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

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:

$\text{Hemoglobin} + O_2 \rightarrow \text{Oxyhemoglobin}$

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