Blood - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 3

the first stage of defense against invading pathogens and signals for the development
of adaptive immunity. 2
Blood limits its own loss through damaged vessel walls by blood coagulation,
or clotting (Chap. 9 ). Blood coagulation is a component of hemostasis, i.e.,
cessation of blood flow through a damaged vessel wall. Clot formation covers
the injury site by a platelet and fibrin-containing material to stop hemorrhage and
begin repair of the damaged vessel. Platelets immediately form a plug (primary
hemostasis) that is simultaneously reinforced by fibrin strands, which result from
the coagulation cascade of activation of coagulation factors (or clotting factors;
secondary hemostasis).
Blood contains living cells and plasma (Table 1.1 ). Eight to twelve hours after a
meal, 100 ml of blood contains 19 to 23 g of solids and 77 to 81 g of water. Blood
cells include red blood cells (RBC), or erythrocytes, white blood cells (WBC), or
leukocytes, and platelets, or thrombocytes.
Leukocytes operate in immunity. Five main classes of leukocytes exist: 3 types
of granulocytes that have about the same size — neutrophils, eosinophils, and
basophils — and 2 types of agranular leukocytes — lymphocytes and monocytes.
Leukocyte lifespan in blood is several days.

1.2 Plasma

Plasma constitutes a body fluid subcompartment. The body fluids indeed form
several compartments. The 2 major compartments include intra- and extracellular
fluids. The extracellular fluids comprise: (1) interstitial fluids; 3 (2) plasma; and

2 The detection of both commensal and pathogenic microbes that trigger mutualistic or antagonistic
interactions with host cells relies on host receptors and microbial sensing pathways. Two types
of immunity — innate and adaptive — are used to protect the host from infections. The innate
immune system is genetically programmed to detect invariant features of invading microbes. Non-
specific innate responses are mainly mediated by natural killer and myelomonocytic cells. They are
activated on engagement of pattern recognition receptors expressed by microorganisms and host
cells (Vol. 3 - Chap. 11. Receptors of the Immune System). The initial inflammation (Chap. 11 )
aims at increasing the cytotoxic function of immunocytes and clearance of eliciting agents and,
later, to tissue repair and immunity regression. Cells of the innate immunity — dendritic cells,
macrophages, and neutrophils, among others — trigger specific adaptive immune responses. They
internalize pathogens and dying cells and process them into peptidic antigens that are presented
with major histocompatibility complex (MHC) molecules to conventional T lymphocytes. They
integrate danger and inflammation signals and stimulate proper B- and T-cell responses. Cells of
adaptive immunity and B and T lymphocytes employ antigen receptors that are not encoded in
the germ line, but are generated owing to immunological memory that relies on expansion of T-
and B-cell clones, yield stronger and faster responses after repeated exposure to a given eliciting
antigen, after engagement of B- (BCR) and T-cell (TCR) receptors.
3 The interstitial fluid located between cells corresponds to the interstice or interstitium.