Human circulatory system

In a nutshell

Humans have a double circulatory system and the heart is the main organ that pumps blood around the body. The cardiac output can be calculated using a simple formula. Blood is made up of four main components and the flow of blood is affected by the total cross-sectional area of the blood vessels.

The double circulatory system

In humans, the circulatory system is made up of the heart, blood vessels and blood itself. Humans have a double circulatory system, with two separate circuits joined together.

Biology; Exchange and transport in animals; KS4 Year 10; Human circulatory system — 1. Blood vessels supplying the head and upper body, 2. Right lung; 3. Left lung, 4. Blood vessels supplying the organs, trunk and legs.

First Circuit	The heart pumps deoxygenated blood to the lungs. At the gas exchange surfaces, the blood takes in oxygen. The oxygenated blood returns back to the heart.
Second Circuit	The heart pumps oxygenated blood around the body to all the other organs. Once at the tissues, the blood gives up its oxygen to the cells. The deoxygenated blood returns to the heart to be pumped out to the lungs again.

An advantage of having a double circulatory system is that oxygenated blood from the lungs can be pumped out around the body at a much higher pressure. This allows more oxygen to be delivered to the cells, as there is increased blood flow to the tissues.

The heart

The heart is one of the most important organs as it keeps blood flowing around the body. It is made up of cardiac muscle. The coronary arteries supply blood to the heart to make sure it keeps beating. Inside the heart, there are valves which make sure blood flows in the right direction and prevent backflow. There are four chambers within the heart: two ventricles and two atria.

1.	Superior vena cava	8.	Inferior vena cava
2.	Aorta	9.	Pulmonary veins
3.	Pulmonary artery	10.	Right atrium
4.	Right atrium	11.	Mitral (bicuspid) valve
5.	Pulmonary valve	12.	Cardiac muscle
6.	Tricuspid valve	13.	Aortic valve
7.	Right ventricle	14.	Left ventricle

The cardiac cycle

Biology; Exchange and transport in animals; KS4 Year 10; Human circulatory system

1.	Atria contract	Blood from the body flows from the vena cava and pulmonary vein into the two atria. The atria contract. This pushes the blood through the tricuspid and bicuspid valves into the ventricles.
2.	Ventricles contract	The ventricles contract. This pushes the blood out of the heart through the aorta and the pulmonary artery, opening the aortic and pulmonary valve. The blood flows to the organs through the arteries.
3.	Atria and ventricles relax	The atria and ventricles relax. Blood flows back into the atria from the veins and the whole cardiac cycle starts over.

Curiosity: The left ventricle has a thicker wall than the right ventricle because it needs more muscle to pump blood around the entire body at high pressure, but the right ventricle only needs to pump blood to the lungs.

Cardiac output

The cardiac output is the amount of blood the heart pumps in a minute. To calculate the cardiac output, this equation can be used:

$cardiac\space output\space (cm^3/min) = heart\space rate \space (bpm) \times stroke\space volume\space (cm^3)$

Cardiac output	The total volume of blood pumped by a ventricle per minute.
Heart rate	The number of times the heart beats per minute.
Stroke volume	The volume of blood pumped by a ventricle each time it contracts.

Example

What is the cardiac output of a person with a heart rate of $85\space bpm$  and a stroke volume of $73\space cm^3$ ?

Substitute into formula:

$=85\times 73$

$= \underline6\underline2\underline0\underline5\space cm^3/min$

$\underline{Therefore,\ the\ cardiac\ output\ is\ 6205\ cm^3/min}$ .

Blood vessels

There are three different types of blood vessels.

Blood vessel	Feature
Arteries	These carry blood away from the heart at high pressure. The walls are strong and elastic to maintain this pressure. The walls are also thick in comparison to the lumen.
Capillaries	The arteries branch into capillaries. These allow the exchange of important substances in the tissues. The capillaries are really tiny and very narrow. This allows them to squeeze into gaps between the cells. Their walls are permeable. This allows substances to diffuse into and out of them. Their walls are also only one cell thick, increasing the rate of diffusion even more.
Veins	The capillaries eventually join back up to form the veins. Veins take the blood back to the heart. The walls are less thick than arteries, as blood is at a lower pressure. To keep blood flowing in the right direction, veins have valves. The lumen of the veins is bigger than the arteries. This also helps blood flow.

Cross-sectional area of vessels

Definition

The cross-sectional area of a blood vessel is the area of the plane of a blood vessel when it is cut in half. Blood flow is affected by the total cross-sectional area of blood vessels.

As the total cross-sectional area increases, the mean velocity (speed) of the blood flowing through the vessels decreases. In the diagram below, you can see the relationship between the total cross-sectional area and the mean velocity at the different blood vessels.

Biology; Exchange and transport in animals; KS4 Year 10; Human circulatory system — A. Mean velocity, B. Total cross-sectional area, 1. Arteries, 2. Capillaries, 3. Veins.

Blood flows more slowly through the capillaries compared to the arteries or veins. Although capillaries are very small, there are so many of them that their total cross-sectional area is really big. This allows more time for the exchange of important substances, as the blood flows through the capillary beds more slowly. On the other hand, the mean pressure of the blood is highest in the arteries, as they are closest to the heart.

Blood

Blood is a tissue that acts as a huge transport system. There are four main components of blood.

Component	Function
Red blood cells	These cells carry oxygen from the lungs to all the cells in the body. Their shape is of a biconcave disc which gives a large surface area. They contain haemoglobin, which has iron inside of it. This gives the blood its red colour. To allow more oxygen into red blood cells, they don't have a nucleus.
White blood cells	Two of the main types of white blood cells are phagocytes and lymphocytes. Phagocytes engulf microbes via phagocytosis. Lymphocytes produce antibodies and antitoxins against microbes.
Platelets	Platelets are small fragments of cells that help the blood to clot at a wound. This stops blood from pouring out and prevents microbes from getting in.
Plasma	Plasma is a straw-coloured liquid. It carries the blood cells, platelets, glucose, amino acids, carbon dioxide, urea, hormones, proteins and more.