Learning Objectives: 1. To understand how blood pressure and velocity changes during exercise. 2. To know the mechanisms that aid venous return. 3. To be able to explain the transport of O 2 and CO 2 at rest and during exercise.

Blood travels through a series of blood vessels when going to and from the muscles (or any other body part). * Arteries * Arterioles * Capillaries * Body part (e.g. muscle) * Capillaries * Venules * Veins

* Both veins and arteries have a structure containing an inner endothelium, a middle layer of smooth, elastic fibres and an outer fibrous layer. * In arteries the middle layer is thicker to allow them to withstand higher blood pressure. * Veins have valves to prevent backflow.

* Blood is shunted to the working muscles (and away from other organs) during exercise due to their greater demand for oxygen. * Blood supply to the skin also increases. * This is done through the vasodilation (widening) of arterioles supplying muscles and the vasoconstriction (narrowing) of arterioles supplying other organs. * This is controlled by the sympathetic nervous system as factors such as blood acidity, O 2 levels etc are detected.

* Systolic pressure: blood pressure when heart contracts. * Diastolic pressure: blood pressure when heart relaxes. * Blood pressure reduces as blood moves further from the heart due to friction and an increase in surface area (blood gets more spread out). * Blood velocity also reduces as blood moves further from the heart, however it increases when blood moves from capillaries to venules and veins as surface area decreases.

Blood pressure in large veins is so low that mechanisms are required to allow the blood to return to the heart against the effects of gravity. These include: * Valves (found only in veins) * Skeletal muscle pump – when muscles contract they squeeze veins forcing blood back to the heart. Exercise increases this. A sudden stop will result in ‘pooling’. * Respiratory pump – breathing movements forcing blood back to the heart.

* Oxygen is transported in haemoglobin in red blood cells (oxyhaemoglobin). * Where O 2 concentration is high (e.g. the lungs), haemoglobin becomes fully (100%) saturated. * Where O 2 concentration is lower (e.g. working muscles), the percentage saturation of haemoglobin is lower.

* The amount of 0 2 released from haemoglobin is affected by the partial pressure concentration of oxygen. * It is also affected by blood acidity levels. As more CO 2 and lactic acid are produced, oxygen splits more readily from haemoglobin. * This is known as the Bohr Shift. * Increases in temperature have a similar effect. * Myoglobin in the muscle tissues takes up oxygen even more readily than haemoglobin and acts as an oxygen store in muscles. Bohr shift video clip

The carbon dioxide that enters the body via the alveoli is transported in three ways: * 7% in blood plasma * 23% in haemoglobin * 70% as bicarbonate ion.

* Gas exchange at the muscles is mainly effected by the partial pressure of gases involved. * Other factors such as an increase in temperature and decrease in pH also encourage O 2 to diffuse from blood into muscles. * The aterio-venous difference (a-vO 2 ) is the difference between the O 2 content of arterial blood and venous blood (i.e. the amount of O 2 that the muscle uses). * At rest a-vO 2 is low, during exercise it is high. * Training increases a-vO 2 as individuals can extract more O 2 from blood.