Capillaries are microscopic vessels that consist of a single, semipermeable layer of endothelial cells and they are particularly important to homeostasis because they provide the sites needed for substances to be exchanged between the blood and the cells (Adragna et al 1990). Oxygen moves out of the capillary, through the interstitial fluid and into the cell, through diffusion which is a passive process that involves substances moving down a concentration gradient until equilibrium is reached (Adragna et al 1990). Lipid soluble substances, such as oxygen, are able to dissolve into the phospholipid membrane of the cell. Other nutrients, such as sodium and glucose, also move from the capillaries and into the cells through diffusion but there are different routes for the different substances. Small ions and molecules (sodium) must travel through the fenestrae or pores that are found in fenestrated capillaries (Seeley, 1992) whereas large lipid insoluble molecules (glucose) must be moved across the plasma membrane by specific carrier proteins.
Capillaries are ideally suited to their function because the capillary wall is only one endothelium cell thick which creates a very short diffusion pathway and thus ensures diffusion remains efficient. This is supported by Fick’s Law which states that by increasing the surface area and concentration difference, and by minimising the distance between the two areas (the diffusion pathway), the rate of diffusion will be increased (Boyle 2000).
An efficient diffusion pathway is also important for the removal of metabolic waste that builds up in the cells, if the waste builds up the cell can no longer function properly and may become necrotic. Waste products are able to passively diffuse out of the cells and into the capillary since there is a higher concentration of waste substances in the cell than in the capillary (Marieb 2004).
Water molecules rely on the hydrostatic and osmotic pressures in the capillaries to enter and leave the tissue cells (Adragna et al 1990). At the arterial end of the capillary they are filtered out of the capillary and into the interstitium. This is because the hydrostatic force, created by the capillary blood pressure, is higher at the arterial end of the capillary than at the venous end and so fluid is forced out (Adragna et al 1990). At the venous end of the capillary the hydrostatic pressure is much lower, this coupled with the oncotic pressure caused by the high concentration of large proteins in the venous end of the capillary, means that water is drawn back into the capillary by osmosis (Karch, 2006). If the pressure isn’t maintained by the cardiovascular system then the cells either lack the necessary fluid or are overwhelmed by too much fluid are not able to function effectively and as a result biological health can not be maintained.
So far this essay has discussed that for a cell to perform optimally the cardiovascular system needs to provide the cell with access to vital nutrients, remove the waste products from the cell and maintain capillary fluid pressure around the cell. Another task the cardiovascular system is involved in to maintain the internal conditions needed for biological health, is to maintain the core body temperature. It is important the body’s temperature is kept within the normal boundaries which Marieb (2004) recognises are between 35.6’C and 37.8’C. An increase in temperature above the homeostatic range will cause bonds in the tertiary structure of proteins and enzymes to break, thus causing them to be denatured and therefore unable to function properly (Boyle 2000). Heat is produced within the body as a by-product of respiration and the amount we retain or release is controlled by a mechanism called negative feedback. Negative feedback is a mechanism used to ensure that homeostasis is maintained throughout the body, as well as temperature control, blood pH and blood sugar levels are also controlled this way (Waugh, 2001).
The medulla oblongata contains the vasomotor centre which controls the size of the lumen of the small arteries and arterioles (Waugh, 2001). The vasomotor centre will detect any changes in body temperature and affect a response by either decreasing sympathetic nerve stimulation causing vasoconstriction of the small arteries and arterioles which will help to prevent loss of heat, or increasing sympathetic nerve stimulation causing vasodilatation of the arteries and arterioles which will help promote heat loss (Marieb 2004). The two responses combined ensure that the body’s core temperature always remains within the homeostatic range, ensuring the body’s proteins and enzymes remain able to function and the composition of the internal environment is maintained.
In conclusion the cardiovascular system helps to maintain a constant homeostatic temperature, as well as optimum glucose and pH levels. The cardiovascular system also delivers vital nutrients that the cell needs and helps to remove and dispose of the metabolic waste that each and every cell produces. It is clear that the cardiovascular system plays a vital part in maintaining an internal environment in which the cells of the body are able to work optimally and as a result biological health is attained.
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Reference List
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Boyle, M. (2000) AQA (B) Biology: core principles. London: HarperCollins
Karch, A.M. (2006) Focus on Nursing Pharmacology. 3rd Ed. Philadelphia: Lippincott Williams and Wilkins.
Marieb, E.N. (2003) Human Anatomy and Physiology. 6th Ed. San Francisco: Pearson.
Seeley, R. (1992) Anatomy and Physiology. 2nd Ed. St Louis: Mosby Year Book.
Tortora, G.J. (2005) Principles of Human Anatomy. 10th Ed. New York; Chichester: Wiley
Turner, P.P. (1976) the Cardiovascular System. Edinburgh: Churchill Livingstone.
Waugh, A. (2001) Ross and Wilson Anatomy and Physiology in health and illness. 9th Ed. Edinburgh: Churchill Livingstone.
