32.4Blood

Blood

In vertebrates, it is considered a connective tissue

Composed of various types of cells suspended in a liquid matrix called the plasma (more on that later)

The average human body contains between 4-6L of it

Average pH of 7.4 in humans

Separation

If a blood sample is taken, the cells can be separated from the plasma by spinning the whole blood in a centrifugewhole blood is blood with all its components

intact

• Formed elements, the cells that occupy 45% of the volume of blood, settle to the bottom and leave a red pellet at the bottom with the plasma suspended above

Plasma Water accounts for 90% of the plasma and has a

variety of solutes dissolved in it Electrolytes- inorganic salts in the blood

present in the plasma in the form of ions The concentration of these ions is important factor in

maintaining osmotic balance between the blood and interstitial fluid

Nerves and muscles require a concentration of key ions in the interstitial fluid in order to function properly, which reflects the concentration in the plasma

The kidney is responsible for maintaining plasma electrolytes at precise concentrations, which is an example of homeostasis

Plasma (cont’d)

Plasma proteins are another important class of solutes and have several functionsAct as buffers to help maintain constant pH Help determine osmotic strength of bloodContribute to blood viscositySome also have specific functions

Some serve as escorts for lipids (remember?)

Plasma (cont’d)

Immunoglobulins are a class of proteins that serve as antibodies that help combat viruses and other foreign agents that invade the body

Fibrinogens are a class of proteins that that serve as clotting agents that help to plug leaks when blood vessels are injuredBlood plasma that has the fibrinogens removed

is called serum

Plasma (cont’d)

Also contains various substances that are traveling from one part of the body to another Nutrients Metabolic waste products Respiratory gases Hormones

NB: blood plasma and interstitial fluid are similar in composition, but blood plasma has a higher concentration of proteins Because capillary walls are not permeable to proteins

Blood Cells

3 classes of cellsred blood cells, which transport oxygenwhite blood cells, which function in

defenseplatelets, which are involved in blood

clotting

Red Blood Cells

Red cells, also known as erythrocytes, are the most numerous blood cellsEach cubic mm of human blood contains

about 5 million of these cells, and there are about 25 trillion of them in the average 5L of human blood

RBC Structure

Structure fits functionBiconcave disc, flatter at the center than at

its edgesNB: mammalian erythrocytes have no

nuclei, which is unusual for living cells (other vertebrate classes have nucleated erythrocytes)

Lack mitochondria and generate ATP only through anaerobic metabolism

RBC Structure (cont’d)

Small -> great for oxygen transportFor oxygen to be transported, it needs to be

able to diffuse across the plasma membrane of the RBC

The smaller the cell, the greater the total area of plasma membrane in a given volume of blood

Biconcave shape also adds to the surface area

RBC: Small but mighty!

Contains 250 million molecules of hemoglobin, a protein containing iron has heme’s that the oxygen bind to

Heme’s are the deep red, nonprotein, ferrous component of hemoglobin

As red blood cells pass through the capillary beds of lungs, gills, or other respiratory organs, oxygen diffuses into the erythrocytes and hemoglobin binds the oxygen

Reversed in the capillaries of the systemic circuit with hemoglobin unloading the oxygen

RBC Life

Formed in the red marrow of bones, esp. the ribs, vertebrae, breastbone, and pelvis

Marrow has stem cells (remember those?) that can develop into any type of blood cell

Red cell production is stimulated by a negative feedback mechanism that is sensitive to the amount of oxygen reaching the tissues via blood

If the tissues aren’t receiving enough oxygen, the kidney secretes a hormone called erythropoietin, which stimulates the production of erythrocytes in the marrow

If blood is supplying more oxygen than the tissues are using, the level of erythropoietin is reduced and RBC production is slowed

RBC end

On average, RBC’s live 3-4 months before they are destroyed by phagocytic cells primarily in the liverThe hemoglobin is digested and the amino

acids are incorporated into other proteins made in the liver

Most of the iron in the hemoglobin is cycled back to bone marrow, where it is reused in erythrocyte production

White Blood Cells

White cells, aka leukocytes, fight infectionsThere are five types: basophil, neutrophil,

eosinophil, lymphocyte, monocyteSome of these are phagocytes, which eat

bacteria and debris from our own dead cellsLymphocytes give rise to the cells that

produce antibodies, the plasma proteins that react against foreign substances

WBC (cont’d)

The leukocytes we see in blood are in transit

White cells spend most of their time outside of the circulatory system, battling pathogens in the interstitial fluid

Lots of white cells, esp lymphocytes, in lymph nodes and other parts of the lymphatic system (more later)

WBC Life

Arise in bone marrow from the same stem cells that can differentiate into erythrocytes

Some lymphocytes, after leaving the marrow, mature in the spleen, thymus, tonsils, adenoids, and lymph nodes – all of which are called lymphoid organs

Normally a cubic mm of human blood has 5000 to 10,000 leukocytes, but this number increases when the body is fighting an infection

Platelets

NB: NOT ACTUALLY CELLSChips of cells about 2-3 µm in diameterNo nucleiOriginate as pinched-off cytoplasmic

fragments of larger cells in the bone marrowEnter the blood and help with clotting

Blood Clotting

Fibrinogen is the inactive form of the sealant our body uses to plug leaks in our vessels

Its active form, fibrin, aggregates into threads that form the fabric of the clot

Clotting begins with the release of clotting factors from platelets and involves a complex chain of reactions

More than 12 clotting factors have been discovered and they are still not fully understood

Blood Clotting (cont’d)

Hemophilia is a disease characterized by excessive bleeding from even minor cuts and bruises Caused by an inherited defect in any step of the clotting

process

Anticlotting factors in the blood normally prevent spontaneous clotting in the absence of injury Sometimes platelets clump and fibrin coagulates within

a blood vessel, blocking the flow of blood This clot is called a thrombus More likely to form in someone with cardiovascular disease

The Lymphatic System

Returns fluid to the blood and aids in body defense Fluids and some blood proteins that leak from the

capillaries into the interstitial fluid are returned to the blood via the lymphatic system

Fluid enters this system by diffusing into tiny lymph capillaries intermingled among capillaries of the cardiovascular system

Once inside the lymphatic system, the fluid is called lymph, with a composition similar to the interstitial fluid

The lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium

The Lymphatic System (cont’d) Lymph vessels, like veins, have valves that prevent the

backflow of fluid toward the capillaries Rhythmic contraction of the vessel walls helps draw fluid

into lymphatic capillaries Like veins, lymph vessels depend mainly on the movement

of skeletal muscle to squeeze fluid toward the heart Along lymph vessels are organs called lymph nodes The lymph nodes filter the lymph and attack viruses and

bacteria Inside a lymph node is a honeycomb of connective tissue

with spaces filled with white blood cells specialized for defense

When the body is fighting an infection, these cells multiply, and the lymph nodes become swollen

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