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Posts Tagged ‘physiology

what is this thing called lymph?

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Canto: So in the last post, lymph glands, or nodes or whatever, got a passing mention, and I realise I’ve lived a pretty full lifetime without having much of an idea of this substance – is it a solid, liquid or gas, or is it delightfully ethereal, like qi?

Jacinta: Okay, let’s explore. The Better Health Channel, an Australian website, manages to give a point by point summary of the lymphatic system without really explaining what lymph actually is. For example, here are a couple of points that come close, but not very….

  • The lymph nodes monitor the lymph flowing into them and produce cells and antibodies which protect our body from infection and disease.
  • It maintains fluid levels in our body tissues by removing all fluids that leak out of our blood vessels.

From which we can deduce that it’s a fluid, since it flows.

Canto: The book we’ve been reading on CFS and its symptoms gives, en passant, this useful information on lymph nodes:

The lymph nodes are tender in multiple areas, such as in the front and back of the neck, armpits, elbows and groin…. One of the most characteristic symptoms is pain in the sub-auricular lymph nodes, the nodes located under the ear and behind the angle of the jaw.

Jacinta: Wow, they bin everywhere. And yes it does sound a bit like qi, some energy force that just needs to be needled at the nodes.

Canto: Time for some science. Lymph comes from Latin, lympha, ‘water’. So, very fluid. Here’s what Wikipedia says on its structure:

Lymph has a composition similar but not identical to that of blood plasma. Lymph that leaves a lymph node is richer in lymphocytes than blood plasma is. The lymph formed in the human digestive system called chyle is rich in triglycerides (fat), and looks milky white because of its lipid content.

Which sounds like the lymph nodes are where lymphocytes are produced. Lymphocytes are a type of leukocyte or white blood cell.

Jacinta: Well, here’s what I’ve come up with, to start things off.

The lymphatic system is the system of lymphoid channels and tissues that drains extracellular fluid from the periphery via the thoracic duct to the blood. It includes the lymph nodes, Peyer’s patches, and other organized lymphoid elements apart from the spleen, which communicates directly with the blood.

And what, you might ask, is the thoracic duct? Not to mention Peyer’s patches. The thorax, I think, is basically that part of the body covered by the rib cage, which includes the heart, the lungs and other organs, perhaps the spleen, perhaps the pancreas, the liver, the stomach, I’m very vague about it all. Anyway, the thoracic duct is an essential part of the lymphatic system, so here’s some more essential info about it:

The lymph from most of the body, except the head, neck, and right arm, is gathered in a large lymphatic vessel, the thoracic duct, which runs parallel to the aorta through the thorax and drains into the left subclavian vein. The thoracic duct thus returns the lymphatic fluid and lymphocytes back into the peripheral blood circulation.

So from this it’s clear that blood and lymph seem to circulate and work together in some respects.

Canto: It’s annoying that lymph is described as the ‘stuff of the lymphatic system’ or in the lymph nodes/vessels, etc etc. It reminds me of dormative virtue, somehow. Then again, it’s a bit like blood. What’s blood? It’s the stuff that comes out of us when we cut ourselves. Most people don’t know much beyond that – except for one key fact. It’s red, and it pools all over the floor in murder dramas. What colour is lymph? Have we ever seen a pool of it? Do we every lymph to death? Why can’t we turn lymph into a verb?

Jacinta: Okay, enough of the deepities. This really is a fascinating topic, and tracing the discovery of lymph, chyle, and the lymphatic system, starting with Hippocrates some 2400 years ago, would be the best, or at least the most interesting way to learn about the stuff, IMHO. I’ve found a recent series of pieces, The discovery of lymphatic system in the seventeenth century, which I’d love to read, but they’re behind a paywall, because we impoverished dilettantes need to be kept from accessing such things. They do give us access to the abstracts though. Here’s the abstract from part one:

The early history of lymphatic anatomy from Hippocrates (ca. 460–377 B.C.) to Eustachius (1510–1574). The presence of lymphatic vessels and lymph nodes was reported by ancient anatomists without any accurate knowledge of their true functions. Lymph nodes were described as spongy structures, spread over the whole body for the support of vulnerable body parts. Digestion was explained as being the resorption of clear chyle from digested food by the open endings of chyle vessels. The first insights into the place of lymphatic components within nutrition emanated from the medical school of Alexandria (fourth century B.C.) where vivisection was a common practice. Herophilus and Erasistratus described mesenteric veins [relating to the mesentery, a fold of membrane that attaches the intestine to the abdominal wall] full of clear liquid, air or milk. For Galen of Pergamum, (104–210) mesenteric lymph nodes also had a nutritional function. He described three different types of mesenteric vessels, namely, the arterial vessels, for the transport of spirituous blood to the intestines; the venous side branches of the portal vein, for the transport of nutritive blood from the liver to the intestines; and small vessels, from the intestines to the mesenteric lymph nodes (serous lymph vessels?). According to Galen, chyle was transported via the above-mentioned mesenteric venous vessels from the intestines to the portal vein and liver, where it was transformed into nutritive blood. This doctrine would be obliterated in the seventeenth century by the discovery of systemic circulation and of the drainage of chyle through a thoracic duct to the subclavian veins.

Canto: Hmmm. Chyle? Peritoneum? Subclavian?

Jacinta: Chyle’s a milky, fatty fluid (containing lymph), formed in the small intestine during digestion. It flows into those lymph vessels known as lacteals. These are special ‘lymph capillaries’ where the lipids ‘are colloidally suspended in chylomicrons’ My guess is that ‘chylomicrons’ are itty-bitty chyle bits. Colloidal suspension is ‘a stable phase showing little tendency to aggregate and separate from the aqueous phase’, according to ScienceDirect. The peritoneum is ‘the serous membrane that lines the abdominal cavity’. Other serous membranes are the pleura and the pericardium. They are two-layered membranes ‘lubricated by a fluid derived from serum’. The subclavian veins (and arteries) are those running from the neck down the left and right arms.

Canto: Serum?

Jacinta: Comes from the blood, and rich in proteins.

Canto: So it seems that lymph, or the lymphatic system, has a few functions. Three in particular are highlighted by a NIH website relating to cancer. First, it returns interstitial fluid – fluid that leaks from blood capillaries into the spaces between cells – to the venous blood. This is a sort of recycling process – a regular leakage and a regular return. The returned fluid is called lymph. The second function connects it to the digestive system. Fats and fat soluble vitamins are absorbed and transported to the venous circulation. This happens through those aforementioned lacteals. The small intestines are lined with villi, little finger-like projections, in the centre of which are blood capillaries, and lacteals, aka lymph capillaries. The blood and the lymph thus act together, with the blood capillaries absorbing most of the nutrients and the lymph capillaries absorbing the fatty stuff. And this high fat content lymph is called chyle. And the third function – the most well-known function according to my source – is immunological:

Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles. Lymphatic organs contain lymphocytes that destroy invading organisms.

Jacinta: A reasonably good dummies intro to lymph and the lymphatic system, IMHO, and it’s not really surprising that it took a while to work out what it was all about. We certainly don’t know ourselves, but we know a bit more than we did.

Canto: Yes, much more to learn, about lymphoid tissue, capillaries, vessels and that big thoracic duct. And since much of this info comes from the National Cancer Institute (in the US), the connection with cancer, positive or negative, might be worth exploring….


David Bell, The disease of a thousand names, 1991










Written by stewart henderson

March 31, 2023 at 8:30 pm

the amazing physiology of hummingbirds

with 4 comments

The smallest bird on our planet is the bee hummingbird, of Cuba. The average adult weight ranges between 2 and 2.5 grams, with females being slightly larger than males. There are other tiny hummingbirds, including the bumblebee, from Mexico, and the calliope, of Canada and the US. Basically the adults of all these birds weigh little more than a couple of paper clips. Yet, as Jim Robbins reports in The wonder of birds, these featherlight birds are incredibly robust. Calliopes fly from the northern US down to Mexico every winter, often through powerful head-winds and raindrops as big as their ‘eads. They fly back north in spring, early arrivals, living on insects (their principal source of nutrients) until the flowers start blooming (providing nectar, their principal source of energy). It’s an annual journey of nearly 3000 kms.

adult male bee hummingbird

It takes heart to undertake such a journey, and hummingbirds have plenty. The hummingbird heart is the largest of any known animal relative to its size, and its rate has been measured to reach over 1200 beats per minute (in the blue-throated hummingbird). There are some 350 species of hummingbird, all living in the Americas. 

But it’s not just their long-distance flights that astonish, it’s their everyday manoeuvres. They can fly upside-down, change speed and direction smartly, and hover for long periods, even in strong winds, while collecting sweet nectar in vast quantities – as much as 12 times their body weight daily. Their wing-beat speed, which can reach 100 beats per second, is about ten times that of a pigeon, and they have the largest pectorals for their size of any bird. Birds’ pectorals, which power their flight, are always proportionally massive, taking up some 80% of their weight, but hummingbirds are clearly built for flight more than any other, which allows them to remain in the air more or less constantly. ‘It’s their default setting’, says Bret Tobalske of the University of Montana, who studies the mechanics of flight in birds, bats and insects. Tobalske has studied their flight using ultra high-speed cameras and atomised olive  oil illuminated by lasers, so that the revealed air-flow around their wings can help in understanding the mechanical processes involved. He’s also used wind tunnel experiments to investigate how well the birds can withstand wind forces. In a 20mph headwind, they simply increase their wingbeat rate, and can remain hovering for up to an hour and a half. 

calliope hummingbird

Hummingbirds are very trainable and human-friendly, especially if you reward them with sugar water, their favourite energy hit, though the more food is laid on for them the less they’ll visit and pollinate flowers. Their beaks and long tongues are adapted to extracting nectar. The tongues themselves are an extraordinary adaptation. They’re forked at the tip, and when retracted they coil up inside their tiny heads like a garden hose. For years it was thought that the nectar was drawn out of the flowers by capillary action, like a blotter soaks up ink (showing my age), but Margaret Rubega of the University of Connecticut quickly recognised this was a crock, on first hearing of the hypothesis in the 1980s. Capillary action is a slow process, especially with more viscous liquids, but hummingbirds stick their tongues into flowers at a rate of up to 16 times a second. How their tongue works has been revealed by slow-motion photography, another example of technological advances leading to advances in knowledge – though the ingenuity of Rubega and her colleague Alejandro Rico-Guevara in working out the process played a large part. Ed Yong provides a good account here, and the more detailed original paper is also online. The hummingbird’s tongue appears to be a unique evolutionary invention, a bespoke tongue, so to speak. At its tip, where it forks, it curls up at the edges, creating two tubes. Here’s how it works, from Yong:

As the bird sticks its tongue out, it uses its beak to compress the two tubes at the tip, squeezing them flat. They momentarily stay compressed because the residual nectar inside them glues them in place. But when the tongue hits nectar, the liquid around it overwhelms whatever’s already inside. The tubes spring back to their original shape and nectar rushes into them.

The two tubes also separate from each other, giving the tongue a forked, snakelike appearance. And they unfurl, exposing a row of flaps along their long edges. It’s as if the entire tongue blooms open, like the very flowers from which it drinks.

When the bird retracts its tongue, all of these changes reverse. The tubes roll back up as their flaps curl inward, trapping nectar in the process. And because the flaps at the very tip are shorter than those further back, they curl into a shape that’s similar to an ice-cream cone; this seals the nectar in. The tongue is what Rubega calls a nectar trap. It opens up as it immerses, and closes on its way out, physically grabbing a mouthful in the process.

As Rubega and Rico-Guevara suggest in their abstract, such a unique fluid-trapping mechanism may well have biomimetic applications. As the researchers have shown, the tongue mechanism works even after the bird has died, showing that it’s in some sense independent of the bird itself, and requires none of the bird’s energy. 

It shouldn’t be too much of a surprise to find that hummingbirds have the highest metabolism of any creature (excluding insects). Apart from their record heart rate, they take around 250 breaths a minute, even resting – which they rarely do. Their oxygen intake (per gram of muscle) during flight is ten times higher than that of the most elite human athletes, and they get almost all of their energy for this hyperactive life through ingested sugars – compared to a maximum of 30% for humans. They can utilise sugars for flight within 35 minutes of consumption, which requires a very rapid oxidation rate. Though it isn’t precisely known how this rapid oxidation occurs, it does explain how they can maintain flight while feeding – they’re essentially refuelling while in flight. This raises questions, though, about long-haul flights, for example across the Gulf of Mexico – a distance of 800 kms. It appears they’re able to store fat as a fuel reserve, like other migratory birds, thus almost doubling their weight before the big journey. 

Hummingbird songs and calls are highly varied, and some are even ultrasonic – at a frequency above that of human hearing. These may be used to disturb the flight patterns of small edible insects. Most interestingly, neurological and genetic expression studies suggest that they are capable of vocal learning, something rare among birds as well as mammals. Research in this area is something I hope to explore more fully in future posts – it involves brain design, development and epigenetic factors. 

blue-throated hummingbird, a larger species – only the male has the blue throat

A few other interesting points in closing. Hummingbirds do rest at night, and when there’s no available food – they can enter a state something like hibernation, when their metabolism slows almost to a full stop. They can lose about 10% of their body weight during these states. It’s also notable that they have surprisingly long life-spans for such hyperactive creatures.  Average life-spans have been difficult to measure, but individuals of different species have been known to live for eleven or twelve years at least. 

My growing interest in birds and other creatures, especially with regard to intelligence, has inevitably led me to the load of videos available online, displaying all sorts of amazing traits, as well as profound human-animal relations. There are too many to recommend, but I would strongly suggest to any reader that they sample some of them. Watching them is somehow uplifting, and inspires a sense of hope. Life is nothing if not ingenious, even if accidentally. 





Written by stewart henderson

November 15, 2018 at 10:09 am