Body membranes and tissue repair

Serous membranes (the pleura, pericardium and peritoneum) line closed body cavities. Their parietal and visceral layers slide on a thin film of serous fluid.

• A tube that opens to the outside of the body:
• Correct: A closed body cavity, such as the pleural cavity:
• The space inside a single cell:

Fibroblasts make the fibers and ground substance of connective tissue, including collagen. Macrophages engulf debris by phagocytosis, and adipocytes store fat.

• Macrophage:
• Correct: Fibroblast:
• Adipocyte:

In vasodilation the smooth muscle in the vessel wall relaxes, the vessel widens, and resistance falls, so more blood flows through it.

• Correct: Diameter increases and flow increases:
• Diameter decreases and flow increases:
• Diameter increases and flow decreases:

A lining of a tract open to the outside, with epithelium over a lamina propria of loose connective tissue, is a mucous membrane (mucosa). The goblet cells keep it coated with mucus.

• A serous membrane: A serous membrane lines a closed cavity and has simple squamous epithelium, not columnar epithelium with goblet cells.
• The cutaneous membrane: The cutaneous membrane is the skin, whose surface is keratinized stratified squamous epithelium.
• A synovial membrane: A synovial membrane has no epithelium and lines the space inside a movable joint.
• Correct: A mucous membrane: Correct. Epithelium plus lamina propria, lining a tube open to the outside, is a mucosa.

Histamine opens gaps between the endothelial cells of capillaries and the smallest veins. Water and proteins from plasma leak into the interstitial fluid, and the tissue swells.

• Vasoconstriction of the small arteries nearby: Inflammation causes vasodilation, not vasoconstriction. Narrower arteries would bring less blood and would not cause swelling.
• Fibroblasts laying down collagen at the site: Collagen is laid down days later during repair. It does not explain swelling within an hour.
• Correct: Leakier small vessels letting fluid out into the tissue: Correct. Fluid leaking out of leaky vessels into the interstitial fluid is what makes the tissue swell.
• Mast cells dividing rapidly at the site: Mast cells release histamine within minutes; they do not multiply to cause swelling. The swelling comes from fluid, not extra cells.

• Amount of histamine released by mast cells: no change. The drug acts on the receptor proteins of the target cells, not on the mast cells, so release is unchanged.
• Blood flow into the area: down. With its receptor proteins blocked, histamine causes less vasodilation of the small arteries, so less blood flows in and the area is less red and warm.
• Swelling: down. Histamine can no longer make capillaries and the smallest veins as leaky, so less fluid leaks into the interstitial fluid.

Cardiac muscle regenerates very little because its cells rarely divide. Fibroblasts filled the gap with collagen (fibrosis). Scar tissue holds the wall together but cannot contract, so that region stays still.

• Correct: Scar tissue has replaced the dead heart muscle: Correct. The muscle cells of the heart rarely divide, so the heart repairs by fibrosis, and scar tissue does not contract.
• The dead muscle regenerated but is still immature: Cardiac muscle regenerates very little. Months later, the region is scar, not immature muscle.
• Inflammation is still going on in that part of the wall: Inflammation after an injury like this settles within weeks. A thin, still wall months later reflects scar tissue.
• Apoptosis removed the cells without any repair: Cells that die from lost blood supply die by necrosis, not apoptosis, and the gap they leave is filled by repair.

Swelling, membrane rupture, many neighboring cells dying together, and inflammation around them are the marks of necrosis, cell death caused by injury.

• Apoptosis: In apoptosis the cell shrinks and breaks into sealed pieces, and no inflammation follows.
• Mitosis: Mitosis is cell division, not cell death.
• Regeneration: Regeneration replaces lost cells; it is not a way cells die.
• Correct: Necrosis: Correct. Swollen, ruptured cells with inflammation around them point to necrosis.

Epidermis regenerates well, but it needs an intact framework to rebuild the right shape. In a shallow scrape the deeper connective tissue survives, so stem cells regrow normal skin. In a deep cut the framework is destroyed and collagen fills the gap as scar tissue.

• The deep cut: it forms a larger clot: A bigger clot does not make regeneration more likely. The deep cut destroys the framework, so it is the one that scars.
• Correct: The scrape: its framework survives: Correct. Surviving framework plus dividing stem cells in the epidermis allows regeneration without a scar.
• Both heal alike: skin regenerates well: Both involve skin, but depth decides the outcome. The deep cut loses its framework and scars.
• Neither: skin injuries all heal by fibrosis: Shallow injuries of the epidermis regenerate without scarring. Fibrosis fills only deeper gaps.

The space inside a movable joint such as the knee is lined by a synovial membrane. It is a connective tissue membrane, and its lining cells release the thick, slippery joint fluid.

• A serous membrane made of simple squamous epithelium: Serous membranes line the closed body cavities, such as the pleural and peritoneal cavities, not joint spaces.
• A mucous membrane made of epithelium and lamina propria: Mucous membranes line tracts open to the outside of the body. The space inside the knee is sealed.
• Correct: A synovial membrane made of connective tissue: Correct. A synovial membrane, made of connective tissue, lines the joint and releases its fluid.
• The cutaneous membrane made of epidermis and dermis: The skin covers the knee from outside. It does not line the joint space or release joint fluid.