1. protected by 12 pairs of ribs and situates



1.1  Basics. The
cardiovascular system is essential to life. It is comprised of a pump, the
heart, and vessels which carry blood around the body. Blood consists of 3 major
components, red blood cells (45%), plasma (55%) and the buffy layer which is
mostly white blood cells (<1%). Blood transports oxygen and glucose to tissues and gets rid of waste produces such as carbon dioxide. In addition, blood carries many other nutrients and hormones which are vital for normal bodily function.   1.2  Heart. The heart is situated in the centre of the chest. It is protected by 12 pairs of ribs and situates in between the left and right lung. Anatomically, it is made up of 4 chambers, 2 atria lying above the 2 ventricles. The vena cava is the main vein of the body which drains into the right atrium. From the right ventricle arises the pulmonary artery which transports deoxygenated blood to the lung. From the lung are the 4 pulmonary veins which enter the left atrium. Blood travels from the left atrium into the left ventricle before finally entering the aorta. Structurally, the heart consists of muscle which is myogenic in nature in addition to fibrous heart valves. Within the wall of the right atrium lies the sino-atrial node (SAN) which produces the heartbeat. The muscle of the heart also has electrical fibres which conduct and transport electricity which eventually causes the ventricles to beat and produce a heart rate. Supplying the heart muscle oxygen and glucose are the smaller coronary arteries which come off the aorta. These can commonly get blocked by fatty plaques and can lead to a heart attack. Risk factors for heart attack include high blood pressure, increasing age, smoking and obesity.   1.3  Arteries. Arteries are the principal carrier of oxygenated blood to tissues (apart from the pulmonary artery). They are characterised by a thick, tough wall with lots of collagen. The thick wall is vital ad blood travelling in arteries is under high pressure and the strong wall ensures that the vessel is capable of holding the blood in the artery.   1.4  Veins. Veins carry deoxygenated blood back to the heart and lungs where carbon dioxide is removed and oxygen is replaced. Unlike arteries which have the thick wall, veins have a thinner, less structurally sound wall. This is a physiological adaptation as the blood in veins is of a lower pressure, thus, the wall of veins does not need to be as thick.   1.5  Capillaries. Capillaries are only 1 cell thick allowing for the transport of nutrients and oxygen from blood to cells and waste from cells back to the blood. Its diameter is as wide as a single red blood cell. Capillaries connect arteries to veins. They often form a capillary bed, increasing the surface area by which nutrient and waste exchange can occur. Because of the narrow lumen, red blood cells travel in single file. The pressure of the capillaries is lower than that of the arteries, due to the 1 cell thick wall it has.   2.    Respiratory   2.1  Upper Respiratory tract 2.2  Lower respiratory tract 2.3  Lungs and diaphragm   3.    Nervous   3.1  Brain. The brain is about 4.5kg in weight and has a consistency of jelly in real life. It is divided into several regional lobes, the frontal lobe which is concerned with motor control, personality and executive functions. The parietal lobe is the centre of the somatosensory cortex. The temporal lobe is where the primary auditory cortex is. Finally, the occipital lobe is where the visual cortex is located. Structurally, the brain is divided into 2 cerebral hemispheres, connected only by a bundle of white mater, the corpus callosum. There is also the cerebellum, situated under the occipital lobe of the two cerebral hemispheres. The brain is connected to the spinal cord by the brainstem which itself is divided into 3 parts, the midbrain, pons and medulla oblongata. The brain is protected by 3 coverings, the dura, arachnoid and pia mater.  The brain is arguably the most important organ in the body as it is the centre of our consciousness and the organ in which we interpret are surroundings and interact with our environment. In addition, the brain has many 'hidden' functions which we do not have conscious control over such as our heart rate and hormonal axis   3.2  Nerves and spinal cord. The spinal cord is where peripheral nerves interact with the brain. The spinal cord is a continuous jelly like substance. The spinal cord is protected by individual vertebral bones, 7 cervical, 12 thoracic, and 5 lumbar. Spinal nerve roots emerge from every segmental level. The spinal cord ends at the level L1/L2. This is the reason why a lumbar puncture happens at the level L3/L4.   3.3  Autonomic nervous system. The autonomic system is part of the nervous system that we don't exert any conscious control over. It is divided mainly into sympathetic and parasympathetic systems. The sympathetic system is the 'fear, flight or flight' system which gets activated when you are stressed. It increases the heart rate, causes pilo-erection, vasodilates arterioles in the skin while constricting blood vessels leading to gut. Parasympathetic is the 'rest and digest' system. It is active when you are relaxed. It increases gut activity, decreases the heart rate and allow you to pass urine.   3.4  Eyes, ears and skin. The skin is the largest organ in the body. It is divided into the epidermis and dermis. The epidermis is further divided into the stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Beneath the epidermis is the dermal layer where the vessels and nerves are located. Beneath this is the subcutis fat layer. The skin has many functions, it acts as a barrier against pathogens, involved in the first stages of vitamin D synthesis, insulates you, contains sensory nerves and helps regulate temperature and water balance. The eyes are in many senses a continuation of the brain. It is the only organ other than the brain and spinal cord which contains the same 3 protective coverings that the brain has. Most anteriorly on a mid-sagittal section of the eye is the cornea, a transparent serous membrane protecting the eye. Then there is the iris, a muscle which can constrict and dilate depending on the environmental light conditions. The pupil the space in which light enters and gets to the retina. The lens which is posterior to the pupil is next. It is connected to the cilary muscles by the suspensory ligaments, when you want to look at something close, the cilary muscles contract, which relaxes the suspensory ligaments and allows the lens to bulge, increasing it's refractive index and causing the light to hit the retina. Commonly, people are short sighted, whereby they cannot see things far away from them, this can be due to an over-powerful cilary muscle which should normally relax to see things far away, making the ligaments tort and the lens flat, or more commonly, the eye is too long and the image is projected in front of and not on the retina. The most important structure of the eye is the retina which sits most posteriorly. It contains photoreceptors which detect different wavelengths of light. Cone cells detect colour and rod cells detect black and white. Cone cells give you the most accurate vision are located in high density at the macula. Rod cells are more numerous at the periphery and produce sub-optimal vision. This is why your peripheral vision is worse than your central vision.   4.    Digestive   4.1  Upper GI 4.2  Stomach 4.3  Liver 4.4  Pancreas 4.5  Small intestine 4.6  Large intestine 4.7  Rectum   5.    Endocrine   5.1  Concepts. Unlike many of the other systems of the body, the endocrine system does not have structural contact with all of its constituent parts. It is made up of several endocrine organs which secrete hormones that are small chemical messengers which travel in blood and have an effect on distant target organs. The majority of endocrine organs are governed by the master gland, the pituitary. Exceptions to this rule are the pancreas and other non-classical endocrine glands.   5.2  Pituitary gland. As mentioned in 6.1, the pituitary gland controls many other endocrine glands. The pituitary gland is a small, pea shaped organ located at the base of the brain. The pituitary itself is controlled higher up by the hypothalamus. The pituitary is divided into 2 parts, the anterior and posterior lobe. The anterior lobes secretes TSH, ACTH, GH, FSH, LH and prolactin. The posterior lobe releases ADH and oxytocin.   5.3  Thyroid. The thyroid is located in the neck, in front of the larynx. It is a lobular gland made up of a left and right lobe, connected by the isthmus. The thyroid releases thyroid hormone. The release of this is under the influence of TSH from the pituitary. Thyroid hormone has many wide-ranging effects and is a peptide hormone. It increases the body's metabolism and too much and too little of the hormone can lead to characteristic set of symptoms.   5.4  Adrenal. There are 2 adrenal glands lying on top of each of the kidneys. The adrenal gland is subdivided in the medulla which makes adrenaline for the 'fight or flight' response and the cortex which makes cortisol, aldosterone and other sex steroids. Cortisol and aldosterone regulates salt homeostasis and blood volume. Aldosterone has effects on the kidney while cortisol has many wide-ranging effects including body composition and overlaps with adrenaline in the fight or flight response.   5.5  Pancreas. Anatomically, the pancreas is made up of the tail, body, neck, head and uncinated process and is located deep behind the stomach. Unlike the majority of the endocrine organs, the pancreas is not under the influence of the pituitary. The pancreas is made up of 98% exocrine function and 2% endocrine function. The exocrine part makes digestive enzymes and watery secretions which aids the function of the digestive system. However, the endocrine part is of significance. The endocrine part of the pancreas is made up of Islets of Langerhans. Within the islets are many cells, 2 of which are of importance. The beta cells secrete insulin and alpha cells secrete glucagon. These two hormones are vital in the regulation of glucose homeostasis. Glucose is needed for every cellular process in the body and thus the pancreas has an indirect relationship with every other cell in the body. Mitochondria which are the powerhouse of the cell need glucose to work. Insulin makes glucose enter cells to be utilised by the mitochondria. It can therefore be said the insulin lowers blood glucose. Glucagon on the other hand raises blood glucose by increasing glucose production of the liver and switching off insulin secretion. Type 1 diabetes is a condition where the body's immune system destroys the beta cells. This results in a condition where there is a total lack of insulin. As insulin lowers blood glucose, in a type 1 diabetic, they have high blood glucose and require daily insulin injections for life.   5.6  Testes.   5.7  Ovaries. Ovaries are specific to humans with a XX chromosome. The majority of these identify as female. The ovaries are the size of a large grape and lie laterally to the uterus on the posterior abdominal wall. They are held to the abdominal wall by the suspensory ligament which contains the ovarian artery, vein and nerves. They are held to the side of the uterus by the ovarian ligament. The ovaries produce oestrogen and progesterone under the influence of FSH and LH from the pituitary. Oestrogen is fundamental in the development of female sexual characteristics and the maintenance of female bone density. Oestrogen, progesterone, LH and FSH are vital hormones in the menstrual cycle. Day 0 is the start of the cycle and is when a girl is on her period. The falling progesterone stimulates the pituitary to make FSH, the FSH recruits follicles to grow and mature. As the follicles grow, they produce oestrogen which negatively inhibit the FSH. The fall in FSH selects 1 dominant follicle to grow more. This produces more oestrogen which paradoxically causes a rise in LH. The LH surge causes the ovum to be ovulated. The remaining follicle becomes the corpus luteum and produces progesterone which maintains the thick endometrium, ready for pregnancy. If no pregnancy occurs within 14 days, the corpus luteum dies, a fall in progesterone happens, menses occurs and the cycle begins again.   5.8  Non-classical organs. Finally, it is worth mentioning that many other organs have endocrine function such as bone and fat.

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