Which of the following would increase cardiac output to the greatest extent?

A.Increased heart rate and increased stroke volume.

B.Decreased heart rate and decreased stroke volume.

C.Decreased heart rate and increased stroke volume.

D.Increased heart rate and decreased stroke volume.

Diagram of human heart
Diagram of a human heart showing the path of blood flow from ventricles to arteries

The correct answer is A. Increased heart rate and increased stroke volume.

The cardiovascular system of humans consists of a muscular pump, the heart, and several blood vessels. The heart is a four-chambered organ which contracts to move blood around the body.

Heart muscle cells are specially adapted to contract rhythmically, and there is a specialized natural pacemaker region of the heart known as the sinoatrial node.

The top chambers are the atria and the lower chambers are the ventricles. These lower chambers are much bigger in size with thick muscular walls. This is because the ventricles have to contract more forcefully than the atria.

Valves are present between the atria and ventricles and between the ventricles and blood vessels leaving the heart. These valves are important in ensuring that blood moves in the correct direction.

The cardiac output is a measure of how forcefully the heart is contracting and how often it is contracting. It is based on both stroke volume and heart rate, which can be influenced by hormones and how active you are.

There are two types of circulation which occur simultaneously in the heart. The systemic circulation pumps oxygenated blood around the body to all the cells and tissues. The pulmonary circulation pumps deoxygenated blood to the lungs so we can breathe out the carbon dioxide.

The heart

The heart is a muscular pump that contracts to force blood into blood vessels that convey the blood all around the body to all the cells and tissues. It is a four-chambered structure which has two types of circulation: systemic and pulmonary.

The four chambers consist of two atria and two ventricles, with the atria being the smaller of the two chambers. Atria are located above the ventricles and are separated from the ventricles by heart wall and valves.

Cardiac muscle cells are specially designed to contract in synchrony to ensure coordinated contraction of the chambers. There are also special contractile cells called the sinoatrial node which is found in the top part of the right atrium. These cells can propagate a signal to start the contraction of heart muscle.

Blood flow through the heart

Blood first flows into atria then into the ventricles. The last bit of blood left in atria is forced into the ventricles by a contraction of the atrial muscle.

There are valves between the atrial and ventricular chambers. These are atrioventricular valves which snap shut after all the blood has entered the ventricles.

The ventricular contraction is much more forceful than the atrial contraction because the blood has to be forced upwards into the aorta or pulmonary arteries and has to be forced through the semilunar valves. These valves occur between the blood vessels and the ventricles.

Once all the blood is forced out of the ventricles these semilunar valves snap shut to stop blood flowing back into the chambers.

The circulations

Blood vessels that carry blood to the heart are known as veins while those that carry blood away from the heart are known as arteries.

The systemic circulation concerns the left side of the heart while pulmonary involves the right side of the heart. In the systemic system, blood is received via veins known as the pulmonary veins which carry oxygenated blood from the lungs.

Blood flows from the left atrium into the left ventricle. When the ventricle contracts it pushes blood into the aorta which forms an arch as it leaves the heart. This vessel branches to form three main arteries that convey oxygenated blood to all the tissues of the body.

The pulmonary circulation is the function of the right chambers of the heart. Deoxygenated blood from the cells and tissues of the body are carried into the heart by large veins known as the superior vena cava and inferior vena cava.

This blood drains into the right atrium and then into the right ventricle. When the right ventricle muscle contracts it is pushed into the pulmonary arteries. These arteries carry this oxygen-poor blood to the lungs so that we can breathe out the carbon dioxide.

Although there are two circulations the reality is that both sides of the heart fill with blood and contract at the same time.

Blood vessels

Arteries are vessels that have very thick muscular walls compared with veins because they have to withstand the pressure of the contracting heart.

Arteries become progressively smaller to form arterioles. These then become capillaries which then lead into venules. Venules are smaller-size veins which enlarge to form the larger veins.

Oxygen and nutrients are offloaded from the blood into cells at the capillary beds, and at the same time, carbon dioxide and wastes move from cells into the blood. Capillaries, therefore, have only one layer of cells so that this diffusion of substances can occur easily.

Cardiac output

The cardiac output is a way to measure how much blood moves out of the ventricular chambers each minute. It is based on liters of blood pumped out of the heart and on the heartbeat.

The stroke volume is the volume that is transported out of the ventricles each minute. The heartbeat is measured as heart rate which is the number of contractions of the heart per minute.

Thus, cardiac output is the heart rate times the stroke volume, which means an increase in both of these would cause an increase in your cardiac output.

Our output can adjust according to the needs of the body for oxygen and nutrients. This means that during exercise we have a greater cardiac output than when at rest since our cells need oxygen more quickly.

Hormones such as noradrenaline and adrenaline that are released during exercise and stress, work to increase the heart rate and the output from the heart.

References

  1. Editors of Encyclopedia Britannica (2018). Cardiac output. Retrieved from Encyclopedia Britannica.
  2. Editors of Encyclopedia Britannica (2018). Systemic circulation. Retrieved from Encyclopedia Britannica.
  3. JG Betts, et al. (2017). Human Anatomy and Physiology. Houston: USA, Rice University.
  4. Editors of Encyclopedia Britannica (2018). Heart. Retrieved from Encyclopedia Britannica.
  5. MF Oliver, BE Mathews, ME Rogers (2018). Circulatory system. Retrieved from Encyclopedia Britannica.

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