Question

Compare blood flow (perfusion) and air/water flow (ventilation) in fish gills, frog lungs, bird lungs and mammalian lungs. Consider how terms of the Fick Diffusion Equation (write out) are manifest in the design of the exchange surfaces.

Answer 1

Answer)

Fish- Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle (figure a). The atrium collects blood that has returned from the body, while the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation.

The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.

Frog- have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish. The two atria receive blood from the two different circuits (the lungs and the systems). There is some mixing of the blood in the heart’s ventricle, which reduces the efficiency of oxygenation.

The advantage of this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit where gas exchange occurs in the lungs and through the skin. For this reason, frogs are often described as having double circulation.

Birds and Mammals: In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart.

Air enters the body through the nose, is warmed, filtered, and passed through the nasal cavity. Air passes the pharynx (which has the epiglottis that prevents food from entering the trachea). The upper part of the trachea contains the larynx. The vocal cords are two bands of tissue that extend across the opening of the larynx. After passing the larynx, the air moves into the bronchi that carry air in and out of the lungs.

Fick's Law describes the relationship between the rate of diffusion and the three factors that affect diffusion. It states that 'the rate of diffusion is proportional to both the surface area and concentration difference and is inversely proportional to the thickness of the membrane'.

The rate of diffusion will double if:

1. surface area or concentration difference is doubled or
2. the thickness of the exchange membrane is halved.

All of the exchange surfaces consist of cell membranes which are very thin. This explains why diffusion is very fast across membranes.