We know the heart is a vital organ. If it stops working, the body’s cells quickly die due to a lack of nutrients. Blood flows through lungs to pick up oxygen and the digestive tract to pick up food molecules. Once the nutrient-rich blood reaches the heart, arteries carry the blood out from the heart to the tissues, then veins bring it back to the heart. This cycle continues our whole life without any conscious effort from us.

Why this is important…The ability to move blood around the body quickly is what makes our life possible. Controlling blood flow is key to regulating our different activities throughout each day.

Foundations

The heart isn’t just any pump, it is THE PUMP – the one that sustains life. The familiar “lubb-dupp” sound of a heart beating is an unmistakable sign of life. Not until the 1960’s, with the development of cardio-pulmonary resuscitation (CPR), did people have a chance to keep living after their heart stopped beating.

The forceful circulation of blood through the body makes most animal life possible. It speeds up the distribution of materials to make activities like walking, thinking and singing possible. Otherwise, we’d live at a much slower pace like plants or fungi, who move materials slowly from cell to cell.

Details

Blood flow through the circulatory system follows a series of pressure gradients. The forceful contraction of the heart creates very high pressure in the heart chamber. Blood moves out into the arteries for two reasons: (1) valves prevent blood from flowing backward in the heart and (2) the blood in the arteries experience lower pressure. The pulses of pressure send blood flowing through arteries and you can feel the pulse by pressing against an artery with your fingertips. As blood moves to smaller arteries, then to capillaries, then to veins, the pressure gets lower and lower. The blood follows these flow-down gradients of pressure until it returns to the heart.

Stop & Think #1: Which would lose blood faster if it were cut – an artery or a vein? Explain this using the principles of flow-down gradients.

Try this experiment while you read the next couple paragraphs: hold one hand below your waist and the other hand above your head. Keep them there while you read.

Blood flow is affected by resistance in the blood vessels. Blood flow decreases when there is more resistance and increases when there is less. There are several ways resistance can be altered, such as:

Diameter of arteries - dilation of arteries (vasodilation) decreases resistance, while constriction of arteries (vasoconstriction) increases resistance
Distance from the heart – the longer the vessels, the greater the resistance due to friction
Gravity – moving blood against gravity creates greater resistance

Blood moves energy. The more blood that flows to an organ, the more energy is available. By adjusting the size of our arteries, energy is diverted to where it is most needed. For example, when you stand up gravity would pull blood down to your legs and away from your brain causing you to faint, but this doesn’t happen because our arteries effectively squeeze the blood up to our head.

If you are still holding your hands in position, put them both in front of you to compare. Take a look at them…The lower hand is probably a lot redder (and maybe slightly swollen) compared to the hand that was over your head. In each case, blood flowed down its pressure gradient but resistance was different due to gravity. This caused more blood to collect in the lower hand since it returned to the heart more slowly.

Our body alters blood flow when we change activities. More blood flows to muscles when we exercise, but after a meal more blood flows to our digestive tract. Blood vessels in the skin can also dilate or constrict to adjust how much heat radiates from our skin (as discussed in section 2e). Changes are controlled by muscles in the artery walls. This works like pinching a hose to slow the flow of water (Figure 6-2).

Branched tube with fluid flowing out of two openings. Less fluid exits tube that is constricted at one point.

FIGURE 6-2. Restricted flow. The constriction in the bottom tube decreases flow because there is more resistance.
(“Restricted flow” by J. Buegge is in the Public Domain)

Stop & Think #2: Endurance sports like running or cycling require sustained, high blood flow to muscles. How would the heart respond to regular endurance training?

Since water retains heat well, it keeps body temperature stable. We have studied temperature regulation already, but now that we understand how blood flow is controlled, it is easier to understand how this works. Since we can control where blood flows, we can also control where heat flows. This is how the skin helps regulate body temperature. Blood flow also increases to injured tissue to deliver nutrients and speed healing. Because of this, injured areas of the body often feel warm, whether that is a cut, strained muscle, or sunburned skin.

Increased resistance can also cause trouble. Fatty deposits in the arteries make them narrower, increasing resistance. The heart must pump harder to overcome this resistance and get blood to flow around the entire loop. This puts a lot of extra strain on the heart.

Arteries deliver blood to the body cells, but to exchange materials they must branch into extremely fine capillaries. The capillary walls are very thin, making diffusion of oxygen, glucose and other nutrients easy. As in other exchange organs, diffusion is helped by having a large surface area. Capillaries cover over 6000 square meters in our body and no cell is more than 8 cells away from a capillary.

Learning objectives

Explain how flow-down gradients explain the direction of blood flow.
Explain how resistance alters blood flow.
Explain how blood helps regulate body temperature.

Stop & Think Questions Explained

Arteries would lose blood faster. Arteries have higher pressure than veins so blood would flow out faster. Fortunately, arteries are usually located in deeper tissues, so they are at less risk of being cut.
Regular endurance exercise conditions the heart, meaning that the heart gets stronger and more efficient.