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.

Lung structure and gas exchange

Air enters the lungs through the trachea, which divides into two bronchi, one leading to each lung. The bronchi divide into smaller airways that carry air to tiny air sacs called alveoli. Each alveolus is surrounded by a network of blood capillaries.

Ventilation moves air into and out of the lungs. At the alveoli, gases move between the air and the blood by diffusion: each gas moves from a region of higher concentration to a region of lower concentration.

• Oxygen diffuses from the air in the alveoli into the blood.
• Carbon dioxide diffuses from the blood into the alveoli and is breathed out.

How alveoli support gas exchange

The lungs contain many alveoli, giving them a large surface area across which gases can diffuse. The alveolar walls are moist, so gases can dissolve at the exchange surface. Both the alveolar walls and the capillary walls are thin, providing a short diffusion distance between the air and the blood.

Ventilation continually brings fresh air into the alveoli and removes carbon dioxide. Meanwhile, the continuous blood supply carries oxygen away and brings carbon dioxide to the alveoli. Together, these maintain concentration gradients that allow oxygen to diffuse into the blood and carbon dioxide to diffuse in the opposite direction.

Cardiac pacemakers

The heart needs a regular signal to coordinate its contractions. A group of cells in the right atrium provides this signal, acting as the heart’s natural pacemaker.

These cells produce regular electrical impulses that spread through the heart. The impulses coordinate the contractions of the heart muscle, setting its natural resting rate.

Artificial pacemakers

If the heart’s own control produces an irregular rate, an artificial pacemaker can be used. This electrical device sends impulses to the heart to regulate its contractions and help maintain a regular rate.

Arteries

Arteries are blood vessels that carry blood away from the heart. Blood enters them under relatively high pressure as the heart pumps, so artery walls are thick and strong. The space through which blood flows, called the lumen, is relatively narrow.

Artery walls contain elastic tissue that stretches as blood is pumped into the artery. Between heartbeats, the elastic tissue recoils, helping to maintain blood pressure as blood continues to flow.

Artery walls also contain muscle. When this muscle contracts or relaxes, it changes the artery’s diameter and so affects the flow of blood through it.

Veins

Veins carry blood towards the heart. The blood in them is at relatively low pressure, so veins have adaptations that help blood return to the heart and prevent it from moving backwards.

Vein structure and blood flow

Veins have thinner walls and a wider lumen than arteries. The lumen is the space inside a blood vessel through which blood flows. A wide lumen offers less resistance to flow, helping blood return to the heart at low pressure.

Many veins contain valves that allow blood to move towards the heart but prevent it from flowing backwards. When muscles surrounding a vein contract, they squeeze the vein and push blood along it. The valves prevent the blood from moving backwards, helping it continue towards the heart.

Capillaries

Capillaries are tiny blood vessels that form networks close to body cells. They connect the arterial side of the circulation to the venous side. As blood passes through these networks, substances can be exchanged between the blood and surrounding tissues.

Capillary structure and exchange

Capillaries have features that make exchange by diffusion effective:

• Their walls are one cell thick, so substances have a short distance to diffuse between the blood and surrounding tissues.
• Their lumen—the space inside the vessel—is narrow, bringing blood close to the wall.
• Their extensive networks provide a large surface area across which substances can diffuse.

Substances diffuse down their concentration gradients, from a region of higher concentration to a region of lower concentration. The direction of movement depends on the concentration of each substance in the blood and the surrounding tissues.

Blood flow rate calculations

Blood flow rate is the volume of blood passing a particular point in a given time. For example, a flow rate of $$400\ \text{mL/min}$$ means that $$400\ \text{mL}$$ of blood passes the point each minute.

To calculate flow rate, divide the volume of blood by the time taken:

If $$1200\ \text{mL}$$ of blood passes a point in $$3\ \text{min}$$, the flow rate is:

Finding volume or time

The same relationship can be rearranged. To find the volume passing a point at a constant flow rate, multiply the flow rate by the time:

To find the time taken for a volume to pass a point at a constant flow rate, divide the volume by the flow rate:

For example, at $$400\ \text{mL/min}$$, a volume of $$800\ \text{mL}$$ takes $$2\ \text{min}$$ to pass the point.

Blood flow rate units

Flow rate is a compound measure: its unit combines a volume unit and a time unit. In $$\text{mL/min}$$, the slash means “per”, so the unit is read as millilitres per minute.

Make sure the volume and time units match the rate unit required. For a rate in $$\text{mL/min}$$, use millilitres and minutes. For example, convert $$180\ \text{s}$$ to $$3\ \text{min}$$ before calculating, or calculate in $$\text{mL/s}$$ and convert the resulting rate afterwards.