6 HUMAN HEALTH AND PHYSIOLOGY 1 135 Our circulatory system provides a delivery and collection service
for the whole body. The heart, blood and blood vessels make up a most effi cient transport system that reaches all cells, bringing the substances they need and taking away their waste. Humans and other mammals have what is known as a closed circulatory system with blood contained inside a network of arteries, veins and capillaries.
Atria are separated from ventricles by atrioventricular valves, which prevent the blood fl owing backwards into the atria. A second set of valves in the aorta and pulmonary arteries – the semilunar valves – prevent backfl ow into the ventricles as they relax after a contraction.
Heart muscle works continuously, beating about 75 times per minute when a person is resting, and so it has a large demand for oxygen. Coronary arteries extend over the surface of the heart and penetrate deep into the muscle fi bres to supply oxygen and nutrients for this unremitting activity (Figure 6.5).
The cardiac cycle
The cardiac cycle is the sequence of events that takes place during one heart beat (Figure 6.6 overleaf ). As the heart’s chambers contract, blood inside them is forced on its way. Valves in the heart and arteries stop the blood fl owing backwards.
Control of the heart beat
Heart tissue is made of a special type of muscle that is diff erent from other muscles in our bodies. Cardiac muscle is unique because it contracts and relaxes without stimulation from the nervous system. It is said to be myogenic. Natural myogenic contractions are initiated at an inbuilt pacemaker, which keeps cardiac muscle working in a coordinated, controlled sequence. The pacemaker, or sinoatrial node (SAN), is special region of muscle cells in the right atrium that sets the basic pace of the heart. The rate set by the SAN is also infl uenced by stimulation from the nervous system and by hormones.
At the start of every heart beat, the SAN produces an impulse that stimulates both atria to contract. A second structure, the atrioventricular node (AVN) at the base of the right atrium, is also stimulated. It delays the impulse briefl y until the atrial contraction fi nishes and then transmits it on down a bundle of modifi ed muscle fi bres – the bundle of His and Purkinje fi bres – to the base of the ventricles. Impulses radiate up through the ventricles, which contract simultaneously about 0.1 seconds after the atria.
The natural rhythm of the pacemaker is modulated by the nervous system so that the heart rate is adjusted to our activity levels. It speeds up when we are exercising and need extra oxygen and nutrients, and slows down as we sleep. Changes to our heart rate are not under our conscious control but result from impulses sent from a control centre in the part of the brain stem known as the medulla. Impulses to speed up the heart pass along the sympathetic nerve, which stimulates the pacemaker to increase its rate. Impulses sent along the parasympathetic (vagus) nerve cause the heart rate to slow down. The medulla monitors blood pressure and carbon dioxide levels using information it receives from receptors in arteries.
Emotions such as stress, as well as increases in activity level, can cause an increase in heart rate. During periods of excitement, fear or stress the adrenal glands release the hormone adrenalin, which travels in the blood to the pacemaker and stimulates it to increase the heart rate.
Cardiac muscle will contract rhythmically in tissue culture if it is supplied with oxygen and glucose.
Its normal resting rate in culture is about 50 beats per minute.
Sympathetic and parasympathetic nerves Sympathetic and parasympathetic nerves are part of the autonomic nervous system, which controls activities, such as heart rate, that are not under our conscious control.
Figure 6.5 A human heart. Clearly visible are the coronary arteries, which supply oxygen to the heart muscle.
6 HUMAN HEALTH AND PHYSIOLOGY 1 137
Blood and blood vessels
Arteries are blood vessels that carry blood away from the heart. They branch and divide many times forming arterioles and eventually the tiny capillaries that reach all our tissues. Arteries have thick outer walls of collagen and elastic fi bres (Figure 6.7, overleaf ), which withstand high blood pressure and prevent vessels becoming overstretched or bursting.
Just beneath the outer covering is a ring of circular smooth muscle that contracts with each heart beat to maintain blood pressure and keep blood moving along. Inside an artery, the lumen is narrow to keep blood pressure high. The lumen’s lining of smooth epithelial cells reduces friction and keeps blood fl owing smoothly.
Capillaries are the smallest vessels – the lumen of a capillary is only about 10 μm in diameter and some are so small that red blood cells must fold up in order to pass along. Networks of these tiny capillaries reach
pulmonary artery
left atrium
left ventricle
semilunar valves
atrioventricular valve 1 The muscles of the atrium wall contract,
pushing blood through the atrioventricular valves into the ventricles. Both atria contract at the same time. This is called atrial systole.
5 The whole cycle is repeated when the atria contract again.
2 Blood forced into the ventricles causes the blood pressure inside them to rise, so the atrioventricular valves snap closed. When the ventricles are full, ventricle muscles contract, generating the pressure that drives blood through the semilunar valves into the aorta and the pulmonary artery. This is ventricular systole. A pulse is produced that can be felt in arteries in other parts of the body.
4 Blood flows into the atria from the veins, opens the atrioventricular valves, and begins to fill the ventricles.
Blood from the body enters the right atrium via the vena cava. Blood from the lungs entersthe left atrium from the pulmonary artery.
3 Ventricles and atria now relax, and the pressure inside them is low. The semilunar valves are closed by the back pressure of blood in the arteries.
This part of the cycle is called diastole.
Key
pressure exerted by contraction of muscle movement of blood
Figure 6.6 The events of a heart beat – the cardiac cycle. The heart normally beats about 75 times per minute, and a complete heart beat takes about 0.8 seconds.
TS small artery
TS capillary
TS small vein
endothelium: a very smooth, single layer of cells
lumen, just big enough for a red cell to squeeze through
wall made of endothelium one cell thick endothelium: thinner
than that of the artery
relatively large lumen
very thin layer containing some smooth muscle and elastic fibres
collagen fibres
10 μm
0.7 mm
relatively narrow lumen
elastic fibres, collagen fibres and smooth muscle collagen fibres and some elastic fibres
almost every body cell. Blood fl ow here is very slow, at less than 1 mm per second, but capillary walls are only one cell thick so the distance for diff usion of materials in and out of them is as small as possible. Some capillaries have spaces between their cells enabling plasma and phagocytes to leak out into the tissues.
Veins carry blood back towards the heart from body tissues. Small veins called venules join up to form large veins, which can be distinguished from arteries by their much thinner walls, which contain few elastic and muscle fi bres. Blood inside a vein does not pulse along and the lumen is large to hold the slow-moving fl ow. The relatively thin walls can be compressed by adjacent muscles and this helps to squeeze blood along and keep it moving. Many veins contain valves to prevent blood fl owing backwards, a problem which can arise if fl ow is sluggish.
Table 6.3 summarises the diff erences and similarities between the three types of blood vessel.
Figure 6.7 The structure of arteries, veins and capillaries.
Artery Vein Capillary
thick walls thin walls walls one cell thick
no valves valves present no valves
blood pressure high blood pressure low blood pressure low
carry blood from the heart carry blood to the heart link small arteries to small veins Table 6.3 Comparing arteries, veins and capillaries.
Lymphatic system
The lymphatic system collects tissue fl uid that leaks from the capillaries and returns it to the large veins close to the heart.
Plasma that leaks out of tiny capillaries bathes the nearby tissues and supplies oxygen and nutrients to the cells. Once out of the capillary, the fl uid is known as tissue fl uid.
6 HUMAN HEALTH AND PHYSIOLOGY 1 139
Composition of blood
Blood plasma is a pale yellow liquid that makes up 50–60% of our blood volume. Suspended in plasma are three important groups of cells:
•
erythrocytes (red blood cells), whose job is to carry oxygen•
leucocytes (white blood cells), which fi ght disease•
platelets (cell fragments), which are needed for blood clottingFigure 6.8 shows the appearance of blood when examined under a light microscope.
Functions of blood
Blood has two important roles: it is a vital part of the body’s transport network, carrying dissolved materials to all cells, and it helps to fi ght infectious disease. Table 6.4 (overleaf ) summarises the important
substances the blood carries. Its role in infection control will be explored in the next section.
Figure 6.8 Light micrograph of a smear of healthy human blood. Two types of leucocyte can be seen – a lymphocyte (lower left) and a phagocyte (upper right). Lymphocytes produce antibodies, and phagocytes engulf foreign particles, including bacteria, that enter the body. Many erythrocytes can also be seen.