Of the three buffering mechanisms in the body, which is the strongest?

A. Renal system.

B. Chemical buffer system.

C. Respiratory system.

Diagram of a kidney
Diagram of a kidney

The correct answer is A. Renal system.

The acid-base balance of the blood and cells has to be kept within certain limits. This is because enzymes that are present can only function within a certain limited range of pH values. In fact, enzymes are proteins which means they can be denatured when conditions are not right.

The renal system and respiratory system both help to control the pH values of the blood by acting as buffers. Of these two organ systems, the renal system is the most important and has the greatest effect on blood pH.

The respiratory system includes the lungs and airways which allow us to breathe in oxygen and breathe out carbon dioxide. Carbon dioxide in blood combines with water to form carbonic acid which thus drops the pH of the blood.

Chemical receptors in the blood vessels and brain can detect high levels of carbon dioxide. Signals are then sent from the brain to increase the breathing rate to expel more of this gas. The opposite happens when there is too little carbon dioxide in the blood.

The nephron of the kidney regulates pH by using hydrogen ions to form bicarbonate in the cells lining the tubules. These bicarbonate ions can then enter the bloodstream to help buffer acidic levels in the blood.

Acid-base balance

It is very important that the acid-base balance of the body is carefully regulated. In fact, the pH of blood needs to be maintained between a narrow range of values of pH 7.35 and 7.45.

The cells of our body depend on the activity of several enzymes which catalyze various reactions. Enzymes are proteins which work best at specific temperatures and pH values and are badly affected at pH and temperatures that are far from optimal.

In fact, the molecules are denatured, which means they are unfolded and the shape is destroyed. Enzymes work by binding with a substrate that has a specific shape, so clearly if the shape of the enzyme is destroyed then they can no longer function.

Various systems in the body help to regulate the acid-base balance by acting as buffer systems to stop the pH of the blood and other fluids in the body, from becoming too basic or too acidic. A buffering system is a way to stop extreme alterations in pH values.

This is often accomplished by a buffer taking up hydrogen ions or hydroxyl ions, depending on what the pH of the fluid happens to be.

Buffering mechanisms

There are a variety of mechanisms that the human body uses to buffer fluids. Some proteins such as hemoglobin in the blood can help to buffer pH changes, and other molecules like phosphates can also play a role by binding with hydrogen ions.

The respiratory and renal systems are two important buffering systems of the body. Of these two organ systems, the renal system is the strongest. However, the importance of the respiratory system should not be underestimated.

It is very important that homeostasis is maintained in the human body through the action of these different organ systems in order to ensure pH is carefully regulated. Various feedback mechanisms involving the brain, hormones, and organs work together to ensure that homeostasis is maintained.

Respiratory system buffering

The respiratory system is how we breathe and includes our respiratory passages and our lungs.  This is the system in which oxygen is brought into the body and carbon dioxide is expelled. To accomplish this blood has to diffuse between blood capillaries and the air sacs (alveoli) of the lungs.

Carbon dioxide in the blood readily combines with water to produce carbonic acid. The presence of an acidic substance could lower the pH value of the blood plasma, which can occur if too much carbonic acid is present.

This means that excess levels of carbon dioxide in the blood would be expected to result in a drop in blood pH. When blood becomes too acidic the body is said to be entering acidosis, while when it becomes too alkaline it is said to be in a state of alkalosis.

Feedback control mechanism

When there is too much carbon dioxide and pH is too low, special receptors in certain arteries and the brain are triggered. Receptors are found in the carotid arteries and the aorta, which detect changes in carbon dioxide levels in the blood.

The result is that the brain then sends signals causing the breathing rate to increase. The result is carbon dioxide is expelled more quickly from the body and thus the blood pH increases since less carbonic acid molecules are formed.

This effect is also apparent when you exercise strenuously and are in danger of accumulating too much carbon dioxide.

Hyperventilation, where breathing is too fast can cause too much carbon dioxide to be expelled causing the pH to rise too quickly. Now the receptor proteins send messages to the brain which then signals a decrease in the respiratory rate.

The consequence of this is that blood pH now decreases to a more normal level. Certain diseases or medications can significantly impact the system and make individuals prone to having problems of alkalosis or acidosis.

Renal system buffering

The kidneys have many important functions in the body, including reabsorbing substances, regulating blood pressure and excreting metabolic wastes. A very important role of the renal system which few people realize is that of a chemical buffering system.

The kidneys are able to buffer the blood by regulating the levels of bicarbonate ions that are present. The basic microscopic unit of the kidney is the nephron, which contains a Bowman’s capsule and tubules. Filtrate passes through the capsule first and then enters the tubules.

It is in the tubules that bicarbonate ions can be reclaimed but the process is quite complex. Sodium ions are taken up out of the proximal tubule lumen and at the same time, hydrogen ions are secreted into the tubule.

This is done via a cotransport system involving a membrane protein and is an active transport process. The hydrogen ions that are secreted into the lumen of the tubule then react with bicarbonate ions that are already present in the filtrate.

The raw materials to make bicarbonate ions move into the cells lining the tubule, and then these cells are able to make these ions which can be transported into blood capillaries. A certain amount of bicarbonates is also taken up in the collecting duct.

Certain medical conditions such as diabetes mellitus can impair the kidney’s ability to buffer the pH of blood, leading to problems such as acidosis.


  1. OpenStax College (2013). Anatomy and physiology. USA: Houston, OpenStax College Rice University.
  2.  RL Dorit, WF Walker, RD Barnes (1991).  Philadelphia: USA, Saunders College Publishing.
  3. Editors of Encyclopedia Britannica (2018). Kidney. Retrieved from Encyclopedia Britannica.
  4. Editors of Encyclopedia Britannica (2018). Nephron. Retrieved from Encyclopedia Britannica.
  5. JS Robson, GAG Mitchell, D LeVay (2018). Renal system. Retrieved from Encyclopedia Britannica.


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