Cardiovascular system

I begin here for two reasons. First, it was my own specialty, when I worked as a physician for humans only. Second, it is the most obvious difference between humans and Time Lords: they have two hearts. Not only two hearts, but two completely independent cardiovascular systems, each of which (with very little exception) supplies the whole body with blood.

Anatomy

Hearts

The addition of another organ in the same size body necessitates that something be smaller. In humans, the heart is roughly the size of a two-handed fist, but each of a Time Lord's hearts is only the size of a one-handed fist. The decrease in stroke volume leads to a slightly higher pulse rate, generally around 80-90 beats per minute.

Ignoring the blood vessels, the left heart is identical to humans, except for the decreased size, with four chambers of which the right two pump deoxygenated blood from the body to the lungs, and the left two pump oxygenated blood from the lungs to the body. The right is a mirror image; in fact, the entire systems are mirror images of each other. Each heart pumps blood to itself in roughly the same configuration of coronary arteries as in humans. It also provides blood to the opposite heart, through a secondary coronary artery branching off the descending aorta. This blood is returned through veins that cross over and empty directly into the medial atrium (ie, the right atrium of the left heart, and the left atrium of the right heart). The chest cavity Because of the non-central positions of the hearts and the doubling of major blood vessels, the lungs are also different. They are symmetrical, and are divided into posterior and anterior lobes, between which the secondary brachial arteries and veins pass. The hearts are separated by the thymus gland, which I will discuss further when I get to the lymphatic system. The apparently empty space below the hearts is not. It is part of a dense connective tissue sac, outside the pericardium, that also extends in front of the hearts. The sacs contain not just fluid but areolar connective tissue, which provides additional protection, necessary because the hearts are not behind the sternum. The major blood vessels entering and leaving the heart fit together in a rather complex pattern that I only hope this drawing makes clear. Vessels of the right heart are shown intact; note that the pulmonary artery begins most anterior, then crosses behind both the aorta and the superior vena cava (which also changes position; at the heart, it is posterior to the aorta, but closer to the clavicles, it is anterior). Vessels of the left heart are cut to show the crossing patterns. Where they cross, the vessels connected to the left heart are all anterior to their counterparts from the right heart.

Figure 1: Location of structures in the thoracic cavity 1. External carotid artery and      internal jugular vein 2. Dotted line indicates location of      anterior lung (not shown) 3. Pulmonary veins 4. Pulmonary arteries 5. Right bronchi 6. Internal carotid artery and      deep jugular vein 7. Trachea 8. Thyroid gland 9. Left lung, superior lobe 10. Superior vena cava (cut) 11. Ascending aorta (cut) 12. Right lung, inferior lobe 13. Diaphragm 14. Inferior vena cava 15. Descending aorta 16. Thymus gland 17. Coronary veins

Blood vessels

Like the hearts, the major blood vessels are significantly smaller than those of humans. They do not have to be as large, because the hearts are smaller; also, since each system covers the entire body, there are additional vessels in most locations. Where there are not, as with some peripheral veins, they are not reduced in size.

For each heart, the arteries on the opposite side of the body follow roughly the same path as those of humans. Arteries on the same side of the body as the heart are different (see figure 2). Recall that the second system in a mirror image; both arrangements of vessels are found on both sides of the body, but the different locations of vulnerable points reduce the likelihood that both systems will be injured at once.

The aortas are a significant exception. The human aorta curls tightly over the left bronchus and continues down along the spine. Time Lord aortas follow a smoother curve, as they cross each other at the base of the trachea before hooking over the bronchi. This otherwise very vulnerable point is protected by the sternum and cushioned by fat.

Figure 2: Major arteries This drawing shows only the vessels from the left heart, with a few exceptions in the axial regions. Vessels from the right heart are shown in darker red. 1. Arteries of the neck. This artery follows the same path as the human carotid artery, and also contains the baroreceptors the body uses to monitor and regulate blood pressure. On the left side of the neck, this artery comes from the right heart. Secondary (deep) carotid artery. Beneath the sternocleidomastoid muscle instead of running along it, this vessel is finer still than the primary one, though both supply blood to the brain via the circles of Willis (yes, there are two). No pressure receptors, but the nerves that monitor oxygen saturation are here, instead of in the primary carotid artery as with humans. 2. Pulmonary arteries. Note that there are three branches instead of the four humans have. The branch that follows the aorta to the other side of the body then splits further to go to the separate lobes. 3. Coronary arteries. Each heart sends itself blood, as in humans. Note here that they also send blood to each other. 4. Right spleen and kidney. Humans have only one spleen, on the left. Note that each spleen only processes blood from one system. However, both are connected to the single lymphatic system. The kidneys, however, each filter blood from both cardiovascular systems. 6. Femoral arteries. Both are near the surface at the groin, but they are actually about 2 inches apart, thus decreasing the risk of damage to both. 7. Secondary brachial artery, posterior carpal pulse point. The secondary brachial artery, like the deep carotid, runs beneath muscle (in this case, the biceps brachii). Note that at the wrist, where the radial and ulnar arteries (split from the primary brachial) pass at the palmar side, the secondary brachial artery passes behind the wrist bones. There is a groove in the bones to offer some protection. After the thumb branch splits off, the artery passes between the metacarpals before branching again for the fingers. 8. The arteries at the ankle. On the tibial (medial) side, the primary (human analog) artery passes in front of the malleolus, and the secondary passes behind and under. On the fibular (lateral) side, it is the opposite: primary artery under and behind the ankle bone, and secondary in front.

Figure 3: Major veins Like the artery drawing above, this one shows primarily the veins of the left heart. 1. Veins of the neck.  Jugular veins. Humans have two jugular veins on each side: the external, running down the outside of the neck, and the internal, parallel to the carotid artery along the sternocleidomastoid muscle. Gallifreyans have both of these, as well as a third that runs parallel to the deep carotid artery. The more superficial pair return blood to the opposite heart, while the deep returns it to the near heart. 2. Right subclavian vein. 3. Right superior vena cava. Note that it is the left subclavian vein that runs into the right vena cava. 4. Left azygous vein. Azygous veins collect blood from the ribs and intercostal muscles. This is another example where humans already have a pair of veins, and in the Gallifreyan they are split between the systems. 6. Liver. Slightly smaller than the human liver to make room for the second spleen. Also, each lobe processes blood from only the vascular system with the heart on the same side. Note that the splenic veins cross; this is necessary because each spleen processes blood from the opposite heart.  7. Pulmonary veins. Three to each heart; one from the same-side posterior lobes of the lung, one from the same-side anterior lobe, and one from the opposite side. 8. Mesenteric vein. Collects blood from the intestines, and combines with the splenic vein to form the hepatic portal vein. 9. Left inferior vena cava. On the veins of the arms: These are almost entirely analogous to human veins, and are simply split between the two systems. Like the jugular veins, they are significantly larger than their attendant arteries. Only the veins of the hands are doubled, possibly because the smaller diameter affords more protection in a heavily used area. On the veins of the legs: Here, there are several extras, though not a complete doubling as with the cerebral veins. Veins of the feet are doubled, and in the ankle, they cross the bones in the same locations as their attendant arteries.

Capillaries

Although both vascular systems bring blood to the entire body, not every small area is served by both. Rather, capillary beds from each system alternate, but they are small enough that the neighboring beds could compensate if necessary.

Blood composition

The red cell count is noticeably higher than in most humans; it is comparable to those who live at high altitudes. Under normal conditions, white cell count is similar, but it can change very rapidly in the face of infection. More on this under Lymphatic System.

Physiology

Results of blood loss

Unless both vascular systems were damaged, even severe blood loss would not kill a Time Lord. The second system could supply enough oxygen and nutrients to keep all organs alive, though not up to optimum functioning. Loss of as little as a pint of blood (the amount humans donate in my time) from either system can produce symptoms, from giddiness with a small loss, through reduced function of all organs, possibly progressing as far as coma if loss is severe.

The lymphatic system will work to correct an imbalance between the two vascular systems. Fluid balance is corrected quite readily, though replacement of lost cells takes longer. White cells are returned to the damaged system more quickly because extra ones are stored in the lymphatic vessels when not needed. Red cells are more difficult; they must be replaced by new ones just as in humans.

Protection of peripheral arteries

Because there are more points at which arteries approach the surface, one might think they are more vulnerable to damage. However, they are protected in several ways. First, they are significantly smaller in diameter and thus not only present a smaller target but also lose less blood if cut. Second, as in humans, veins run parallel and slightly superficial to many arteries, but this is even more significant in Time Lords because most veins are so much larger than the arteries. Third, there are actually sphincters proximal to the pulse points which will close in response to damage, minimizing (though not completely stopping) blood flow through the wound.

With some exceptions, the veins do not have these same protective factors. Those of the head, hands, and feet are doubled and therefore smaller, providing some protection relative to humans, and major veins have sphincters where they approach the surface.

Hemoglobin

Gel electrophoresis suggests four-part hemoglobin molecules as in humans. It also suggests that 50% of the components are alpha-chains. However, beta-chains do not make up the entire remaining 50% as they do in adult humans; only 45% are beta-chains. I believe the remaining 5% are gamma-chains, which would mean that Time Lords retain (and maintain, since red cells are replaced every few months) 10% of their fetal hemoglobin (humans retain none, under normal circumstances). Fetal hemoglobin has a much higher affinity for oxygen - necessary to be able to take the oxygen from mother's blood, but problematic in free-living organisms because it does not readily give up the oxygen to the tissues unless they are in distress. This likely aids a respiratory adaptation I will discuss later, and it may explain the need for more red cells (so that the same amount of oxygen is always available to the tissues).

Vital signs

As I mentioned above, the heart rate is higher than in humans. Also, the two heart rates are not synchronized, and thus electronic monitoring devices will likely read the rate as twice what it actually is. This is the reason I unintentionally killed the Time Lord who would later become my friend; we used defibrillation to treat non-existent tachycardia and instead stopped his hearts.

The pulse can be measured by auscultation of one heart at a time, and by palpation of arteries. Despite the increased number of surface pulse points, there are fewer readable ones than in humans because the arteries are so much smaller. Only the femoral and external carotid arteries are consistently palpable. Those with a light touch and sensitive fingers may be able to detect the pulse at the radial, ulnar, and posterior carpal sites, as well as the temporal arteries (one along the base of the temple, the other about one inch higher). Those with a heavier touch will not be able to feel these by hand because the pressure they need would obliterate the pulse.

Most pulse points analogous to humans as well as other places the arteries approach the surface can be read with a Doppler. Deeper sites, such as the popliteal, can not be read at all.

Blood pressure tends to be comparable to that of healthy humans. Although the arteries are smaller and thus offer more resistance to blood flow, the cardiac output is proportionally decreased.

Blood pressure can be measured by auscultation of the brachial arteries as in humans. However, it is essential to place the stethoscope carefully so only the medial artery can be heard. Testing the left arm will give the blood pressure from the right heart, and vice versa. Generally, they would be the same, unless one vascular system were damaged.

Although Gallifreyans have a higher basal metabolic rate than humans, their core temperature is surprisingly low. A tympanic thermometer will appear to read only room temperature under normal circumstances. (Time Lords can survive much higher and even slightly lower temperatures with minimal effect; I can only assume that their enzymes have a wider functional range, or they have multiple forms of enzymes that can act under different temperatures.) I have not dared to consult a physicist or biochemist about this unusual fact; my best guess as to how they can maintain such a low temperature despite such a high metabolism is that there are endothermic reactions going on - that some of their body processes actually absorb heat instead of creating it.

Resuscitation

CPR compressions are quite obviously impossible without breaking numerous ribs, but I believe that defibrillation would work, assuming that a) the pads were placed correctly to affect only one heart, b) the heart was in either ventricular fibrillation or true tachycardia, and c) the subject did not regenerate before you could get the pads in place and deliver a shock.

Index by system

Cardiovascular  - Respiratory - Genetics - Life Cycle