Using the heart again as our starting point, we can classify vessels by whether they carry blood away from the heart, the arterial side, or return blood back to the heart, the venous side. Specifically, blood is pumped from the heart, into the arteries (also known as efferent vessels), transported to peripheral tissues via smaller arteries known as arterioles, and through the capillaries where diffusion occurs between blood and interstitial fluids. After the capillaries, blood flows into the venules and finally the veins (also known as afferent vessels) on its way back to the heart. This relationship is true whether the blood is being carried through the pulmonary circuit on its way to and from the gas exchanging surfaces of the lungs or its being carried along the systemic circuit throughout the rest of the body.
As blood courses through the vasculature, the vessel diameters tend to go from large near the heart (the aorta, the largest artery, is typically around 2.5 cm, while the vena cava, the largest vein, can be up to 3 cm) to their smallest point within the capillaries (this relationship can be observed in Figure 1.6(a)). This decrease in individual vessel size is accompanied by an increased number of vessels branching further along the pulmonary and systemic circuits. The effect of this vessel branching causes the total cross-sectional area of the vessels to increase as blood is pumped to and from the capillary beds (Figure 1.6(b)). As previously mentioned, blood will only flow down a pressure gradient and this true throughout the vasculature where the average blood pressure continues to decrease from its peak, just exiting the aorta, to its lowest point, returning through the vena cava (as seen in Figure 1.6(c)). Blood flow, being proportional to the ratio of pressure to cross- sectional area, decreases through the arterial system, reaches a minimum necessary for sufficient exchange and perfusion to occur throughout the capillary beds, then begins to rise throughout the venous system as blood returns to the heart (see Figure 1.6(d)).
Anatomically, arteries differ from veins in a number of significant ways. Arteries typically have thicker, more muscular walls than veins enabling arteries to resist the large pressure change generated by the heart as it forces blood through the circulatory system by passively responding to changes in pressure through elastic strain. Given their contractility, arteries may also actively change their diameter (via the sympathetic autonomic nervous system) by either decreasing their size, a process known as vasoconstriction, or by increasing it, through vasodilation. This active control affects a number of different parameters including peripheral blood pressure, capillary blood flow, and afterload of the heart. Veins, in contrast, have thinner walls, are more elastic, often tend to have larger diameters than their arterial counterparts, and tend not to be able to tolerate as much force. The venous system also contains valves to prevent blood from flowing backward.
Between the two systems lie the capillaries. These very small, very delicate vessels are woven through us, feeding our muscles, surrounding our connective tissues, allowing our skin to breath. Bundled in networks called capillary beds, it is only through capillaries that blood can exchange what it needs (water, solutes, gases, etc.) with interstitial tissues and fluids. To make this possible, the vast majority of the walls of these vessels are so thin (~5 m) and so small (~8m, around the size of a single red blood cell) that diffusion can occur across them. Capillaries of this sort that allow for continuous passive diffusion are known as continuous capillaries and they are located in all tissues except for epithelia and cartilage. In contrast, fenestrated capillaries contain small pores in their endothelial lining that allow for rapid exchange of fluids and small peptides. Fenestrated capillaries are located in a number of areas, including hormone secreting regions (the hypothalamus, the pituitary gland, and the thyroid), absorptive areas (the intestines), and in areas of filtration, most notably the kidneys. Once the blood in these beds has done its work it must come back to the heart. From all of the branches the vessels have broken into, they must recombine. This joining of two (or more) vessels is known as an anastomosis. Anastomoses serve many functions throughout the body, acting as backup routes for blood flow (as is the case in arterial anastomosis) and as connections from the arterioles to venules (as is the case in arteriovenous anastomoses). The role of artificial anastomoses in clinical practice will be discussed at length later in and will lie at the foundation of later work.
Blood does not reside equally across the body. Typically, about one third of total blood volume (~1.5 L) is contained in the heart, arteries, and capillaries of the pulmonary and systemic circuits. The other two thirds (~3.5 L) is retained in the venous system with a large portion of that amount (~1 L) circulating within the extensive venous networks of the liver, bone marrow, and the skin. Blood is retained in this low-pressure to serve as a reservoir for many compensatory mechanisms necessary for homeostasis. Because of their generally thinner walls and lower proportion of smooth muscle, veins are able to distend (expand) far more than arteries, accommodating large changes in blood volume. In this sense, veins have a greater capacitance — the relationship between a vessels volume and the pressure at which it can hold that volume — than arteries. That is, because it takes less pressure to expand veins than it does arteries, they have a great capacitance. Herein, to avoid confusion with the electrical term, I will refer to this quantity—represented by the ratio of the change in volume to the change in pressure ∆V/∆P—as the compliance of the vessel. The compliance of arteries and veins will play a key role in each of the clinical investigations.
The venous system is also able to contract to compensatory for changes throughout the cardiovascular system. This contraction, venoconstriction, reduces the amount of blood in the venous system to attempt to increase the volume within the arterial system and the capillaries. With such a large reservoir (and such a fast ability to respond), the venous system can help maintain arterial blood volume at near-normal levels despite potentially large changes in volume blood volume. In cases of large volume loss, such as in hemorrhage and overly aggressive hemodialysis treatment, the extreme venous response can have the adverse consequence of increasing vascular resistance, making it difficult for blood to return to the heart and, aggravating already delicate situations.
It would be fruitful now to remember that pressures and volumes throughout the cardiovascular system are by their nature dynamic. The nature of the cardiac cycle itself ensures that there are peak pressures, shifting volumes, and constantly changing flows. From a control theory perspective, the body constantly undergoes excitation from these varying parameters and attempts to reconcile their effects into a cohesive homeostasis. How the bodys systems maintain or fail to maintain the feedback loops necessary to achieve these ends is a crucial component of diagnosing pathological states and, in the end, for treating or improving them. The work outlined here is meant to be a tool to help diagnose such states, by estimating volume status, a key parameter of hemodynamic stability. But before we can understand any sort of signal from any sort of system, we must first understand its context, its environment, and the way in which it is collected. Therefore, it will be necessary to first understand the manner in which blood flows through the vessels, current means of measuring certain hemodynamic principles, and finally how each of the signals are modified in normal and abnormal states.s are modified in normal and abnormal states.