7.2 Circulatory systems in animals

Open and closed circulation systems (ESG8Y)

There are two types of circulatory systems found in animals: open and closed circulatory systems.

Open circulatory systems

In an open circulatory system, blood vessels transport all fluids into a cavity. When the animal moves, the blood inside the cavity moves freely around the body in all directions. The blood bathes the organs directly, thus supplying oxygen and removing waste from the organs. Blood flows at a very slow speed due to the absence of smooth muscles, which, as you learnt previously, are responsible for contraction of blood vessels. Most invertebrates (crabs, insects, snails etc.) have an open circulatory system. Figure 7.1 shows a schematic of an open circulatory system delivering blood directly to tissues.

Closed circulatory systems

Closed circulatory systems are different to open circulatory systems because blood never leaves the blood vessels. Instead, it is transferred from one blood vessel to another continuously without entering a cavity. Blood is transported in a single direction, delivering oxygen and nutrients to cells and removing waste products. Closed circulatory systems can be further divided into single circulatory systems and double circulatory systems.

Single and double circulation systems (ESG8Z)

The circulatory system is a broad term that encompasses the cardiovascular and lymphatic systems. The lymphatic system will be discussed later in this chapter. The cardiovascular system consists of the heart (cardio) and the vessels required for transport of blood (vascular). The vascular system consists of arteries, veins and capillaries. Vertebrates (animals with backbones like fish, birds, reptiles, etc.), including most mammals, have closed cardiovascular systems. The two main circulation pathways in invertebrates are the single and double circulation pathways.

Single circulatory pathways

Single circulatory pathways as shown in the diagram below consist of a double chambered heart with an atrium and ventricle (the heart structure will be described in detail later in this chapter). Fish possess single circulation pathways. The heart pumps deoxygenated blood to the gills where it gets oxygenated. Oxygenated blood is then supplied to the entire fish body, with deoxygenated blood returned to the heart.

Double circulatory systems

Double circulation pathways are found in birds and mammals. Animals with this type of circulatory system have a four-chambered heart.

The right atrium receives deoxygenated from the body and the right ventricle sends it to the lungs to be oxygenated. The left atrium receives oxygenated blood from the lungs and the left ventricle sends it to the rest of the body. Most mammals, including humans, have this type of circulatory system. These circulatory systems are called 'double' circulatory systems because they are made up of two circuits, referred to as the pulmonary and systemic circulatory systems.

Human circulatory systems (ESG92)

The human circulatory system involves the pulmonary and systemic circulatory systems. The pulmonary circulatory system consists of blood vessels that transport deoxygenated blood from the heart to the lungs and return oxygenated blood from the lungs to the heart. In the systemic circulatory system, blood vessels transport oxygenated blood from the heart to various organs in the body and return deoxygenated blood to the heart.

Pulmonary circulation system

In the pulmonary circulation system, deoxygenated blood leaves the heart through the right ventricle and is transported to the lungs via the pulmonary artery. The pulmonary artery is the only artery that carries deoxygenated blood. It carries blood to the capillaries where carbon dioxide diffuses out of the blood into the alveoli (lung cells) and then into the lungs, where it is exhaled. At the same time, oxygen diffuses into the alveoli, and then enters the blood and is returned to the left atrium of the heart via the pulmonary vein.

Systemic circulation

Systemic circulation refers to the part of the circulation system that leaves the heart, carrying oxygenated blood to the body's cells, and returning deoxygenated blood to the heart. Blood leaves through the left ventricle into the aorta, the body's largest artery. The aorta leads to smaller arteries that supply all organs of the body. These arteries finally branch into capillaries. In the capillaries, oxygen diffuses from the blood into the cells, and waste and carbon dioxide diffuse out of cells and into blood. Deoxygenated blood in capillaries then moves into venules that merge into veins, and the blood is transported back to the heart. These veins merge into two major veins, namely the superior vena cava and the inferior vena cava (figure:doublecirculation). The movement of blood is indicated by arrows on the diagram. The deoxygenated blood enters the right atrium via the the superior vena cava. Major arteries supply blood to the brain, small intestine, liver and kidneys. However, systemic circulation also reaches the other organs, including the muscles and skin. The following diagram (Figure 7.4) shows the circulatory system in humans.

The heart and associated blood vessels (ESG93)

External structure of the heart

The heart is a large muscle, about the size of your clenched fist, that pumps blood through repeated rhythmic contractions. The heart is situated in your thorax, just behind your breastbone, in a space called the pericardial cavity. The heart is enclosed by a double protective membrane, called the pericardium. The region between the two pericardium layers is filled with pericardial fluid which protects the heart from shock and enables the heart to contract without friction.

The heart is a muscle (myocardium) and consists of four chambers. The upper two chambers of the heart are called atria (singular= atrium). The two atria are separated by the inter-atrial septum. The lower two chambers of the heart are known as ventricles and are separated from each other by the interventricular septum. The ventricles have more muscular walls than the atria, and the walls of the right ventricle, which supplies blood to the lungs is less muscular than the walls of the left ventricle, which must pump blood to the whole body.

Clench your fist - the size of your fist is more or less the size of your heart.

In addition, there are a number of large blood vessels that carry blood towards and away from the heart. The terms `artery' and `vein' are not determined by what the vessel transports (oxygenated blood or deoxygenated) but by whether the vessel flows to or from the heart. Arteries take blood away from the heart and generally carry oxygenated blood, with the exception of the pulmonary artery. Veins transport blood towards the heart and generally carry deoxygenated blood, except the pulmonary vein. On the right side of the heart, the superior vena cava transports deoxygenated blood from the head and arms and the inferior vena cava transports deoxygenated blood from the lower part of the body back to the heart, where it enters the right atrium. The pulmonary artery carries deoxygenated blood away from the right ventricle of the heart towards the lungs to be oxygenated. On the left side of the heart, the pulmonary vein brings oxygenated blood from the lungs towards the left atrium of the heart and the oxygenated blood exits the left ventricle via the aorta and is transported to all parts of the body.

Since the heart is a muscle, and therefore requires oxygen and nutrients itself to keep beating, it receives blood from the coronary arteries, and returns deoxygenated blood via the coronary veins.

In humans, the left lung is smaller than the right lung to make room in the chest cavity for the heart.

Internal structure of the heart

As previously mentioned, the heart is made up of four chambers. There are two atria at the top of the heart which receive blood and two ventricles at the bottom of the heart which pump blood out of the heart. The septum divides the left and right sides of the heart. In order to make sure that blood flows in only one direction (forward), and to prevent backflow of blood, there are valves between the atria and ventricles (atrioventricular valves). These valves only open in one direction, to let blood into the ventricles, and are flapped shut by the pressure of the blood when the ventricles contract.

The tricuspid valve is situated between the right atrium and the right ventricle while the bicuspid/ mitral valve is found between the left atrium and the left ventricle. Strong tendinous cords (chordae tendineae) attached to valves prevent them from turning inside out when they close. The semi-lunar valves are located at the bottom of the aorta and pulmonary artery, and prevent blood from re-entering the ventricles after it has been pumped out of the heart.

In the previous sections we have discussed pulmonary and systemic circulation, and we have described the four chamber structure of the heart as well as some of the major arteries and veins that transport blood towards and away from the heart. In order to summarise all this information, study the flow diagram below which describes the passage of deoxygenated blood through one full cycle.

Major organs and systemic circulation (ESG94)

All the organs of the body are supplied with blood. This is necessary so that the cells can obtain oxygen, which is required for cellular respiration, as well as essential nutrients. Each organ has an artery that supplies it with blood from the heart. Metabolic wastes, including carbon dioxide, need to be removed from cells and returned to the heart. These move into the capillaries which enter into veins that eventually enters either the superior or inferior vena cava which then enters the right atrium.

Arteries and veins have been named according to the organ to which they supply blood. The liver receives oxygenated blood from the heart via the hepatic artery. This artery runs alongside the hepatic portal vein. The hepatic portal vein contains nutrients that have been absorbed by the digestive system. This nutrient-rich blood must first pass through the liver, so that the nutrient composition of the blood can be controlled. Blood passes from the liver to the heart through the hepatic vein. Metabolic waste is circulated in the blood, and if allowed to accumulate, would eventually reach toxic levels. The kidneys are supplied with blood (which contain waste) via the renal arteries. The kidneys filter metabolic waste from the blood, passing it to urine to be excreted safely. Blood leaves the kidney via the renal vein.

The brain is supplied with blood via the carotid arteries and the vertebral arteries. The blood from the brain is drained via the jugular veins. The brain is supplied with \(\text{15}\%\) of the total amount of blood pumped by the heart. The heart is also a muscle (myocardium) that requires blood flow to work. Blood is supplied to the heart via two coronary arteries, and leaves the heart via four cardiac veins.

The cardiac cycle (ESG95)

A cardiac cycle refers to the sequence of events that happens in the heart from the start of one heartbeat to the start of the subsequent heartbeat. During a cardiac cycle the atria and the ventricles work separately. The sinoatrial node (pacemaker) is located in the right atrium and regulates the contraction and relaxing of the atria.

• At rest, each heartbeat takes approximately \(\text{0,8}\) seconds.
• The normal heart rate at rest is approximately \(\text{72}\) beats per minute.
• During systole the heart muscle contracts.
• During diastole the heart muscle relaxes.

The phases of the cardiac cycle will be broken down and explained in the following section:

Phase 1: Atrial systole (Atrium contracts)

• Blood from the superior and inferior vena cava flows into the right atrium.
• Blood from the pulmonary veins flows into the left atrium.
• The atria contract at the same time.
• This contraction lasts for about \(\text{0,1}\) seconds.
• Blood is forced through the tricuspid and bicuspid valves into the ventricles.

Phase 2: Ventricular systole (Ventricle contracts)

• Ventricles relax and fill with blood.
• The ventricles contract for \(\text{0,3}\) seconds.
• Blood is forced upwards, closing the bicuspid and tricuspid valves (lubb sound).
• The blood travels up into the pulmonary artery (on the right) and the aorta (on the left).
• The atria are relaxed during ventricular systole.

Phase 3: General diastole: (General relaxation of the heart)

• The ventricles relax, thus decreasing the flow from the ventricles.
• Once there is no pressure the blood flow closes the semi-lunar valves in the aorta and the pulmonary artery (dubb sound).
• General diastole lasts for about \(\text{0,4}\) seconds.

Electrical activity

The electrical activity in the heart is so strong that it can be measured from the surface of the body as an electrocardiogram (ECG). A normal heart has a very regular rhythm. Arrhythmia is a condition where the heart has an abnormal rhythm, as shown in the figures. Tachycardia is when the resting heart rate is too fast (more than \(\text{100}\) beats per minute), and bradycardia is when the heart rate is too slow (less than \(\text{60}\) beats per minute).

Blood vessels (ESG96)

We will now examine the structure and function of arteries, capillaries, veins and valves.

Arteries

Arteries carry blood Away from the heart. The pressure created by the pumping heart forces blood through the arteries. Arteries have three layers. They have an outside layer made up of connective tissue; a middle layer made up of smooth muscle, to allow contraction of the arteries in order to regulate the pressure of blood flow, and an inside layer of tightly connected simple squamous endothelial cells. The large arteries close to the heart branch into smaller arterioles (smaller arteries) and eventually branch into capillaries.

Laughing is good exercise for your heart. Whenever you laugh, the blood vessels dilate (open up), causing the blood flow to increase, thus keeping your heart healthy.

Capillaries

Capillaries are little more than a single layer of endothelial cells. Capillaries form intricate networks throughout the tissues. They allow water, nutrients and gases to diffuse out of the blood and waste materials to diffuse into the blood. This exchange occurs between the blood and the tissue fluid. The tissue fluid is the fluid surrounding the cells. The blood cells never come into contact with the cells. The blood and tissue fluid exchange material, and the tissue fluid then exchanges material with the cells.

Veins

The intricate networks formed by the capillaries eventually converge to form venules, (small veins). The venules then converge to form veins which return the blood to the heart. Vein walls only consist of two layers. The outer layer is made up of connective tissue whereas the inner layer is made up of endothelial cells.

Valves

Once the blood has passed through the capillaries very little blood pressure remains to return blood to the heart.Instead of pressure from the heart veins use a series of valves to force blood to return to the heart. Contraction of the muscles squeezes the veins, pushing the blood through them. The valves cause the blood to flow in only one direction, back to the heart.

Comparison between arteries, veins and capillaries (ESG97)

The figure and table below summarise the differences between arteries, capillaries and veins.

Arteries	Capillaries	Veins
blood moves away from the heart	blood supply at tissue level	blood returned to the heart
thick middle layer of involuntary muscle to increase or decrease diameter	one layer of endothelium with very small diameter	thin middle layer as pressure is reduced
inner layer of endothelium which reduces friction	only endothelium layer present	larger diameter of inner cavity, lined with endothelium to reduce friction
situated deeper in the tissue to maintain body temperature	situated at tissue level only	situated near the surface of the skin to release heat
no valves except in the base of the aorta and the pulmonary arteries	no valves present	semi-lunar valves are present at intervals, to prevent back flow of blood
blood always under high pressure	blood is under high pressure where red blood cells are forced to flow through in single file	blood is under low pressure
a pulse can be felt as blood flows	no pulse	no pulse can be detected

Table 7.1: Table comparing arteries, capillaries, and veins

The average adult heart beats:

• \(\text{72}\) times a minute
• \(\text{100 000}\) times a day
• \(\text{3 600 000}\) times a year
• A billion times during a lifetime.