Happy St. Patrick’s day too!
        
                    
             
        
        
        
The answer is respiration.
The burning of fossil fuels is known as combustion. It is often compared with cellular respiration. 
The general equation of cellular respiration is:
glucose + oxygen → <span>water + carbon dioxide
</span>
The general equation of combustion is:
fuel + oxygen → <span>water + carbon dioxide
Both reactions involve breaking down of large molecules (glucose and fuel) using oxygen into water and carbon dioxide. In both reactions, chemical energy is transformed into more useful form.</span>
        
             
        
        
        
Answer: The Heart
Explanation:
The blood circulatory system (cardiovascular system) delivers nutrients and oxygen to all cells in the body. It consists of the heart and the blood vessels running through the entire body. The arteries carry blood away from the heart; the veins carry it back to the heart. The system of blood vessels resembles a tree: The “trunk” – the main artery (aorta) – branches into large arteries, which lead to smaller and smaller vessels. The smallest arteries end in a network of tiny vessels known as the capillary network.
There are two types of blood circulatory system in the human body, which are connected: The systemic circulation provides organs, tissues and cells with blood so that they get oxygen and other vital substances. The pulmonary circulation is where the fresh oxygen we breathe in enters the blood. At the same time, carbon dioxide is released from the blood.
Blood circulation starts when the heart relaxes between two heartbeats: The blood flows from both atria (the upper two chambers of the heart) into the ventricles (the lower two chambers), which then expand. The following phase is called the ejection period, which is when both ventricles pump the blood into the large arteries.
In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery (aorta). The blood travels from the main artery to larger and smaller arteries and into the capillary network. There the blood drops off oxygen, nutrients and other important substances and picks up carbon dioxide and waste products. The blood, which is now low in oxygen, is collected in veins and travels to the right atrium and into the right ventricle.
This is where pulmonary circulation begins: The right ventricle pumps low-oxygen blood into the pulmonary artery, which branches off into smaller and smaller arteries and capillaries. The capillaries form a fine network around the pulmonary vesicles (grape-like air sacs at the end of the airways). This is where carbon dioxide is released from the blood into the air inside the pulmonary vesicles, and fresh oxygen enters the bloodstream. When we breathe out, carbon dioxide leaves our body. Oxygen-rich blood travels through the pulmonary veins and the left atrium into the left ventricle. The next heartbeat starts a new cycle of systemic circulation. Below is an attachment of a diagram that explains the connection between pulmonary and systemic circulation from google.
 
        
             
        
        
        
I don’t see any labels, but remember that humans are a great example of a k-selected species. High rates of parental care leads to high survivorship of offspring, and thus negates the need to have large amounts of offspring. 
R-selected species are quite the opposite. They produce large amounts of offspring at many intervals in an attempt to maximize survivorship, as they don’t have strong parental involvement with their young. Think of insects.