Haemodynamics is the study of blood flow, including the physical forces that influence the movement of blood through the circulatory system. This system is responsible for transporting oxygen and nutrients to all tissues and removing waste products, a process fundamental to sustaining life. The continuous and efficient circulation of blood ensures that cells receive what they need to function, while also helping to regulate body temperature and transport hormones.
Fundamental Elements of Blood Circulation
A primary driver of circulation is blood pressure, which is the force exerted by blood against the walls of blood vessels. This pressure is not constant; it fluctuates with each heartbeat, reaching a peak during contraction (systolic pressure) and a low point during relaxation (diastolic pressure). This pressure gradient is what propels blood from areas of higher pressure to areas of lower pressure.
Blood flow is the volume of blood that moves past a specific point in the circulatory system over a given period. For instance, muscles require significantly more blood flow during exercise than at rest. The rate of flow is directly related to the pressure gradient and inversely related to the resistance it encounters.
Vascular resistance is the opposition to blood flow caused by friction between the blood and the vessel walls. The diameter of the blood vessels is a significant factor in determining this resistance; smaller vessels, like arterioles and capillaries, offer more resistance than larger arteries and veins.
The heart’s contribution to this system is measured as cardiac output, which is the total volume of blood the heart pumps per minute. It is calculated by multiplying the heart rate (beats per minute) by the stroke volume (the amount of blood pumped with each beat). Another physical property influencing flow is blood viscosity, or its thickness. Blood that is more viscous creates more friction and requires more pressure to move.
How the Body Regulates Blood Flow
The autonomic nervous system plays a large part in this regulation. Its sympathetic division can increase heart rate, the force of heart contractions, and constrict blood vessels, all of which increase blood pressure. Conversely, the parasympathetic division generally has the opposite effect, slowing the heart rate.
Hormonal control is another layer of regulation. Hormones such as adrenaline (epinephrine) and noradrenaline (norepinephrine), released from the adrenal glands, can mimic the effects of the sympathetic nervous system, preparing the body for “fight or flight” by increasing cardiac output and redirecting blood flow. Other hormones, like angiotensin and vasopressin, cause blood vessels to constrict, which helps to raise blood pressure when it falls too low.
Beyond systemic controls, blood flow is also managed at the local level within tissues. Active tissues produce metabolic byproducts, such as carbon dioxide and lactic acid, which act as signals to dilate nearby arterioles. This process, known as autoregulation, increases blood flow to the specific area that needs it most. Nitric oxide is another important locally acting molecule that relaxes blood vessel walls, further enhancing flow.
Assessing Haemodynamic Status
The most common and non-invasive measurement is blood pressure, typically taken with a device called a sphygmomanometer. This provides a snapshot of the systolic and diastolic pressures within the large arteries, offering clues about the overall state of circulation. Monitoring these values over time can reveal important trends.
In hospital settings, particularly in critical care, more advanced techniques are used for a detailed assessment. Arterial lines, which are thin catheters inserted directly into an artery, allow for continuous, beat-to-beat blood pressure monitoring. Central venous catheters, placed in large veins near the heart, can measure venous pressure and provide information about the body’s fluid status.
For a direct look at the heart’s performance, an echocardiogram uses ultrasound waves to create images of the heart, allowing clinicians to see how well its chambers are pumping and to calculate cardiac output. In some cases, a pulmonary artery catheter is used to measure pressures within the heart and pulmonary artery, providing a comprehensive picture of cardiac function and fluid balance.
Haemodynamic Dysfunctions and Health
Shock is a life-threatening condition defined by inadequate blood flow and oxygen delivery to the body’s tissues. This can occur for several reasons, such as severe blood loss (hypovolemic shock), heart failure (cardiogenic shock), or widespread infection causing massive vasodilation (septic shock).
Hypertension, or high blood pressure, is a chronic haemodynamic disorder where the force of blood against the artery walls is consistently too high. Over time, this increased pressure can damage blood vessels and lead to complications like heart attack and stroke.
Heart failure occurs when the heart muscle is weakened and cannot pump enough blood to meet the body’s needs. This reduces cardiac output, leading to a backup of pressure in the veins and lungs, and poor perfusion of vital organs.