Kidney Function, Blood Composition, and Circulatory System Basics

How Kidneys Work: Filtration and Waste Elimination

Solid waste products need to be dissolved in water to be eliminated from the body.

The Nephron: The Kidney's Functional Unit

Each kidney contains over a million functional units called nephrons, where the filtering process takes place.

Parts of the Nephron

• Bowman’s Capsule: Surrounds a tiny network of capillaries.
• The Tubule: Shaped like a handle, several nephrons empty into a large tube called a collecting duct.
• The Collecting Duct: Moves urine into the renal pelvis.

Kidney Function Process

1. Blood moves through the capillary networks where filtration occurs. Small molecules such as water, salts, glucose, and urea exit the bloodstream and enter Bowman’s capsule, forming the filtrate.
2. Some of the molecules it contains are reabsorbed by the nearby capillaries. Glucose and 99% of water return to the bloodstream, while only a small part of urea and salts return to the blood.
3. The result is urine, which will be excreted.

Understanding Blood: A Vital Connective Tissue

Blood is considered a type of connective tissue because its cells are surrounded by abundant liquid extracellular material.

Blood Plasma

Blood plasma is the liquid part, has a yellow appearance, and is mostly water (91%), containing a variety of dissolved molecules.

Blood Cells: Formed in Bone Marrow

Blood cells are formed by red bone marrow, a tissue found within large bones. There are three basic types:

• White Blood Cells (Leukocytes)

  • Contain a nucleus.
  • Approximately 8,000 per mm³ of blood.
  • Lifespan: Approximately 1 day.

• Platelets (Thrombocytes)

  • Cell fragments that form small groups.
  • Approximately 250,000 per mm³ of blood.
  • Lifespan: Approximately 1 week.

• Red Blood Cells (Erythrocytes)

  • Do not have a nucleus.
  • Have the appearance of doubly-concave sacs.
  • Approximately 5 million per mm³ of blood.
  • Lifespan: Approximately 4 months.

Functions of Blood Components

• Plasma: Transports nutrients and waste.
• Red Blood Cells: Contain hemoglobin, which transports oxygen.
• White Blood Cells: Protect the body from infection.
• Platelets: Help to stop bleeding.

The Blood Vessels: Pathways of Circulation

Blood vessels are passageways through which the blood travels. All vertebrates have a closed circulatory system.

Types of Blood Vessels

• Arteries

  Carry blood from the heart to the organs. Their walls are strong and elastic as the arterial blood circulates at high pressure.

• Veins

  Carry blood back to the heart from the organs. They have thinner walls than arteries and are not under high pressure.

• Capillaries

  Have a small diameter, in some places less than a hair's width. They form dense networks within the organs and have extremely thin walls, allowing substances to be exchanged between the blood and the cells.