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Physiology of the Circulatory System
Physiology of the Circulatory System


Conditions in the external environment of an organism can change rapidly and drastically. To survive, organisms must maintain a fairly constant internal environment, which involves continuous regulation of temperature, pH, and other factors. This balanced state is known as homeostasis, which describes the processes by which organisms maintain their optimal internal conditions. To maintain homeostasis, organisms have developed structures with distinct functions. Physiology is the study of the normal functions and mechanisms of the different systems in the body. For example, if the external environment becomes warmer than the ideal internal temperature, the organism will activate physiological processes that will keep the body from heating to the external temperature. Humans and many other animals attain homeostasis through distinct physiological processes in specialized cells.

Organ Systems in Humans

One or more cell types with supporting functions form tissues, which in turn make up organs with specialized bodily functions. Organ systems consist of two or more organs that work together to provide a common function. The vertebrate physiological system comprises 11 major organ systems. While all of the organ systems are interconnected, they do function somewhat independently of one another.

The body is covered by the integumentary system, which consists of skin, hair, nails, sensory receptors, and various glands. Besides protecting the internal structures, integumentary organs sense many features of the external environment and helps regulate body temperature. Internally, organs are protected and supported by the skeletal system, which comprises bones, cartilage, tendons, and ligaments. The skeletal system also provides attachment for the muscles that make up the muscular system. Muscles can move the body by moving the skeleton or contract to move substances through hollow organs. The nervous system consists of the brain, spinal cord and peripheral nerves. It interprets sensory stimuli and directs behavior of the organism to control physiological processes together with other systems. The endocrine system is made up of hormone-secreting glands and organs including the pituitary, thyroid, pancreas, ovaries and testes. It regulates growth, metabolism and reproduction together with other systems. The respiratory system controls gas exchange to supply the body with oxygen as it removes carbon dioxide in the lungs after air passage through the nasal cavity, pharynx, larynx, trachea and bronchus. The digestive system processes and breaks down food which is taken in through the oral cavity and esophagus, and then moved through the stomach, small and large intestines before excretion through the rectum and anus. Nutrients are absorbed in the small and large intestines and then processed by the liver. The urinary system concentrates and eliminates nitrogenous waste via the kidney, bladder, and urethra. It also rids the body of excess water. The cardiovascular or circulatory system consists of the heart, blood vessels and blood, and delivers oxygen and nutrients to the tissues, while removing carbon dioxide and waste products throughout the body. The lymphatic system maintains the body’s immune response via white blood cells or lymphocytes (housed in red bone marrow), thymus, lymphatic vessels, thoracic duct, spleen, and lymph nodes. Finally, the reproductive system primes reproductive cells of organisms. In males, testes and the penis comprise the reproductive system while in females the uterus, ovaries, and vagina comprise the reproductive system.

Physiology of the Circulatory System

The physiology of single-celled organisms and basal multicellular animals, such as sponges, is often simple. For example, the small size and large surface to volume ratio of microorganisms allows them to attain regulation by diffusion across the cell membrane. Similarly, seawater circulates through the pores of sponges, carrying nutrients and waste products to and from its cells. More complex animals have developed circulatory systems to move blood throughout the body to transport nutrients, waste products, hormones and other molecules, while the respiratory systems allow gas exchange between the circulatory system and the external environment.

The circulatory system in animals can be open or closed. Open circulatory systems are typically present in many invertebrates and consist of one or more simple hearts, a vessel network and interconnected spaces that directly bathe internal organs in a fluid that allows exchange of materials. Vertebrates have closed circulatory systems, in which blood is confined inside a closed vessel system which branching extensively into the tissues to ensure material exchange. This closed vessel system is connected to a heart with veins carrying blood from tissues towards the heart and arteries carrying blood from the heart to the rest of the body. Four-chambered hearts, such as in humans, are associated with two loops of vessels. In humans, oxygen-depleted blood from organs enter the heart through the right atrium, which contracts to push blood to the right ventricle, which in turn sends blood to the lungs. Following gas exchange in the lungs, oxygen-rich blood returns to the left atrium and is later pushed into the left ventricle. This final chamber is more muscular than the others and with a strong contraction, is able to pump the blood to the rest of the body.

Closed circulatory systems allow rapid circulation of blood, which in turn enables fast and efficient transport of substances throughout the body as well as higher blood pressure than in open systems. The blood pressure is generated by the contraction of the heart ventricles as blood is forced into the arteries. As the ventricles of the heart relax, the blood pressure decreases.

Blood Pressure

In humans, the functioning of the circulatory system can be assessed by measuring the blood pressure and the corresponding heart rate in an individual. Blood pressure is measured in millimeters of mercury (mmHg), which is the height in millimeters that mercury in a column is raised to due to the pressure exerted on it. Heart rate is measured as beats per minute. Because of the heart’s contraction and relaxation movements, blood pressure readings consist of two numbers—systolic and diastolic. Systolic pressure is measured during contractions of the ventricles and diastolic pressure is the minimum pressure in the arteries during rest between contractions. Generally, systolic pressures of 90-120 mmHg and diastolic pressures of 60-80 mmHg are considered healthy. When it comes to heart rate, 60-100 beats per minute is considered healthy for adults. Athletes generally have lower heart rates as cardiovascular exercises elevate the heart rate and condition the heart to pump more efficiently, which eventually reduces the resting heart rate1.

Elevated blood pressure levels for extended periods or hypertension can damage blood vessels and has been linked to heart attack and stroke2. Researchers discovered that the cardiovascular effects of systolic and diastolic pressure are different, such that the rates of cardiovascular events are strongly associated with systolic pressure. Therefore, the number of patients with systolic hypertension who suffer from cardiovascular events is higher than the number of patients with diastolic hypertension3. Genetic factors, as well as lifestyle and environmental factors can cause hypertension and cardiovascular disease. For instance, consumption of high amounts salt causes retention of excess water in the body, elevating blood pressure and straining blood vessels. Any insults of blood vessels make them prone to injuries, which provides surfaces for plaque buildup, eventually stiffening blood vessels and reducing the efficiency of blood flow.

Blood pressure measurements

Sphygmomanometers are used to measure blood pressure. They are composed of an inflatable cuff which is connected to a pump (manual or automatic) and a pressure gauge. The most efficient way to use a sphygmomanometer is on the brachial artery on the upper arm, which is in level with the heart. Sphygmomanometers are used in conjunction with a stethoscope, which is an acoustic medical device used to listen to internal sounds via a metal disc, or resonator. The stethoscope is held just underneath the sphygmomanometer, just above the inside of the individual's elbow to measure the sounds of the systolic and diastolic blood pressure. The cuff is inflated to 200 mmHg stopping blood flow by pinching the blood vessels and is a safe amount of pressure to be applied to the arm. As the cuff deflates, the blood vessels begin to open, and blood is allowed to flow through again. The systolic blood pressure is signified by the first noise heard and the diastolic pressure is determined by the last noise heard. These noises are called Korotkoff sounds, that are the sound of blood being forcibly pushed through vessels by the heart4.

Respiratory System

The circulatory system works closely with the respiratory system to provide oxygen to tissues while removing carbon dioxide. Different organisms have developed distinct respiratory structures for gas exchange. For instance, many aquatic animals exchange gases through gills. Gill movements are easily observed and can be used to calculate the respiration rate of aquatic organisms by counting the number of times the organism moves its gill cover or operculum. Respiration rate may change with temperature because oxygen molecules are transported at different rates depending on how warm or cool an environment is. In an aquatic environment, the amount of available dissolved oxygen in the water decreases with increased temperature. The decreased oxygen has effects on the respiration rate of aquatic organisms, given their ability to diffuse oxygen throughout their bodies. On the other hand, terrestrial animals have internal respiratory structures, such as lungs. In humans, breathing involves inhalation by contracting the diaphragm to pull in air. When the diaphragm relaxes, the air is passively released from the lungs.

Smoking is the leading cause of lung cancer, responsible for 80-90% of lung cancer deaths. Every year, more than 120,000 Americans die of smoking-associated lung cancer and represent a large portion of preventable deaths5. Taken together, lifestyle contributes to health of both circulatory and respiratory systems and a significant amount of deaths can be prevented by lifestyle changes.


  1. Wilson, MG, Ellison, GM and Cable, NT. Basic science behind the cardiovascular benefits of exercise. Br J Sports Med . 2016, 50(93-99).
  2. Psaty, BM, et al. Association between blood pressure level and the risk of myocardial infarction, stroke, and total mortality: the cardiovascular health study. Arch Intern Med. 2001 , Vol. 161, 9 (1183-92).
  3. Kannel, WB. Elevated systolic blood pressure as a cardiovascular risk factor. Am. J. Cardiol. 2000, Vol. 85, 2 (251-5).
  4. Perloff, D, et al. Human blood pressure determination by sphygmomanometry. Circulation. 1993, Vol. 88, 5 (2460-70).
  5. U.S. Department of Health and Human Services. . The Health Consequences of Smoking: A Report of the Surgeon General. 2004.


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