8.1
The Main Blood Vessels
The pulmonary and systemic circulations are features of a double circulatory system, where blood passes through the heart twice in one full circuit around the body
The pulmonary circulatory system transports deoxygenated blood to the lungs for gas exchange
The systemic circulatory system transports oxygenated blood to the rest of the body
The diffusion of oxygen is reliant on:
The concentration gradients that exists between oxygen in the air in the alveoli of the lungs and oxygen in the blood (net diffusion into red blood cells)
The concentration gradient that exists between oxygen in red blood cells and the respiring tissues of the body (net diffusion into the mitochondria of cells)
Closed Double Circulatory System
The need for a circulatory system
The cells of all living organisms need a constant supply of reactants for metabolism, e.g. oxygen and glucose
Single celled organisms can gain oxygen and glucose directly from their surroundings, and the molecules can diffuse to all parts of the cell quickly due to short diffusion distances
Larger organisms, however, are made up of many layers of cells, meaning that the time taken for substances such as glucose and oxygen to diffuse to every cell in the body would be far too long
The diffusion distances involved are too great
To solve this problem their exchange surfaces are connected to a mass transport system, for example
The digestive system is connected to the circulatory system
The lungs are connected to the circulatory system
Mass transport is the bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes
The circulatory system in mammals is a well-studied example of a mass transport system; the one-way flow of blood within the blood vessels carries essential nutrients and gases to all the cells of the body
Open & closed systems
Circulatory systems are either described as being open or closed
In a closed circulatory system, blood is pumped around the body and is always contained within a network of blood vessels
All vertebrates and many invertebrates have closed circulatory systems
In an open circulatory system, blood is not contained within blood vessels but is pumped directly into body cavities
Organisms such as arthropods and molluscs have open circulatory systems.
Humans have a closed double circulatory system: in one complete circuit of the body blood passes through the heart (the pump) twice
The right side of the heart pumps blood deoxygenated blood to the lungs for gas exchange; this is the pulmonary circulatory system
Blood then returns to the left side of the heart, so that oxygenated blood can be pumped efficiently (at high pressure) around the body; this is the systemic circulatory system
Arteries, Veins & Capillaries: Observing & Drawing
Arteries, veins and capillaries have distinctive structures which reflect their differing roles throughout the body
The walls of arteries and veins contain the same components; but in differing proportions and with different wall thicknesses
The walls of the capillaries are formed from a single layer of cells
Plan diagrams show the structures of arteries and veins; these can be drawn in transverse section (TS) and longitudinal section (LS)
Arteries
Arteries are blood vessels that carry blood at high pressures away from the heart
Arteries have relatively thick walls which allow them to withstand the high pressure of blood as it surges through with each ventricular contraction of the heart
The walls of arteries are composed of elastic and muscular tissue, as well as collagen fibres
Arteries closer to the heart contain a higher proportion of elastic fibres – the walls of these arteries must be able to stretch and recoil to accommodate blood surging through, preventing them from bursting or from the blood pressure dropping
These arteries are described as being elastic
Arteries further from the heart contain less elastic and more smooth muscle tissue - the diameter of these arteries can be adjusted to alter the blood flowing to different tissues
These arteries are described as being muscular and they branch into smaller arteries (arterioles)
The blood pressure in the arterioles is lower than that of the arteries
The lumen of the arteries is relatively narrow; this ensures that blood remains at relatively high pressure for efficient delivery to the tissues whilst also providing resistance to blood flow to allow gas exchange as blood passes through the tissues
Capillaries
Arterioles branch into the smallest blood vessel – the capillaries – which form networks throughout most tissues of the body (where they are described as capillary beds)
Capillaries have a diameter of between 5-10 μm and most cells of the body are no more than a few μm from one
The diameter of a typical red blood cell is 7 μm
Blood flowing through the capillaries is brought close to the cells of the body to allow efficient exchange of materials (particularly the diffusion of oxygen)
The endothelial wall of the capillaries is only one-cell thick, which ensures that substances can diffuse easily between the capillary and neighbouring cells
The walls are also “leaky” – there are small gaps between individual squamous epithelial cells that form the wall to allow small substances to leak out of the blood into the fluid surrounding the cells of the body
Veins
Capillaries join together to form larger blood vessels called venules which join to form veins
The outer layer of the veins is relatively tough, composed largely of collagen fibres
Conversely, the middle layer of the veins is relatively thin in comparison and contains only a small amount of smooth muscle and elastic fibre
This is because the blood flowing through veins is under very low pressures so the walls of the veins do not have to stretch and recoil to accommodate blood flow
The lumen of veins is characteristically large
Skeletal muscle contraction helps raise blood pressure temporarily within the veins, and the presence of one-way valves keeps blood moving back towards the hearth
Micrographs
A photomicrograph is a photograph taken of a specimen observed using a light microscope
An electron micrograph is a photograph taken of a specimen observed using an electron microscope
Cells of the Blood
Blood is a tissue composed of a number of important specialised cells
Red blood cells, monocytes, neutrophils and lymphocytes all have distinguishable structures which enable them to be recognised on microscope slides, in photomicrographs and in electron micrographs
Red blood cells
Red blood cell
There are approximately 5 million red blood cells per mm3 of blood
Red blood cells contain haemoglobin, a protein with a quaternary structure that contains haem iron groups which can bind reversibly to oxygen
Distinctive features of erythrocytes when viewed under a microscope, are their distinctive biconcave disc shape (caused by their lack of nucleus)
Red blood cell micrograph
Monocytes
Monocyte
Monocytes are identifiable by their size – they are the largest of the leukocytes and have a nucleus shaped like a kidney or a bean
The nucleus of monocytes tends to appear lighter after staining than other leukocytes
The nucleus should appear a light blue colour, while the chromatin inside is distinct and fine
Monocyte micrograph
Neutrophils
Neutrophil
Neutrophils are distinguished by their multi-lobed nuclei
Up to 70% of all leukocytes are neutrophils – this makes them easy to spot on a micrograph
The granules of neutrophils typically stain pink or purple-blue
Neutrophil micrograph
Lymphocytes
Lymphocyte
Lymphocytes are small leukocytes that are identifiable by their very large nuclei, which typically stains a dark colour
Lymphocytes constitute around 20-25% of all leukocytes
Lymphocytes are around the size of red blood cells
olvent action
Water is the main component of blood (where it constitutes 95% of plasma, a straw-coloured liquid) and tissue fluid
Tissue fluid is formed when plasma passes through capillaries and some of it leaks into the spaces between the cells in the walls of the capillary. Tissue fluid is therefore mainly water, too
Water’s properties as a solvent make it ideal for transport in mammals
For example, glucose is transported in solution from the small intestine to every cell of the body for respiration. In addition, urea is transported in solution from the liver to the kidneys
Specific heat capacity
Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1 oC
Water has a high specific heat capacity of 4200 J / Kg oC – a relatively large amount of energy is required to raise its temperature
This means that water is able to absorb a lot of heat without big temperature fluctuations
This is vital in maintaining temperatures that are optimal for enzyme activity
Water in blood plasma is also vital in transferring heat around the body, helping to maintain a fairly constant temperature
As blood passes through more active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant
Water in tissue fluid also plays an important regulatory role in maintaining a constant temperature
Blood, Tissue Fluid & Lymph
Plasma is a straw-coloured liquid that constitutes around 55% of the blood
Plasma is largely composed of water (95%) and because water is a good solvent, many substances can dissolve in it, allowing them to be transported around the body
As blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body
This results in the formation of tissue fluid
The composition of plasma and tissue fluid are virtually the same, although tissue fluid contains far fewer proteins
Proteins are too large to fit through gaps in the capillary walls and so remain in the blood
Tissue fluid bathes almost all the cells of the body outside of the circulatory system
Exchange of substances between cells and the blood occurs via the tissue fluid
For example, carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then diffuse into the capillary
Tissue fluid formation
How much liquid leaves the plasma to form tissue fluid depends on two opposing forces
When blood is at the arterial end of a capillary, the hydrostatic pressure is great enough to push molecules out of the capillary
Proteins remain in the blood; the increased protein content creates a water potential between the capillary and the tissue fluid
However, overall movement of water is out from the capillaries into the tissue fluid
At the venous end of the capillary, less fluid is pushed out of the capillary as pressure within the capillary is reduced
The water potential gradient between the capillary and the tissue fluid remains the same as at the arterial end, so water begins to flow back into the capillary from the tissue fluid
Overall, more fluid leaves the capillary than returns, leaving tissue fluid behind to bathe cells
If blood pressure is high (hypertension) then the pressure at the arterial end is even greater
This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues. This is called oedema
Formation of tissue fluid
Formation of lymph
Some tissue fluid reenters the capillaries while some enters the lymph capillaries
The lymph capillaries are separate from the circulatory system
They have closed ends and large pores that allow large molecules to pass through
Larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph
Small valves in the vessel walls are the entry point to the lymphatic system
The liquid moves along the larger vessels of this system by compression caused by body movement. Any backflow is prevented by valves
This is why people who have been sedentary on planes can experience swollen lower limbs
The lymph eventually reenters the bloodstream through veins located close to the heart
Any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries
If plasma proteins were not removed from tissue fluid they could lower the water potential (of the tissue fluid) and prevent the reabsorption of water into the blood in the capillaries
After digestion lipids are transported from the intestines to the bloodstream by the lymph system
Muscular Artery:
• Cannot stretch and recoil
• Narrow lumen
• Blood flows under high pressure
• Thick tunica media with smooth muscle fibers
• Capable of vasoconstriction and vasodilation
Elastic Artery:
• Thinner tunica media with elastin and collagen
• Ability to stretch in response to each pulse
• Relatively few smooth muscle fibers
• Cannot perform vasoconstriction or vasodilation
• Narrow lumen
• Blood flows under high pressure
Vein:
• One-way valves prevent backflow of blood
• Wide lumen
• Blood pressure reduced, no surges
• Less smooth muscle and elastin
• Abundant collagen for increased strength and structure
Capillary:
• Very small diameter
• Slow blood flow, allowing for diffusion
• Branches between cells
• Rapid diffusion between blood and tissues
• Thin walls, lacking elastic fibers, smooth muscle, or collagen