The cardiovascular system circulates blood throughout the body, delivering oxygen and nutrients to tissues and removing carbon dioxide and metabolic waste. It consists of the heart (the pump), the blood vessels (the network), and the blood (the medium). Airway management exists because the cardiovascular system’s capacity to deliver oxygen depends on the respiratory system’s capacity to acquire it — and both fail together when either fails alone.
The heart
The heart is a muscular organ roughly the size of a fist, located in the center of the chest. It has four chambers:
- Right atrium — receives deoxygenated blood returning from the body through the superior and inferior vena cava
- Right ventricle — pumps deoxygenated blood to the lungs through the pulmonary arteries
- Left atrium — receives oxygenated blood returning from the lungs through the pulmonary veins
- Left ventricle — pumps oxygenated blood to the entire body through the aorta. The left ventricle has the thickest muscular wall because it must generate sufficient pressure to push blood through the systemic circulation.
The heart beats approximately 60-100 times per minute at rest, generating a cardiac output of approximately 5 liters per minute — the body’s entire blood volume circulates in about one minute.
Blood vessels
Arteries carry blood away from the heart. They have thick, muscular walls that can constrict (vasoconstriction) or relax (vasodilation) to regulate blood pressure and direct blood flow. Arteries branch into progressively smaller vessels — arterioles, then capillaries.
Capillaries are microscopic vessels with walls only one cell thick. This is where the actual work of the cardiovascular system happens: oxygen and nutrients diffuse from the blood through the capillary wall into the surrounding tissue, and carbon dioxide and waste diffuse from the tissue into the blood. Gas exchange in the lungs occurs across alveolar capillaries. Drug absorption from the gut occurs across intestinal capillaries.
Veins carry blood back to the heart. They have thinner walls than arteries and contain one-way valves that prevent backflow. Venous return to the heart is assisted by skeletal muscle contraction (the “muscle pump”) and by negative intrathoracic pressure during breathing.
Blood
Blood is a fluid tissue composed of:
- Plasma (~55%) — water, dissolved proteins (albumin, clotting factors, immunoglobulins), electrolytes, nutrients, hormones, and dissolved gases. Drugs circulate in plasma, either dissolved freely or bound to plasma proteins (especially albumin). Only the unbound fraction is pharmacologically active — protein binding is a key factor in drug distribution.
- Red blood cells (erythrocytes) (~45%) — contain hemoglobin, the protein that binds and transports oxygen. Each hemoglobin molecule binds four oxygen molecules. Pulse oximetry (SpO2) measures the percentage of hemoglobin that is oxygen-saturated.
- White blood cells (leukocytes) (<1%) — immune cells (neutrophils, lymphocytes, monocytes, eosinophils, basophils) that defend against infection and mediate inflammation.
- Platelets (<1%) — cell fragments involved in blood clotting.
Oxygen delivery
The cardiovascular system’s primary function — and the function that airway management exists to support — is oxygen delivery. The chain of oxygen delivery has several links:
- Ventilation — breathing moves fresh air into the alveoli
- Oxygenation — oxygen diffuses from the alveoli into the pulmonary capillary blood
- Hemoglobin binding — oxygen binds to hemoglobin in red blood cells
- Cardiac output — the heart pumps oxygenated blood to the tissues
- Tissue perfusion — blood flow reaches the capillary beds
- Oxygen unloading — oxygen dissociates from hemoglobin and diffuses into cells
Failure at any point in this chain produces tissue hypoxia — inadequate oxygen delivery. Airway problems affect steps 1-2. Anemia (low hemoglobin) affects step 3. Heart failure affects step 4. Shock (inadequate tissue perfusion) affects step 5.
Blood pressure
Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is generated by cardiac contraction (systolic pressure) and maintained by arterial tone (diastolic pressure). Blood pressure is regulated by:
- Cardiac output — heart rate x stroke volume
- Peripheral vascular resistance — the diameter of arterioles (controlled by smooth muscle tone, influenced by the sympathetic nervous system and hormones)
- Blood volume — maintained by the kidneys through fluid and sodium regulation
Many drugs affect blood pressure through these mechanisms. Beta-blockers reduce heart rate and contractility. ACE inhibitors and ARBs reduce vascular resistance. Diuretics reduce blood volume. Vasopressors (epinephrine, norepinephrine, vasopressin) increase both cardiac output and vascular resistance — they are used in shock and in post-intubation hypotension during airway management.
Cardiovascular system in TCM
In traditional Chinese medicine, the Heart (心) is a Zang-Fu organ system that governs blood circulation but also houses Shen — awareness, coherence, and relational presence. The biomedical heart is a pump. The TCM Heart is both a pump and the seat of reflective consciousness. This dual function is not as foreign to biomedicine as it might seem — the relationship between cardiovascular health and cognitive function, between heart rate variability and emotional regulation, between cardiac events and psychological outcomes is well-documented, even if Western medicine has no single concept that unifies them.