Describe the process of regulation of bulk fluid flow across capillaries. How would heart failure disrupt this process resulting in oedema?

Introduction
The cardiovascular system comprises of the heart and blood vessels which both work together in keeping the continuous circulation of blood through the body (Marieb & Hoehn, 2014). This ensures a continuous supply of oxygen and nutrients to tissue cells and also prevents waste build up, a critical homeostatic need (Marieb & Hoehn, 2014). As well as gas and nutrient exchange the capillaries facilitate bulk fluid flow across capillary walls, important in regulating the relative fluid volume in the blood stream and interstitial space (tissue) (Marieb & Hoehn, 2014). When this homeostatic balance is disrupted it can lead to excess fluid draining into tissue and causing oedema (Marieb & Hoehn, 2014). This overload of fluid into the tissue can be caused by heightened pressure in the veins and capillaries as a result of heart failure (Hughes, 2013).
Discussion
To achieve homeostatic regulation of fluid between the blood and interstitial space the capillaries form a capillary bed where bulk fluid is forced out of intercellular clefts at the arterial end of the bed and into the interstitial space (Marieb & Hoehn, 2014). The fluid will eventually be returned to the blood via the venous end of the capillary bed (Marieb & Hoehn, 2014). Hydrostatic pressure and osmotic pressure are two processes which influence the rate and volume of fluid that is leaked from the capillary into interstitial space (Hughes, 2013). Hydrostatic pressure, and in this case ‘capillary blood pressure’ is the forceful action of blood against the capillary wall and therefore filtration of fluid from the capillary into the tissue (Marieb & Hoehn, 2014). Once fluid is pushed through the capillary walls, colloid osmotic pressure acts to ‘pull’ fluid back into the capillary. This occurs due to large plasma proteins which are too large to cross the capillary wall drawing water back to them, thus developing capillary colloid osmotic pressure (Marieb & Hoehn, 2014). This push-pull mechanism means that fluid is forced out of circulation at the arterial end of the capillary bed and returns to circulation at the venous end thus maintaining comparative fluid volume in the blood and the tissue (Marieb & Hoehn, 2014). Any excess fluid remaining in the tissue that is not returned to the blood is carried away by the lymphatic vessels which transport it back to the vascular system (Marieb & Hoehn, 2014).

Patients who suffer heart failure often present with pulmonary oedema, peripheral oedema or both (Clark & Cleland, 2013). Oedema occurs due to a disruption of the normal transudation of fluid in the lungs or tissue of the lower extremities (Clark & Cleland, 2013). Clark and Cleland describe where the heart is weakened or not pumping effectively by either ventricle there is a rise in ventricle filling pressure and thus followed by a rise in atrial, vein and capillary pressure. When hydrostatic capillary pressure is increased, filtration of fluid through capillary walls is intensified often to a point where it exceeds the capability of the lymphatic vessels to drain fluid from the tissue or alveoli of the lungs (Clark & Cleland, 2013). The result of accumulated fluid in the tissue leads to oedema (swelling) of the area such as the ankles or shortness of breath due to fluid sitting in the alveoli or ‘air sacs’ of the lungs (Clark & Cleland, 2013). Following is a concept map illustrating the processes involved during bulk flow of fluid in capillary beds.

References
Clark, A., & Cleland, J. (2013). Causes and treatment of oedema in patients with heart failure. Nature Reviews. Cardiology, 10(3), 156-170. doi: 10.1038/nrcardio.2012.191
Hughes, R. (2013). Treatments to achieve fluid balance in heart failure. British Journal of Cardiac Nursing, 8(11), 537-540.
Marieb, E. N., & Hoehn, K. N., (2014). Human Anatomy & Physiology. Boston, Unites States of America: Pearson.