The Cardiovascular System

Introduction

The cardiovascular system has four main functions:

1. transport and deliver oxygen, nutrients and hormones to the body
2. remove waste products such as carbon dioxide and lactic acid
3. protection against disease and infection
4. maintain body temperature

The cardioivasacular system has three components: the heart, blood and blood vessels. 

The heart is located in the middle of the chest and slightly towards the left. It is a large muscular pump and is divided into two halves - the right-hand side and the left-hand side.The right-hand side of the heart is responsible for pumping deoxygenated blood to the lungs.The left-hand side pumps oxygenated blood around the body.Each side of the heart consists of an atrium and a ventricle which are two connected chambers.

Blood Pressure-

When the heart contracts it pushes blood into blood vessels which creates blood pressure.

A blood pressure reading consists of two values:

• systolic value – blood pressure while the heart is squeezing
• diastolic value – blood pressure while the heart is relaxing

The average blood pressure for an adult is 120/80 mmHg. The first number is the systolic value and the second number is the diastolic value.

Blood pressure is determined by Q (cardiac output) and the resistance to the blood flow (R). Resistance to blood flow is caused both by the diameter of the blood vessels and by the thickness of the blood. Furthermore, if a person has a condition called atherosclerosis (plaque in the arteries), their resistance to blood flow will increase and so will blood pressure. This can have serious health implications such as causing chronic high blood pressure, angina or even heart attack or stroke.

Maintaining Body Temperature-

In the heat, blood vessels close to the surface of the skin enlarge. This process is called vasodilation. This allows more heat to be lost from the blood.

When a person takes part in exercise their face can become pink due to vasodilation of the blood vessels close to the skin's surface.

In the cold, blood vessels at the skin's surface close. This process is called vasoconstriction and takes blood away from the surface of the skin to help prevent it from losing heat.

The Double Circulatory System

The cardiovascular system in humans is actually a double circulatory system- it comprises of two separate circuits in which blood circulates and blood passes through the heart twice. You can also consider the heart as working as a dual action pump – two pumps that work at the same time to pump blood in two different directions. The name of two different circuits that bloood travels through are: pulmonary and systemic.

The right-hand side of the heart collects deoxygenated blood from the body and pumps it to the lungs (to collect more oxygen). This is called pulmonary circulation.

The left-hand side of the heart collects oxygenated blood from the lungs? and pumps it round the body. This is called systemic circulation.

Blood-

Blood has several functions but their main function is to transport nutrients and oxygen to cells in the body.

Blood is made up of four components:

• red blood cells – these transport oxygen around the body
• white blood cells - these fight infection
• platelets - these clot to prevent blood loss during injury
• plasma - this is the liquid part of blood

Red blood cells are very important for sport and physical activity because they contain haemoglobin. Haemoglobin allows them to carry oxygen from the lungs to the working muscles. Haemoglobin bonds with oxygen to form oxyhaemoglobin which then travels to the body's tissues and releases the oxygen. Red blood cells also deliver carbon dioxide to the lungs so that it can be removed.

Red blood cells are disc-shaped cells with no nucleus. They are very small but their flattened shape gives a relatively large surface area which allows rapid diffusion of oxygen.

The Pathway of Blood

Right- hand side

De-oxygenated blood enters the heart through the vena cava. The blood then enters the right atrium, passes through the tricuspid valve and into the right ventricle. The ventricle pumps the blood through the semilunar valve to the pulmonary artery where the blood travels to the lungs to get oxygenated.

Left-hand side

The oxygenated blood returns to the heart through the pulmonary vein and enters the left atrium. It then passes through the bicuspid valve into the left ventricle. The ventricle pumps the blood through the semilunar valve into the aorta where it is then distributed to the working muscles and organs around the body. 

The Cardiac Cycle-

The filling of the heart and the subsequent emptying follows a particular sequence. There are two stages to each heart beat:

1) Diastole- the heart fills with blood whilst it relaxes

2) Systole- the heart contracts and empties the blood (it pumps blood to the body)

Blood Vessels

Blood is carried through three different types of blood vessels in the body:

1. arteries
2. capillaries
3. veins

All blood vessels are specifically structured to perform their function. For example, a capillary is microscopically thin to allow gases to exchange, the arteries are tough and flexible to cope with high pressure blood flow and the veins contain valves to prevent the blood from travelling backwards when at low pressure. All vessels feature varying lumen size. The lumen is the hollow opening or the space inside the blood vessel.

Arteries carry blood away from the heart (usually oxygenated blood, except for the pulmonary artery).They have thick muscular walls to withstand blood flowing at high pressure as it leaves the heart; the fact that the walls of arteries contain smooth muscle tissue, enables the vessels to increase or decrease their diameter and ,therefore, control the blood flow and pressure of the blood.They also have small lumen and the largest artery in the body is the Aorta. You can locate your pulse using arteries and blood travelling through them usually travels in spurts.

Veins carry blood from various regions of the body towards the heart (usually deoxygenated blood, except for the pulmonary vein) . They have thinner walls, large lumen and contain valves to prevent the backflow of blood. Blood travelling through veins typically travels smoothly and veins carry blood under a low pressure.

Capillaries allow diffusion of gases and nutrients between blood and body cells. They are exchange points where oxygen and carbon dioxide cross into the tissue cells (muscles) from the arterioles.They have very thin walls (one cell thick) and these walls are made of semi-permeable membrane to allow transport of gases and nutrients into and out of the blood. They also have very small lumen which means they only allow blood to pass through one cell at a time.They occur in large quantities around the muscels wich enables effective exchange of gases. 

Heart Components

The atria (plural of atrium) are where the blood collects when it enters the heart.

The ventricles pump the blood out of the heart to the lungs or around the body.

The septum separates the right-hand and left-hand side of the heart.

The tricuspid valve separates the right atrium and right ventricle. It opens due to a build-up of pressure in the right atrium, and prevents back flow of blood from the right ventricle to the right atrium.

The bicuspid valve separates the left atrium and left ventricle and prevents back flow of blood from the ventricle to the atrium. It likewise opens due to a build-up of pressure, this time in the left atrium.

The semilunar valves stop the back flow of blood into the heart. There is a semilunar valve where the aorta leaves the left ventricle and another where the pulmonary artery leaves the right ventricle.

The Main Blood Vessels

There are four main blood vessels that take blood into and out of the heart.

The aorta is the largest artery in the body. It carries oxygenated blood away from the left ventricle to the body.

The vena cava is the largest vein in the body. It carries deoxygenated blood from the body back to the heart.

The pulmonary artery carries deoxygenated blood away from the right ventricle to the lungs.

The pulmonary vein returns oxygenated blood from the lungs to the heart.

Key Definitions-

The heart's function is to pump the blood and circulate it round the body. We assess the heart's performance by measuring how much blood it pumps out each minute. This is called cardiac output. To calculate cardiac output, we also need to know about heart rate and stroke volume.

Heart rate- the number of times the heart beats (or the ventricles pump blood out) in one minute. This heartbeat is produced because the heart contracts and relaxes in a rhythm. The average resting HR is approximately 70 beats per minute (bpm).

Stroke volume- the amount of blood pumped out of the ventricles each time they contract. The average resting SV is approximately 70 ml.

Cardiac output (Q) - the amount of blood pumped from the heart every minute. It can be calculated by multiplying heart rate (HR) by sroke volume (SV.) It is typically measured in ml.

Heart Rate-

Resting heart rate- the number of times your heart beats per minute while at complete rest

MHR = Maximum heart rate

MHR= 220 - Age

Using your MHR you can work out your best rate to train. This can directly affect the results of your training sessions.

The Effects of Exercise on the CV System-

When a person takes part in exercise the cardiovascular, respiratory, energy and muscular systems all work together to supply energy to the working muscles and remove waste products.

During exercise the heart rate increases so that sufficient blood is taken to the working muscles to provide them with enough nutrients and oxygen. An increase in heart rate also allows for waste products to be removed.

Stroke volume increases which means more blood is pumped out of the heart each time it contracts.

At rest a person's cardiac output is approximately 5 litres per minute, while during exercise it can increase to as much as 30 litres per minute as both their heart rate and stroke volume increase.

As exercise increases, cardiac output (Q) also increases. This has the effect of increasing blood pressure. A typical blood pressure reading for a person at the start of exercise would be around 160/85 mmHg.

The Vascular Shunt Mechansim-

The vascular shunt mechanism occurs during exercise and it refers to the redistribution of blood around the body by the cardiovascular system so that more of it goes to the working muscles and less of it goes to other body organs (such as the digestive system.) It works a bit like a railway terminal; the blood vessels allow lots of blood to travel to the working muscles by dilating but they dont allow much to travel to other organs as they constrict. As a result of this mechanism, more oxygen is transported to the areas with the greatest demand for it (the working muscles.) When the muscles stop working, the blood distribution returns to its normal route.