Question

Explain the physiological concept of flow down gradients and illustrate your understanding by presenting three examples from at least two different body systems. In your answer include the terms gradient and resistance.

Need more examples about resistance.

Answer 1

Flow-Down Gradients.

Flow is the movement of stuff (small molecules, ions, blood, air, heat) from one point in a system to another point in a system. Flow occurs because of the existence of an energy gradient between two points in the system.

Examples :

1.Blood :

In order for blood to flow through a vessel or across a heart valve, there must be a force propelling the blood. This force is the difference in blood pressure (i.e., pressure gradient) across the vessel length or across the valve . At any given pressure gradient , the flow rate is determined by the resistance (R) to that flow. The factors determining the resistance are described by the Poiseuille relationship.

The most important factor, quantitatively and functionally, is the radius of the vessel, or in the case of a heart valve, the orifice area of the opened valve. Resistance is inversely related to the fourth power of the radius of a blood vessel. For heart valves, it is not possible to use orifice radius because the opening is not circular. Therefore, the area of the valve orifice is used to compute resistance instead of radius, where area (A) is proportional to the square of the radius , For a heart valve, therefore, the resistance to flow is inversely proportional to area.

2. Air in the lungs :

Concentration gradient in lungs (gas exchange)

Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration.

Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture is the sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture.

The structure of the lung maximizes its surface area to increase gas diffusion. Because of the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large . Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.

3. ion gradient (exchange of ions )

    Neurons spend a huge amount of energy – about 20-25% of all the body’s calories, in humans – pumping potassium into their cells, and sodium out. The result is an extremely high concentration of potassium inside of nerve cells, and a very high concentration of sodium outside.

When cells communicate, they open ion gates that allow sodium and potassium to pass through. The sodium/potassium concentration differences are so strong that the ions “want” to instantly rush out of the cell. Because ions are electrically charged, this actually changes the electrical charge of the cell.

This “electrochemical” signal travels much faster than a merely chemical signal would, allowing us to perceive, think, and respond rapidly.