Chapter 20 - Anatomy and Physiology

Human anatomy refers to the various structures that make up the body. The body's organs are formed by specialised cells and tissues that perform specific functions How these organs function and interact is known as physiology. The anatomy and physiology of each organ and organ system are highly complex, with only a superficial, essential level of knowledge presented in this chapter.

CARDIOVASCULAR SYSTEM

The cardiovascular system facilitates the movement of blood through the body, allowing organs to receive both oxygen and nutrients as well as the removal of waste products. The heart serves as a pump that drives blood through the arteries (pipes) that terminate in the organs. From the organs and tissues, blood continues to circulate then returns to the heart via the veins.

What is the anatomy of the heart?

The heart is located within the thoracic (chest) cavity of the body. It is a muscular organ, roughly the size of a fist and contains four heart chambers - two atria (singular: atrium) and two ventricles (singular: ventricle) (Figure 159). The left and right atria are separated from the left and right ventricles by heart valves. The muscle that forms the heart walls are supplied with blood by vessels known as coronary arteries. The heart also contains an electrical system that controls the rate at which the heart contracts (heart rate) and ensures this contraction is coordinated correctly.

How does the heart perform its function?

The role of the heart is to pump blood around the body. Blood returns to the heart via veins that form a single large vein known as the vena cava that connects to the heart. The vena cava directly fills the right atrium with blood. Next, the blood moves from the right atrium to the right ventricle through a heart valve (tricuspid valve). For blood to collect in the right ventricle, the heart muscle must be relaxed (known as diastole). Once enough blood has collected in the right ventricle, the muscular walls of the right ventricle contract (known as systole), which pumps the blood into the pulmonary artery which supplies the lungs. This same process occurs simultaneously on the left side of the heart. Blood returns to the heart from the lungs in the pulmonary vein. The pulmonary vein is connected to the left trium, where blood collects before moving to the left ventricle through another heart valve (mitral valve). Again, this occurs during diastole, where the muscular wall of the left ventricle is relaxed. When the left ventricle is full of blood, it contracts (systole) and pumps blood in the aorta.

What is the vascular system?

The vascular system refers to the blood vessels (arteries and veins) that circulate blood around the body.

After passing through the tissues, deoxygenated blood (blood that has a low oxygen content) and metabolic waste products are carried through the venous capillary network of pipes into the venules. These venules come together to form larger structures called veins. Veins are low-pressure vessels that have thin walls. Blood moves through them in an almost passive fashion due to pressure gradients in the body. Veins form the vena cava just before connecting to the heart to be pumped around the circulation again. Venous blood is, therefore, generally deoxygenated as the tissues of the organs have consumed the oxygen present in the blood before returning to the heart. The exception is the pulmonary vein that returns blood to the heart from the lungs, where blood becomes oxygenated.

The arterial system generally carries oxygenated blood away from the heart to supply the tissues of the body. The pulmonary artery is the exception as it carries deoxygenated blood away from the heart to the lungs. The aorta is the main artery that leaves the heart as a large, high-pressure vessel that goes on to supply all other arteries within the body (Figure

161). Compared to the right ventricle, the left ventricle has to work harder and generate higher pressures, and therefore it is thicker and more muscular to achieve this. The arteries themselves are also thick and muscular to be able to tolerate the relatively high pressures.

Therefore, when you take your blood pressure, it is the pressure within the arteries that is measured. A blood pressure measurement has two readings - either high or low. Each one refers refer to pressure within the artery when the left ventricle contracts (high reading, systolic) and when the left ventricle relaxes (low reading, diastolic). The arteries all eventually terminate in a microscopic pipe network known as capillaries within the tissues and organs.

It is there that the cells can take up oxygen and nutrients from the blood.

RESPIRATORY SYSTEM

What is the anatomy of the respiratory system?

The respiratory system includes the airways and the lungs (Figure 161). The airways enable the transfer of air from the external environment into the lungs. The nose and mouth provide the external orifices that allow air to enter the body. As this occurs, the nose and mouth humidify and clean the air to protect the lungs from foreign material and infective organisms.

The air then travels down the throat (pharynx) and voice box (larynx) to enter the trachea (windpipe). This trachea is made of cartilage and runs down the neck into the chest (thoracic) cavity, where it splits into a left and right bronchus. The left and right bronchi supply the left and right lungs, respectively. The main bronchi split further into smaller and smaller bronchioles within each lung and form a tree-like network of branches made up of airways within the lungs. The bronchioles terminate with multiple tiny inflatable pockets called alveoli.

The alveoli are the functional unit of the lung, and this arrangement leads to a vast surface area where gas exchange can occur (Figure 162).

How does air from the atmosphere get into the lungs?

The respiratory system enables the uptake of oxygen from the atmosphere into the blood and the disposal of carbon dioxide from the blood to the atmosphere. To perform this function, both mechanical and chemical processes are required.

The first process is a mechanical process, known as breathing or ventilation, where air moves from the atmosphere into the alveoli. When a person inhales, the volume of the thoracic cavity increases as the diaphragm moves down and the rib cage moves outwards. This creates a slightly negative pressure within the thoracic cavity, so air is drawn through the nose and mouth down into the alveoli.

Inspiration is an active process that requires energy and can occur voluntarily (taking a deep breath in) or involuntarily (breathing at rest). In contrast, expiration is a passive process as the diaphragm and ribs move back to their resting positions, causing the thoracic volume to decrease and moving air from the alveoli through airways to the atmosphere.

How does oxygen from the atmosphere get into the blood?

Once air enters the alveoli, the second process, a chemical process that is known as gas exchange, occurs. As the name implies, gas exchange involves exchanging one gas for another. In this context, oxygen within the alveoli is exchanged for carbon dioxide within the blood of the capillaries that surround them.

Atmospheric air that is drawn into the alveoli has a high partial pressure of oxygen. Partial pressure means the pressure exerted by a specific gas when it is in a mixture of gases, if it were to occupy the same volume. This is a complicated concept that involves various physical gas laws and is beyond the scope of this text. It is akin to the concentration or amount of the gas but is not strictly true. Venous blood that returns from the body is relatively deoxygenated and has a relatively low partial pressure of oxygen. This blood is pumped from the right ventricle into the pulmonary artery, which branches into smaller vessels. These eventually become microscopic capillary vessels that intimately surround the alveoli.

The air within the alveoli contains inhaled atmospheric air. Atmospheric air in ISA conditions has a total air pressure of 760mmHg (medicine tends to use mmHa as primary units of pressures, this is equivalent to 1013hPa or 1013hPa). Oxygen occupies 21% of this air, so inhaled air has an oxygen partial pressure of 159mmHg. Due to the humidification process that occurs as inhaled air passes through the airways, the partial pressure of oxygen reduces to 101mmHg when it reaches the alveoli. The venous deoxygenated blood within the capillaries that surround the alveoli have a low partial pressure of oxygen, approximately 40mmHg. Therefore, there is a partial pressure difference in oxygen between the alveoll and the venous blood surrounding it. This eauses oyvaen to move via a process known as diffusion, where a substance moves from where it's in high concentration to where it is in lower concentration (Figure 164). Hence, it moves from the alveoli into the blood of the capillaries. This process is highly efficient due to the very thin alveoli wall, the one-cell-thick capillary blood vessels and the surfactant solution that cover the alveoli, taking approximately 0.25 seconds. Once it has diffused across the alveolar and capillary wall, oxygen will bind to a molecule known as haemoglobin. Haemoglobin is a protein molecule that is contained within the red blood cells of the blood. The red blood cells can carry the oxygen through the arterial system and supply the various tissues and cells of the body.

Carbon dioxide is a waste product produced by all metabolically active tissues which needs to be removed from the tissues and transported to the lunas to be exhaled into the atmosphere.

Carbon dioxide is carried within the blood of the veins and ends up in the alveolar capillaries via the pulmonary vein. Gas exchange of carbon dioxide occurs in the opposite direction to oxygen, where carbon dioxide moves from the capillaries into the alveoli. This is again due to a partial pressure gradient that exists between the venous blood within the capillaries and the alveoli. The carbon dioxide produced within the capillaries results in a higher partial pressure of carbon dioxide than the near-zero carbon dioxide partial pressure within the inspired air of the alveoli. This allows carbon dioxide to enter the alveoli and be expelled to the atmosphere during the exhalation process.

THE EYES AND VISION

What are the main anatomical features of the eye?

Figure 165 shows the kev anatomical components of the eve: the clear cornea which allows light to enter the eve, the lens that focuses this light and the retina that receives light signals.

How does vision work?

Light that enables vision is an electromagnetic wave within the visual spectrum of frequencies.

Different colours have different wavelengths ranging from short (red) to long (blue). Light is reflected from an object towards the eye, where it is enters the eye through the cornea and the iris. The iris controls the amount of light that enters the eye with the ability to constrict and dilate the hole in the iris (the pupil). As a light ray travels through the cornea, it begins to bend (refract). The lens further refracts the light rays to ensure that they are focused (come together) precisely on the retina. The ability to focus light accurately onto the retina enables us to have clear vision. Light reflected from closer objects requires greater focus (higher refraction) than light reflected off distant objects (lower refraction). The lens can achieve this as it can change shape depending upon the contraction state of the ciliary muscles. The resting state of focus occurs when the ciliary muscles are relaxed, resulting in a concave-shaped lens. In this state, the lens will focus on distant objects better. To focus on distant objects, the ciliary muscles contract and produce a more convex lens. Short-sightedness occurs when light rays are focused short of the retina which results in distant objects becoming unclear and harder to see. Conversely, long-sightedness results in light focused past the retina which makes near objects appear unclear (Figure 166).

The retina contains cells that can sense light. These cells can then convert the focused light rays into an electrical signal that is subsequently transmitted to and interpreted by the brain.

The fovea is an area of the retina that contains the highest number of retinal cells, and so the highest degree of visual sharpness can be achieved if light rays are focused in this area.

The retina consists of two types of cells:

• Rod cells - These types of cells can only sense black/white and are situated peripherally in the retina. They function both during the day and night but have a particular role in night vision. As they are located in the periphery of the retina, it can be easier to see objects at night if they are not directly looked at and causing the light rays to be focused on the peripheral retina.

• Cone cells - These cells can sense both colour and detail. There are three types of cone cells that have peak sensitivity to light of different frequencies and enable the perception of different colours. Cone cells are most effective when the object is directly looked at, causing light to be focused on the fovea of the retina.

What are the limitations of human eyesight during the day?

The ability to discern objects in daylight is generally related to the function of cone cells.

Visual acuity refers to the ability to see clearly without distortion and is tested as part of an aviation medical. Having good visual acuity when flying is essential to identifying fine objects and is dependent upon a healthy retina and accurate light focus. Contrast is also a necessary feature of visual perception, as a lack of contrast leads to an object being camouflaged and unable to be seen.

During the daytime, glare can be problematic for our ability to clearly see objects. The pupil is responsible for altering the amount of light that enters the eye by constricting and dilating the pupil. If excessive or unfocussed light enters the eye, it will lead to poor visual acuity. In very bright conditions, the pupil cannot constrict enough to prevent this and results in glare. Wearing sunglasses is an effective counter-measure to avoid glare. Flying sunglasses need to be comfortable, UV-protective and not polarised. Polarised lenses block out light entering the glasses at certain angles. While this is helpful going about your day-to-day life, it is not suitable for aviation as those angles may be the ones necessary for your instruments and you will not be able to see them correctly!

Where the optic nerve enters the retina, there are no rod or cone cells to sense light.

Therefore, any light focused on this area cannot be sensed, interpreted and seen - it is a blind spot. Each eye has a blind spot that can be perceived when testing uniocular (single eye) vision. However, with binocular vision (both eyes), the blind spot is not evident.

Another potential limitation of vision is in relation to the perception of colour. Some individuals have a genetically pre-determined reduced function (or in extreme absence) of one or more of the three types of cone cells. People who lack all three types cannot see colour, but it is very rare. Most commonly, all three types of cone cells are present, but one type has a reduced sensitivity to its specified frequency of light. Reduced function in the red-green spectrum is the most common and these individuals can sense colour but will be perceived as different to what individuals will normal colour vision would perceive. Safe colour vision to conduct aviation will also be determined during your aviation medical examination, however colour blindness does not necessarily exclude a person from holding a medical clearance and attaining a pilot licence.

What are the limitations of human eyesight at night?

Compared to other animals, humans have relatively poor night vision. Reduced lighting impairs our ability to maintain good visual acuity. Our cone cells become ineffective at night, causing us to become reliant on our rod cells. There is a period of adaption of the visual system when converting to night vision. It takes approximately 30 minutes to adapt to night vision and this period is extended if exposed to bright light during that time. Before a night flight, avoiding significant glare by wearing sunglasses and not viewing the sunset directly will help to reduce the night adaptation time for optimal night vision. When night flying, dimming the cockpit lights dim will also help to preserve night vision adaptation.

THE EARS AND HEARING

What are the main structures of the auditory system?

The auditory svstem comprises the outer, middle and inner ear. The pinna and ear canal form the outer ear. The ear canal terminates at the tympanic membrane (eardrum) and forms the anatomical boundar with the middle ear, which contains small bones that transmit the auditor sional to the round window. Beyond the round window is the inner ear that contains the cochlea and the balance organs of the semi-circular canals and the otolith organs (Figure 167).

How are sounds heard?

Sound is produced by vibrations of air particles transmitted from one air particle to the next. This creates a soundwave where the wave frequency dictates the pitch and the wave amplitude dictates the volume. The soundwave travels to the ear, where it is channeled by the pinna down the ear canal to the tympanic membrane. When it reaches the tympanic membrane, it causes it to vibrate. This vibration is transmitted through the ossicles to the round window and causes the fluid within the cochlea to vibrate in synchrony. The movement of the fluid within the cochlea is sensed by specialised cells that line the cochlea wall. When activated. an electrical signal is generated and transmitted via the auditory nerve to the brain for processing, which is then interpreted as a sound (Figure 168).

How can hearing be damaged?

Prolonged noise exposure can reduce the sensitivity of the cochlea wall cells and cause noise-induced hearing loss. This particularly affects the cells sensitive to the higher frequencies initially, but eventually all frequencies will be affected. If severe enough, it can lead to difficulty in speech comprehension, even at conversational tone and volume. The effect of noise exposure that can damage hearing depends on the volume (amplitude) and duration of exposure. The impact of noise damage is both cumulative and gradual, and so audiogram tests of hearing are conducted routinely as part of aviation medicals. When you are around aircraft, it is advisable to wear earplugs. When you are within the aircraft, wearing noise-cancelling headsets reduces the cumulative damaging effects of noise.

What are the organs of balance and motion?

Vision is the primary sense we use to orientate ourselves. Our secondary organs are the two signals to the brain about gravitational force, head position and acceleration. vestibular organs within the inner ear. The otolith organs and the semicircular canals send The two otolithic organs are the utricle and sacculus and are present in both ears. These organs sense acceleration in the vertical plane - gravity (sacculus) and the horizontal plane (utricle). They lie adjacent to the semicircular canals within the inner ear and contain a jellv-like coating that moves and changes shape with body orientation and exposure to accelerative forces. As the jelly changes shape, specialised cells within the jelly sense this and transmit signals to the brain. When the head is tilted and off centre, gravity will act upon the left and right otolithic organs differently. This discrepancy can be sensed and interpreted as a head position. As the balance organs have evolved from life on land in normal gravitational acceleration conditions, exposure to abnormal gravitational acceleration can be easily misinterpreted. During flight, exposure to high positive, minus and zero g-force accelerative forces are all possible and contribute to the disorientation of body position.

The three fluid-filled semicircular canals sense angular motion across three planes. Each canal is orientated to feel this three-dimensional movement during the pitch, roll and yaw motions. A change in acceleration in any of these axes moves the fluid within the loop that are sensed by tiny hairs that lie within the canal. When these hairs are activated, they generate an electrical signal that is transmitted via nerves to the brain for perceptions. The hairs only respond to movement and not to constant gravity as an endolymph fluid constantly surrounds them. It is the inertia of the hairs moving through the endolymph fluid that activates the cells.

The other means to sense position are the kinesthetic senses, based on nervous signals from skin and muscle and can sense their position and pressure.