Topic 1 Unit 1 Snab Edexcel

  • Created by: Tamsin
  • Created on: 12-05-10 16:12

Capillary Bed

At the atrial end of a capillary, blood is under high pressure. This forces fluid out through the capillary walls into the intercellular spaces forming tissue fluid. This contains water and small molecules found in plasma. The capillary walls prevent blood cells and larger plasma proteins from passing through.

At the venous end of the capillary blood pressure is lower so fluid isn’t being forced outwards. Due to the presence of blood cells and plasma proteins and loss of fluid the blood is more concentrated. Therefore fluid moves back into the capillary by osmosis. In addition around 20% of the tissue fluid returns to circulation via the lymph vessels.

Cardiovascular Disease

CVD’s: diseases of heart and circulation

CHD: form of CVD

In small organisms diffusion moves digestive products oxygen and carbon dioxide around the body. However, most multicelluar organisms are too large for diffusion so they have a circulatory system.

Open Circulatory System

Blood circulates large open spaces from a single heart. Substances can diffuse between blood and cells. When the heart relaxes, blood is drawn from the cavity back into the heart, through small valved openings along its length.

Closed Circulatory Systems

Blood is enclosed with tubes. Higher blood pressure as blood is forced along narrow tubes. Blood travels faster so system is more efficient. Heart>Arteries>Arterioles>Capillaries. There are a large number of capillaries and they are close to cells where substances can be exchanged. Capillaries>Venules>Veins>Heart. Valves make sure that blood flows in one direction.

Single Circulatory Systems: Closed circulatory system. Found in fish. Heart pumps deoxygenated blood to the gills, this is the site of gas exchange, and blood leaves gills and flows around the body back to the heart.

Double Circulatory System: Birds and mammals. Right ventricle pumps deoxygenated blood to lungs to get oxygen. Oxygenated blood returns to the heart to be pumped out by left ventricle to the rest of the body. This means that there is a high metabolic rate due to the substances being delivered quicker.

The Transport Medium

The oxygen in water is slightly negative and the hydrogen is slightly positive so overall the molecule is polar due to an unevenly distributed charge. The negative oxygen’s attracting to the hydrogen’s and forming hydrogen bonds.

Polar molecules dissolve easily in water=hydrophilic


If water wasn’t polar that plasma wouldn’t be able to dissolve and more substances such as proteins, amino acids, urea, salts, enzymes, hormones and antibodies.

Arteries and Veins

Collagen: tough fibrous protein which makes arteries and veins durable and strong

Elastic fibres: allows them to stretch and recoil

Smooth muscle: allows them to constrict and dilate



Narrow Lumen

Wide Lumen

Thicker Walls

Narrow Walls

More collagen, elastic fibres and smooth muscle

Less collagen, elastic fibres and smooth muscle

No Valves


Capillaries join small arteries and venules. They are only one cell thick (endothelium)

How does blood move through the vessels?

Contract: Systole

Dilate: Diastole

Heart contracts>blood forced into arteries>elastic walls stretch to contain blood>elasticity of artery walls


Cecilia Bawden


This is brilliant, thank you! There's just one bit with the arteries and veins where they're mixed up? 

Chris Brown


sideman ^



can any one help me to find a good AS revision note.  Please !!!!