Even with specialised exchange surfaces, the size of larger organisms means that they must still have a system to transport substances between the exchange surface and the cells of the body. In humans and large animals, this is achieved through the circulatory system.

The circulatory system

The circulatory system consists of:

• the heart - which is the muscular pump that keeps the blood moving around the body

• the blood - which carries the substances around the body

• the arteries - which carry blood away from the heart

• the veins - which return blood to the heart

• the capillaries - which are tiny blood vessels that are close to the body’s cells where exchanges can happen

The heart

The heart is the organ responsible for pumping blood around the circulatory system. The walls of the heart are made from muscle tissue which can contract to put the blood under pressure, forcing its movement.The heart consists of two separate sides, and the blood does not mix between the two. The right-hand side of the heart only pumps blood to the lungs to pick up oxygen, whilst the left-hand side of the heart pumps blood to the rest of the body. This double circulation allows the oxygenated blood to become re-pressurised before being sent around the body.Each side of the heart is made up of two chambers - meaning that there are four in total. The atrium is at the top and the ventricle is at the bottom. Blood enters the heart through a vein and collects in the left atrium (remembering that you always describe the heart from the perspective you view it from). The first part of a heart beat causes the atrium wall to contract, which puts the blood under pressure - forcing it through a one-way valve into the ventricle. The second part of a heart beat then causes the muscular wall of the ventricle to contract - forcing the blood out through an artery under pressure. The one-way valve prevents the blood flowing back to the atrium. The artery also contains a valve to stop blood flowing back to the ventricle when the ventricle relaxes.

The passage of blood through the heart

Deoxygenated blood arrives at the left-hand side of the heart:

1. It enters the heart through the vena cava.

2. Blood flows into the right atrium.

3. Blood is pumped into the right ventricle.

4. Blood is pumped out of the heart, along the pulmonary artery, to the lungs.

Oxygenated blood arrives at the right-hand side of the heart:

1. It enters the heart through the pulmonary vein.
2. Blood flows into the left atrium.
3. Blood is pumped into the left ventricle.
4. Blood is pumped out of the heart, along the aorta, to the rest of the body.

Artificial heart valves

Occasionally, some people’s heart valves become stiff or leaky, which prevents the valves from functioning properly to prevent the backflow of blood. In these circumstances, it is possible to replace the faulty valves with either valves from a biological source (eg human donor or animal) or by using mechanical (man-made) valves. Both types of artificial heart valve have advantages and disadvantages. The table below shows some of the main pros and cons.

 AdvantagesDisadvantages Biological valves

• Do not damage red blood cells as they pass through the open valves

• Prone to becoming hardened over the course of several years

• For patients with long life expectancy, there is a higher chance of further operations to replace the valves (any operation carries risks)

Mechanical valves

• Very strong and durable - able to last a lifetime

• Damage red blood cells as they pass through the open valves

• Require the patient to take anti-blood clotting drugs for the rest of their life

• Some people say they can hear the valves opening and closing

Artificial hearts

In cases where a patient has severe heart disease/damage/failure, a heart transplant is necessary. However, there is often a shortage of compatible heart donors available - meaning that many people die while on the waiting list.

Artificial (man-made) hearts provide an alternative as they replicate the function of the heart. But current designs have not proved to be very successful in the long term, and are prone to blood clotting within them. Therefore, artificial hearts are only used as a short-term measure to keep patients alive until a biological donor heart can be found.

Arteries and veins

Blood flows from the heart to the body’s other organs through arteries. In the organs, the arteries repeatedly branch into a network of smaller blood vessels called capillaries. These then branch back together to form veins, which then carry blood back to the heart.

Remember it like this:

Artery – carries blood Away from the heart

VeIN – carries blood back INto the heart

With both arteries and veins, their structure is related to their function.

Arteries

Blood in the arteries is under high pressure generated by the heart. The arteries therefore have:

• thick walls - to resist the high pressure of the blood
• a thick layer of elastic fibres – to allow the artery to stretch when a surge of blood passes through it, and then recoil in between heart beats to maintain blood pressure
• a thick layer of muscle within the wall – to allow blood to be diverted to where it is needed in the body

Veins

Blood in the veins is under less pressure. The veins therefore have:

• thin walls as they have blood with a lower pressure flowing through them

• one-way valves in them to prevent blood flowing back in the opposite direction

Stents

In order to keep beating, the heart muscle has its own artery called the coronary artery, which supplies the heart with glucose and oxygen. For patients who have heart disease, arteries can become narrower due to the build-up of fatty deposits within the wall of the artery. This has the effect of narrowing the lumen of the artery, reducing the amount of oxygenated blood that can be supplied to the heart muscle.Stents are metal grids which can be inserted into an artery to maintain blood flow by keeping the artery open. To insert a stent, a catheter with a balloon attached to it is inserted into a blood vessel in the leg. The balloon has the metal stent on it. The catheter is directed to the coronary artery. When the narrowed section of artery is found, the balloon is inflated which causes the stent to expand, and it becomes lodged in the artery. The stent then acts to keep the artery open so that the heart continues to receive enough oxygen to function effectively.

Stents are good alternatives to more risky operations, like by-pass surgery, providing the patient’s heart disease is not too serious. However, fatty deposits may build up on the stent over time - meaning that blood flow to the heart muscle may be reduced again.

Capillaries

Capillaries are the smallest type of blood vessel, and are adapted to allow the effective exchange of substances between the blood and the tissues of the body.

Capillaries are made of thin cells, meaning that some parts of the blood can easily leave the capillary, bathing the cells in a fluid known as tissue fluid.

Useful substances within the tissue fluid - including glucose, oxygen and amino acids - can then diffuse into the cells down a concentration gradient. The concentration gradient is always maintained as the useful substances are constantly being used up by the cell.

Waste substances generated by the cell diffuse out of the cell, and back into the tissue fluid. Most of the tissue fluid is then reabsorbed back into the blood, and with it the waste substances – such as carbon dioxide and urea – which are taken away to be excreted.

A concentration gradient is always maintained as the cell constantly generates more waste substances, and the blood constantly takes them away.

Blood is made of four constituent parts - red blood cells, white blood cells, platelets and plasma. Each part plays a vital role in ensuring that blood can meet its two primary roles, to transport substances around our body and to defend against infection by potential pathogens.

Blood is used to transport materials around the body and to protect against disease.

Blood is a tissue which includes liquid, cells, cell fragments and solutes.

Red blood cells

Red blood cells are tiny, nucleus-free cells which carry oxygen from the lungs to tissues. Oxygen transport is efficient because:

• there are huge numbers of red blood cells
• the cells are tiny so they can pass through narrow capillaries
• the cells have a flattened disc shape to increase surface area - allowing rapid diffusion of oxygen
• the cells contain haemoglobin - which transports oxygen and carbon dioxide around the body

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Blood appears bright red when oxygenated and dark red when deoxygenated. In oxygen-rich environments (ie the lungs), haemoglobin combines with oxygen to form oxyhaemoglobin. In low-oxygen environments (such as body cells), oxyhaemoglobin releases the oxygen to become haemoglobin again.

This process is summarised here:

White blood cells

Different types of white blood cells exist. Some white blood cells can engulf bacteria and other pathogens by phagocytosis. They can change shape easily and produce enzymes which digest the pathogens.

Other types of white blood cell secrete antibodies and antitoxins to help destroy pathogens.

Blood plasma

Plasma is a straw-coloured liquid which makes up about 55 per cent of blood. It transports dissolved substances around the body. These include:

• hormones

• antibodies

• nutrients - such as water, glucose, amino acids, minerals and vitamins

• waste substances - such as carbon dioxide and urea

The table below gives more detail about the transportation of nutrients and waste products in plasma.

Substance typeSubstanceMoved fromMoved to Nutrients Soluble products of digestion Small intestine Organs of the body Waste Carbon dioxide Organs of the body Lungs Waste Urea Liver Kidneys

Platelets

Platelets are small fragments of cells, but they do not possess a nucleus. They are involved in the process of forming clots at sites where there is a wound, eg a cut or graze.

Plants have two systems for the transportation of substances - using two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, while phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.

Xylem and phloem

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

Xylem

Xylem vessels are involved in the movement of water through a plant - from its roots to its leaves via the stem.

During this process:

1. Water is absorbed from the soil through root hair cells.

2. Water moves by osmosis from root cell to root cell until it reaches the xylem.

3. It is transported through the xylem vessels up the stem to the leaves.

4. It evaporates from the leaves (transpiration).

The xylem tubes are made from dead xylem cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. The rest of the xylem cell has a thick, reinforced cell wall which provides strength

Phloem

Phloem vessels are involved in translocation. Dissolved sugars, produced during photosynthesis, and other soluble food molecules are moved from the leaves to growing tissues (eg the tips of the roots and shoots) and storage tissues (eg in the roots).

In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down, but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Transpiration

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what has been lost. Water molecules have a tendency to stick together – so as water leaves the xylem to enter the leaf, more water is pulled up behind it. This produces a continuous flow of water and dissolved minerals moving up the xylem tube from the roots, up the stem, and into the leaves. This is known as the transpiration stream.

Movement of water through the roots

The movement of water up the xylem means more water must be drawn in through the roots from the soil. To do this, water passes from root cell to root cell by osmosis.As water moves into the root hair cell down the concentration gradient, the solution inside the root hair cell becomes more dilute. This means that there is now a concentration gradient between the root hair cell and adjacent root cells, so water moves from the root hair cell and into the adjacent cells by osmosis.This pattern continues until the water reaches the xylem vessel within the root - where it enters the xylem to replace the water which has been drawn up the stem.

Factors that affect transpiration rate

• light; Transpiration increases in bright light, The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis. More water is therefore able to evaporate.
• temperature; Transpiration is faster in higher temperatures, Evaporation and diffusion are faster at higher temperatures
• wind; Transpiration is faster in windy conditions, Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf.
• humidity; Transpiration is slower in humid conditions, Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. If the loss of water is faster than the rate at which it is being replaced by the roots, then plants can slow down the transpiration rate by closing some of their stomata. This is regulated by guard cells, which lie on either side of a stoma.If the guard cells are turgid, then they curve forming ‘sausage-shaped’ structures with a hole between them. This is the stoma. However, if the guard cells are flaccid due to water loss, they shrivel up and come closer together, closing the stoma. This is turn reduces the water loss due to transpiration, and can prevent the plant from wilting.