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Blood and the Cardiovascular System: Structure, Function, and Clinical Relevance

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Components of the Cardiovascular System

Main Components

The cardiovascular system is responsible for transporting blood, nutrients, gases, and wastes throughout the body. Its main components include:

  • Blood

  • Heart

  • Blood Vessels

    • Arteries

    • Arterioles

    • Capillaries

    • Venules

    • Veins

Blood Components

Liquid and Formed Elements

Blood is a connective tissue composed of liquid plasma and formed elements (cells and cell fragments).

  • Plasma: 55% of blood volume; contains proteins and solutes

  • Formed Elements:

    • Erythrocytes (RBCs): 44% (transport respiratory gases)

    • Leukocytes (WBCs): <1% (fight infection)

    • Thrombocytes (Platelets): <1% (help form clots)

Functions of Blood

Key Roles

  • Transport: Moves nutrients, wastes, gases, hormones, and other substances throughout the body.

  • Regulation: Maintains body temperature, pH, and fluid balance.

  • Protection: Provides immunity (WBCs) and blood loss protection (platelets).

General Characteristics of Blood

Normal Values and Properties

Characteristic

Normal Values

Color

Oxygenated = bright red; Deoxygenated = deep red

Volume

About 5L (4-5L females, 5-6L males)

Viscosity

About 5x thicker than water

Temperature

38°C (100.4°F) – warmer than rest of body

pH

About 7.4 (range: 7.35–7.45)

Blood Plasma

Components

  • 92% water

  • 7% proteins

    • Albumins (58%): Maintain osmotic pressure, transport hormones

    • Globulins (37%): Antibodies, transport lipids/hormones/metals

    • Fibrinogen (4%): Blood clotting

    • Regulatory proteins (1%): Enzymes, hormones

  • Solutes: Electrolytes, nutrients, gases, wastes

Function

  • Maintains osmotic pressure (mainly via albumin)

  • Buffers pH

  • Transports nutrients, hormones, and wastes

  • Prevents fluid loss from blood vessels

  • Helps maintain blood volume and BP

Additional info: Excessive fluid in interstitial spaces is called edema.

Hematopoiesis: Formation of Blood Cells

Process Overview

Hematopoiesis is the process of making formed elements (RBCs, WBCs, platelets) in the red bone marrow.

  • All blood cells originate from hemocytoblasts (stem cells)

  • Requires colony-stimulating factors (CSFs) and hormones

  • Thrombopoiesis: Platelet formation

  • Erythropoiesis: RBC formation (stimulated by erythropoietin, EPO)

  • Leukopoiesis: WBC formation

Formed Elements

Erythrocytes (Red Blood Cells)

  • Most numerous blood cells (99% of formed elements)

  • Biconcave discs, lack nucleus when mature

  • Main function: transport oxygen and carbon dioxide

  • Contain hemoglobin (protein that binds O2 and CO2)

Hemoglobin Structure

  • Composed of 4 polypeptide chains (2 alpha, 2 beta)

  • Each chain has a heme group with iron (Fe2+)

  • Each hemoglobin binds 4 O2 molecules

Equation:

Additional info: Carbon dioxide binds to globin chains, not to iron.

EPO Homeostasis

  • Erythropoietin (EPO) is secreted by kidneys in response to low oxygen levels

  • Stimulates RBC production in bone marrow

  • Higher in males due to testosterone

Leukocytes (White Blood Cells)

  • Defend the body against pathogens

  • Classified as granulocytes or agranulocytes

Type

Function

Appearance

% of WBCs

Neutrophil

Phagocytose bacteria

Small, grayish granules

50–70%

Eosinophil

Phagocytose allergens, kill parasites

Reddish, with a bilobed nucleus

1–4%

Basophil

Release histamine (inflammation)

Large, dark granules

0.5–1%

Lymphocyte

Produce antibodies, regulate immune response

Large, central nucleus

20–40%

Monocyte

Phagocytose pathogens, debris

Largest WBC, kidney-shaped nucleus

2–8%

Platelets (Thrombocytes)

  • Cell fragments from megakaryocytes

  • Essential for blood clotting (hemostasis)

  • Form temporary plugs at injury sites

Hemostasis

Process of Stopping Bleeding

Hemostasis is the process that stops bleeding after blood vessel injury. It involves three main steps:

  1. Vascular Spasm: Blood vessels constrict to limit blood loss

  2. Platelet Plug Formation: Platelets stick to exposed collagen fibers and to each other

  3. Coagulation: Fibrin forms a mesh that stabilizes the platelet plug

Additional info: Platelets release chemicals that attract more platelets to the area.

Blood Typing: ABO and Rh Systems

ABO Blood Types

Blood type is determined by antigens (A, B) on the surface of RBCs.

Blood Type

Antigens Present

Antibodies in Plasma

A

A

Anti-B

B

B

Anti-A

AB

A, B

None

O

None

Anti-A, Anti-B

Rh Factor: If present, blood type is positive (+); if absent, negative (–).

Blood Donations

Compatibility depends on antigens and antibodies. Receiving incompatible blood can cause agglutination (clumping) and serious complications.

Recipient Blood Type

Can Receive From

Can Donate To

AB+

All types

AB+

O–

O–

All types

A+

A+, A–, O+, O–

A+, AB+

B+

B+, B–, O+, O–

B+, AB+

AB–

AB–, A–, B–, O–

AB+, AB–

A–

A–, O–

A+, A–, AB+, AB–

B–

B–, O–

B+, B–, AB+, AB–

O+

O+, O–

O+, A+, B+, AB+

Additional info: AB+ is the universal recipient; O– is the universal donor.

Clinical Relevance

Blood Disorders and Transfusion Reactions

  • Anemia: Low RBC count or hemoglobin, leading to reduced oxygen delivery

  • Leukemia: Cancer of WBCs, causing abnormal proliferation

  • Hemophilia: Deficiency of clotting factors, leading to excessive bleeding

  • Transfusion Reaction: Occurs when incompatible blood is transfused, causing agglutination and hemolysis

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