by blood haemoglobin. Haemoglobin is a red-coloured protein found in red blood cells. Each haemoglobin molecule has four sites to which O2 atoms can bind. One haemoglobin molecule can carry up to four molecules of oxygen.

Types of respiration

Respiration is the sequence of events that results in the exchange of oxygen and carbon dioxide between the atmosphere and the body's cells. Every three to five seconds, nerve impulses stimulate the breathing process, or ventilation, which moves air through a series of passages into and out of the lungs. After this, there is an exchange of gases between the lungs and the blood. This is called external respiration. The blood transports the gases to and from the tissue cells. The exchange of gases between the blood and tissue cells is called internal respiration. Finally, the cells utilise the oxygen for their specific activities. This is cellular metabolism, or cellular respiration. Together these activities constitute respiration.

Respiratory system organs

Respiration is achieved through our mouth, nose, trachea, lungs and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (the area of the throat containing the vocal cords and used for breathing, swallowing and talking) and the trachea (the airway that leads from the larynx to the lungs). In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to alveoli. Alveoli are sac-like air spaces in the lung where carbon dioxide and oxygen are exchanged. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into the alveoli. Carbon dioxide follows the same path out of the lungs when you exhale. See image 1.

The diaphragm's job is to help pump carbon dioxide out of the lungs and pull oxygen into the lungs. The diaphragm is a sheet of muscles that lies across the bottom of the chest cavity. As the diaphragm contracts and relaxes, breathing takes place. The contractions of the diaphragm make us breathe in, or in other words, pull oxygen into the lungs. When the diaphragm relaxes we breathe out and carbon dioxide is pumped out of the lungs.

When we breathe air we remove oxygen from it and add carbon dioxide and water vapour.
See animation 1.

Circulatory system

The circulatory system contains the heart and the blood vessels and moves blood throughout the body. Our body has about five litres of blood continually travelling through the circulatory system. All our circulatory organs work together, forming a very efficient pipe network for our bodies.

The body's circulatory system has three distinct parts: pulmonary circulation, coronary circulation and systemic circulation.

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. Coronary circulation refers to the movement of blood through the tissues of the heart. Systemic circulation supplies nourishment to all our body tissues. Each of our circulatory systems works independently.

Heart - is the hollow muscular organ that pumps blood through our body. See image 2.

Blood vessels are hollow blood-carrying tubes that form a closed network. There are three types of blood vessels: arteries, veins and capillaries.

Arteries are blood vessels that carry oxygen-rich blood away from the heart.

Veins are blood vessels that carry blood back towards the heart.

Capillaries are thin blood vessels that connect arteries to veins. Different nutrients, oxygen and waste products pass in and out of our blood through the capillary walls.

The Lungs are two sponge-like, cone-shaped structures that fill most of the chest cavity and are protected by the flexible rib cage. Our lungs form one of the largest organs of our body. We need lungs to provide oxygen from inhaled air to the capillaries and to exhale the carbon dioxide delivered from them.

The Circulatory system is also called the cardiovascular system. Besides moving different substances between body cells, it also maintains our body temperature. See image 3.

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Question 1/5

1. The respiratory system supplies our body with _______ and removes the metabolism waste products.


Carbon dioxide




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