Objectives


  1. List briefly the main components of blood and outline its functions.
  2. Describe how blood loss stops (haemostasis) after wounding.
  3. Outline the processes by which blood clots (coagulation).
  4. Illustrate how control and abnormalities of clotting are important in medicine.



Blood



Functions



The blood is a liquid connective tissue, and has many important functions. The three main general functions are as follows:
  • Transportation
– Blood transports oxygen and carbon dioxide to and from the respiratory system to the cells of the body. It carries nutrients from the G.I. tract to the body cells and hormones from endocrine glands around the body. It also transports heat around the body, and waste products for elimination from the body.
  • Regulation
– The blood helps maintain homeostasis of all bodily fluids. Body temperature, pH and osmotic potential of all body cells and tissues.
  • Protection
– Blood is able to clot, and thus the cardiovascular system is protected from excessive injury when an injury occurs. The blood is also responsible for transporting white blood cells around the body, and these protect the body from infection by phagocytosis.



Blood Composition



  • Red Blood Cells
  • White Blood Cells
  • Platelets
  • Plasma


If a full blood sample is centrifuged, the sample separates as the heavier elements fall to the bottom of the test tube. The sample forms 3 distinctive layers , the Plasma (55%), the Buffy Coat (~1%), and Red Blood Cells (45%)



The Top layer, the blood Plasma (55%) forms solid clots and a clear liquid, known as Serum if left for enough time. The Plasma thus composes Blood Serum + clotting factors.

The middle layer, known as the Buffy Coat (1%) contains the bloods platelets and leucocytes. In cases of leukemia, the buffy coat layer almost dominates the centrifuged blood sample, showing a massive overproduction of white blood cells. BUFFY COAT =PLATELETS + LEUCOCYTES
Blood.gif
Red Blood Cells make up approximately 45% of the full blood sample by volume. This value is known as the Hematocrit. That is, the HEMATOCRIT = the % of the sample taken up by Red Blood Cells. This value is typically around 45, but may be lower in cases of anaemia






Red Blood Cells (RBCs) Redbloodcells.jpg



Red blood cells, also known as RBCs or erythrocytes, are the most common type of blood cell and are the body's principal means of delivering oxygen from the lungs to body tissues.
RBCs transport oxygen around the body to be distributed through capillary walls for energy production. RBCs consist mainly of haemoglobin, a complex molecule containing heme groups whose iron molecules temporarily link to oxygen molecules in the lungs and release them throughout the body. In doing so the RBCs alter in colour from Scarlet Red (when bound to oxygen) to Dark Blue (When not bound.)

RBCs absorb oxygen in the pulmonary veins through diffusion and thus are biconcave to maximise the surface area over which the diffusion can occur. This biconcave (flattened and depressed in the centre) design also allows RBCs to flex to fit through tiny capillaries, which may be less than 1 RBC wide.
The average life cycle of a red blood cell is 120 days and they are constantly regenerated through erythropoeisis (SEE Block 1 Scenario 5.)
The diameter of a typical human erythrocyte disk is 6–8 µm; they are thus much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups.


Relative Cell Numbers:


Cell

Number per mm3 (microlitre)


Men

Women
RBC
5-6 Million

4-5 Million
White Cells

4,000-11,000

Platelets

150,000-400,000


Thus it is obvious that Red Blood Cells are FAR more abundant than any other cell in the blood. BE WARY OF UNITS!!!





Blood Plasma



Blood Plasma is a watery liquid extracellular matrix that contains several substances:

  1. Water – 91.5% of blood plasma is water; it acts as a solvent and suspending medium for the other components of blood, as well as absorbing, transporting and releasing heat.
  2. Proteins – 7% of plasma is protein. Protein is responsible for exerting colloid osmotic pressure, thus helping to maintain the water balance between the blood and tissues and regulating blood volume. There are three main types of protein in the blood:
    1. Albumins (54%) – Smallest plasma proteins; produced in the liver. Function as transport proteins for several steroid hormones and fatty acids
    2. Globulins (38%) – Also produced by liver. Immunoglobulins fight infection, α and β globulins transport iron, lipids and fat soluble vitamins
    3. Fibrinogens (7%) – Produced by liver. Play a vital role in blood clotting (see question 3 below)
  3. Other solutes – 1.5% of plasma. These include the following:
    1. Electrolytes – Inorganic salts i.e. Na+ ions. Required for maintaining osmotic pressure, and vital functions in cells
    2. Nutrients – Building blocks for cells, metabolites, vitamins and minerals
    3. Gases – Oxygen, carbon dioxide and nitrogen.
    4. Regulatory Substances – Enzymes, hormones and vitamins; these regulate chemical reactions in the blood.
    5. Waste Products – Includes: urea, uric acid, creatine, creatinine, bilirubin and ammonia.

A mnemonic for remembering the components of plasma (excluding water, as it is 95% of it, you shouldn't need help remembering!!) is:

All - Albumins
Goats - Globulins
From - Fibrinogens
England - Electrolytes
Need - Nutrients
Garish - Gases
Red - Regulatory Substances
Waistcoats - Waste Products

Formed Elements of blood are those which are actually bound by a plasma membrane themselves. These elements thus consist of three principle components:

  1. Red Blood Cells – these make up the largest proportion of the formed elements of the blood, with approximately 40% of total blood volume (haematocrit) consisting of RBCs.
  2. White Blood Cells – These cells are responsible for the immune response in the body (see scenario 6). They are in fewer numbers than the RBCs with approximately 5-10,000 cells per μL of blood.
  3. Platelets – These are cell fragments which are involved in the process of haemostasis.



2. Describe how blood loss stops (haemostasis) after wounding

Haemostasis is a sequence of responses which stops bleeding in the body. When blood vessels are damaged, then the response must be swift, localized to the region of damage, and carefully controlled in order to be effective. In the human body three mechanisms reduce blood loss:
  1. Vascular Spasm – When arteries or arterioles are damaged, the circularly arranged smooth muscle in their walls contracts immediately. This can reduce blood loss for several minutes or several hours. This allows the other haemostatic mechanisms to go into action. Although the exact cause of vascular spasm is not known, it is probably triggered by damage to smooth muscle, substances released by activated platelets, and reflexes initiated by pain receptors.


2. Platelet Plug Formation – This is the process of blocking the hole in the damaged wall of a blood vessel and it occurs in three main stages (diagram below):

  1. Initially, platelets contact and stick to parts of a damaged blood vessel, such as collagen fibres of the connective tissue underlying the damaged endothelial cells. This process is known as platelet adhesion.
  2. Due to adhesion, the platelets become activated, and their characteristics change dramatically. They extend many projections that enable them to contact and interact with one another, and they begin to liberate the contents of their vesicles. This phase is known as the platelet release reaction. (Liberated ADP and thromboxane A2 play a major role by activating nearby platelets. Serotonin and thromboxane A2 function as vasoconstrictors, causing and sustaining contraction of vascular smooth muscle, which decreases blood flow through then injured vessel.)
  3. The release of ADP makes other platelets in the area sticky, and the stickiness of the newly recruited and activated platelets causes them to adhere to the originally activated platelets. This gathering of platelets is called platelet aggregation.
Eventually, the accumulation and attachment of large numbers of platelets form a mass called the platelet plug.

platelet_plug_formation.jpg


3.Blood Coagulation/Clotting - This is the most complex mechanism in the prevention of blood loss from the body, and is also the most effective. It can however, be simplified into three main parts: