Blood: Plasma, Formed Elements, Circulation, and Capillary Exchange – Key Terms

Key vocabulary terms covering blood components, heart and valve anatomy, circulation pathways, capillary exchange, and pressures that govern fluid movement.

Plasma

The liquid component of blood (~55% of blood volume) that contains water, electrolytes, nutrients, hormones, and plasma proteins.

Albumin

A major plasma protein that helps maintain blood volume and oncotic pressure in the capillaries.

Formed elements

The cellular components of blood: red blood cells (RBCs), white blood cells (WBCs), and platelets (thrombocytes).

Erythrocytes (RBCs)

Red blood cells; biconcave disks containing hemoglobin that transport oxygen and some carbon dioxide.

Hemoglobin

Iron-containing protein in red blood cells that binds oxygen in the lungs and releases it to tissues; also helps carry carbon dioxide back to the lungs.

Iron

Mineral essential for hemoglobin’s ability to bind oxygen; required for proper oxygen transport.

Leukocytes (WBCs)

White blood cells; immune cells that defend against infections; can be granular or agranular.

Granular leukocytes

White blood cells with visible granules (e.g., neutrophils, eosinophils, basophils).

Agranular leukocytes

White blood cells without visible granules (e.g., lymphocytes, monocytes).

Thrombocytes (Platelets)

Small cell fragments that aid in blood clotting and wound repair.

RBC shape – Biconcave

Disc-shaped RBCs with a concave surface on both sides, increasing surface area for gas exchange.

ABO blood groups

Blood types determined by the presence or absence of A and B antigens on RBCs (A, B, AB, O).

Rh factor

An antigen on RBCs; positive if present, negative if absent; combines with ABO typing to determine blood type.

Antigen

A molecule capable of triggering an immune response; on RBCs, A, B, and Rh antigens define blood type.

Pulmonary circulation

Pathway of blood from the right ventricle to the lungs for gas exchange and back to the left atrium.

Systemic circulation

Pathway of blood from the left ventricle through the aorta to the body and back to the right atrium.

Pericardium

The double-walled sac surrounding the heart that provides lubrication and protection.

Tricuspid valve

Valve between the right atrium and right ventricle with three cusps.

Pulmonary valve

Valve between the right ventricle and pulmonary artery; semilunar valve.

Mitral valve

Valve between the left atrium and left ventricle with two cusps.

Aortic valve

Valve between the left ventricle and the aorta; semilunar valve.

Aorta

The main artery that carries oxygenated blood from the left ventricle to the systemic circulation.

Arteries

Blood vessels that carry blood away from the heart; thick-walled and under high pressure.

Arterioles

Small arteries that lead to capillary beds and regulate blood flow into tissues.

Veins

Blood vessels that carry blood toward the heart; typically under lower pressure and often with valves.

Venules

Small veins that collect blood from capillaries and feed into larger veins.

Pulmonary veins

Veins that return oxygenated blood from the lungs to the left atrium.

Capillaries

The smallest blood vessels where exchange of nutrients, gases, and wastes occurs between blood and tissues.

Capillary bed

Network of capillaries within a tissue through which blood flows and exchanges occur.

Capillary sphincter

Circular smooth muscle that constricts or dilates to regulate blood flow into capillaries.

Thoroughfare channel

A central pathway through a capillary bed allowing direct blood flow when full capillary exchange isn’t needed.

Interstitial space

The fluid-filled space between cells where diffusion of nutrients and wastes occurs.

Filtration

Movement of fluid and solutes from capillaries into the interstitial space, driven by hydrostatic pressure.

Reabsorption

Movement of fluid and solutes from the interstitial space back into capillaries, driven by osmotic pressures.

Starling forces

Forces (hydrostatic and oncotic) across capillary walls that determine filtration and reabsorption.

Capillary hydrostatic pressure (CHP)

Pressure exerted by blood inside capillaries pushing fluid outward; promotes filtration.

Plasma oncotic pressure (POP)

Osmotic pressure due to plasma proteins drawing water into capillaries; promotes reabsorption.

Interstitial oncotic pressure (IOP)

Osmotic pressure in the interstitial space drawing fluid out of capillaries.

Interstitial hydrostatic pressure (IHP)

Pressure in the interstitial space that can push fluid back into capillaries.

Hydrostatic pressure

Pressure exerted by fluid (blood) against vessel walls; drives filtration and flow.

Systole

Phase of the cardiac cycle when the ventricles contract; higher blood pressure reading.

Diastole

Phase of the cardiac cycle when the ventricles relax; lower blood pressure reading.

Blood pressure (mmHg)

Hydrostatic pressure of blood in the arteries; typical healthy value around 120/80.

Hypertension

High blood pressure (commonly 140/90 or higher); a risk factor for cardiovascular disease and, in massage, a clinical consideration.

Endothelium

Thin layer of cells lining blood vessels that reduces resistance and aids smooth blood flow.

Elasticity of vessel walls

The ability of arteries to stretch and recoil; decreases with age, contributing to higher blood pressure.