16.2 Cardiovascular and Respiratory Systems

The cardiovascular and respiratory systems work as a unit to deliver oxygen to your cells and remove carbon dioxide. The heart pumps blood through a closed loop of vessels, while the lungs handle gas exchange with the outside air. Together, they maintain the constant supply of oxygen your tissues need to produce energy through cellular respiration.

Cardiovascular System

more resources to help you study

practice questions

Heart and Blood Vessels

The heart is a muscular pump that drives blood through a closed circulatory system, meaning blood never leaves the vessels. It has four chambers arranged in two pairs:

• Right atrium receives deoxygenated blood returning from the body (via the superior and inferior vena cava)
• Right ventricle pumps that blood to the lungs through the pulmonary arteries
• Left atrium receives oxygenated blood returning from the lungs via the pulmonary veins
• Left ventricle pumps oxygenated blood out to the entire body through the aorta

The left ventricle has the thickest muscular wall because it needs to generate enough pressure to push blood all the way through the systemic circuit.

Three types of blood vessels form the network:

• Arteries carry blood away from the heart. Most arteries carry oxygenated blood, but the pulmonary arteries are an exception: they carry deoxygenated blood to the lungs.
• Veins carry blood toward the heart. Most veins carry deoxygenated blood, but the pulmonary veins carry oxygenated blood back from the lungs.
• Capillaries are microscopic vessels that connect the smallest arteries (arterioles) to the smallest veins (venules). Their walls are only one cell thick, which allows nutrients, oxygen, and waste products to diffuse between the blood and surrounding tissues.

Blood Pressure

Blood pressure is the force that circulating blood exerts against the walls of blood vessels. It's measured with two numbers:

• Systolic pressure (the top number): pressure during a heartbeat, when the ventricles contract
• Diastolic pressure (the bottom number): pressure between beats, when the heart relaxes and refills

Normal adult blood pressure is around 120/80 mmHg. Values consistently above 130/80 mmHg are classified as hypertension (high blood pressure), which can damage vessel walls over time and increase the risk of heart disease and stroke. Hypotension (low blood pressure) can reduce blood flow to the brain and organs, causing dizziness, fainting, or in severe cases, shock.

Respiratory System

Lungs and Alveoli

The lungs are the primary organs of gas exchange. Air enters through the trachea, which splits into two bronchi (one for each lung). These bronchi branch repeatedly into smaller and smaller bronchioles, which finally end in clusters of tiny air sacs called alveoli.

A few structural details matter here:

• The right lung has three lobes; the left lung has only two (to make room for the heart).
• There are roughly 300 million alveoli in your lungs, giving a total surface area of about 70 square meters for gas exchange.
• Alveolar walls are extremely thin (one cell thick) and wrapped in a dense network of capillaries. This short diffusion distance is what makes gas exchange so efficient.
• Alveoli are coated with surfactant, a substance that reduces surface tension in the fluid lining the air sacs. Without surfactant, alveoli would collapse during exhalation and stick shut.

Gas Exchange and Hemoglobin

Gas exchange happens by diffusion, driven by concentration gradients across the alveolar and capillary walls:

1. Inhaled air fills the alveoli with a high concentration of $O_2$ and a low concentration of $CO_2$.
2. Blood arriving at the alveolar capillaries from the body is low in $O_2$ and high in $CO_2$.
3. $O_2$ diffuses from the alveoli into the blood (high to low concentration).
4. $CO_2$ diffuses from the blood into the alveoli (high to low concentration), then gets exhaled.

Hemoglobin is the protein inside red blood cells that makes oxygen transport practical. Each hemoglobin molecule contains four heme groups, so it can bind up to four $O_2$ molecules at once. Oxygen-rich blood appears bright red because of oxyhemoglobin, while oxygen-poor blood appears darker red.

Carbon dioxide travels back to the lungs in three forms:

• Dissolved directly in plasma (about 7–10% of total $CO_2$)
• Bound to hemoglobin as carbaminohemoglobin (about 20–23%)
• Converted to bicarbonate ions ($HCO_3^-$) in the plasma (about 70%), a reaction catalyzed by the enzyme carbonic anhydrase inside red blood cells

The bicarbonate pathway is the most significant. Once blood reaches the lungs, the reaction reverses: bicarbonate is converted back to $CO_2$, which then diffuses into the alveoli and is exhaled.