Heart and circulation ECG and Blood vessels

The ECG trace for each heartbeat displays a P wave, a QRS wave or complex and a T wave Q R S P wave T wave The ECG Trace

Q R S P wave T wave The P wave is the result of depolarisation spreading across the atria from the SA node; it coincides with atrial contraction or systole The QRS wave or complex is the result of depolarisation of the ventricles and coincides with ventricular systole The T wave is the result of repolarisation of the ventricles as the ventricles begin to relax; repolarisation of the atria is not detected as the small voltage changes involved are masked by the QRS wave

Blood vessels Arteries carry blood away from the heart, each organ has a main artery supplying blood e.g. renal artery to the kidneys, hepatic artery to the liver. The arteries branch into smaller vessels called arterioles These branch into tiny vessels called capillaries

Structure of arteries A tough outer layer of collagen for strength. Elastic fibres which recoil after each pulse of blood. This evens out the blood flow and maintains blood pressure. Elastic recoil is particularly important in the major arteries like the aorta. Smooth muscle which contracts to narrow(constrict) the artery, reducing blood flow. Endothelium a single layer of squamous epithelial cells which provide a smooth surface for blood to flow over. (smooth surface =less friction)

Arterioles Smooth muscle in the arteriole walls enables different quantities of blood to be directed to different organs. During exercise the arterioles in the muscles dilate to increase the blood flow bringing more oxygen to muscle cells for respiration to release energy. In a cold environment, smooth muscle in the walls of the arterioles bringing blood to the skin contracts. This narrows(constricts) the arterioles in the skin so less blood flows through the capillary networks in the skin and less heat is lost from the body Muscle in the artery walls DOES NOT PUSH THE BLOOD ALONG!!!

Capillaries Capillaries are about 8- 10µm in diameter (about 0.01mm) Capillaries have walls which are one cell thick, made of squamous epithelial cells that have small gaps or pores between them. There is no smooth muscle or elastic tissue. Blood plasma escapes from the capillaries and forms tissue fluid which bathes all the body tissues

Blood cells in a capillary

Veins Have a wider lumen Thin layers of elastic tissue and smooth muscle –veins do not need to constrict to reduce blood flow. Blood pressure in veins is lower

Veins continued Valves in veins prevent back flow of blood

Pressure changes in blood vessels In the aorta blood flows in surges. Pressure is high as the ventricles in the heart contract (systole) Pressure falls when the ventricles relax (diastole)

Changes in blood flow across capillaries Although individual capillaries have a tiny diameter, adding the diameter of all the capillaries together provides a total wide diameter in a capillary network. The wide diameter leads to a drop in blood pressure and a slowing of blood flow, allowing time for the exchange of gases and nutrients between the tissues and the blood

Constituents of blood

Formation of tissue fluid