The cardiovascular system delivers blood to the body providing both oxygen and nutrients.
It is organised into two circuits, pulmonary and systemic.
The heart is a dual pump that sits at the centre of the cardiovascular system.
It is composed of four chambers:
Right side: blood is received from the body to be delivered to the lungs.
Left side: blood is received from the lungs to be delivered to the body.
The flow of blood is mediated by a network of vessels:
Capillaries are microscopic single-celled blood vessels that form networks around tissue where exchange of oxygen & nutrients occurs.
Capillaries are connections between the arterial and venous systems. In contrast to the plasma membrane of most cells, the capillary wall is relatively leaky. The extent of 'leakiness' varies between tissues; the liver is particularly permeable, whilst in the brain, free movement of solutes is considerably less.
Precapillary sphincters help to control and direct flow.
The exchange of substances between capillaries and extracellular fluids is achieved by two main mechanisms:
The capillary wall permits free passage of water and solutes less than a particular size. As a consequence, small molecules achieve equal concentrations on both sides, and there is no net contribution to the effective osmotic pressure. Since larger plasma proteins cannot cross the capillary wall, they produce an effective osmotic pressure (oncotic pressure).
Hydrostatic pressure refers to the pressure that any fluid in a confined space exerts.
Describes the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes.
Formulated in the 19th-centrury by the British physiologist Ernest Starling (also known for the Frank-Starling law of the heart).
Bulk flow is controlled by a number of factors termed ‘Starling's forces’:
Essentially, net pressure = hydrostatic pressure - oncotic pressure:
At the arteriolar end, this net difference results in ultrafiltration with fluid leaving the capillary. At the venular end, however, the hydrostatic pressure is much less and so the net difference results in reabsorption.
Interestingly, this apparent equilibirum is not total; slightly more fluid is lost into the interstitial space than is reabsorbed. This additional fluid is drained by the lymphatic system.
The vascular system delivers blood by a network of vessels, this process is regulated by the dynamics of fluid flow. Blood flows in concentric streams, known as laminar flow. At higher velocity, laminar flow may become disrupted and turbulent.
Describes flow’s dependence on pressure gradient & resistance.
Describes factors that may affect flow; these include: pressure, radius of the vessel, fluid viscosity & vessel length.
Stroke volume: the amount of blood pumped out of the heart from each contraction.
Cardiac output: the amount of blood pumped out of the heart in one minute, equivalent to HR x SV.
The heart is responsible for maintaining adequate cardiac output through rate and stroke volume. With an average of 70 bpm and 70 ml per beat, cardiac output can be estimated at 5L / min.
Heart rate is predominantly determined by the sympathetic nervous system and parasympathetic nervous system (autonomic nervous system).
Acts on the SAN, AVN and contractile cells to reduce heart rate.
Acts on the SAN, AVN, conduction fibres and contractile cells to increase heart rate.
Stroke volume is determined by the sympathetic nervous system and the preload.
The primary determinants of stroke volume are:
The Frank-Starling law describes the relationship of ventricular stretching (preload) & myocardial contractility. See Heart Failure notes for more detail.
Venous return (and so preload) is influenced by numerous factors:
Blood pressure = cardiac output x peripheral vascular resistance
Dependent upon two major variables: arteriolar radius and blood viscosity.
Numerous interconnected systems contribute to the regulation of blood pressure.
Baroreceptors allow the body to make rapid, short-term changes to blood pressure.
Baroreceptors, a specialised mechanoreceptor, are located in the aortic arch and carotid sinus. These locations give vital information on blood pressure to the body and brain.
An increase in arterial pressures leads to an increase in baroreceptor firing. The cardiovascular control centre in the medulla may then augment the autonomic nervous system.
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