Tissues: Blood (part 1)

A quick bit on beginner's blood:

The average adult should contain around 5-6 litres of blood which transports a range of substances around the body.

Around 55% of blood volume is plasma, a water-based fluid within which various substances such as proteins, nutrients and hormones are dissolved or suspended. Also suspended in plasma are the blood cells of which there are 3 types:

• Erythrocytes - red blood cells responsible for transporting oxygen and carbon dioxide between the lungs and the rest of the body. Erythrocytes are the most numerous of the blood cells with 2 million dying and being replaced every second.
  
• Leukocytes - white blood cells responsible for protecting the body from damage by microbes and other dangerous substances. They come in various forms with different functions. They're less numerous than erythrocytes, but much larger.
  
• Thrombocytes - platelets. Tiny cell fragments which are central to the blood clotting process.

The blood is the primary transport system of the body ensuring every cell receives the oxygen, nutrients and waste removal it needs in order to survive. A network of blood vessels allow blood to be pumped around the body by the heart.

Arteries transport blood away from the heart whilst veins transport blood back towards the heart. Small networks of capillaries are found throughout and these allow for the exchange of substances between blood and body tissues.

At rest the heart should contract between 65 and 75 times per minute pumping blood either to the lungs to be oxygenated or to the rest of the body to transport that oxygen. In this time all blood should have passed through the heart once.

Homeostasis

Homeostasis almost literally means "staying the same". It describes the miraculous talent the body has for maintaining various aspects of the internal environment within very narrow limits so that life may continue unabated.

Such feats of stability include:

Maintaining a temperature range of roughly 36.1°C to 37.5°C when measured orally.
Retaining appropriate concentrations of water in and around cells.
Keeping the pH of bodily fluids balanced.
Maintaining healthy blood pressure...

... and much more.

There are a variety of systems used to achieve the necessary standards, but all can be defined as either a positive feedback mechanism or a negative feedback mechanism. All feedback mechanisms function with at least 3 components:
A receptor which senses a change in the environment (stimulus) such as reduced body temperature.
A control centre such as the hypothalamus which sets limits on what the normal temperature range should be and determines appropriate responses to a change.
An effector such as a muscle which behaves according to signals sent by the control center and begins "shivering".

Negative Feedback Mechanisms:
These are the most common and follow the pattern described above. A stimulus is detected by a receptor which notifies the control centre. The control centre then initiates a series of mechanisms (via effectors) which will eventually reduce or remove the original stimulus once homeostasis is restored.

Positive Feedback Mechanisms:
This system is slightly different and is found only in a limited number of contexts. It involves a constantly increasing response to the stimulus so that levels are temporarily pushed beyond the normal limits. The best example is that of childbirth. After an initial release of the hormone oxytocin, contractions force the baby's head against the cervix. This stimulates stretch receptors in the cervix which in turn prompt the release or more oxytocin so that contractions increase. This cycle of stimulation followed by oxytocin release and contraction continues until the baby is born at which point the source of stimulation has been removed and the cycle stops. Blood clotting is another example of a positive feedback mechanism.


The mechanisms involved in homeostasis are truly remarkable and frequently taken for granted. Minor homeostatic imbalances can give rise to a range of health problems and can be potentially life threatening. Many aspects of health care are aimed at supporting the body's efforts at maintaining homeostasis although it could also be said that many drugs place undue stress upon mechanisms such as those mentioned above causing side effects.

Body complexity levels

Here is a breakdown of the levels of structural complexity within the body from lowest to highest:

• Atoms: The building blocks of life. An atom can be one of 92 different elements. The most common are Hydrogen, Oxygen, Nitrogen and Carbon, the mainstay of organic chemistry.
• Molecules: Made up of groups of atoms.
• Cells: The smallest units of independent living matter. Specialised to carry out particular functions.
• Tissues: Collections of cells with the same specialism.
• Organs: Collections of different tissues which work together to achieve certain outcomes.
• Systems: Groups of organs designed to work together to achieve a particular survival need.
• Human Body: Consists of several interdependent systems.

Anatomy, Physiology and Pathology

This sister-site to the Herbal Matters blog will cover topics relating to Anatomy, Physiology and Pathology as and when I learn (or revise) them.

Anatomy refers to physical structures of the body and their relationship to one another.

Physiology studies the physical processes within the body, the way different parts work and the ways in which they function interdependently.

Pathology is the study of dysfunction or abnormality and the causes of ill-health.