Hold your breath and within a minute or so you feel an urgent need to breathe. Nearly every cell in your body is constantly using oxygen to release energy from glucose (cellular respiration) and producing carbon dioxide as waste. Cells deep inside you are far from the air, so two organ systems team up. The respiratory system brings oxygen into the lungs and removes carbon dioxide; the circulatory system carries oxygen from the lungs to every cell and brings carbon dioxide back. Neither system could keep you alive without the other.
The heart: a double pump
The heart is made mostly of cardiac muscle and has four chambers. The two upper chambers (atria) receive blood; the two lower chambers (ventricles) pump it out. The right side pumps oxygen-poor blood to the lungs, and the left side pumps oxygen-rich blood to the rest of the body, so blood passes through the heart twice on each complete trip: this is called double circulation. The left ventricle has the thickest wall because it must push blood all the way around the body. Valves between the chambers and at the exits act like one-way doors, so blood cannot flow backward; the closing of the valves makes the lub-dub sound of a heartbeat.
| Blood vessel | Structure | Job |
|---|---|---|
| Arteries | Thick, muscular, elastic walls | Carry blood away from the heart under high pressure; you feel the pressure surges as your pulse |
| Capillaries | Tiny vessels with walls only one cell thick | Connect arteries to veins; materials diffuse between the blood and body cells here |
| Veins | Thinner walls, wider space inside; many (especially in the limbs) have valves | Carry blood back to the heart at low pressure; valves stop backflow |
| Part of blood | Description | Job |
|---|---|---|
| Plasma | Yellowish liquid, about 55% of blood, mostly water | Carries dissolved nutrients, wastes (including much of the carbon dioxide), hormones and blood cells |
| Red blood cells | Biconcave discs with no nucleus, packed with iron-containing hemoglobin; about 4 to 6 million in each cubic millimetre of blood | Carry oxygen from the lungs to the cells; each lasts about 120 days |
| White blood cells | Larger cells with a nucleus; several kinds | Defend the body by destroying microbes and making antibodies |
| Platelets | Small cell fragments | Help blood clot to seal wounds |
The respiratory system
Air enters through the nose (where it is warmed, moistened and filtered by hairs and mucus) and passes through the pharynx (throat), the larynx (voice box) and the trachea (windpipe), which is held open by rings of cartilage. The trachea splits into two bronchi, one for each lung, which branch again and again into narrow bronchioles. These end in clusters of hundreds of millions of tiny air sacs called alveoli. To inhale, the dome-shaped diaphragm muscle under the lungs contracts and flattens while rib muscles lift the ribs; the chest gets bigger, air pressure inside the lungs drops, and air flows in. When these muscles relax, the chest gets smaller and air is pushed out.
Gas exchange: diffusion at the alveoli
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Blood arriving at the lungs from the body is low in oxygen and high in carbon dioxide. Air in the alveoli is the opposite. So oxygen diffuses from the alveoli into the blood in the surrounding capillaries, where it binds to hemoglobin in the red blood cells, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. Alveoli are built for fast diffusion: there are enormous numbers of them (a huge total surface area), their walls and the capillary walls are each only one cell thick, they are moist so gases can dissolve, and a dense net of capillaries keeps carrying oxygen away, which keeps the concentration difference large.
| Disorder | What goes wrong |
|---|---|
| Atherosclerosis (a cause of heart disease) | Fatty deposits build up inside arteries, narrowing them; a blocked coronary artery starves heart muscle of oxygen and causes a heart attack |
| High blood pressure | Blood pushes too hard on artery walls, straining the heart and damaging vessels over time |
| Anemia | Too few red blood cells or too little hemoglobin (often from lack of iron), so less oxygen is carried and a person feels tired |
| Asthma | Bronchioles become inflamed and narrow, making breathing difficult; inhalers relax the muscles around them |
| Emphysema (a form of COPD) | Alveoli walls break down, mostly from smoking, so surface area for gas exchange falls |