Physiology basics: how your body works

Physiology is the science of how living organisms and their parts actually function — from a single cell to the entire cardiovascular system. This article covers the fundamentals of physiology: nervous system control, muscle contraction, blood composition, circulation, breathing, digestion, hormones, kidneys and thermoregulation. Think of it as a foundation for further study in biology or medicine.

What physiology is and why homeostasis matters

Physiology aims to explain the physical and chemical factors that life depends on. A central concept is homeostasis — the stability of the internal environment. Your body keeps specific parameters (temperature, pH and blood composition) within narrow limits by balancing the effects that increase and decrease them.

Because the processes that affect these parameters occur in different locations, regulation must span the entire body. Cells communicate in several ways to make this possible: an autocrine signal acts on the cell that releases it, a paracrine signal acts on neighboring cells, and an endocrine signal travels through the blood to distant organs. Neurons also use rapid electrical signals, and the whole system operates through long signaling cascades in which each molecule activates the next.

Nervous system physiology: autonomic control

The autonomic nervous system (ANS), also known as the vegetative nervous system, controls your internal organs involuntarily. Its effectors are cardiac muscle, smooth muscle and glands. The ANS uses a 2-neuron pathway: a preganglionic fiber from the central nervous system synapses with another neuron in a ganglion. That neuron’s axon — the postganglionic fiber — then reaches the target organ. The main integration center is the hypothalamus.

The ANS has 2 divisions, which often supply the same organ but have opposing effects:

• The sympathetic nervous system dominates in emergencies (“fight or flight”). Heart rate and stroke volume increase, blood vessels in the skin and internal organs constrict, and blood pressure rises. Blood vessels supplying working skeletal muscles and the heart, however, dilate. The bronchi widen, the pupils dilate and sweat is released. The sympathetic nervous system slows digestion, controls thermoregulation in hot conditions and triggers renin release from the kidney.
• The parasympathetic nervous system dominates when your body is recovering its strength — during sleep and digestion. Heart rate slows, digestive activity and excretion increase, and the bladder empties more easily.

Muscle physiology: how muscles actually contract

The outside of a muscle cell membrane is positively charged (with more Na+), while the inside is negatively charged. This difference in electrical potential, or polarization, is essential for generating an action potential. The point where a motor neuron meets a muscle fiber is the neuromuscular junction. The motor neuron releases acetylcholine, which causes depolarization of the motor end plate, producing an end-plate potential (EPP). A single motor neuron and the muscle fibers it supplies form a motor unit.

A single muscle contraction has 3 phases: the latent period, lasting a few milliseconds after the stimulus; contraction, when cross-bridges are active and the muscle shortens; and relaxation, when Ca2+ is pumped back into the sarcoplasmic reticulum and tension returns to baseline.

Muscle structure follows a nested arrangement: the endomysium covers individual muscle fibers, the perimysium surrounds bundles of fibers, and the epimysium encloses the entire muscle. The muscle fiber’s membrane is the sarcolemma, and its cytoplasm is the sarcoplasm. The contractile units are sarcomeres, which consist of thin actin filaments (with troponin and tropomyosin) and thick myosin filaments. Actin slides past myosin, shortening the muscle.

At the molecular level, the process works like this: an action potential travels along the sarcolemma and transverse tubules. Ca2+ is released from the sarcoplasmic reticulum, binds to troponin and shifts tropomyosin, exposing the active sites on actin. Myosin heads attach to actin and “row” along it, breaking down ATP in the process. Once the action potential has passed, Ca2+ is pumped back into the reticulum.

In smooth muscle, actin and myosin are not arranged into sarcomeres. These muscles contract more slowly and can generate about 1/3 of the force of striated muscle, but they do not fatigue as quickly. Most smooth muscle contracts spontaneously, without a nerve impulse; it is innervated by the ANS, with acetylcholine and noradrenaline acting as neurotransmitters. Cardiac muscle (the myocardium) is also innervated by the ANS, but its impulses originate in pacemaker cells and spread between cells without a neurotransmitter. The myocardium has a long repolarization period, which prevents the heart from remaining in a sustained “cramp.”

Actual body movement almost always involves tetanic contraction, in which individual twitches summate, rather than a single contraction. If each subsequent stimulus arrives during the final phase of the preceding contraction, unfused (partial) tetanus occurs. In human muscles, this is triggered at frequencies of 5–10 Hz. Fused (complete) tetanus occurs at frequencies of 15–20 Hz. Slow motor units reach fused tetanus at just 20 Hz, while fast motor units require 35–40 Hz. During low-force efforts, only slow motor units are active (in unfused tetanus); as force increases, fast motor units are also recruited.

An electromyograph records the electrical potentials generated by a muscle. A muscle at rest shows no electrical activity; a voluntary contraction produces a potential whose amplitude increases with the force exerted. The membrane potential is approximately -70 mV, and the frequency range is 7–20 Hz.

Body fluids and general blood physiology

Body fluids are divided into 3 main fluid compartments. Interstitial fluid accounts for 4/5 of extracellular fluid, and blood plasma for 1/5. Intracellular fluid is the cytosol. Transcellular fluid includes cerebrospinal fluid, exocrine gland secretions and fluid in the chambers of the eye.

Interstitial fluid contains high concentrations of Na+, Cl- and bicarbonate ions — its composition remains stable thanks to the balance between food and fluid intake and excretion. Intracellular fluid is dominated by K+, Mg2+, phosphates and sulfates; it contains little Na+ and almost no Ca2+. Active pumps in the cell membrane transport Na+, Ca2+ and Cl- out of the cell, and K+ and Mg2+ into it. Protein synthesis takes place continuously within cells, and proteins are transported from the interstitial fluid into the lymphatic vessels.

Blood itself is a fluid connective tissue and accounts for about 7% of body weight, or ~5 l. Plasma makes up ~55% of blood volume, and cells ~45%. Blood plasma is 90–92% water, 6–8% proteins and 1–2% low-molecular-weight compounds. Of the plasma proteins, albumin accounts for 60%, and globulins and fibrinogen for up to 40%. Most are synthesized in the liver; the exception is antibodies, which are produced by plasma cells.

Among blood cells, red blood cells (erythrocytes) number 4–5 × 10¹² per liter — 1 in every 4 cells in the body is a red blood cell. They have no nucleus, hemoglobin accounts for 1/3 of their mass, and their main function is to transport oxygen. Blood groups (the ABO system) are clinically important. White blood cells, or leukocytes, are divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (T and B lymphocytes, monocytes); their role is immune defense. White blood cells can leave a blood vessel without damaging its wall — this is called leukocyte diapedesis.

Blood clotting begins when platelets stick together (aggregation), forming an initial “plug”. Aggregating platelets release thromboxane and other substances, which in turn trigger contraction of the muscles in the blood vessel wall. More substantial bleeding activates cascades of clotting factors: activation of 1 factor leads to cleavage of the next, which in turn cleaves a 3rd. Blood flow and the consumption of clotting factors limit the spread of the clot. A healthy blood vessel wall produces prostacyclin, which inhibits platelet aggregation — keeping the inside of the vessel “slippery”.

Circulatory physiology: the heart as a pump

The circulation consists of 2 circuits connected in series — the systemic circulation and the pulmonary circulation. The blood vessels (arteries, capillaries, veins), together with the heart, form the cardiovascular system, which supplies cells with oxygen, nutrients and signaling molecules and removes metabolic waste. Blood also transports hormones and distributes heat evenly throughout the body.

Blood always flows from higher to lower pressure. The volumetric flow rate depends on the pressure gradient and resistance to flow. If a blood vessel’s diameter decreases by a factor of 2, blood flow decreases by a factor of 16 — resistance increases with the inverse of the diameter raised to the power of 4. Arterioles and capillaries account for the greatest resistance. Linear flow velocity, however, decreases as the total cross-sectional area of a given section of the circulation increases; it is highest in the aorta, at 30–50 cm/s.

Blood pressure is expressed as 2 numbers: systolic ~120 mmHg (the peak of the left ventricular ejection phase) and diastolic ~80 mmHg (after the semilunar valves close). The venous pulse occurs when pressure changes during the cardiac cycle cause the walls of nearby veins to oscillate. The arterial pulse is a pressure wave generated during systole that travels along the arterial walls.

The cardiac impulse generation and conduction system

The heart’s impulse generator is the sinoatrial node (SA node) in the right atrium — a group of slightly modified cardiac muscle cells. Although every cardiac cell could, in principle, generate an impulse, the SA node takes the lead because its rhythm is faster than that of the other cells. If the SA node fails, the atrioventricular node (AV node) takes over.

The impulse spreads from the SA node across the right and left atria (the P wave), traveling along specialized pathways (internodal tracts) to the AV node. The AV node introduces a crucial delay — without it, the atria and ventricles would contract simultaneously, leaving no time for blood to flow from the atria into the ventricles. This delay forms the PR interval on the ECG.

The distal part of the AV node divides into right and left bundles (bundle branches), which activate the right and left ventricles, respectively. The left bundle further divides into anterior and posterior fascicles. The left posterior fascicle is short and broad and has a dual blood supply, making it particularly resistant to ischemia. The bundles narrow into Purkinje fibers, which stimulate individual cardiac muscle cells. Ventricular depolarization produces the QRS complex on the ECG, and repolarization produces the T wave.

The heart’s pumping cycle

The heart is actually 2 pumps connected in series. Valves ensure that blood flows in 1 direction: the atrioventricular valves lie between the atria and ventricles (the bicuspid, or mitral, valve on the left and the tricuspid valve on the right); the semilunar valves lie between the ventricles and the major arteries. Maximum pressure in the right ventricle is 25 mmHg, compared with 120 mmHg in the left — which explains the thickness of the left ventricular wall.

Atrial systole lasts about 0.1 s and adds another ~10% of blood to the ventricle. By this point, 70–80 ml of blood has flowed into each ventricle during diastole (filling volume), and the atrial contribution brings the ventricular blood volume to about 150 ml — the end-diastolic volume.

Ventricular systole begins with an asynchronous contraction phase (0.05 s), during which the AV valves are still open. Rising pressure closes these valves (heart sound I), preventing blood from flowing back into the atria. This is followed by an isovolumetric contraction phase (0.05 s) — pressure inside the ventricles rises sharply without any change in volume. When left ventricular pressure exceeds the aortic pressure of 80 mmHg and right ventricular pressure exceeds the pulmonary artery pressure of 8 mmHg, the semilunar valves open and the ejection phase begins (0.25–0.27 s). Ventricular systole lasts 0.35–0.37 s in total. At rest, each ventricle ejects 70–80 ml of blood into the aorta or pulmonary artery — this is the stroke volume. The amount left in the ventricle is the end-systolic, or residual, volume.

At the end of the ejection phase, the muscle relaxes and pressure begins to fall. Protodiastole lasts 0.04 s, ending when the semilunar valves close and produce heart sound II. This is followed by an isovolumetric relaxation phase (0.05 s), during which pressure inside the ventricles falls to almost zero. As soon as ventricular pressure falls below atrial pressure, the AV valves open and the filling phase begins (0.5–0.6 s).

Regulation of circulation and regular relaxation of the heart muscle

Circulation is controlled through a combination of local and systemic regulatory processes. Various receptors monitor the state of the system and send impulses along afferent fibers to the cardiovascular centers in the medulla oblongata. From there, efferent signals are sent back to the heart and blood vessels, as well as to other parts of the central nervous system that mediate neurohumoral mechanisms.

The central task of systemic circulatory regulation is to coordinate peripheral resistance and cardiac output — these determine the pressure gradient that drives blood flow. The relationship between vascular capacity and blood volume also matters, as it determines static pressure. During diastole — the phase of regular relaxation of the heart muscle — the ventricle refills, and this window determines how much blood is ejected during the next systole. If relaxation is impaired, the efficiency of the entire circulation suffers.

Measuring blood pressure and potential errors

Blood pressure can be measured indirectly using the Korotkoff method. A stethoscope is placed on the inside of the elbow, distal to the cuff, to listen for sounds. Cuff pressure is initially raised above the expected systolic pressure — this compresses the artery and stops blood flow. The pressure is then lowered slowly. When cuff pressure falls below systolic pressure, a brief, sharp Korotkoff sound is heard with each pulse — the pressure at this point is the systolic pressure. As cuff pressure falls further, the sounds initially grow louder, then suddenly become muffled and disappear — the pressure at this point is the diastolic pressure.

Several factors can compromise the measurement. With the Riva-Rocci method, the doctor’s own pulse in their fingers or a lapse in concentration can interfere. With the Korotkoff method, the doctor’s hearing, background noise and the expected result can all affect the reading. In children, in certain medical conditions or immediately after physical work, Korotkoff sounds may persist below the actual diastolic pressure. Common sources of error include an incorrectly sized cuff, incorrect arm position, white coat syndrome, a full bowel or bladder, and recent coffee or alcohol consumption or cigarette smoking. When measuring blood pressure in a finger artery, the hands must be warm and the fingers at heart level.

Respiratory physiology: gas exchange step by step

In its broadest sense, respiration is the exchange of gases between the body and the environment: oxygen is delivered to the tissues, and CO2 produced by metabolism is removed. This takes place in 4 stages: gas exchange in the lungs (alveolar ventilation), diffusion of gases between the alveoli and the blood, transport of gases in the blood, and diffusion of gases between the tissues and the blood. Within the cells themselves, internal respiration takes place, using oxygen for the biological oxidation of nutrients.

Gas exchange between the outside air and alveolar gas keeps the partial pressure of CO2 lower and that of O2 higher in the alveolar gas than in venous blood. As a result, CO2 moves from the blood into the alveoli, while O2 moves from the alveoli into the blood — the blood becomes arterialized. In the absence of diffusion impairment, partial pressures in the alveolar gas and arterial blood equalize within about 0.3 s. Blood flow and ventilation are matched locally so that blood flows through the alveoli that are ventilated, and ventilation reaches the alveoli whose capillaries carry blood.

Oxygen is transported by hemoglobin. It consists of 4 polypeptide chains, each containing a heme group with a divalent iron atom. O2 binds to heme in a readily reversible reaction without changing the valence of the iron — hemoglobin becomes oxyhemoglobin.

Breathing is regulated through neurohumoral mechanisms. The respiratory center is located in the medulla oblongata and consists of inspiratory and expiratory centers, which send impulses through the spinal cord to the respiratory muscles. Ventilation is influenced both by the chemical composition of the blood (the humoral pathway) and by signals from receptors (the reflex pathway). The respiratory center is highly sensitive to changes in CO2 and less sensitive to changes in O2. Lactic acid and shifts in blood pH also play a role. In addition to the direct effect of CO2, the reflex pathway from vascular chemoreceptors is important: these receptors are even more sensitive to excess CO2 and insufficient O2 than the respiratory center itself. Afferent signals also arrive via the vagus nerve from stretch receptors in the lungs and proprioceptors in the respiratory muscles.

Spirography and lung volumes

An adult’s lungs hold an average of about 6 liters of air. The world record belongs to British rower Peter Reed — 11.68 liters. A spirometer measures the volume and flow rate of inhaled and exhaled air; it was invented by John Hutchinson in 1846. A spirogram can be used to calculate vital capacity, tidal volume, breathing rate and minute ventilation (tidal volume × breathing rate).

In a closed-circuit spirograph, a known volume of air circulates through the system, with oxygen added in controlled amounts as needed to prevent the test subject from suffocating. In an open system, the subject breathes outside air. The lungs’ residual volume is the amount of air that remains in the lungs after a maximal exhalation — this air can never be exhaled. To measure residual volume, an inert gas that is not immediately absorbed into the blood is introduced into the closed system; residual volume is then calculated from the change in gas concentration.

During physical exertion, pulmonary ventilation increases through a rise in both breathing rate and tidal volume. Tidal volume usually does not exceed half of vital capacity. In a well-trained athlete, ventilation can reach 120–130 l/min. Conditions for gas diffusion in the lungs improve, and alveolar ventilation accounts for a greater share of total ventilation. Cardiac output increases as both stroke volume and heart rate rise, and blood pressure increases. Red blood cells are released into circulation from blood reservoirs. In endurance athletes, O2 consumption during exercise can reach 6–7 l/min — up to 20 times resting consumption. In working muscles, the number of capillaries increases, temperature rises and the partial pressure of CO2 increases, allowing arterial blood to release oxygen more readily.

Digestive physiology: from mouth to gut

The main processes of digestion are food intake, transport through the digestive tract at an optimal rate, secretion of fluids and enzymes, breakdown of food, absorption of the breakdown products and elimination of indigestible material. Carbohydrate digestion begins in the mouth (amylase) and continues in the small intestine. Protein digestion begins in the stomach and continues in the small intestine. Lipid digestion begins with the salivary glands, with pancreatic enzymes continuing the process in the stomach and small intestine.

In the mouth, chewing breaks food into smaller pieces, while saliva moistens it and begins carbohydrate digestion. The stomach temporarily stores food, mechanically breaks it down and regulates its passage into the small intestine. Protein digestion also begins in the stomach.

The small intestine does most of the work. Pancreatic juice contains several enzymes: trypsinogen and chymotrypsinogen (protease precursors), pancreatic lipase (fats) and amylase (starch). The gallbladder is a hollow, pear-shaped organ in the upper right abdomen, attached to the underside of the liver; when you eat, its muscular wall contracts, forcing bile through the common bile duct into the duodenum.

The liver produces bile continuously — 0.5–1 liters per day (about 400 ml). Bile itself contains no enzymes, but it activates lipases from the pancreas and intestinal glands. Bile also emulsifies fats, promotes the absorption of fat-soluble vitamins and stimulates intestinal motility. Together, intestinal secretions complete the breakdown of proteins, carbohydrates and lipids, and the nutrients are absorbed into the blood and lymph.

Endocrine physiology: hormones as signaling molecules

The endocrine system controls growth and development, energy regulation, internal homeostasis, reproduction and the stress response. Hormones act in 3 main ways: endocrine signaling (hormones dissolved in the blood bind to target cells), paracrine signaling (local effects on nearby cells) and autocrine signaling (effects on the same cell that produced the hormone).

Hormones fall into several chemical classes. Peptide hormones are synthesized as prohormones and preprohormones, stored in granules, relatively polar and water-soluble, and make up the largest group. They range in size from 3 amino acids to hundreds. Because they cannot cross the cell membrane, they use membrane receptors and second messenger cascades — mostly G protein-coupled receptors, although some (such as insulin) use receptor tyrosine kinases.

Amine hormones are derivatives of L-tyrosine: thyroid hormones and catecholamines. Steroid hormones are all synthesized from cholesterol and are fat-soluble — they include glucocorticoids, mineralocorticoids, androgens (testosterone), estrogens and progesterones. Fatty acid derivatives remain active for a very short time and have autocrine or paracrine effects.

The stages of a humoral response are detection of a need, hormone synthesis and release, transport to the target cell, a response in the target cell and hormone breakdown. The hypothalamus and pituitary gland regulate hormones from the thyroid gland, adrenal glands and gonads, and control somatic growth, lactation and water metabolism. Pituitary activity depends on the hypothalamus. This system uses neural control, chronotropic (time-based) control and, most extensively, feedback mechanisms. In the reproductive system, sex is determined by the SRY gene, which initiates the pathways of oogenesis and spermatogenesis.

Renal physiology: from filtration to concentrated urine

Blood flow through the kidneys varies greatly by region: cortical perfusion is 5.3; outer medullary perfusion is 1.4; papillary perfusion is 0.4. The afferent arterioles and glomeruli are all located in the cortex. A distinctive feature is that resistance vessels and capillaries are connected in series in 2 successive arrangements. At rest, the kidneys receive 0.12 l/min, or 2.4% of total cardiac output (~5 l/min).

The nephron is the structural and functional unit of the kidney. It consists of the glomerulus, proximal convoluted tubule, loop of Henle (descending and ascending limbs), distal convoluted tubule and collecting ducts. Filtration pressure, or effective filtration pressure (Pef), is the difference between hydrostatic and oncotic pressures: Pef = Pkap − PBow − Ponk. Low blood pressure reduces filtration; ADH, cholesterol and toxins increase it.

The glomerular filtration barrier consists of 3 layers: a highly fenestrated capillary endothelium, a basement membrane (which filters out larger protein molecules) and the tightest layer, the epithelium of Bowman’s capsule, with its podocytes (filtration slits). The result is an ultrafiltrate: it contains no blood cells or platelets and almost no protein, while the concentration of dissolved small molecules is the same as in plasma.

Passive diffusion, facilitated diffusion (ion channels, uniport, symport and antiport), active transport (which requires energy to move substances against an electrochemical gradient) and endocytosis all occur in the renal tubules. Their purpose is to maintain water and electrolyte balance.

The proximal convoluted tubule reabsorbs 2/3 of the filtered water, Na+, Cl-, K+ and other substances; glucose and amino acids are reabsorbed almost completely. The key enzyme is basolateral Na-K-ATPase — almost all reabsorption is linked to its activity. In the 1st half of the tubule, Na+ is reabsorbed together with glucose, amino acids, phosphate, lactate and HCO3-; in the 2nd half, it is reabsorbed mainly with Cl-.

The loop of Henle reabsorbs approximately 20% of the filtered Na+, Cl- and K+, as well as Ca2+, HCO3- and Mg2+. These processes occur almost exclusively in the ascending limb. Approximately 20% of the water is also reabsorbed, mainly in the descending limb — the ascending limb is impermeable to water.

The distal convoluted tubule and collecting ducts reabsorb approximately 12% of the filtered Na+ and Cl- and secrete varying amounts of K+ and H+. The initial portion of the distal tubule is also impermeable to water, continuing the dilution of tubular fluid that began in the ascending limb of the loop of Henle. The formation of final urine relies on the sodium chloride pump in the thick ascending limb of the loop of Henle, regulation by ADH and the countercurrent principle: flow in opposite directions through the descending and ascending limbs makes the surrounding environment increasingly hypertonic toward the tip. Depending on the amount of ADH (antidiuretic hormone), concentrated final urine of reduced volume emerges from the tip of the renal papilla.

Acid–base balance and the physiology of thermoregulation

The distal tubule helps regulate acid–base balance by excreting hydrogen ions and producing ammonia from glutamine and other amino acids. Ammonia binds H+, allowing more acid to be excreted in the urine. Reabsorbed bicarbonate ions contribute to the same effect.

Enzymes have an optimal pH at which their activity is greatest. A shift in the balance toward acidity is called acidosis (which occurs more frequently), while a shift in the opposite direction is called alkalosis. Respiratory acidosis occurs when the removal of CO2 through the lungs is impaired — for example, by airway obstruction, pneumonia or emphysema — and blood pH falls. Respiratory alkalosis occurs during hyperventilation: too much CO2 is eliminated, pH rises and symptoms caused by changes in Ca levels may develop. Metabolic acidosis and alkalosis result from dysfunction of the liver, digestive tract or kidneys.

Hemolysis is the rupture of red blood cells and the release of hemoglobin into the surrounding fluid. Osmotic hemolysis occurs when the osmotic pressure of the fluid surrounding red blood cells is significantly lower than that inside the cells — water enters the cells and causes them to burst. An isotonic solution has the same concentration of dissolved substances as the reference solution; a hypertonic solution has a higher concentration, and a hypotonic solution has a lower concentration.

Thermoregulation involves the body controlling both heat loss and heat production at the same time. The actual regulator is the thermoregulatory center in the hypothalamus, which is influenced mainly by blood temperature and, to a lesser extent, by thermoreceptors in the skin. Human body temperature does not remain exactly constant — it fluctuates throughout the day. A fever is essentially a resetting of the temperature “set point”: the body starts warming itself (shivering + peripheral vasoconstriction) as though responding to cold surroundings. As the fever subsides, the pattern reverses — sweating and vasodilation occur. During a sustained fever, the mechanisms that compensate for temperature disturbances continue to work, simply at a higher set point.

Electrocardiography (ECG) and the heart’s electrical physiology

An ECG is a graph showing how electrical potential differences at the body’s surface change over time. These arise from the electrical field generated as excitation spreads through the heart and subsides. Remember: an ECG records excitation, not contraction.

An ECG has 2 main parts. The atrial component includes the P wave (the spread of excitation across both atria) and the PQ segment, during which both atria are fully depolarized. The ventricular component extends from the beginning of Q to the end of T: the QRS complex represents the spread of excitation across both ventricles, while the T wave represents ventricular repolarization. Between them lies the ST segment, which (like the PQ segment in the atria) indicates that the entire ventricular myocardium is depolarized. Sometimes the T wave is followed by a U wave, which is thought to represent repolarization in the terminal branches of the cardiac conduction system. An interval comprises a wave and the segment that follows it — for example, the PQ interval extends from the beginning of P to the beginning of Q.

Frequently asked questions (FAQ) about physiology

What is the difference between the autonomic and somatic nervous systems?

The somatic nervous system is under conscious control and controls skeletal muscles. The autonomic, or vegetative, nervous system (ANS) is not under voluntary control — its effectors are cardiac muscle, smooth muscle and glands. The ANS consists of sympathetic and parasympathetic divisions, which generally have opposing effects.

How much blood is in the human body, and what does it contain?

Blood accounts for approximately 7% of body weight — an average of ~5 l in an adult. Plasma makes up 55% of blood volume, and cells make up 45%. Plasma consists of 90–92% water, 6–8% proteins (albumin 60%, globulins and fibrinogen up to 40%) and 1–2% low-molecular-weight compounds.

Why is diastole — the regular relaxation of the heart muscle — so important?

During diastole, the ventricles fill with blood: the end-diastolic volume is about 150 ml. If relaxation is impaired, filling is incomplete, and the stroke volume of the next systole (70–80 ml) cannot be optimal. Regular relaxation therefore determines how effectively the heart pumps.

Author: Marek Morozov

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