Blood vessels of the heart

Four main vessels connect the heart to the body and the lungs. To identify a vessel, follow its connection and determine whether blood enters or leaves the heart through it.

Vessels entering the heart

• The vena cava brings blood from the body into the right atrium.
• The pulmonary vein brings blood from the lungs into the left atrium.

Vessels leaving the heart

• The aorta carries blood from the left ventricle to the body.
• The pulmonary artery carries blood from the right ventricle to the lungs.

Identifying the vessels on a heart diagram

Trace each vessel to the chamber it connects to: the vena cava and pulmonary vein enter atria, while the aorta and pulmonary artery leave ventricles. The heart’s right and left are the person’s right and left; on a diagram viewed from the front, the right side of the heart usually appears on the left of the page.

A diagram may show more than one vena cava or pulmonary vein. Follow each vessel’s connection to the heart and its route to or from the body or lungs.

Mammalian heart structure

The mammalian heart is a muscular organ that pumps blood. Its muscular wall contracts to force blood out of the heart. A wall called the septum separates the heart into right and left sides, preventing blood from mixing between them.

Chambers and valves

Each side has two chambers. An upper chamber, called an atrium, receives blood entering the heart. A lower chamber, called a ventricle, pumps blood out. Blood passes from the right atrium to the right ventricle, and from the left atrium to the left ventricle.

One-way valves keep blood moving in the correct direction. Valves between the atria and ventricles, and at the exits from the ventricles, prevent blood from flowing backwards when the chambers contract.

Recognising the heart’s structures

In a heart diagram, look for the septum separating the two sides, then identify the atrium above the ventricle on each side. The labels left and right refer to the animal’s own left and right. In a front-view diagram, the heart’s left side therefore often appears on the right of the image.

The coronary arteries run over the surface of the heart and supply its muscle with oxygen and nutrients. This supply allows the heart muscle to keep contracting.

Double circulation

In a double circulatory system, blood passes through the heart twice during one complete circuit. It travels from the heart to the lungs and back, then from the heart to the rest of the body and back.

Pulmonary circuit: heart and lungs

The right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs. There, the blood gains oxygen and loses carbon dioxide. The oxygenated blood returns to the left atrium through the pulmonary vein. This journey between the heart and lungs is the pulmonary circuit.

Systemic circuit: heart and body

Blood passes from the left atrium to the left ventricle, which pumps it through the aorta to the rest of the body. After delivering oxygen, the deoxygenated blood returns to the right atrium through the vena cava. It then passes to the right ventricle, ready to travel to the lungs again. The journey between the heart and the rest of the body is the systemic circuit.

Gas exchange in the lungs

Air travels through the trachea and bronchi into the lungs, eventually reaching tiny air sacs called alveoli. Each alveolus is surrounded by a network of blood capillaries, so air and blood are close together but separated by the alveolar and capillary walls.

Gas exchange occurs as gases diffuse from a region of higher concentration to a region of lower concentration. Oxygen diffuses from the air in the alveoli, through the walls, into the blood. At the same time, carbon dioxide diffuses from the blood into the alveoli and is then breathed out.

Alveoli and efficient diffusion

The lungs contain many alveoli, providing a large surface area across which gases can diffuse. The alveolar walls and capillary walls are both thin, giving gases a short diffusion distance between the air and the blood.

Ventilation continually refreshes the air in the alveoli, bringing in oxygen and removing carbon dioxide. Blood flow carries oxygen away from the alveoli and brings carbon dioxide to them. Together, ventilation and blood flow maintain the concentration differences that allow both gases to keep diffusing.

Heart-rate control by pacemakers

The heart needs regular electrical signals to coordinate its contractions. A group of cells in the right atrium produces these signals and acts as the heart’s natural pacemaker.

The natural pacemaker generates electrical impulses that spread through the heart and coordinate its contractions. The rate at which it produces impulses controls the heart’s natural resting rate.

An artificial pacemaker is an electrical device used to correct an irregular heart rate. It sends electrical impulses to the heart, helping its contractions occur at an appropriate rate.

Arteries

Arteries are blood vessels that carry blood away from the heart. Blood enters them at high pressure as the heart pumps, so their structure helps them withstand and maintain this pressure.

Arteries have thick, strong walls containing muscle and elastic tissue. The walls withstand the high pressure, while the elastic tissue allows them to stretch as blood is pumped into them and recoil between heartbeats. This stretch and recoil helps maintain pressure as blood flows through the artery.

The lumen is the space inside a blood vessel through which blood flows. An artery has a relatively narrow lumen, which also helps maintain the pressure of the blood.

Veins

Veins are blood vessels that carry blood towards the heart. The blood in veins is at relatively low pressure, so their structure helps it flow towards the heart without flowing backwards.

Vein walls are thinner than artery walls and contain less muscle and elastic tissue. Veins also have a wide lumen: the space inside the vessel through which blood flows. This wide space provides less resistance to blood flow.

Veins contain valves that prevent blood from flowing backwards. The valves allow blood to continue towards the heart even though it is at relatively low pressure.

Capillaries

Capillaries are tiny blood vessels that form networks through tissues. They allow substances to be exchanged between the blood and surrounding cells. For example, oxygen and nutrients move from the blood towards cells, while carbon dioxide moves from cells into the blood.

Capillary structure and exchange

Capillary walls are one cell thick. This gives substances a short distance to diffuse across the wall, helping exchange occur quickly.

The lumen is the space inside a blood vessel through which blood flows. A capillary has a narrow lumen, which keeps the blood close to the vessel wall and reduces the distance substances must travel between the blood and the wall.

Capillaries form extensive networks throughout tissues. Together, their walls provide a large surface area across which substances can be exchanged.

Blood-flow rate calculations

A rate describes a quantity in relation to the time taken. For blood flow, the quantity may be a volume of blood or a distance travelled. These give two different rates: volume flow rate and blood-flow speed.

Volume flow rate

Volume flow rate is the volume of blood passing a point, such as a point in a blood vessel, divided by the time taken. It can be measured in millilitres per minute ($$\text{mL/min}$$).

The relationship can be rearranged to find the volume or the time:

For example, if $$150,\text{mL}$$ of blood passes a point in $$30$$ seconds, convert the time to $$0.5$$ minutes to calculate the rate in $$\text{mL/min}$$:

At this rate, the volume passing the point in $$2,\text{min}$$ is $$300,\text{mL/min} \times 2,\text{min} = 600,\text{mL}$$. Conversely, $$600,\text{mL}$$ takes $$600,\text{mL} \div 300,\text{mL/min} = 2,\text{min}$$ to pass the point.

Blood-flow speed

Blood-flow speed describes the distance blood travels in a given time. It is not the same as volume flow rate: speed measures distance per time, whereas volume flow rate measures volume per time.

For example, blood travelling $$12,\text{cm}$$ in $$3,\text{s}$$ has a speed of $$4,\text{cm/s}$$.

Consistent units in blood-flow calculations

Use matching time units throughout a calculation. For example, a rate in $$\text{mL/min}$$ must be multiplied by a time in minutes, not seconds. When converting a rate, convert both the volume or distance unit and the time unit as needed. Always give the answer with the appropriate units.