Control of ventilation in humans

Ventilation of the respiratory system in humans is primarily controlled by the breathing centre in the medulla oblongata. The ventral portion of this centre controls inspiratory movements and is called the inspiratory centre; the remainder controls breathing out and is called the expiratory centre. Control also relies on chemoreceptors in the carotid and aortic bodies of the blood system. These are sensitive to minute changes in the concentration of carbon dioxide in the blood. When the carbon dioxide level rises, increased ventilation of the respiratory surfaces is required. Nerve impulses from the chemoreceptors stimulate the inspiratory centre in the medulla. Nerve impulses pass along the phrenic and thoracic nerves to the diaphragm and intercostal muscles. Their increased rate of contraction causes faster inspiration.

As the lungs expand, stretch receptors in their walls are stimulated and impulses pass along the vagus nerve to the expiratory centre in the medulla. This automatically ‘switches off’ the inspiratory centre, the muscles relax and expiration takes place. The breathing centre may also be stimulated by impulses from the forebrain resulting in a conscious increase or decrease in breathing rate. The main stimulus for ventilation is therefore the change in carbon dioxide concentration and stimulation of stretch receptors in the lungs.

Speaking

1. Imagine that your younger sister (brother) doesn’t like walking. Explain the importance of fresh air and its role in the functioning of our organism.

2. How would you persuade someone smoking that he damages his own health.

Chapter 3. Blood and circulatory system.

Did you know?

The heart pumps out 13 000

litres of blood each day.

As all cells are bathed in an aqueous medium, the delivery of materials to and from these cells is carried out largely in solution. The fluid in which the materials are dissolved or suspended is blood. While a number of ideas on blood were put forward by Greek and Roman scientists, it was the English physician William Harvey (1578-1657) who first showed that it was pumped into arteries by the heart, circulated around the body and returned via veins.

The purpose of the mammalian circulatory system is to carry blood between various parts of the body. Each organ has a major artery supplying it with blood from the heart and a major vein which returns it. These arteries and veins are usually named by preceding them with the adjective appropriate to that organ, e.g. each kidney has a renal artery and a renal vein. The flow of blood is maintained in three ways:

1. The pumping action of the heart - This forces blood through the arteries into the capillaries.

2. Contraction of skeletal muscle - The contraction of muscles during the normal movements squeeze the thin-walled veins, increasing the pressure of blood within them.

3. Inspiratory movements - When breathing in, the pressure in the thorax is reduced. This helps to draw blood towards the heart, which is within the thorax.

Did you know?

We each produce 200 billion

new red blood cells every day.

Essential Vocabulary

Apex to adhere artificial

arteriole to arrest biconcave

beat to attach bicuspid

chamber to circulate excretory

cranium to contract mitral

cycle to dilate

erythrocyte

extremity

leucocyte

lymphocyte

node

output

platelet

pump

thrombocyte

valve

ventricle

Analytical reading

Heart structure and action

A pump to circulate the blood is an essential feature of most circulatory systems. These pumps or hearts generally consist of a thin-walled chamber - the atrium (auricle) - and a thick-walled pumping chamber - the ventricle.

The mammalian heart consists largely of cardiac muscle, a specialized tissue which is capable of rhythmical contraction and relaxation over a long period without fatigue. The muscle is richly supplied with blood vessels and also contains connective tissue which gives strength and helps to prevent the muscle tearing.

The mammalian heart is made up of two thin-walled atria which are elastic and distend as blood enters them. The left atrium receives oxygenated blood from the pulmonary veins while the right atrium receives deoxygenated blood from the venae cavae. When full, the atria contract together, forcing the remaining blood into their respective ventricles. The right ventricle then pumps blood to the lungs. Owing to the close proximity of the lungs to the heart, the right ventricle does not need to force blood far and is much less muscular than the left ventricle which has to pump blood to the extremities of the body. To prevent backflow of blood into the atria when the ventricles contract, there are valves between the atria and ventricles. On the right side of the heart these comprise the tricuspid valves. On the left side of the heart the bicuspid or mitral valves are present.

All vertebrate hearts are myogenic, that is, the heart beat is initiated from within the heart muscle itself rather than by a nervous impulse from outside it. Where it is initiated by nerves, as in insects, the heart is neurogenic.

The initial stimulus for a heart beat originates in a group of cardiac muscle cells known as the sino-atrial node (SA node). This is located in the wall of the right atrium. The SA node determines the basic rate of heart beat and is therefore known as the pacemaker. In humans, this basic rate is 70 beats per minute. A wave of excitation spreads out from the SA node across both atria, causing them to contract. Then the wave reaches the atrio-ventricular node (AV node) which lies between the two atria. The ventricles contract from the apex upwards. These events are known as the cardiac cycle.

The rate of heart contractions can be varied from 50 to 200 beats per minute. Another important factor in controlling blood pressure is the diameter of the blood vessels. When narrowed - vasoconstriction - blood pressure rises; when widened - vasodilation - it falls. Vasoconstriction and vasodilation are also controlled by the medulla oblongata. From this centre nerves run to the smooth muscles of arterioles

throughout the body. Pressure receptors, known as baroreceptors, in the carotid artery detect blood pressure changes. If blood pressure falls, the vasomotor centre sends impulses along sympathetic nerves to the arterioles. The muscles in the arterioles contract, causing vasoconstriction and a consequent rise in blood pressure. A rise in blood pressure causes the vasomotor centre to send messages via the parasympathetic system to the arterioles, causing them to dilate and so reduce blood pressure.

Summary

Structure and functions of blood

Blood comprises a watery plasma in which are a variety of different cells. The majority of cells present are erythrocytes or red blood cells which are biconcave discs about 7 µm in diameter. They have no nucleus and are formed in the bone marrow. The remaining cells are the larger, nucleated white cells or leucocytes. Most of these are also made in the bone marrow. There are two basic types of leucocyte. Granulocytes have granular cytoplasm and a lobed nucleus; they can engulf bacteria by phagocytosis. Agranulocytes have a non-granular cytoplasm and a compact nucleus. Some of these also ingest bacteria but the lymphocytes, made mainly in the thymus gland and lymphoid tissues, produce antibodies. More sparsely distributed in the plasma are tiny cell fragments called platelets. These are important in the process of blood clotting.

The best known and most efficient respiratory pigment is haemoglobin. It occurs in most animal. The haemoglobin molecule is made up of an iron porphyrin compound - the haem group - and a protein -globin. The haem group contains a ferrous iron atom, which is capable of carrying a single oxygen molecule. Different haemoglobins have a different number of haem groups and so vary in their ability to carry oxygen. A single molecule of human haemoglobin, for example, possesses four haem groups. Therefore it is capable of carrying four molecules of oxygen.

Formation of blood. In the fetus red blood cells are formed in the liver, but in adults production moves to bones, such as the cranium, sternum, vertebrae and ribs, which have red bone marrow. White cells like lymphocytes are formed in the thymus gland and lymph nodes whereas other types are formed in bones, e.g. the long bones of the limbs, which have white bone marrow.

Blood performs two distinct functions: the transport of materials and defence against disease. Summary of the transport functions of blood is given in the Table:

Materials transported	Examples	Transported from	Transported to	Transported in
Respiratory gases	Oxygen	Lungs	Respiring tissues	Haemoglobin in red blood cells
	Carbon dioxide	Respiring tissues	Lungs	Haemoglobin in red blood cells. Hydrogen carbonate ions in plasma
Organic digestive products	Glucose	Intestines	Respiring tissues / liver	Plasma
	Amino acids	Intestines	Liver/body tissues	Plasma
	Vitamins	Intestines	Liver/body tissues	Plasma
Mineral salts	Calcium	Intestines	Bones /teeth	Plasma
	Iodine	Intestines	Thyroid gland	Plasma
	Iron	Intestines /liver	Bone marrow	Plasma
Excretory products	Urea	Liver	Kidney	Plasma
Hormones	Insulin	Pancreas	Liver	Plasma
Heat	Metabolic heat	Liver and muscle	All parts of the body	All parts of the blood

Did you know?

The entire length of arteries, veins

and capillaries in the human body