- The circulatory system comprises of the heart, the blood vessels and blood.
- It can be thought of as the body's internal transport system, carrying messages and materials from one part of the body to another.
- The system is also central to the maintenance of homoeostasis, being closely integrated with the processes which control nutrients levels and waste disposal as well as water and temperature regulation.
- The need for an internal transport system arose many millions of years ago when an increase in the size of multicellular animals became limited by the rate at which gasses and nutrients could be obtained from the external environment by simple diffusion.
- The development of a simple circulation meant that exchange could be managed by specialist organs such as the gut and lungs and the products of exchange transported to all of the cells of the body.
- It would be reasonable to suggest that without the development of the circulation system, animal life as we know it today would not exist - including ourselves…!
The main functions of the circulatory system can be summarised as follows:
- Transport of respiratory gasses.
- Oxygen is transported from the lungs to the tissues for use in energy production and carbon dioxide
- A waste product of this process is transported from the tissues to the lungs for disposal to the atmos phere.
- Transport of nutrients. Glucose, amino acids and other useful substances such as vitamins and minerals are absorbed by the gut are transported in the blood to tissues or sites of storage such as the liver.
- Fats absorbed in the digestive process are initially transported from the gut by the lymphatic system that eventually feeds into the circulatory system.
- Nutrients released from storage are also transported in the blood to tissues that need them.
- Transport of metabolic wastes.
- During metabolic processes, tissues produce waste products such as creatinine and urea.
- These diffuse into the blood and are transported to the kidneys where they are excreted in the urine.
- Transport of metabolites. Cholesterol, phospholipids and vitamins are required for the functioning of body cells and need to be transported from their site of synthesis to the site in the body which needs them.
- Immunological functions. Leucocytes (white blood cells) and plasma proteins such as antibodies and complement circulate in the blood and provide a rapid response against the invasion of the body by pathogens.
- Transport of hormonal messengers. Hormones produced in the various endocrine glands are transported in the blood to target tissues in other parts of the body.
- Thermoregulation. By varying the amount of blood in the tissues at the surface of the body, the amount of heat lost to the surrounding environment can be controlled.
- Damage repair. Injuries need to be repaired quickly to restrict the loss of body fluids and prevent the invasion of pathogens.
- Platelets and clotting proteins circulate in the blood and stop blood loss by forming a clot.
Overview of the circulatory system
- At the centre of the system is the heart, a large, muscular pump which beats seventy or so times each minute from a few weeks after conception to the last minute of the individual's life, a simple calculation indicates that at the age of seventy, the heart has beat over 2.5 billion times.
- The heart is not a single pump but two separate pumps joined together and beating in unison.
- The right hand pump sends blood to the pulmonary circulation, around the alveolar capillaries in the lungs, from where it returns to the left hand heart pump.
- From the left hand pump the blood is sent around the systemic circulation to the capillaries which supply blood to the tissues and organs of the body.
- The blood then returns to the right hand pump from where it is again sent around the pulmonary circulation
- The body needs this dual circulation because the pulmonary system is much smaller than the systemic system and the lungs are situated adjacent to the heart.
- When an individual is engaged in strenuous exercise, the heart rate increases, the volume of blood pumped each beat increases and blood pressure also increases slightly.
- This could easily rupture the delicate capillaries surrounding the alveoli in the lungs but fortunately, the right hand side of the heart pumps at lower pressure compared to the left side.
- Heart anatomy reflects this difference, the left hand side of the heart is larger and has thicker, more muscular walls than the smaller right hand side.
The vessels which carry the blood can be divided broadly into three types:
- Arteries these thick, muscular tubes carry high-pressure blood away from the heart.
- The arteries supply blood to organs and muscles where they branch into smaller arterioles.
- Arterioles are the main resistance vessels of the circulatory system.
- They are under sympathetic nervous control that adjusts their diameter to increase or decrease their resistance to the flow of blood
- This is an important function because it enables the body to respond to changes in blood pressure and also to divert blood to particular organs and tissues during exercise or stress.
- Veins thinner and more fibrous than arteries, the veins carry low-pressure blood back to the heart.
- Veins are the capacitance vessels of the circulatory system and over 60% of the body’s blood is retained in the veins.
- The low pressure in the veins is insufficient in itself to move venous blood back to the heart and so various strategies are used to accomplish this task.
- Simple pocket valves in the veins prevent backflow and the contraction of skeletal muscles helps push the blood through the veins.
- Capillaries these are the smallest of the blood vessels, only one cell thick they permeate the tissues of the body and allow the exchange of materials between the blood and the body's cells.
The structure of arteries, veins and capillaries
The Circulatory System and Gas Exchange
- The process of cellular respiration where metabolic fuels such as glucose and fatty acids are broken down to produce adenosine triphosphate (ATP) requires a good supply of oxygen.
- Carbon dioxide, is a waste produced during cellular respiration and needs to be removed from the cells, otherwise, its accumulation could inhibit inhibit metabolic activity.
- The transport of oxygen to the tissues and the removal of carbon dioxide is a very important function of the circulatory system and must be maintained constantly throughout the life of the individual.
- Any trauma which halts the exchange of gasses especially the supply of oxygen quickly results in death of cells or even the individual.
- The structure of the lungs allows rapid gas exchange between the alveolar capillaries and the atmosphere.
- The blood pumped to the lungs by the right hand side of the heart is low in oxygen and high in carbon dioxide. In the alveolar capillaries, blood is brought into close proximity with the air in the alveoli which is high in oxygen and low in carbon dioxide \
- This difference in the concentration of gasses is called a diffusion gradient and gas exchange takes place as oxygen diffuses into the blood from the air and carbon dioxide diffuses out of the blood into the air.
- The diffusion gradient is maintained by regular breathing and the flow of blood.
Gas exchange between an alveolus and adjacent capillaries
- The oxygenated blood flows from the lungs to the left side of the heart that sends it around the body tissues via the arteries to the capillaries.
- Here, gas exchange takes place opposite to what happened in the lungs.
- Blood high in oxygen and low in carbon dioxide is brought into close proximity with respiring cells which are low in oxygen and high in carbon dioxide.
- Oxygen therefore diffuses out of the blood into the cells and carbon dioxide diffuses out of the cells and into the blood.
- The blood now flows out of the capillaries and into the veins on its return journey to the right hand side of the heart which sends it on to the lungs where the carbon dioxide is removed and more oxygen collected -
Schematic diagram of gas exchange in the circulatory system
- Ask most people where their heart is situated and they will point vaguely to the left side of their chest.
- The heart is in fact situated in the centre of the chest, directly under the sternum or breastbone.
- The heart is however on a slightly tilted axis so that the bottom pointed part of it or apex is on the left hand side.
- This means that when the heart is beating strongly, the heart beat can be heard more clearly on the left hand side where you can also feel it by placing your fingers just to the left of the sternum.
The Cardiac Cycle
- Apart from the difference in size, the left and right hand side of the heart are structurally very similar.
- We have seen an outline how the blood flows around the pulmonary and systemic circulations, now let us examine the sequence of events in the heart itself, usually called the cardiac cycle.
- The sequence is the same in the right hand and left hand side.
- The two atria (singular atrium) receive blood from the veins and hold it until the valves to the ventricles open when the heart relaxes.
- The two ventricles are the main pumping chambers, as they contract, blood is forced out of the heart and into the arteries.
- The atria and ventricles are connected by a fibrous skeleton containing one-way valves, collectively called the atrioventricular valves which allow blood to pass from the atria to the ventricles but not from the ventricles to the atria.
The cycle can be divided into two stages, diastole when the heart is relaxed and filling with blood and systole when the heart is contracting and pumping blood into the arteries.
- During diastole, the heart is relaxed, the atrioventricular valves are open and both the atria and ventricles fill with blood emptying into the atria from the venous division. Note that 80% of ventricular filling takes place during diastole.
- In the left hand side of the heart, blood arrives via the pulmonary veins (which are the only veins in the body to carry oxygenated blood). In the right hand side of the heart, blood arrives via the superior and inferior vena cava, large veins carrying blood from the upper and lower body respectively.
- During systole, the heart begins to contract. The atria contract first, forcing blood through the atrioventricular valves into the ventricles. Rings of muscle around the pulmonary valves and vena cava contract to prevent a back-flow of blood into the venous system.
- When the ventricles are full, the atrioventricular valves close as the ventricles contract and blood is forced into the pulmonary artery and aorta through another set of one-way valves collectively called the semi-lunar valves.
- Whilst the ventricles have been contracting, the atria have returned to a diastolic state and have been filling with blood. The whole heart will now relax and the cycle starts over again.
The Valves of the Heart
- The structure of the atrioventricular valves allows an easy flow of blood into the ventricles during diastole but is able to withstand the considerable backpressure of blood when the ventricles contract in systole.
- The valve cusps themselves are made of a thin, tough membrane.
- They are held in place by valve tendons called the chordae tendineae or sometimes, with a slightly romantic overtone, the heart-strings.
- These are attached to the valve cusps at one end and to small papillary muscles at the other end
- The left atrioventricular valve has two cusps and is called the bicuspid valve whereas the right atrioventricular valve has three cusps and is called the tricuspid valve.
- When the ventricle is full of blood and going into systole, the papillary muscles contract so tensioning the tendons and the valve cusps which can withstand the increase in blood pressure as the ventricle contracts.
- The remaining two heart valves, the pulmonary and aortic valves are found where the pulmonary artery and the aorta leave the ventricles.
- These valves do not have tendons attached to them and rely on their structure to prevent a back-flow of blood from the arteries into the ventricles.
- The arterial blood pressure to which these valves are subjected is somewhat less than that acting on the atrioventricular valves.
The Blood Supply to the Heart
- The blood which is being pumped through the heart chambers does not in any significant way supply nutrients or exchange respiratory gasses with the cardiac tissue.
- The myocardial muscle is far too thick to allow nutrient supply and gas exchange by simple diffusion so the heart has its own blood supply via its own vessels and this is called the coronary circulation
- The blood supply for the coronary circulation comes from the two coronary arteries which emerge from the ascending aorta, just above the semi lunar valve.
- These coronary arteries wrap themselves around the heart, the right coronary artery supplying the right atrium and right ventricle and the left coronary artery supplying the left atrium and left ventricle.
- Most of the blood flow through the coronary circulation occurs during diastole, when the heart is relaxed, this is because the blood vessels in the heart are constricted whilst the heart is contracting in systole.
- During exercise when the diastolic period is of very short duration, this leaves very little time for oxygenated blood to flow through the heart.
- Fortunately, there is an excess of oxygen in the blood to meet the respiration needs of myocardial tissue and there is also evidence that the vessels of the coronary circulation dilate during exercise to permit a greater flow of blood.
The Pericardium and the Wall of the Heart
- The heart is contained within a bag like structure called the pericardial sac, (peri = around, cardium = heart).
- The outer layer of the pericardial sac is a tough, inelastic membrane called the fibrous pericardium which protects the heart and anchors it within the thoracic cavity.
- The inner layer of the pericardial sac is the epicardium which is also the outer layer of the heart wall.
- Between the fibrous pericardium and the epicardium is the pericardial cavity filled with pericardial fluid which is secreted by the cells which line the cavity.
- This fluid prevents friction between the beating heart and the surrounding tissue.
The wall of the heart itself can be divided into three layers
- Myocardium. This tissue is composed of muscle tissue and forms the bulk of the heart wall. It is responsible for the contraction of the heart.
- Endocardium. This is a thin layer of endothelial cells which lines the heart and all other vessels of the circulatory system.
- Epicardium. The thin membrane covering the heart, the innermost layer of the epicardium which secretes pericardial fluid into the pericardial cavity.
The Cellular Structure of the Myocardium
Layers of the heart wall
- The myocardium consists almost entirely of cardiac muscle cells which are unique to the heart and are found in no other part of the body
- The cardiac muscle fibres are aligned in a spiral around the heart which is due to the complex twisting which takes place during embryological development.
- This arrangement means that contraction of the heart is in a "wringing" motion which ensures that blood is emptied from the heart with the maximum efficiency.
- Unlike smooth and striated skeletal muscle, the cardiac muscle cells are branched with the ends of the branches joined to adjacent cells by structures called intercalated discs
- These contain desmosomes which anchor the cells together securely during muscular contraction.
- Also found within the intercalated discs are gap junctions, a sort of synapse which allows action potentials (nervous impulses) to pass from cell to cell.
- As might be expected in a muscle mass which may beat continuously for over seventy years, energy supply is of prime importance.
- The cells contain many ATP producing mitochondria fuelled by fatty acids, triacylglycerols being the hearts preferred fuel.
- The heart muscle also has a good supply of myoglobin which can store a small amount of oxygen and aid the transfer of oxygen from haemoglobin to the mitochondria.
- To simplify a fairly complex and dynamic series of pressure changes within the heart and aorta
- The following description divides the cardiac cycle into four sequential stages, starting with the general diastolic or period of relaxation.
- Stage 1. The heart is in diastole, it is relaxed and the atrioventricular valves are open and both the atria and ventricles are filling with blood returning from the venous division. In the aorta, the pressure is still high after ventricular systole but begins to fall steadily as blood moves through the capillaries. At this point the semilunar valves are closed so arterial blood pressure is isolated from ventricular blood pressure.
- Stage 2. The heart’s pacemaker, the sinoatrial node generates a nervous impulse that sweeps in a wave across the atria causing them to contract and force their remaining blood into the ventricles which causes ventricular pressure to increase slightly. In the aorta, the semilunar valve is still closed so the steady fall in blood pressure continues.
- Stage 3. Ventricular systole starts and the increase in pressure in the ventricle causes the atrioventricular valves to close. The pressure in the ventricles increases rapidly causing the semilunar valves to open with a corresponding increase in aortic pressure. The atria which are isolated from the ventricles by the closed atrioventricular valves continue to receive blood from the venous division.
- Stage 4. Ventricular systole finishes and the pressure in the ventricles falls to the point where the semilunar valves close causing a small increase in arterial pressure due to the elastic recoil of the arteries forcing blood back towards the valves. This small increase can be seen as a blip on the graph called the dicrotic notch. As ventricular pressure falls below atrial pressure, the pressure of blood in the atria open the atrioventricular valves and blood flows into the ventricles. The heart is now in general diastole and the cycle of pressure changes starts again.
The Nature of Arterial Blood Pressure
- Arterial pressure is pulsatile in nature in that it fluctuates between a high and a low as the heart contracts and relaxes.
- The maximum pressure of blood on the arterial walls is the systolic pressure and the lowest pressure is the diastolic pressure
- The average pressure in the arteries is the mean arterial pressure (MAP).
- The pulse that can be monitored in certain arteries is the expansion of those arteries during systole.
- As the heart contracts, blood is forced from the ventricles into the arteries.
- The pressure in the arteries is thus increased to their maximum systolic pressure.
- When the heart goes into its diastolic phase and relaxes, the pressure in the arteries is locked in by the closure of the semi-lunar valves.
- During this phase, pressure in the arteries falls steadily as the blood percolates its way through the tiny arterioles and capillaries.
- It is important to note that pressures in the heart fluctuate much more than pressures in the arteries.
- There is always a minimum pressure in the arteries of (typically) 70-80 mm Hg, whereas in the heart, the pressure drops to near zero during ventricular diastole (Figure Pressure changes in aorta and left ventricle during cardiac cycle).
- Thus, the arteries act as pressure reservoirs during the heart’s diastolic period, their elastic walls recoiling to ensure an uninterrupted flow of blood through the capillaries (Figure Systole and diastole in the arteries).
- Pressures differ between blood vessels, being highest in the arteries and falling as the blood progresses through the smaller arterioles and into the capillaries.
- Blood pressure is lowest in the veins. Note that blood flow in the capillaries (and veins) is non-pulsatile (Figure Blood pressure in the blood vessels).
- It is important that arterial pressure is maintained at a sufficiently high level to drive the blood through the millions of capillaries which provide nutrient supply, gas exchange and waste removal to the cells of the body’s tissues.
- However, pressure should not be so high that it ruptures the small blood vessels or places undue strain on the heart.
- Blood pressure is maintained in the short term by the complex interaction of systemic vascular resistance (SVR) and cardiac output (CO).
- Longer-term regulation also calls for adjustment to blood volume.
- Cardiac output is the amount of blood pumped by the heart each minute.
- Systemic vascular resistance is the resistance to blood flow, determined by blood vessel diameter, blood viscosity and total length of blood vessels.
- The body has short-term control over blood vessel diameter via sympathetic nerves that cause the smooth muscle surrounding the arterioles to contract.
- The smaller the diameter of the lumen, the greater the resistance (Figure Effect of sympathetic stimulation of arterioles on vascular resistance).
- The relationship of blood pressure, cardiac output and SVR can be best expressed by a simple formula…
Blood pressure = cardiac output x systemic vascular resistanceBP = CO x SVR
- If either cardiac output or systemic vascular resistance increase then blood pressure will increase.
- If either cardiac output or systemic vascular resistance decrease then blood pressure decreases.
- Cardiac output is expressed by another simple formula…
CO = HR x SVCardiac output = heart rate x stroke volume*
- If either heart rate or stroke volume increase then cardiac output will increase (and so BP increases).
- If either heart rate or stroke volume decrease then cardiac output will decrease (and so BP decreases). *stroke volume = the amount of blood ejected from the left ventricle each cardiac cycle (around 70 ml at rest).
- Arterial blood pressure is monitored by pressure sensors in the circulatory system called baroreceptors, the most important of which are found on the aortic arch and the carotid sinus.
- These stretch receptors monitor changes in blood pressure and send neural signals to the cardiovascular control centre in the medulla.
- The medulla, via the autonomic nervous system regulates the blood pressure by adjustments to the cardiac output of the heart and the peripheral resistance of the arterioles.
- This process is called the baroreceptor reflex.
- We will now examine the response of the baroreceptor reflex to decreases and increases in blood pressure:
- A decrease in blood pressure can be caused by haemorrhage, dehydration, heart attack and even just standing up too quickly.
- The drop in BP is detected by the baroreceptors as a reduction in arterial stretch which reduces the rate of signals sent to the control centre in the medulla via the afferent neurons.
- The medulla then increases the sympathetic activity and decreases the parasympathetic activity of the circulatory system.
- This has the effect of increasing both the stroke volume and heart rate which increases the cardiac output.
- The arterioles are subject to sympathetic vasoconstriction which increases the systemic vascular resistance.
- All of these contribute to an increase in blood pressure (Figure Control of short-term blood pressure fluctuations by the baroreceptor reflex).
- An increase in blood pressure is normally and indication of exercise.
- Obviously, a response that reduced cardiac output would be undesirable, as it would limit our ability to exercise.
- Fortunately, complex mechanisms in the central nervous system suppress the baroreceptor response and selective vasodilation occurs that redistributes blood to the exercising muscles.
- This lowers systemic vascular resistance so that blood pressure only increases moderately, even though cardiac output may have doubled.
- Long-term adjustment to blood pressure involves the regulation of Na+ levels in the blood and this is done by the kidney.
- The level of Na+ in the blood will determine its osmolarity and thus the amount of water it will carry.
- The regulation of Na+ is via the hormone aldosterone produced by the adrenal glands which controls the amount of Na+ lost in the urine.
- The regulation of aldosterone itself is quite complicated.
- Receptors in the kidney respond to a fall in blood pressure and Na+ levels by triggering the renin - angiotensin - aldosterone cascade, a complex of hormones and plasma proteins which increases plasma Na+ levels and induces vasoconstriction - so increasing blood pressure.