Human circulation is a closed system that can be thought of as two separate components connected in series. The arterial system is under high pressure and consists of low capacitance vessels. The venous system is a lower pressure system with high capacitance vessels. One can also classify the circulation as either systemic circulation or pulmonary circulation. Forward flow of blood relies on the heart to actively pump blood from venous to arterial system. Blood travels from the Heart → Arteries → Arterioles → Capillaries → Venules → Veins → Heart.
Without the heart, the vascular system would rely on vascular mechanics to determine where blood pools. Due to the high capacitance of the venous system, the majority of the blood would be in the venous circulation. Mean circulatory filling pressure (MCFP), 7 mm Hg, is the mean pressure that exists within the vascular system if pressure is allowed to redistribute in the absence of cardiac output (CO). It is a measure of vascular fullness and elastic recoil, the energy stored in vessel walls. The difference between the MCFP and central venous pressure (CVP) or right atrial pressure, is an important determinant of venous return to the right heart (preload).
Cardiac output and vascular fullness are coupled in the cardiovascular system. CO is both a determinant of, and dependent on preload and afterload, which is a function of vascular tone.
CO = HR × SV = pressure/resistance
HR = heart rate; SV = stroke volume.
The Frank–Starling law of the heart states that increasing ventricular end-diastolic volume (preload) results in increased force production and higher stroke volumes. It is derived from the myocyte’s ability to increase force production by starting at a longer sarcomere length due to increased affinity of troponin C for Ca2+ and more actin–myosin cross-bridges. Stretching sarcomeres further than optimal decreases contractility, secondary to decreased thick and thin filaments overlap (ie, volume overload heart failure). Less sarcomere stretching (decreased preload) diminishes Ca2+ affinity, thereby weakening contractions.
The human body contains a total blood volume of approximately 7 mL/kg of blood, or 5 L for a 70 kg man, with women having slightly less. Blood contains two major components: plasma and cellular structures. Plasma consists of H2O, proteins (albumin), and electrolytes (Na+, K+, Ca2+, and glucose). Cellular structures include white blood cells, red blood cells, and platelets. Of the total body water, two-third remains intracellular, and one-third is extracellular; of the extracellular water, one-third is intravascular and two-third is interstitial. The net movement of fluid between the intravascular and interstitial compartment is governed by the Starling equation:
Jv = Kf ([Pc − Pi] − σ[πc − πi])