Diffusion in Animals

High School ・Biology ・MLA ・4 Sources

Fick’s Law of diffusion mainly states that the rate at which a substance disperses through a membrane is affected by distance, partial pressure (concentration) difference and surface area. Fick’s Law of diffusion is represented by the formula: Where R= rate of diffusion, D= constant of diffusion, A= Membrane’s surface area of diffusion, ∆p= difference in concentration (partial pressures), and d= distance of diffusion (Ade et al. 1). An increase in the surface area and concentration gradient and a decrease in the thickness of the membrane will greatly increase the diffusion rate in animals. In other words, to increase R, which is the rate of diffusion, you need to increase A, which is the membrane’s surface of diffusion. For example, fishes’ gills offer a large surface area for respiration and a countercurrent exchange system. The rate of diffusion increases with increase in concentration gradient. Gases diffusing between membrane sides have different concentrations (Ade et al. 2). The outside concentration of oxygen is higher than in the membranes and vice versa for carbon dioxide, this creates concentration difference. In human beings, the external and internal parts of the alveoli have different oxygen and carbon dioxide concentration, which increases concentration gradient and thus maximizing rate of diffusion. Additionally, the walls of mammalian alveoli, especially in humans is very thin, therefore the distance of diffusion is reduced thus facilitating maximum diffusion.

When one breaths, air containing 79% nitrogen and 21%oxygen enter into the nasal cavity through the nostrils and travel through the pharynx, larynx, trachea, bronchi, bronchioles, lungs, and alveoli. The exchange of gases takes place in the alveoli. The oxygen contained in the air passes into the epithelial cells of alveoli through the membrane. Oxygen molecules will then diffuse into blood capillaries containing red-blood cells. The red-blood cells contain hemoglobin, which binds oxygen molecules and transports them to the mitochondria.

The unloading and loading of oxygen in tissues and lungs is respectively affected by numerous factors such as temperature and pH. The unloading and loading processes are influenced by oxygen hemoglobin affinity changes. When pH is decreased, the affinity of oxygen to hemoglobin decreases while affinity rises when the pH is increased. The influence of pH on affinity is referred to as Bohr Effect (Altman and Dorothy 152). The unloading of oxygen in tissues increases when pH increases and decreases while the pH decreases. On the other hand, the loading of oxygen in lungs decreases with decrease of pH in blood. In case the blood temperature is high then hemoglobin affinity to oxygen decreases, therefore the loading of oxygen in lungs decreases and the unloading in tissues increases. When the temperature is low the affinity increases, thus leading to increase of loading of oxygen in lungs and unloading decrease in tissues (Altman and Dorothy 127).

(A) Reabsorption of water in the kidney could be achieved through a countercurrent system in the loop of Henle. The countercurrent flow of waste and blood in capillaries causes a difference in osmolarity (Olsen and Matthew 3). The water inside the renal tubule is highly concentrated than it is in the blood stream. Due to the difference in concentration gradient water and other useful ions are reabsorbed back into the blood stream through the process of osmosis.

(B) When water is reabsorbed back into the blood stream, urine concentration increases. Due to maximum reabsorption of water and other ions the amount of water and ions in the wastes become more concentrated leading to production of hypertonic urine (Olsen and Matthew 4).


(A) Parathyroid hormone regulates different effects in different organs.





Increases level of calcium by stimulating the breakdown of bones


Stimulates the reabsorption of calcium in kidneys

Digestive tract

Vitamin D activation

(C) One hormone can regulate multiple effects in the body because hormones are divided into groups based on their chemical structure. The chemical groups of hormones affect its distribution, receptor types to bind on to, and other additional functions. Due to this fact, a single hormone can cause multiple effects on body organs or tissues (Strewler 26).


Hydrophilic hormones cannot act as ECDs because they are soluble in aqueous medium and since the ECDs are always non-polar the hydrophilic hormones will be unfit as ECDs. Hydrophilic hormones are soluble in aqueous solution medium. They are unable to cross the cell membrane; therefore, they are bound to receptor molecules on the target cell’s external surface and alters cells’ functions. Examples include glucagon and insulin. On the other hand, hydrophobic hormones are insoluble in aqueous medium but are soluble in lipid medium. They can cross the membrane easily, thus they bind receptors in target cells intracellularly. For example, steroid hormones and thyroid hormones.

Works Cited

Ade, Carl J. et al. "Decreases in Maximal Oxygen Uptake Following Long-duration Spaceflight: Role of Convective and Diffusive O2 Transport Mechanisms". Journal of Applied Physiology, 2017: jap-00280.

Altman, Philip L. and Dorothy S. Dittmer. Respiration and circulation. "Federation of American Societies for Experimental Biology Bethesda MD", 1971, pp. 1-300.

Olsen, Anders, and Matthew S. Gill. "Introduction." Ageing: Lessons from C. elegans. Springer International Publishing, 2017, pp. 1-7.

Strewler, Gordon J. "The physiology of parathyroid hormone–related protein." New England Buckle, John W. Animal hormones. E. Arnold, 1983, pp. 29-36.

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