Gas exchange in large multicellular organisms is achieved by organs which have a large surface area and so are able to successfully provide the substances the organism needs in order to survive. In humans this is achieved by the lungs. But how does the oxygen acquired by the lungs actually reach every single cell of the body? A network of sorts is needed to do that. Many bigger and smaller tubes would come in handy. They would form like a… circulatory system. Oh wait, that’s precisely what mammals have: a circulatory system made of arteries, veins, capillaries, etc.
Plants, too, have a vascular (tubular) system. It is made of xylems and phloems. Yes, complicated names which you will love to learn about in the following topic (Transport systems in plants).
The key thing is that this circulation of a large amount of substances via a system of transportation is called mass flow, hence mass transport. Just more technical terms for you to learn, which describe something that really couldn’t get any simpler. Water, gases chillin’ through tubes in an organism.
Mass transport is required to fulfil the demands of a high basal metabolic rate arising from being multicellular and hence having a lower surface area to volume ratio.
Organisms exchange substances and heat with their environment all the time, and this possibility is crucial to survival. The specific way in which this is achieved is very tightly related to the shape and structure of the specific organism, as well as its environment. For example, unicellular organisms are so small that molecules such as oxygen and water can readily diffuse in and out via the membrane, due to the short diffusion pathway. Could this be achieved by a human, or even a bee? No – they are simply too big.
Two properties are important to consider here: the volume of an organism, and the surface area of an organism. The volume is what determines the amount of substances which need exchanging, while the surface area determines the amount which can be exchanged.
Surface area describes the number of cells in direct contact with the environment. Volume describes the space occupied by all metabolically active cells.
Key principle: as the size of an organism increases, the surface area to volume ratio decreases.
That might seem hard to really understand. Why use a ratio in the first place? Well, the ratio shows the relationship between surface area and volume ratio, i.e. how similar or dissimilar are they?
Surface area = 12 x 6 = 6
Volume = 13 = 1
Surface area : volume = 6:1 = 6.00
Surface area = 22 x 6 = 24
Volume = 23 = 8
Surface area : volume = 24:8 = 3:1 = 3.00
3 is smaller than 6, so as the cube/organism gets larger, the surface area to volume ratio decreases.
What this basically means is that the larger an organism gets, the less surface area is available to serve its increasing needs due to its increasing volume.
In my quest to find a suitable diagram for the heart, this is what I found:
Definitely use your textbook as a guide on this. It only takes a google search to realise the ridiculous number of variations of diagrams for the heart and different annotations.
The heart contains two atria, two ventricles, the septa, AV-valves (tricuspid and bicuspid), chordae tendinae and papillary muscles. Septa describe the dividing walls between the right and left atria and ventricles respectively.
The chordae tendinae a.k.a. heart strings, are attached to the papillary muscles which prevent the walls collapsing onto themselves during heart contraction.
You need to be able to sketch a heart and label the main veins, valves, arteries and aorta, and the ventricles and atria.
There are two types of circulation going on via the heart: pulmonary circulation and systemic circulation. Pulmonary circulation is a short-distance route between the heart and the lungs, where deoxygenated blood is taken to be replenished with oxygen. Although normally veins take blood away, and arteries take blood to, in the case of pulmonary circulation things are the opposite way around. The pulmonary vein brings freshly oxygenated blood into the heart – left atrium -, while the pulmonary artery takes deoxygented blood back from the right ventricle into the lungs.
Here’s a quick nifty video that shows what happens as the bigger picture…
The advantages of double circulation in mammals as opposed to single circulation (such as in bony fish) include the option to have a higher blood pressure and splitting oxygenated and deoxygenated blood. As blood gets oxygenated in the lungs, the diffusion process takes time, which has the blood at lower pressure. On the other hand, the already oxygenated blood can be pumped around the body at higher pressure, allowing for bigger organisms and increased metabolic activity. Splitting deoxygenated blood from oxygenated blood is key to this.
The atrioventricular valves and semilunar valves play an important role in ensuring proper heart function. The former ensure no blood flows back into the atria from the ventricles, while the latter ensure no blood flows from the ventricles into the atria.
Electrical impulses cause heart muscle contraction which creates an increased pressure of blood, resulting in it being pushed in a certain direction, with the valves opening in its way. The sequence of events in heart contraction is this:
1. Both atria contract – atrial systole
2. Both ventricles contract – ventricular systole
3. All chambers relax – diastole
Pressure changes in the heart are similarly representative of the different parts of the heart cycle. Just like the top ECG signal, the largest pressure increase is that of the ventricular systole, coinciding with the pressure increase in the aorta which is the vessel that carries the oxygenated blood around the body as a result of ventricular systole.
The heart muscle contracts without brain stimulation – the brain only controls the speed. Electrical impulses start in the sino-atrial node in the right atrium, travels down to the atrio-ventricular node, which then spreads it across the Purkyne (a.k.a. Purkinje) fibres, which results in the left ventricle contracting.
Cardiac output = heart rate x stroke volume
Heart rate is measured in beats per minute, while stroke volume is measured in cm3 or ml. For example, the cardiac output of someone with a heart rate of 83 beats per minute of stroke volume 54 ml equals:
Cardiac output = 83 x 54
Cardiac output = 4,482 ml per minute
In other words, this person’s cardiac output is around 4.5 L per minute.
Make sure you can interpret graphs showing the sequence of atria and ventricles contracting followed by diastole.
Cells in mammals require a constant supply of nutrients and oxygen, and a way to remove waste products. Blood is great, as it does all that. Blood needs a way of getting to all cells of the body, a way to… circulate. Without that, blood would just get pulled by gravity towards the centre of the earth. Not a pretty sight I’m afraid.
There are two circulations in the body:
1. The pulmonary circulation takes blood from the heart, pumping it to the lungs in order to oxygenate it.
2. The systemic circulation takes blood from the heart to everywhere else. Eyes, legs, hand, bum, you name it.
Heart rate during exercise
Factors affecting heart rate such as illness, psychology, exercise, metabolism, etc. can be investigated in practical experiments. Heart rate can be monitored with fairly simple devices including by feeling the pulse in the wrist or using a heart rate meter e.g. wristwatch; even smartphone cameras can be used to monitor the pulse by seeing how a finger placed against the camera throbs with the pulse.
Different scenarios can be tested e.g. heart rate measured over 5 minutes lying down, sitting or standing; being stationary or during exercises like running on a treadmill (of varying intensity); or while watching a movie with different scenes that can generate different responses from viewers e.g. thrill, fear, horror, love, etc.
Interpreting heart function data
Heart function data can come in many forms including ECG (electrocardiogram) traces and pressure changes. The aim of collecting this data is to monitor the activity of the heart and identify any issues pertaining to the circulatory system.
ECG traces are electrical changes recorded at the skin level using electrodes. Heart beats are recorded, including the stages between them to visualise full cardiac cycle patterns over time.
The largest signal is given by the ventricular systole, with other smaller signals given by the surrounding heart cycle events. The different signals have wave terms, such as the P wave and the T wave. The spacing and duration of the signals can indicate the speed of the heart beats and their regularity, which can be used to asses various pathologies such as rapid heart rate, tachycardia, slow heart rate, bradichardya or various irregular patterns of heart beat, arrythmia.
Arrythmias such as atrial arrythmia and ventricular arrythmia are caused by muscle fibrillation which results in fast and irregular contractions that fail to fulfil the function of the heart in pumping enough blood to reach all body’s tissues. This is when a defibrillator must be used. A defibrillator is also used following cardiac arrest.
First-aid treatment of hearts attack and cardiac arrest involves calling for emergency help and requesting an AED (automated external defibrillator) device, while starting CPR as soon as possible if the patient is unresponsive. CPR involves chest compressions (100-120 per minute) and rescue breaths (2 per minute in between compression sets). This is administered until the emergency team arrive, the patient becomes conscious or the first-aid person becomes too tired to continue.