The cardiovascular system, also called the circulatory system, is the body system responsible for transporting gases and nutrients. It consists of the heart, blood vessels, and blood, working together to deliver oxygen and nutrients to every cell and carry away carbon dioxide and other waste products. At rest, the heart pumps 5 to 6 liters of blood per minute through a vast network of vessels that reaches every tissue in the body.
How the Heart Drives the System
The heart is a muscular pump divided into four chambers. The right side receives oxygen-poor blood from the body and sends it to the lungs, where it picks up oxygen and releases carbon dioxide. The left side receives that freshly oxygenated blood from the lungs and pumps it out to the rest of the body. This creates two linked loops: one between the heart and lungs (pulmonary circulation) and one between the heart and every other organ (systemic circulation).
That 5 to 6 liters per minute is enough to circulate your entire blood volume roughly once every minute while you’re sitting still. During exercise, the heart can increase its output several times over to meet the higher demand for oxygen and fuel in working muscles.
Three Types of Blood Vessels
Blood travels through three main types of vessels, each built differently for a specific job. Arteries carry blood away from the heart under high pressure. Their walls have a thick, elastic middle layer that stretches with each heartbeat and then snaps back, helping to keep blood moving forward. Veins return blood to the heart under much lower pressure. Their walls are thinner and contain one-way valves that prevent blood from flowing backward, especially in the legs where it has to travel upward against gravity.
Capillaries are the smallest vessels, so narrow that red blood cells pass through them in single file. Their walls are just one cell thick, which makes them the transfer stations where gases, nutrients, and waste products actually cross between blood and tissue. Every artery eventually branches down into capillaries, and every capillary eventually feeds into a vein. This means the real work of the circulatory system, delivering and collecting substances, happens at the capillary level.
How Oxygen and Carbon Dioxide Move
Gas exchange happens at two key sites. In the lungs, tiny air sacs called alveoli sit right next to capillaries. Blood arriving at the lungs carries oxygen at a pressure of about 40 mmHg, while the air in the alveoli holds oxygen at about 100 mmHg. That 60 mmHg pressure difference drives oxygen across the thin membrane and into the blood. By the time blood leaves the lungs, its oxygen level has equalized at 100 mmHg.
Carbon dioxide moves the opposite direction, but it needs a much smaller push. Venous blood returning from the body carries carbon dioxide at 45 mmHg, while the alveolar air sits at 40 mmHg. That modest 5 mmHg gradient is enough for carbon dioxide to pass out of the blood and into the lungs, where you exhale it.
Red blood cells are the oxygen carriers. Each one is packed with hemoglobin, a protein that binds oxygen in the lungs and releases it where oxygen levels are low, like in active muscle tissue. Red blood cells survive about 120 days before the body breaks them down and replaces them.
How Nutrients Reach Your Cells
Once nutrients are absorbed from food in the small intestine, they enter the bloodstream and travel to tissues throughout the body. At the capillary level, three mechanisms move those nutrients from blood into the surrounding fluid that bathes your cells.
Diffusion is the most common. Small, water-soluble molecules like glucose, amino acids, and ions pass through tiny channel proteins in the capillary wall. Small fat-soluble molecules slip directly through the cell membranes of the capillary walls without needing a channel at all. Larger molecules, like certain proteins, cross by a process called transcytosis, where they’re packaged into tiny bubbles inside the cell, shuttled across, and released on the other side. Finally, bulk flow pushes water and dissolved substances out of the capillary when blood pressure inside the vessel exceeds the pressure in the surrounding tissue.
Two opposing forces control this bulk flow. Blood pressure pushes fluid outward into tissues (filtration), while proteins trapped inside the capillary pull water back in. The balance between these forces determines how much fluid and nutrients leave the bloodstream at any given point along the capillary.
The Liver Processes Nutrients First
Not all absorbed nutrients go directly into general circulation. Blood leaving the intestines, stomach, pancreas, and spleen flows through a special route called the portal vein, which delivers it straight to the liver before it reaches the rest of the body. By the time this blood arrives, it’s loaded with newly absorbed nutrients and potentially harmful substances.
The liver acts as a processing center. It converts nutrients into forms your body can immediately use or store for later. It also filters out toxins and breaks down substances that could be harmful if they entered general circulation at full concentration. Only after the liver has done its work does this blood join the main bloodstream through the hepatic vein.
The Lymphatic System Handles Dietary Fats
There’s one important exception to the rule that nutrients travel through the bloodstream. Dietary fats take a different route. After intestinal cells absorb fats, they repackage them into large particles called chylomicrons, which contain triglycerides, cholesterol, and specialized proteins. These particles are too large to enter the tiny blood capillaries in the intestinal wall.
Instead, chylomicrons are absorbed by lacteals, which are small lymphatic vessels inside each finger-like projection (villus) of the intestinal lining. From there, the fats travel through the lymphatic system and eventually drain into the bloodstream near the neck, bypassing the liver’s first-pass processing entirely. Research published in Circulation Research has shown that chylomicrons actually regulate their own absorption by triggering structural changes in lacteal junctions, essentially opening the door wider when more fat needs to pass through.
Waste Removal Through Blood Flow
The circulatory system doesn’t just deliver. It also collects metabolic waste and carries it to organs that can remove it. Carbon dioxide is one waste product, handled through the lungs as described above. But cells also produce waste like urea (from protein breakdown) that needs to leave the body through a different exit.
Blood carries these waste products to the kidneys via the renal arteries. Inside each kidney, blood flows into roughly a million tiny filtering units called nephrons. Within each nephron, a cluster of miniature blood vessels called a glomerulus filters smaller molecules, waste products, and water out of the blood and into a tubule. The tubule then reclaims useful substances like glucose and returns them to the blood, while the remaining waste and excess water become urine. Filtered blood exits the kidney through the renal vein and returns to circulation.
This filtering process is continuous. Your kidneys process your entire blood volume many times per day, fine-tuning the balance of water, salts, and waste products to keep your internal environment stable. Without the circulatory system delivering blood to the kidneys at a steady rate, these waste products would accumulate to toxic levels.