Blood

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Dictionary of Biological Psychology

‘Blood’ might at first appear an odd topic to be of interest to biological psychologists, but such a first impression would be wrong. Blood is interesting to biological psychologists for three reasons. First, it is critically involved in all bodily processes, including the maintenance of brain function. Second, damage to the brain’s blood supply can cause profound neuropsychological impairments (see ANOXIA, CEREBROVASCULAR ACCIDENT, ISCHAEMIA and STROKE). Third, the bloodstream is a critically important source of sensory informa-tion to the brain: the state of the body is very effectively indexed by the composition of blood.

What is blood? Vertebrate blood (which is a form of CONNECTIVE TISSUE) is composed of several types of cells suspended in PLASMA. Some 45% of blood volume is made up of cells, the remainder being fluid: plasma, which is 90% water with various ELECTROLYTES present (these maintain the appropriate OSMOTIC PRESSURE of the blood) and PROTEINS (which have several functions: some transport insoluble LIPIDS, some are ANTIBODIES, and some are involved in blood clotting). There are also many molecules present in the blood that are in transit from one place to another—the bloodstream is a major transport system. The majority of the cells present in the blood are ERYTHROCYTES, which are more colloquially known as red blood cells—the colour comes from the molecule HAEMOGLOBIN, a protein which can carry oxygen. Oxygen transport is the major function of erythrocytes. There are also white blood cells (see LEUKOCYTE): these come in five types: MONOCYTE, NEUTROPHIL, BASOPHIL, EOSINOPHIL and LYMPHOCYTE. All are important components of the IMMUNE SYSTEM. PLATELETS are also present in the blood: these are chips of CYTOPLASM and are important in blood clotting. All of these cells need continually to be replaced: STEM CELLS in bone marrow do this.

What is the function of blood? Single-celled or very simple animals are small enough that chemicals—GLUCOSE and oxygen for example—can diffuse effectively to all the required sites. In larger animals with specialized tissues and organs a transport system is required for moving substances (and heat) around the body. A major function is the transport of oxygen, which is collected by blood in the lungs, while carbon dioxide is expelled from the blood to the air that is exhaled. But there are many other chemicals that are transported through the bloodstream, from their point of RELEASE to target sites elsewhere (see below under CNS systems for analysing blood). Blood also has a critical role in maintaining body TEMPERATURE, carrying heat around the body as well as being involved in the mechanisms of body cooling. The function of blood as a transport system is also used very effectively in DRUG ADMINISTRATION. No matter what route of drug administration is chosen, the delivered drug will be transported to sites of active use around the body by the bloodstream.

The organization of the blood supply. The CARDIOVASCULAR SYSTEM includes the heart, blood vessels and blood itself. The heart is essentially a muscular pump with four chambers: two atria (singular: ATRIUM) and two VENTRICLES, separated by valves ensuring that blood flows in only one direction through the heart: the left side receives blood rich in oxygen, the right side blood that is oxygen-depleted. The ventricles serve as pumps and are therefore composed of thicker MUSCLE walls (made of cardiac muscle). The table on page 111 explains briefly the functions of these atria and ventricles.

Three types of blood vessel are found: CAPILLARY, VEIN and ARTERY. Capillaries are the smallest and are composed of a thin layer of ENDOTHELIUM. Within tissues capillaries form networks known as CAPILLARY beds to facilitate interactions between blood and tissues. Veins and arteries are larger than capillaries and are both made in much the same way: they have three layers of tissue: an outer layer of connective tissue, an intermediate layer of smooth muscle and an inner layer of endothelium. Arteries always carry blood away from the heart (so the blood tends to be oxygen-rich) while veins always carry blood towards the heart (so veins tend to be oxygen-depleted). Of course, arteries do not narrow in a single step to become capillaries but instead narrow gradually. The intermediate portions are ARTERIOLES. Similarly, when blood is flowing away from capillary beds into veins, they pass through VENULES first, venules being wider than capillaries but narrower than veins. Arteries effectively pump blood through the body, having valves present to facilitate this. The flow is regular but discontinuous, reflecting the pumping of the ventricles: this pumping is effectively measured by taking one’s pulse. Veins are less actively involved in moving blood, which tends to drain back to the heart rather than being actively pumped. Bodily movement works to keep blood moving in the veins.

Blood flow to the brain. Neurons require a continuous blood supply and the brain has an extensive vascular network supplying all parts (the CEREBRAL VASCULATURE). The blood supply and the brain are separated from direct contact (except at the CIRCUMVENTRICULAR ORGANS) by the BLOOD-BRAIN BARRIER, The importance of the blood supply to the brain is shown by the following: a 1-second interruption of blood flow to the brain is sufficient for all the oxygen present there to be used; a 6-second interruption is sufficient to produce unconsciousness; and interruption of only a few minutes is sufficient to produce neuronal death. Blood is supplied to the brain by two pairs of arteries: the VERTEBRAL ARTERIES and the INTERNAL CAROTID ARTERIES. The vertebral arteries enter the skull at the FORAMEN MAGNUM, giving off branches (the anterior and posterior spinal arteries and the posterior inferior cerebellar artery) before joining to form the BASILAR ARTERY which runs along the MIDLINE on the surface of the PONS, immediately below the brain. The basilar artery divides into the posterior cerebral arteries, anterior inferior cerebellar artery, pontine branches and the superior cerebellar artery. Much of the posterior part of the brain receives its blood supply from the vertebral arteries. The internal carotids enter the skull via the FORAMEN LACERUM and divide at the level of the OPTIC CHIASM into the anterior cerebral arteries. Further branching occurs giving rise to the MIDDLE CEREBRAL ARTERY and striate arteries. It is the anterior parts of the brain that receive their blood supply from the internal carotids. At the base of the brain the carotid and vertebral arteries communicate via the anterior and posterior communicating arteries, which form the CIRCLE OF WILLIS. Blood drains away from the brain via veins, which run into larger veins known as sinuses (singular SINUS) present in the DURA MATER. The superior sagittal sinus and straight sinus join to form the transverse sinus, which flows into the sigmoid sinus which in turn flows into the JUGULAR VEIN, in the neck.

CNS control mechanisms involved in heart rate and blood pressure. HEART RATE can be changed by SYMPATHETIC and PARASYMPATHETIC neurons of the AUTONOMIC NERVOUS SYSTEM. NORADRENALINE rcleased from sympathetic nerve endings increases heart rate by an action on BETA ADRENERGIC receptors. Noradrenaline, again acting at beta receptors, also constricts blood vessels (a process called VASOCONSTRICTION) which will increase blood pressure. ACETYLCHOLINE relcased from para-sympathetic neurons and acting at MUSCARINIC ACETYLCHOLINE RECEPTORS, has the opposite effect: it slows heart rate by an action on cardiac muscle and by changing the function of cardiac pacemaker cells which regulate heart beat. The CENTRAL NERVOUS SYSTEM is also involved in the regulation of blood pressure. BARORECEPTORS detect changes in blood pressure in the carotid artery (which is a major source of blood supply to the brain): these signal via the VAGUS NERVE to the NUCLEUS OF THE SOLITARY TRACT. This connects via a disynaptic pathway to neurons in the rostral ventrolateral MEDULLA and to the motor nucleus of the vagus. Vagal outflow produces a decrease in heart rate. Rostral ventrolateral medulla outflow, via the reticulospinal tract, can effect the heart, but can produce an increase in pressure rather than a decrease. This pathway can also have an action on arterioles to change arterial resistance to blood flow. These medullary sites in brain are therefore able to regulate closely heart rate and blood pressure.

CNS systems for analysing blood. The blood supply to the brain is a very important source of sensory information about the state of the body and there are various systems present in the brain for analysing blood composition and initiating changes on the basis of that monitoring. Such processes are of importance for OSMOREGULATION, ENERGY BALANCE, SEXUAL BEHAVIOUR, responses to STRESS and THERMOREGULATION. For example, detector mechanisms in brain can monitor the amount of water present in the blood and circulating levels of molecules such as ANGIOTENSIN II (important in signalling information about WATER BALANCE); glucose and INSULIN (important in maintaining energy balance and the regulation of FEEDING); CHOLECYSTOKININ (thought to be involved in signalling SATIETY); and various HORMONES involved in stress and sexual behaviour. Brain mechanisms in areas such as the circumventricular organs, PARAVENTRICULAR SYSTEM and HYPOTHALAMUS are especially important for monitoring blood content and temperature.

See also: breathing, neural control of

Campbell N.A., Reece J.B. & Mitchell L.G. (1999) Biology, 5th edn, Addison-Wesley: Menlo Park CA.

Kandel E.R., Schwartz J.H. & Jessell T.M. (1991) Principles of Neural Science, 3rd edn, Appleton & Lange: East Norwalk CT.

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