HEMODYNAMICS

Haemodynamics

Haemodynamics is the study of the physical laws of blood circulation.
It addresses the properties of both the "content" (i.e., blood) and the "container" (i.e., blood vessels).
Physiological principles are the backbone of clinical medicine, and cardiovascular physiology emphasizes general concepts and regulatory mechanisms.

Blood Flow and Pressure

Blood flow is driven by a constant pressure head across variable resistances.
The vascular system is slightly overfilled with blood, which causes the blood to exert an outward lateral force against the vessel walls, known as blood pressure.
In horizontal rigid tubes, blood flow is governed by the driving pressure, which is the pressure difference (Delta P) between the arterial and venous ends of the circulation.
The relationship among flow, pressure, and resistance is described by the hydraulic equation.
Four factors help generate pressure in the circulation: gravity, compliance of the vessels, viscous resistance, and inertia.

Pressure Types

Driving pressure (Delta P along the axis of the vessel) causes blood to flow.
Transmural pressure (Delta P along the radial axis, across the vessel wall) governs vessel diameter, which is a major determinant of resistance.
The greatest resistance to blood flow, and consequently the greatest pressure drop in the arterial system, occurs at the level of the small arteries and the arterioles.
Pulsatile pressure is progressively damped by the elasticity of the arteriolar walls and the functional resistance of the arterioles, ensuring that capillary blood flow is essentially nonpulsatile.

The cardiovascular system is generally viewed as two pumps arranged in series:

Systemic Circulation (The Left Pump/High-Pressure):
- Function: Propels blood to all other tissues of the body. The left side of the heart is often thought of as a constant pressure generator maintaining a steady mean arterial pressure at the aorta.
- Circuitry: Blood flows from the left ventricle, through the arterial distribution system (arteries), the diffusion/filtration system (microcirculation/capillaries), and the collection system (veins) before returning to the right atrium.
- Pressure Example: Mean pressure in large systemic arteries is approximately 95 mm Hg. The driving force involves a pressure gradient from the arterial side (MAP in aorta approx 95 mm Hg) to the venous side (Right atrium approx 0–4 mm Hg).
Pulmonary Circulation (The Right Pump/Low-Pressure):
- Function: Propels blood through the lungs for the exchange of oxygen (O2) and carbon dioxide (CO2).
- Pressure Example: Mean pressure in the pulmonary artery is about 15 mm Hg, while pressure in the pulmonary veins is about 5 mm Hg. The driving pressure gradient is about 12–15 mm Hg.

Circulatory Arrangement

The systemic and pulmonary vascular systems are composed of many blood vessels arranged in series and in parallel.
The overall resistance across a circulatory bed results from parallel and serial arrangements of branches.
Total Peripheral Resistance (TPR) is the total resistance to blood flow of the systemic blood vessels.

Specialized Circulations (Examples)

Specific organs have specialized circulation, such as the coronary circulation, cerebral blood flow, and renal circulation.

1. Disorders of Blood Circulation (Including Blood Composition and Viscosity)

Blood Viscosity: Blood is a non-Newtonian fluid. Rheologic properties of blood, such as viscosity, are key components of hemodynamics.
- In diseases like polycythemia vera (PV), the marked increase in red cell mass can lead to symptoms (e.g., plethora, headaches) due to increased blood viscosity.
Oxygen Transport: Anemia is a condition where the oxygen carrying capacity of blood is reduced. This reduces the effectiveness of the circulatory system's primary function—transport of O2.
Hemorrhage/Shock: Severe hemorrhage reduces intravascular volume, leading to decreased renal perfusion pressure, which stimulates increased renin secretion. Hypotension in shock also reduces cerebral blood flow.

2. Disorders of Air Circulation (Respiratory Hemodynamics)

Pulmonary Blood Flow (PBF): PBF is directly affected by gravity due to the low pressures and high distensibility of pulmonary vessels.
Alveolar Pressure Effects: Different flow patterns exist in the lung depending on hydrostatic level:
- In zones where alveolar pressure exceeds intravascular pressures, capillaries may not be perfused.
- In other zones (Zone B), the critical pressure gradient for flow is the arterioalveolar pressure difference, rather than the arteriovenous pressure difference typical of most vessels.
Hypoxia: Hypoxia (decreased O2 supply) is related to ventilation changes. Hypoxia causes vasoconstriction in the pulmonary vascular bed.

3. Disorders of Fluid Circulation (Fluid Exchange and Pressure Disorders)

Edema (Fluid Accumulation): Edema is swelling caused by fluid accumulation in the body, which is a disorder of fluid exchange across the microcirculation.
- Edema is caused by alterations in Starling forces (hydrostatic pressure and colloid osmotic pressure) across the capillary wall.
- Fluid movement across the capillary wall is determined by the balance between driving force for outward fluid flow (Pc) and inward fluid flow (Pi).
- Hydrostatic Pressure Example: If the capillary hydrostatic pressure (Pc) increases at the venous end of a capillary, it will increase the outward driving force and lead to altered fluid exchange. Conversely, changes in hepatic venous pressure (and central venous pressure) are transmitted to the hepatic sinusoids and profoundly affect transsinusoidal fluid exchange.
Hypertension and Hypotension: Disorders related to systemic arterial pressure:
- Hypertension (HT): Chronic elevation in blood pressure (generally above 140/85). Essential hypertension is linked to high peripheral resistance, which augments the afterload by decreasing peripheral runoff.
- Hypotension (low blood pressure): Often seen in connection with shock, or as postural/orthostatic hypotension upon standing.

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